Lycoming General Operations Article

User Manual: Lycoming-General-Operations-Article

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Lycoming Flyer 

 

 

Lycoming has been pleased and impressed by the number of requests for copies of the “Key Reprints”

— pleased by your many favorable comments concerning it, and impressed by the thirst for knowledge

by operators of General Aviation aircraft engines.

In the event a reader perceives a conflict between the content of these articles and the content of the

current manuals, service bulletins or service instructions, the latter items govern, but the reader should

contact Lycoming Service and bring such a conflict to our attention. The service bulletins and parts of the

manuals are FAA approved; these articles are not.

Some of the articles published are based on information contained in Service Bulletins, Service

Instruction, and Service Letters. In addition, other articles are taken from actual field test data car-

ried on by Lycoming personnel. Furthermore, the Lycoming factory is a central collecting agency

on its aircraft engines in the field. Experiences and information from all over the world provide

feedback that is recorded and analyzed. We share this information with our readers in the Flyer

and Key Reprints.

During the preparation process of each article and as each subsequent re-issue updating of information

is part of the procedure. After articles are written, each one is carefully screened and checked by man-

agement, engineering and service personnel to ensure accuracy before being released for publication.

The Flyer does not have an established publishing date for each issue. The latest issue is Number .

The original concept of the Flyer has not changed since the first issue was published in 9. Approximately

8% of the prepared text deals with technical information related to Lycoming engines, and the remaining

% covers general newsworthy items. Distribution of the Flyer is made to owners and operators of

Lycoming powered aircraft, authorized distributors and others who write us and request that we place

them on our mailing list, with all costs of printing and mailing absorbed by Lycoming. It is the intention

of Lycoming to continue to make available service, maintenance and operational data in the Flyer and

Key Reprints to assist the owner and operator in improving their engine’s performance and reliability.

G E N E R A L

Lycoming Flyer

8 L y c o m i n g F l y e r

If you are not sure of the meaning of such terms as normally

aspirated, turbocharged, supercharged or direct-drive

engines, then perhaps you’ll want to read our simplified defini-

tion of them.

The Normally Aspirated Engine is one that is not turbocharged 

or supercharged. If the airplane has a manifold pressure gauge, 

at full takeoff power at sea level on a standard day, it would indi-

cate an MP reading of approximately 29" of Hg. Takeoff power 

at 5,000 ft. density altitude airport would read about 24" MP. 

The normally aspirated engine uses atmospheric pressure and is 

thereby altitude limited.

Direct-Drive Engines are those piston-powered engines where 

the propeller is bolted on the end of the crankshaft, and the prop 

turns at the same speed as the crankshaft.

Geared Engines are usually the higher powered, more com-

plex engines using a reduction gear on the nose of the aircraft, 

and with the prop attached to it. As a result, the prop will turn 

somewhat slower than the crankshaft, resulting in a lower prop 

noise level. When the engine is geared, we precede the engine 

designation with a “G.” Thus a geared, opposed (O) normally 

aspirated Lycoming engine with a 480-cubic inch displacement 

of the cylinders would be designated a GO-480 model.

Turbocharged Engines as manufactured by Lycoming simply 

consist of a turbocharger unit with a small turbine wheel attached 

by a common shaft to a compressor wheel, and utilizes the engine 

exhaust gas by directing it over the turbine wheel to drive the 

compressor. The horsepower loss in operating the turbocharger is 

negligible. Turbocharging can provide greater utility to the piston 

engine by providing sea-level horsepower, in some models, as high 

as 20,000 feet; or it can be used to add horsepower to the engine 

particularly for takeoffs. The faster the engine runs, the more air 

the turbocharger can pack into the cylinder to compensate for the 

thin air of altitude, or to increase the horsepower. Although this 

definition is somewhat oversimplified, it is a basic definition of 

turbocharging of General Aviation power plants.

Where turbocharging is used with a fuel-injected, opposed  

Lycoming  engine  with  a  540  cubic  inch  displacement, 

we  designate  it  as  a  TIO-540  model.  “T”  represents  the  

turbocharging.

Supercharged Engines as manufactured by Lycoming uses a 

compressor wheel to pack air into the cylinders; but the com-

pressor is driven by the crankshaft through an intricate gearing 

system, which takes considerable horsepower from the engine to 

operate. In comparison with a turbocharged engine, it is a medium 

altitude power plant.

Although supercharged engines could be built by Lycoming, 

new aircraft designs during the past 20 or more years have used 

turbocharging instead of supercharging because of the advantages 

that turbocharging offers.

A supercharged, geared, opposed, fuel-injected Lycoming 

engine with cylinders of 540-cubic inch displacement is desig-

nated an IGSO-540 model. “S” represents supercharging.

 

All publications may be ordered through authorized Lycoming 

distributors, or direct from Lycoming. If ordered by mail direct 

from Lycoming, payment in the form of check or money order 

must be submitted with the request as outlined in the latest revi-

sion of Lycoming Service Letter No. 114. Fax or telephone orders 

will be accepted if payment is made with a credit card: American 

Express, Visa and MasterCard are accepted.

The available maintenance publications for Lycoming engines 

and accessories consist of the following:

OPERATOR’S MANUAL

The Operator’s Manual contains information of use to pilots and 

maintenance personnel. It contains engine specifications, inspec-

tion procedures, operational information, and is used in conjunc-

tion with the Pilots Operating Handbook for the aircraft.

OVERHAUL MANUALS

The Overhaul Manual is a guide for major repair of the engine. It 

contains complete disassembly, inspection, repair, reassembly and 

test procedures for the various Lycoming reciprocating engines. 

When used in conjunction with the applicable parts catalog and 

service bulletins, service instructions and service letters, this 

manual provides an authoritative text for complete overhaul of 

the engine. Overhaul Manuals for current Lycoming engines are 

published in loose-leaf format so that revisions may be easily 

inserted.

PARTS CATALOGS

Lycoming Parts Catalogs are illustrated to permit iden-

tification of parts. A referenced numerical index is also 

included. 

SERVICE BULLETINS, INSTRUCTIONS, LETTERS

These publications are issued as required. Service Bulletins are 

mandatory, and require some modification or inspection to be 

accomplished on  the engine within a specified time. Service  

Instructions cover a variety of subjects; such as repair processes, 

modification procedures, inspection procedures and overhaul 

methods. Service Letters are of an informative nature, usually 

pertaining to service policy or vendor products.

SPECIAL SERVICE PUBLICATIONS

These  publications  are  concerned  with  topics  of  general  

interest or subjects that are too lengthy for inclusion in maintenance 

manuals.

ENGINE SPECIFICATIONS AND INSTALLATION

DRAWINGS

These materials are needed by those who are planning to install 

a particular engine model in their aircraft.

Lycoming Flyer 9

OWNER ADVISORY

This notification may be sent  without  charge to applicable  

aircraft owners listed in the FAA database when a mandatory  

Service Bulletin is issued and it appears that it may affect the 

engine in your aircraft.

NOTE — In this publication, all references to maintenance

publications are to the most recent revision.

 

As an  engine builds operating hours  and approaches  TBO, 

which may be  either the manufacturer’s recommended oper-

ating hours, or a calendar year limit before overhaul, the ques-

tion  arises concerning the decision  to either continue flying, 

or top overhaul, major overhaul or exchange engines. Here is a  

quick-reference checklist to help make such a decision, followed 

by a brief explanation of the nine points.

I. Oil consumption — any unusual increase?

II. Engine history and calendar age.

III. How has the engine been operated?

IV. Pilot’s opinion of the engine.

V. Maintenance — what kind has the engine received?

VI. What does the oil filter tell?

VII. What has been the trend in compression checks?

VIII. What do the spark plugs show?

IX. Refer to the engine manufacturer’s service letter for engine 

life and recommended overhaul periods.

Along with the above quick-reference checklist, as an engine 

manufacturer we would like to share our experience with inter-

ested operators by discussing the nine points:

I. OIL CONSUMPTION

The operator and maintenance people should know what has been the 

general history of oil consumption during the life of an engine.

A possible danger signal concerning engine health is a definite 

increase in oil consumption during the recent 25 to 50 hours of 

flight time. The oil screens and filter should be carefully observed 

for signs of metal. Maintenance should also take a good differ-

ential compression check at this time. They should also look in 

the cylinders with a gooseneck light or a borescope to detect any 

unusual conditions in the combustion chamber.

If you haven’t looked at your air filter lately, it would be a good 

idea to carefully inspect it for wear and proper fit. This is all the 

more important when operating in dusty areas, and definitely 

could be a cause of increased oil consumption.

II. ENGINE HISTORY AND CALENDAR AGE

If a power plant has been basically healthy throughout its life, this 

would be a favorable factor in continuing to operate it as the engine 

approached high time. Alternately, if it has required frequent 

repairs, the engine may not achieve its expected normal life. The 

engine logbook should contain this accumulative record.

Another important aspect of an engine’s history would be its 

calendar age. Engine flight time and calendar age are equally 

important to the operator. We have observed that engines infre-

quently flown do tend to age or deteriorate more quickly than 

those flown on a regular basis. Therefore, Lycoming recommends 

both an operating-hour limit and a calendar-year limit between 

overhauls. Service Instruction 1009 gives these recommenda-

tions, but other items in this checklist will help to determine if an 

overhaul or engine exchange is needed before the engine reaches 

these recommended limits.

III. OPERATION

The basic question here would be how the engine has been oper-

ated the majority of its life. Some engines operating continuously 

at high power, or in dusty conditions, could have a reduced life. 

Likewise, if the pilot hasn’t followed the manufacturer’s recom-

mendations on operation, it may cause engine problems and 

reduce the expected life. This becomes a more critical influence 

on a decision in single engine aircraft, and also for single or twin 

engine planes flown frequently at night or in IFR conditions.

IV. PILOT’S OPINION OF THE ENGINE

The  pilot’s opinion  of the power  plant,  based on  experience  

operating  it,  is  another  important  point  in  our  checklist.  

The pilot’s opinion and confidence in the engine is based on 

whether it has been a dependable power plant. If the pilot lacks 

confidence in an engine as it approaches  the  manufacturer’s  

recommended limits, this could be a weighty factor in the deci-

sion to continue flying or to overhaul it. The pilot should consult 

with his maintenance personnel concerning their evaluation of 

the condition of the power plant.

V. MAINTENANCE

Good maintenance should aid in achieving maximum engine life; 

alternately, poor maintenance tends to reduce the expected life. 

We notice among those power plants coming back to the factory 

to be rebuilt or for an overhaul, that the smaller engines in general 

have had less care and attention, and in a number of instances have 

been run until something goes wrong. The higher powered engines 

have generally had better maintenance and show evidence that 

the operators do not wait until something goes wrong, but tend 

to observe the manufacturer’s recommended operating hour or 

calendar limits to overhaul. The engine logbook should properly 

reflect the kind of maintenance provided the engine or engines. 

The technician who regularly cares for an engine will usually have 

an opinion about its health.

 0 L y c o m i n g F l y e r

VI. WHAT DOES THE OIL FILTER TELL?

Clean oil has consistently been an important factor in aiding 

and extending engine life. A good full-flow oil filter has been 

a most desirable application here. When the filter is exchanged, 

ask the mechanic to open it and carefully examine for any for-

eign elements, just as is accomplished at oil change when the 

engine oil screen is also examined for the same purpose. Just 

as the spark plugs tell a story about what is going  on in  the  

engine, so the engine oil screen and the external oil filter tell 

a story about the health of an engine. Whether the engine is 

equipped with an oil filter or just a screen, oil changes should 

have been accomplished in accordance with the manufacturer’s 

recommendations. These oil changes should have been recorded 

in the engine logbook.

If oil is analyzed, it should be done at each oil change in order 

to establish a baseline. Analysis is a tool which only gives useful 

information when a dramatic departure from the established norm 

occurs. (See “Spectrometric Oil Analysis” later in this section.)

VII. COMPRESSION CHECKS

What has been the trend in compression in at least the last two 

differential compression checks? The differential compression 

check is the more reliable type and should be taken on a warm 

engine. If the differential check reveals 25% loss or more, then 

trouble may be developing.

Some operators are confused by the compression check and 

its  application.  A  compression  test  should  be  made  any-

time  faulty  compression  is  suspected,  anytime  the  pilot  

observes a loss of power in flight, when high oil consumption is  

experienced, or when soft spots are noticed while hand pulling 

the prop.

Many maintenance technicians do a compression check at each 

oil change, and it is also considered part of the 100-hour engine 

inspection and the annual inspection. Most experienced mechanics 

feel that the differential compression check is best used to chart 

a trend over a period of flight hours. A gradual deterioration of 

charted compression taken during maintenance checks would be 

a sound basis for further investigation.

VIII. SPARK PLUGS

The spark plugs when removed and carefully observed, tell the 

skilled mechanic what has been happening in the cylinders during 

flight, and can be a helpful factor in deciding what to do with a 

high-time engine:

1. Copper run out and/or lead fouling means excessive heat.

2. Black carbon and lead bromide may indicate low temperatures, 

the type of fuel being used, and possibly excessive richness of 

fuel metering at idle.

3. Oil fouled plugs may indicate that piston rings are failing to 

seat, or excessive wear is taking place.

4. The normal color of a spark plug deposit is generally     

brownish gray.

5.  I n  h ig h  compre ssion  a nd   supe rcha rged  engi ne s,  

a cracked spark plug porcelain will cause or has been caused  

by preignition.

IX. ENGINE MANUFACTURER’S RECOMMENDED

OVERHAUL LIFE

Service  Instruction  1009  is the Lycoming published recom-

mendation  for operating hour and calendar  year limits until 

engine overhaul as they apply to each specific engine model. 

The amount of total operating time on an engine will be a basic 

factor in any decision to either continue flying, change, top or 

major overhaul the power plant. Operators should be reminded, 

however, that the hours of service life shown in the  service 

instruction are recommendations for engines as manufactured 

and delivered from the factory. These hours can normally be  

expected, provided recommended operation, periodic inspections, 

frequent flights and engine maintenance have been exercised in 

accordance with respective engine operator’s manuals.

If an operator chooses to operate an engine beyond the recom-

mended limits, there are factors to consider. The cost of overhaul 

is likely to be greater as engine parts continue to wear, and the 

potential for failure may also increase.

Operators who have top overhauled their engine at some point in 

the engine life invariably want to know if this extends the life of 

the engine. This is an important question. The chances are that 

if the operator applies the checklist we have been discussing and 

comes up with favorable answers to these questions about his 

engine, he can probably get the hours desired — with only a few 

exceptions. But a top overhaul does not increase the official life 

or TBO of the engine.

We are surprised from time to time to have owners tell us they top 

overhauled their engine at some point less than the major overhaul 

life for no reason other than somebody said it was a good idea. 

Unless the manufacturer recommends it, or there is a problem 

requiring a top overhaul, this is a needless cost. If the engine is 

healthy and running satisfactorily, then leave it alone!

One other point deserves attention here; there is no substitute 

or cheap route to safety in the proper maintenance or correct 

overhaul of an engine.

CONCLUSION:

Apply  all of these basic  nine  points concerning your engine 

or  engines  and  then  make  a  decision  whether  to  top  over-

haul,  major  overhaul, exchange  engines  or  continue  flying. 

QUESTION: I  hear  the  term  “Shower  of  Sparks”  relative 

to  ignition  systems.  What  does  this  term  mean  to  pilots? 

 

ANSWER: It means that while the engine is cranking during 

a start, a prolonged series of sparks is jumping the spark plug 

gap as compared to one single spark. This results in improved 

cold-weather starting.

Lycoming Flyer  

QUESTION: During starts with the shower of sparks ignition 

system, I get some pretty severe kickbacks. Why?

ANSWER: The common cause here is that the retard breaker 

doesn’t close, resulting in a start attempt on the advance points. 

A simple check is to run the engine at about 700 RPM and for a 

fraction of a second, hold the starter switch to the start position. 

The tachometer will indicate an immediate drop in RPM if the 

retard points are operating. There will be no drop in RPM with a 

malfunctioning retard breaker.

CAUTION — This check noT recommended on our

direcT-drive engines using auTomoTive-Type sTarTers.

QUESTION: At what RPM should I check my mags?

ANSWER: Where  the  airplane  manual  says.  For  example, 

checking at a lower than indicated RPM may give a higher than 

normal mag drop.

QUESTION: Is the actual mag drop in RPM very important?

ANSWER: No. We are more concerned that the mag drop is 

less than 50 RPM between the two mags and smooth, rather 

than whether it’s 50-75 or 150. Again, it should be within the 

limits as indicated in the manual.

QUESTION: I noticed some of your engines show practically 

no mag drop. Is this normal?

ANSWER: Yes,  but  give  it  a  little  more  time,  and  I  think 

you’ll note some mag drop. However, if in doubt about any 

mag drop, be suspicious of a hot mag. Reduce engine RPM 

to idle, and turn switch to off and see if engine dies out. If it 

keeps running, beware of hot mag. It’s sad but true that we 

will still have people getting hurt due to hot mags.

QUESTION: Can  I  save  the  engine  any  by  using  less  than  

takeoff power?

ANSWER: Indeed  not.  In  fact, harm to  the engine  can  be 

caused by using less than takeoff power.

 

A condensed version of several articles.

TIME BETWEEN OVERHAUL (TBO)

Lycoming publishes, and updates from time to time as needed, 

Service Instruction 1009. This document sets forth factory recom-

mendations for time between overhaul (TBO). Aircraft owners 

often have questions concerning TBO, and the need for engine 

overhaul or replacement. This condensed article will answer many 

of those questions.

Published in Service Instruction 1009 is a table listing recom-

mended operating-hour limits for all Lycoming engine models. 

Some owners are  inclined to  think that the number of hours 

listed is some magic number that an engine is certain to obtain, 

but after which it will immediately fail to operate.  Actually, 

the recommended number is not magic at all. It is the average  

number of operating hours a particular model is expected to 

achieve, but there are many qualifying factors.

The  recommended  operating  hours  (TBO)  for  Lycoming  

aircraft engines only applies to those engines that incorporate 

genuine Lycoming parts. Reliability and average service life also 

cannot be predicted when an engine has undergone any modi-

fication not approved by Lycoming. Other factors affecting the  

operating life of an engine are operating conditions, frequency 

of operation and the manner in which the engine is operated  

and maintained.

OPERATING CONDITIONS

Engines installed in aircraft that are used to apply chemicals (crop 

dusting) may be affected by those chemicals and therefore are 

subject to shorter than normal recommended TBOs. Operation 

in dusty conditions, or in an atmosphere of salt air near the ocean 

are examples of conditions that may have a detrimental affect on 

engine condition, and on the capability of reaching the recom-

mended TBO.

OPERATING TECHNIQUE

Pilot technique is another factor affecting engine life. Following 

manufacturer recommendations could be categorized as good 

operation. On the other hand, complete disregard for these rec-

ommendations could conceivably cause engine destruction in as 

little as 100 hours. Although this would be extreme and unusual, 

pilots who regularly climb at steep angles, make abrupt throttle 

changes, improperly lean the engine in climb, exceed maximum 

specified manifold pressure and/or RPM, chop throttles abruptly 

and let down rapidly causing rapid contraction of metals that have 

been up to operating temperatures are using techniques that may 

shorten TBO.

THE FOUR-STROKE CYCLE

  L y c o m i n g F l y e r

GOOD VS. IRREGULAR MAINTENANCE

Regular maintenance, performed by qualified personnel using 

factory-recommended inspection procedures and intervals, will 

contribute to engine performance and the capability of reaching the 

manufacturer recommended TBO hours. Stretching inspection or 

oil change intervals may create the illusion of saving money, but is 

really false economy. Regular oil changes and scheduled mainte-

nance play an important role in achieving recommended TBO.

FREQUENCY OF FLIGHT

Frequency of flight also plays an important part in the operating 

history of an engine. Engines flown only occasionally and with 

extended periods between oil changes are subject to corrosion 

because of acids that  build up  in the oil and  attack internal 

metallic parts of the engine. Only regular oil changes can elimi-

nate these acids.

Moisture that enters through the breather or exhaust system can 

cause rusting of cylinders and other steel parts. Rings may take 

a set and stick in the groove. Condensation in the magnetos may 

cause shorting of the breaker points. Flying as often as possible to 

bring the engine temperatures up to their normal operating levels 

will help to eliminate moisture. A ground run of the engine only 

is not considered satisfactory. Frequent flights are needed.

The number of hours that need to be flown each month, and 

the length of time between flights cannot be specified for every 

aircraft and engine. Variables such as geographic location and 

local temperature and humidity must be considered. Inactivity 

and time will cause hardening of gaskets, seals and hoses. Long 

periods between flights can be expected to cause excessive wear 

during engine start due to loss of the protective oil film on bearing 

surfaces during the long periods of inactivity. Regardless of the 

operating hours, those engines that have not reached the recom-

mended number of operating hours for TBO in a 12-year period 

must be overhauled or replaced during that twelfth year.

DECISION TIME

The timing of engine overhaul or replacement is sometimes the 

result of government regulations. Anytime regulations are not 

a factor, the engine owner must make the decision to overhaul 

or replace the engine based on knowledge of the engine and the 

conditions under which it has been operated. This decision may 

come before the engine has reached the recommended number of 

operating hours, or in some cases, after that number of operating 

hours has been achieved.

TBO recommendations apply to the engine, and in some cases, 

engine accessories and propellers. There is the possibility that 

certain components such as magnetos, ignition harness, gover-

nors and other engine-driven accessories may require overhaul 

or replacement prior to engine overhaul. This decision, too, is to 

be made by the party responsible for maintaining the engine, or 

by the accessory manufacturer.

NEW ENGINE

A  new  engine  is  a  product  manufactured  by  Lycoming  

containing  all  new  parts  and  accessories,  and  meeting  all 

production test specifications, quality control tests and regu-

lations  necessary to hold and maintain a “production certifi-

cate” issued by the FAA. When this engine has met each of 

these criteria and  is shipped  to an airframe manufacturer, it 

will be subject to further testing during flight test of the aircraft 

for its certification acceptance. When the ultimate  purchaser 

receives the aircraft, it may have also been subjected to ferry 

time. However, all flight testing and ferry time will be logged.  

The user then receives the engine with the full Lycoming new 

engine warranty, accompanied by an Engine Logbook.

REBUILT ENGINE

To the aircraft engine purchaser, the “rebuilt” engine, as provided 

by Lycoming, offers the opportunity to obtain many of the ben-

efits of a new engine, but at a price savings.

A Lycoming factory-rebuilt engine is defined as an aircraft engine 

originally designed and manufactured by Lycoming that has been 

disassembled, cleaned, inspected, repaired as necessary, reas-

sembled, and tested to the same tolerances and limits as a new 

item, using either new parts or used parts that either conform to 

new part tolerances and limits or to approved oversized or under-

sized dimensions. Tolerances and limits established and published 

by Lycoming, and approved rework procedures, are used during 

the rebuilding of the engine so that the engine is brought back to 

zero time. It is important to note that the Lycoming factory is the 

only agency authorized by the FAA to return a Lycoming engine 

to ZERO time. Such engines retain their original serial number, 

but the letter “R” is added preceding the letter “L” on the data 

plate which designates rebuilt by Lycoming.

Thus, a factory-rebuilt engine that has been returned to zero time, 

by Lycoming’s definition, has all the foregoing, plus the fact that 

it is done by Lycoming at its factory, by factory personnel with 

manufacturing and engineering expertise. This factory-rebuilt 

engine must also meet the same production test specifications 

used  for a new  engine. With each Lycoming  factory-rebuilt 

engine, an Engine Logbook is furnished with Lycoming Form 

No. 489 included on the inside of the first page. Additionally, each 

engine released through the rebuilding system is accompanied by 

a maintenance release that refers to the factory order to which all 

work was performed.

OVERHAULED ENGINE

Overhaul  is  a  term  which  certainly  means  different  things 

to  different  people.  When  the  aircraft  owner  has  run  a  

factory-new  engine  to  TBO, and  then  paid for  an  overhaul, 

that  owner  usually  has  expectations  of  running  the  engine 

until the manufacturer’s  recommended TBO has  again been 

achieved.  These  expectations  may  or  may  not  be  realistic 

depending on what the overhauler puts into the overhaul. There 

is no specific definition of  the term  overhaul  in  the Code of 

Federal Regulations for Aeronautics  and  Space (FAR). FAR 

Part 43 states the following  about  Maintenance, Rebuilding 

and Alteration.

a.  Each  person  maintaining  or  altering,  or  performing  

preventive  maintenance,  shall  use  methods,  techniques 

and practices acceptable to the Administrator. The tools,  

equipment  and  test  apparatus  necessary  to  assure  com-

Lycoming Flyer  

pletion of  the work  shall  be in accordance with accepted  

industry  practices. If special equipment or test  apparatus 

is recommended by the manufacturer involved, equipment  

acceptable to the Administrator must be used.

b. Each person maintaining or altering, or performing preven-

tive maintenance, shall do that work in such a manner and use 

materials of such quality, that the condition of the aircraft, 

airframe, aircraft engine, propeller or appliance worked on 

will be at least equal to its original or properly altered condi-

tions (with regard to aerodynamic function, structural strength,  

resistance to vibration and deterioration, and other qualities 

affecting airworthiness).

The Lycoming overhaul manuals, as supplemented by appro-

priate service bulletins, service instructions,  and service let-

ters, clearly  stipulate the work to  be done to accomplish  an 

overhaul. We at Lycoming do not distinguish  between  major 

overhaul  and  overhaul.  We  prefer  to  use  the  one  word, 

overhaul,  because  we  want  it  to  be  as  broad  as  possible.  

Overhaul means the entire engine must be considered part by part 

as per the applicable overhaul manuals. To accomplish this, there 

must be complete teardown so that all parts can be examined. In 

overhaul, there are certain parts that must be replaced, regardless 

of condition as per the overhaul manuals, and Lycoming Service 

Bulletin No. 240. The remaining parts must then be examined as 

required by one or more of the following: (1) visual examination 

for discrepancy, (2) non-destructive testing or other mechanical 

examination, and (3) dimensional checking. At this point, parts 

must conform to the fits and limits specifications listed as part of 

the Lycoming Overhaul Manual, and the Table of Limits.

By means of overhaul manuals for the various engine models, 

Lycoming makes available all maintenance data and informa-

tion  necessary  to maintain, repair or  overhaul engines that 

are in service. In addition, Service Bulletin 240 lists the parts 

that  are recommended for replacement at overhaul.  Service 

Instruction No. 1009 lists the recommended overhaul periods 

for the various engine models. However, the FAA has no spe-

cific requirements that the repair agency must comply with the 

Lycoming manual, or with the applicable service bulletins and 

instructions. The final decision on the type of maintenance or 

repair accomplished is left  to  the  mechanic doing the work. 

When the mechanic signs for the overhaul of an aircraft engine, 

the signature certifies that the work performed using methods, 

techniques and practices acceptable to the FAA Administrator. 

While the factory-remanufactured engine goes back to zero 

time, previous time on an overhauled engine is carried forward 

in the engine logbook.

In  addition  to  manufacturing  and  rebuilding  aircraft  

engines, Lycoming also overhauls engines at the Lycoming fac-

tory. These overhauled engines exceed industry standards for 

quality through the use of genuine Lycoming parts and full fac-

tory support. Although these engines do not become zero-time 

as when rebuilt, a policy of extensive parts replacement ensures 

a quality product which has a parts and labor warranty against 

defects in material and workmanship.

Customers  may  choose  either  a  “custom  overhaul”  or  an  

“exchange overhaul.” Both of these overhauls provide expert 

workmanship and replacement of all the parts recommended  

for replacement in Service Bulletin 240.

The exchange overhaul has the advantage of very short aircraft 

down time. A freshly overhauled exchange engine can be shipped 

to the aircraft owner’s airport for installation, and the old engine 

core can be returned to Lycoming after it has been removed and 

replaced.

The owner who orders a custom overhaul gets his or her own 

engine back after overhaul. The disadvantage is the time required 

to remove the engine, ship it for overhaul, have the overhaul 

completed and then have the freshly overhauled engine returned 

and reinstalled in the aircraft. To some owners, getting their own 

engine back is worth the wait.

TOP OVERHAUL VS. MAJOR OVERHAUL

The industry originated the terms top overhaul and major over-

haul years ago to identify and make a distinction between the 

degrees of work done on an engine. Lycoming defines a top over-

haul as the repair or overhaul of those parts on the outside of the 

crankcase without completely disassembling the entire engine. It 

includes the removal of the cylinders and deglazing the cylinder 

walls, inspection of the pistons, valve operating mechanism, valve 

guides and replacing piston rings. A major overhaul consists of 

the complete disassembly of an engine, its repair, reassembly 

and testing to assure proper operation. Nevertheless, whether the 

work accomplished is a top or major overhaul, Federal Aviation 

Regulations require that it meet regulations, which were quoted 

at the very beginning of this discussion of overhaul.

OVERHAUL — A SUMMARY

To  summarize,  all  engine  overhauls  are  not  the  same.  

The Lycoming  factory-overhauled engine is a  QUALITY  

product which  assures  long-term  reliability  to  meet  your 

expectations. An overhauled engine carries forward all pre-

vious time in the engine logbook; a factory-rebuilt engine 

goes back to zero time. The cheapest overhaul may not be the 

best. There should be no compromise with safety. Lycoming  

provides overhaul manuals and related service publications 

to aid operators in the field to accomplish a top overhaul, 

or the more complete major overhaul; but the overhauling 

agency must comply with the performance rules set forth in 

Section 43 of the Federal Aviation Regulations. Lycoming 

can supply either a custom or an exchange engine overhaul. 

 

Much is heard these days about the use of oil analysis as a tool 

for helping to determine engine condition. However, the vast 

majority of the general aviation public do not understand how 

this tool is to be used. We will attempt here to set forth a brief 

summary of the subject.

Oil analysis is not new, but it came late to general aviation as a 

maintenance tool. The object is to examine oil samples from an 

  L y c o m i n g F l y e r

engine, and break down the sample in parts per million in order 

to determine the internal health of the engine. This is based on 

the fact that all lubricated engine parts wear and deposit a certain 

amount of metallic particles in the oil. The number of particles 

per million of each metal determines the wear pattern for the 

particular engine being analyzed. It is of the utmost importance 

to understand that the result of the analysis is only pertinent to 

the engine being analyzed, although accumulation of data on 

any specific engine series is a basis for establishing standards for 

that series of engine.

The fact that is important is a sharp rise above normal of the 

amount of a particular metal in the oil. It is imperative then to 

build a case history of each engine, wherein a sharp rise in any 

one metal will indicate abnormal engine wear. The analysis can 

also tell you whether the oil contains other liquid contaminants 

such as gasoline or water. Gasoline contamination of the oil can 

result from blow-by from the combustion chamber caused by poor 

combustion, bad timing, improper fuel mixture, worn rings and 

the like. Water contamination is usually restricted to condensed 

vapor, but this vapor combines with the fuel combustion products 

to form harmful metal-attacking acids. Based on this contami-

nation in the oil, the analysis will be able to pinpoint improper 

mixture, poor maintenance, etc.

Lycoming Service Letter No. L171, entitled “General Aspects of 

Spectrometric Oil Analysis,” provides a guide for the use of oil analysis 

in measuring engine health. The information is in general terms since the 

health of each engine must be determined on its own merits.

Differences in manufacturing processes may cause a variation 

in analysis results for different engine models. The amount of 

tin plating, copper plating, nitriding, etc., performed during 

manufacture has  a  definite relationship  to  the  oil  analysis 

reports. It is not uncommon, for example, to see what seems 

to be high copper content early in the life of an engine, only 

to have this content continually decrease as the engine accu-

mulates time, and then disappear altogether. Poor air filter 

maintenance, running the aircraft on the ground with carbu-

retor/ alternate air on, and holes in the air intake system are all 

factors which will allow an engine to ingest dirt and foreign  

matter. The result of this will show up as high iron (cylinder bar-

rels) and chrome (piston rings) content at the next oil analysis.  

Neither time nor space permits us here to list all of the variables 

involved (indeed we do not profess to know them all) but it should 

be obvious to everyone that a continuing history of each engine is 

the only criteria by which its health can be determined.

Remember that several samples taken at the regular oil change 

intervals must be analyzed to determine the normal characteristics 

of an engine, and also remember that the first few samples on 

factory fresh engines will read high as new parts are wearing in 

and conforming to each other.

Excessively heavy wear of internal engine parts will show up as 

traces in parts per million during analysis long before detrimental 

flaking or scoring takes place, and almost always before any 

outward indication of trouble. This initial departure from normal 

is not usually any reason to tear the engine down. An investiga-

tion and timely and appropriate corrective action (replacing the 

air filter, perhaps) by the operator will usually result in trace 

elements returning to normal at the next oil change. If long 

TBOs are to be achieved, it is most important that clean air be  

provided to the engines.

Basically and briefly, that is the oil analysis story. It is a good 

tool if properly used. Like any other tool, it is only one of many 

things that must be used to determine engine health. 

A Flyer reader wrote to express interest in a Lycoming IO-360 

engine. He went on to say that the engine would be used in an 

aircraft capable or unlimited aerobatics. A statement like this 

indicates a need for explanation of the differences between the 

standard Lycoming engine and the aerobatic Lycoming engine. 

Aerobatic flight with a non-aerobatic engine could  result in  

engine stoppage from either fuel or oil starvation.

It  should  first  be  explained  that  unlimited  aerobatic  flight  

implies that the aircraft may be flown in any altitude with no 

limitations. Although an aircraft may have excellent aerobatic 

capability, every aircraft and engine does have limitations which 

must not be exceeded.

Any  engine  which  employs  a  float-type  carburetor  for  fuel 

metering is immediately eliminated from use in a fully aero-

batic aircraft. Inverted flight for more than a few seconds would 

cause the carburetor to stop metering fuel and the engine to stop 

running. While carbureted engines are used in some aircraft with 

limited aerobatic capability, only positive G maneuvers and very 

brief periods of inverted flight are possible.

To operate correctly, an engine must have fuel which is properly 

metered in proportion to the air entering the engine induction 

system. The fuel injector measures air flow and meters fuel to 

the inlet ports of each cylinder. Unlike the carburetor, a fuel 

injector is not affected by unusual aircraft attitudes. Therefore, 

all Lycoming engines that are designed for aerobatic flight are 

equipped with a fuel injector.

Delivery of metered fuel to the combustion chamber is not the 

only challenge addressed in  designing an aerobatic  aircraft 

engine. It is also necessary to provide lubricating oil to many 

points in an operating engine regardless of the aircraft attitude. 

Two different methods have been used to provide oil for aerobatic 

engines manufactured by Lycoming.

The flat,  opposed cylinder aerobatic engines first offered by  

Lycoming were designated AIO-320 or AIO-360. These engines 

were the dry sump type with appropriate oil inlet and outlet con-

nections as well as two crankcase breather connections. Necessary 

lines and an external oil tank with a revolving pickup capable of 

reaching oil in almost any aircraft attitude were then supplied by the 

aircraft manufacturer. This type of installation provided aerobatic 

capability, but it was complicated enough to be very expensive. A 

simpler, more universally usable system was needed.

Most Lycoming engines are termed “wet sump” engines because 

oil is stored internally in a sump at the bottom of the crankcase. 

When the engine is inverted, the oil will be in the top of the crank-

Lycoming Flyer  

case rather than in the oil sump. To maintain a continuous flow 

of oil during inverted flight, an oil pickup line must be provided 

near the top of the engine as well as in the oil sump. Lycoming 

aerobatic engines carrying an AEIO designation use inverted oil 

system hardware to adapt oil pickup lines at the top and bottom 

of the wet sump engine.

This inverted oil system comprises two major components: the 

oil valve and the oil separator. Several other items of hardware 

adapt the system to the Lycoming engine so that oil is available 

to the oil pump in either the upright or inverted position. These 

hardware items include a standpipe in the sump which acts as the 

engine breather during inverted flight, a special adapter or plug at 

the oil sump suction screen, and other hoses and fittings.

In addition to the inverted oil system, Lycoming makes other 

engine modifications to adapt standard engine models to aerobatic 

use. Some models of the AEIO-540 engine have a baffle added in 

the oil sump to eliminate oil loss through the oil separator. Also 

the flow of oil to the oil pickup in the accessory case is limited in 

the inverted position. To improve this oil flow, holes are machined 

in the upper rear wall of the crankcase.

With these changes completed, the engine is capable of inverted 

flight  in  addition  to  normal  upright  flight. Because  the oil  

pickup points are at the top and bottom of the engine, knife-

edge flight or flight at very high up or down pitch angles have 

some limitations; these limitations do not prevent engines from 

being used in aircraft which perform all the maneuvers required 

for international aerobatic competition. Engines built with the 

inverted oil system and incorporating the other modifications 

discussed earlier are certified by the FAA as aerobatic engines.

Aerobatic  engines  subjected  to  the  exceedingly  stressful  

maneuvers developed in recent years are also limited by pos-

sible  damage  to  the  crankshaft  flange.  Lycoming  Service  

Bulletin No. 465 requires periodic inspections of all crankshafts 

installed in aircraft that are used for aerobatics.

The  me a n i ng s   o f   t he  le t t e r s  a nd   nu m b er s   i n  t h e  

Lycoming engine designation are fully explained elsewhere in 

this publication, but the AE part of the AEIO indicates “aer-

obatic engine.” Lycoming is currently producing AEIO-320,  

AEIO-360, AEIO-540 and AEIO-580 aerobatic engines which 

range from 150 to 320 horsepower. One of these models should 

be installed in a general aviation aircraft which is designed for 

aerobatic flight.

Condensed from two articles on this subject

Many  Lycoming  engines  designated  as  low  compression  

engines  were  originally  certified  to  use  Grade  80 aviation 

gasoline. The fuel was rated at 80 octane when the engine was 

leaned for cruise, and at 87 octane when it was set at rich for 

takeoff and climb. This aviation gasoline contained one-half 

milliliter of lead per gallon. Owners of aircraft that use engines 

certified  to  use  Grade  80 fuel  occasionally have  questions 

about the use of higher leaded fuels.

During the mid-1970s, announcement of a single-grade aviation 

fuel for all reciprocating aircraft engines created a furor which 

gradually  faded away  as  pilots  and mechanics became  more 

knowledgeable of the actual effects of using the new fuel, Grade 

100LL. Grade 100LL has two milliliters of lead per gallon and 

is rated at 100 octane when the engine is leaned for cruise, and at 

130 octane when the mixture is set at rich. The fuel is designated 

as “low lead” because the previous fuel with a 100/130 octane 

rating contained twice as much lead, four milliliters per gallon.

For all practical purposes, Grade 80 fuel with one-half milliliter 

of lead has been phased out and is no longer available. Use of 

Grade 100LL fuel in engines certified for 80 octane fuel can 

result  in increased  engine  deposits in  both  the combustion 

chamber and the engine oil. It may require increased spark plug 

maintenance and more frequent oil changes. The frequency of 

spark plug maintenance and oil drain periods will be governed 

by the type of operation. Operation at full-rich mixture requires 

more frequent maintenance periods; therefore, it is important to 

use approved mixture-leaning procedures.

To reduce or keep engine deposits at a minimum when using 

the leaded  fuel  available today,  it  is  essential  that  the  fol-

lowing  four  conditions  of  operation  and  maintenance  are  

applied. These procedures are taken directly from Service Letter  

No. L185.

A. GENERAL RULES

1. Never lean the mixture from full rich during take-off, climb or 

high-performance cruise operation unless the Pilot’s Operating 

Handbook advises otherwise. However, during takeoff from  

high-elevation airports  or during climb at  higher altitudes, 

roughness or reduction of power may occur at full-rich mixture.  

In such a case, the mixture may be adjusted only enough to  

obtain smooth engine operation. Careful observation of tempera-

ture instruments should be practiced.

2.  Operate  the  engine  at  maximum  power  mixture  for  

performance  cruise  powers  and  at  best  economy  mixture 

for  economy cruise  power;  unless  otherwise specified in the  

Pilot’s Operating Handbook.

3. Always return the mixture to full rich before increasing     

power settings.

4. During let-down and reduced-power flight operations, it may 

be necessary to manually lean or leave mixture setting at cruise 

position  prior to  landing.  During  the landing  sequence, the 

mixture control should then be placed in the full-rich position,  

unless landing at high-elevation fields where operation at a lean 

setting may be necessary.

5. Methods for manually setting maximum power or best     

economy mixture.

a.  Engine  Tachometer  —  Airspeed  Indicator  Method:  

The  tachometer  and/or  the  airspeed  indicator  may  be 

used to  locate,  approximately, maximum  power and best  

economy-mixture ranges. When a fixed-pitch propeller is 

  L y c o m i n g F l y e r

used, either or both instruments are useful indicators. When 

the airplane uses a constant speed propeller, the airspeed 

indicator  is useful.  Regardless of the propeller type, set 

the controls for  the desired cruise power as shown  in the  

Pilot’s Operating Handbook. Gradually lean the mixture from 

full rich until either the tachometer or the airspeed indicator 

are reading peaks. At peak indication the engine is operating 

in the maximum power range.

b. For Cruise Power: Where best economy operation is allowed 

by the manufacturer, the mixture is first leaned from full rich 

to maximum power, then leaning is slowly continued until 

engine operation becomes rough or until engine power is 

rapidly diminishing as noted by an undesirable decrease in 

airspeed. When either condition occurs, enrich the mixture 

sufficiently to obtain an evenly firing engine or to regain 

most of the lost airspeed or engine RPM. Some slight engine 

power and airspeed must be sacrificed to gain a best economy 

mixture setting.

c. Exhaust Gas Temperature Method (EGT): Refer to the  

article on this subject in the Operations section of this book.

Recommended fuel management — manual leaning will not only 

result in less engine deposits and reduced maintenance cost, but 

will provide more economical operation and fuel savings.

B. ENGINE GROUND OPERATION

The engine ground operation greatly influences formation of 

lead salt deposits on spark plugs and exhaust valve stems. Proper 

operation of the engine on the ground (warm-up, landing, taxi 

and engine shut-down) can greatly reduce the deposition rate and 

deposit formation which cause spark plug fouling and exhaust 

valve sticking.

1. Proper adjustment of the idle speed (600 to 650 RPM) fuel 

mixture, and maintenance of the induction air system will ensure 

smooth engine operation and eliminate excessively rich fuel/air 

mixtures at idle speeds. This will minimize the separation of the 

nonvolatile components of the high-leaded aviation fuels greatly 

retarding the deposition rate.

2.  The engine  should  be operated  at engine speeds between 

1000 and 1200 RPM after starting and during the initial warm-

up period. Avoid prolonged closed-throttle idle engine speed 

operation (when possible). At engine speeds from 1000 to 1200 

RPM, the spark plug core temperatures are hot enough to activate 

the lead scavenging agents contained in the fuel which retards 

the formation of the lead salt deposits on the spark plugs and  

exhaust valve stems. Avoid rapid engine speed changes after start-

up, and use only the power settings required to taxi.

3. Rapid engine cooldown from low-power altitude changes, low-

power landing approach and/or engine shut-down too soon after 

landing or ground runs should be avoided.

4. Prior to the engine shut-down, the engine speed should be 

maintained between 1000 and 1200 RPM until the operating 

temperatures  have stabilized.  At this time, the  engine speed 

should be increased to approximately 1800 RPM for 15 to 20 

seconds, then reduced to 1000  to 1200  RPM and shut  down  

immediately using the mixture control.

C. LUBRICATION RECOMMENDATIONS

Many of the engine deposits formed by combustion, regardless of 

the lead content of fuel used, are in suspension within the engine 

oil and are not removed by a full-flow filter. When sufficient 

amounts of these contaminants in the oil reach high temperature 

areas of the engine, they can be baked out, resulting in possible 

malfunctions such as in exhaust valve guides, causing sticking 

valves. The recommended periods of 50-hour interval oil change 

and filter replacement for all engines using full-flow filtration 

system and 25-hour intervals for oil change and screen cleaning 

for pressure screen systems must be followed. If valve sticking is 

noted, all guides should be reamed using the procedures stated 

in latest editions of Service Instruction No. 1116 and/or Service 

Instruction No. 1425, and the time between oil and filter changes 

should be reduced.

D. SPARK PLUGS

The fuel management techniques outlined previously will aid 

in minimizing spark plug fouling. Engine operation, spark plug 

selection and spark plug maintenance are all factors that help to 

keep engines operating smoothly with leaded fuels. 

If the magneto check before or after flight reveals any rough-

ness caused by a fouled spark plug, open the throttle slowly and 

smoothly to cruise RPM, and lean the mixture as far as possible 

(yet with a smooth engine). After several seconds leaned, return 

to the proper mixture position for takeoff and recheck the mag-

neto. If two such attempts do not clear the fouled plug, then return 

to the line and report the problem to maintenance.

Spark plugs should be rotated from top to bottom on a 50-hour 

basis, and serviced on a 100-hour basis. If excessive spark plug 

lead fouling occurs, the  selection  of  a  hotter plug  from  the 

approved list in Service Instruction No. 1042 may be necessary. 

However, depending on the type of lead deposit formed, a colder 

plug from the approved list may better resolve  the  problem. 

Depending on the lead content of the fuel and the type of opera-

tion, more frequent cleaning of the spark plugs may be neces-

sary. Where the majority of operation is at low power, such as 

patrol, a hotter plug would be advantageous. If the majority of 

operation is at high cruise power, a colder plug is recommended.

Spark plug fouling is not limited to engines that were certified 

for 80-octane aviation fuel, but which are using the higher leaded 

100-octane gasoline. Therefore, the techniques recommended 

herein for  operation and maintenance  apply to all Lycoming 

piston  engines,  but with emphasis  on  the  80-octane  engine  

using 100-octane fuel.

E. SUMMARY

When Grade 80  aviation gasoline was first  phased out, the 

highly leaded Grade 100 green  fuel was the only alternative 

for some operators. During that period of time, in the middle 

1970s, exhaust valve erosion was a concern for the operators of  

low-compression engines. There are two reasons why this should 

not cause concern today. First, Grade 100LL does not cause this 

problem, and second, the materials used in Lycoming exhaust 

valves are highly resistant to erosion.

Lycoming Flyer  

In addition, Grade 100LL has proved to be a satisfactory fuel for 

all Lycoming reciprocating aircraft engines. The higher octane 

level does not change engine operating temperatures, and engine 

deposits on the spark plugs and in the oil can be managed by using 

the techniques outlined in previous paragraphs.

 

 

With the increase in recent years of the number of fuel injec-

tors on our engines, there have been a number of complaints 

about a mysterious occasional engine “miss” in flight. We have 

been able to verify that the majority of these  complaints on  

fuel-injected  power  plants  are  from  contamination  in  the  

fuel — principally water. We have been telling operators for many 

years that fuel injectors and their systems are more vulnerable 

to contamination than are carburetors. Since water and other 

contaminants collect on the bottom of the airplane fuel tank, it 

also makes good sense not to make a practice of running a fuel 

tank dry.

Careful draining of fuel sumps for water will help alleviate the 

“miss” problem. A sufficient amount of fuel must be drained 

to ensure getting the water. Experience indicates that draining 

should be accomplished before refueling, because fuel servicing 

mixes the water and fuel, and the water may not have settled 

to the bottom of the tank until the airplane is airborne. Learn 

to identify suspended water droplets in the fuel which causes 

the fuel to have a cloudy appearance; or the clear separation of  

water from the colored fuel after the water has settled to the 

bottom of the fuel tank.

 

 

The  Flyer articles  reprinted  here  provide  product   

information. Informed pilots and mechanics contribute 

to safe flying

 

 

Lycoming does not permit the use of any fuel other than those 

specified in our latest edition of Service Instruction No. 1070. 

Although Supplemental Type Certificates (STC) now make the 

use of automotive fuel, which meets minimum specified stan-

dards, legal for use in some aircraft, reciprocating engine manu-

facturers and most major oil companies do not approve. While 

it is true that octane levels appear adequate, these organizations 

are of the opinion that the varying quality control  standards 

applicable to automobile gasoline produce undue risk when it 

is  used in aircraft. Several specific reasons are given for the  

non-approval of automobile fuel:

1. Its use reduces safety. Although an operator may find that 

the engine runs well on a specific grade of auto fuel, there is no 

assurance that fuel from the same tank will be of the same quality 

when purchased the next time. Risk is increased.

2.  Its use can  void warranty, or result  in cancellation  of the  

owner’s insurance.

3. The storage characteristics of automotive fuel are less desir-

able in comparison with the good storage characteristics of avia-

tion gasoline. After several months, stored automotive fuel may 

suffer loss of octane rating, and tends to deteriorate into hard 

starting, along with forming gum deposits that cause sticking  

exhaust and intake valves, and fuel metering problems, resulting 

in rough running engines. The turnover of automotive fuel is so 

fast that long-lasting storage characteristics are not required.

4. The additives in automotive fuels are chemically different 

from those designed for aviation, and contain auxiliary scav-

engers  which  are  very  corrosive,  and  under  continued  use 

can  lead  to  exhaust valve failures.  They also cause rust and  

corrosion  in the internal parts of  the engine.  The  allowable 

additives for aviation gasoline are rigidly tested and controlled. 

There is no uniform control of additives in automotive gasoline.  

Many different additives are used, depending on the fuel manu-

facturer. For example, one fuel company adds a detergent to clean 

carburetors. This additive creates a significant increase in the 

affinity of the gasoline for water which can cause fuel filter icing 

problems in flight if outside temperatures are cold enough.

5. Automotive fuels have higher vapor pressures than aviation 

fuel. This can lead to vapor lock during flight because the fuel 

companies advise that automotive fuels can  have double the  

vapor lock pressures of aviation gasoline, depending on the sea-

sons of the year and the location because of climatic conditions. 

In  addition, automotive fuel also increases the  possibility of  

vapor lock on the ground with a warm engine on a hot day.

6. Although the fuel octane numbers shown on the pump of auto-

motive and aircraft gasolines may be similar, the actual octane 

ratings are not comparable due to the different methods used to 

rate the two types of fuels. Furthermore, aviation gasolines have 

a lean and rich rating, i.e., 100/130, whereas motor gas is not 

tested for a rich rating.

COMBUSTION DEPOSIT FOULING FUNDAMENTALS

 8 L y c o m i n g F l y e r

7. Automotive fuel used in an aircraft engine may lead to destruc-

tive detonation or preignition and potential engine failure at high 

power conditions.

8. Please review the Mo-Gas fuel requirements in your state or 

destination.

SUMMARY:

Auto fuel is now being used as a substitute for Grade 80 aviation 

gasoline under STCs issued by the FAA. Most major oil com-

panies and engine manufacturers continue to recommend that 

aircraft piston engines be operated only on aviation gasoline. 

Deterioration of engine and fuel system parts have been reported 

in aircraft using auto fuel. Operators should consider the added risk 

of using auto fuel in aircraft. Remember — a pilot can’t pull over 

to the side of the road when fuel creates a problem with the engine. 

 

The advent of the lightweight turbocharger has been called the 

shot of adrenaline which the piston engine needed to remain the 

prime method of powering general aviation-type aircraft. Although 

in some respects this may be an overstatement, it does have much 

merit, and it is the lightweight turbocharger that has enabled 

general aviation aircraft to operate above adverse weather in the 

smooth air of the higher altitudes, and to realize that increased true 

air speed is not possible with normally aspirated engines. So this 

is the “why” of turbocharging, and since it is possible that there 

is a turbocharged Lycoming engine in your present or future, we 

are going to review the very basics of turbocharging and bring the 

reader up to the present “state of the art” of it.

The aircraft engine, as any reciprocating engine, is a heat engine 

which derives its power from the burning of a mixture of air 

and fuel, which has been mixed in the proper proportions by a  

fuel-metering device. The amount of power the engine develops 

will be directly proportional to the total mass of air pumped 

through the engine, providing the fuel/air ratio is kept constant. 

This can be varied in a normally aspirated (unturbocharged) 

engine by changing the throttle setting and/or changing the RPM. 

Let us go over that again. Changing the throttle will vary the mani-

fold pressure available to the cylinder during the intake stroke. 

As a result, the cylinder will develop a given amount of power 

on each power stroke. So if we increase manifold pressure to the 

cylinder, we will in turn receive more power from the engine. 

Now if we keep the manifold pressure constant, but increase the 

number of power strokes by increasing the RPM, we will also 

receive more power from the engine. We see that changing either 

the throttle setting (manifold pressure) or the number of power 

strokes per minute (RPM), will result in varying the total air mass 

flow through the engine and will determine the horsepower the 

engine will develop. So in essence, a reciprocating engine is also 

an air pump, and if the fuel/air ratio is kept constant, the power 

developed will vary directly with the mass of air consumed.

We are limited in the speed at which we can operate the engine 

because of engine and prop mechanical limitations. So the only 

other way to change the mass flow is to increase the manifold 

pressure. We all know, however, that as we ascend in altitude, the 

air becomes less dense which reduces the mass flow through the 

engine with the result of a power loss that is proportional to the 

reduced-mass air flow through the engine. You have noted that 

in climb with a normally aspirated engine, it is necessary to keep 

opening the throttle if you are to keep the air speed and the rate 

of climb constant. So we see that if there were a way we could 

put the engine into a container so it could be kept at sea level 

conditions, we could maintain the same performance regardless 

of ambient conditions and altitude.

A long time ago, a smart engineer who was thinking along these 

same lines reasoned that if he built an air pump into the engine that 

could pump the less dense air at altitude up to the same pressure 

he had at sea level, he would be able to maintain sea level horse-

power. So he designed a centrifugal air compressor and placed it 

between the fuel-metering system and the intake pipes. The pump 

consisted of an impeller, diffuser and collector. The impeller was 

driven at about 12 times crankshaft speed, and this high rota-

tional speed imparted a large velocity of energy to the air passing 

through. Now as the fuel/air charge leaves the impeller, it goes to 

the diffuser where vanes smooth out the air flow while allowing 

the mixture to slow down so that the velocity pressure acquired 

from the rapid rotational speed of the impeller is transformed 

into static pressure. This air mass is then stored momentarily and 

equalized in the collector and is then drawn into the cylinders. Our 

engineer now has his air pump, but how is he to drive it? Well, 

he could drive it from the accessory gear train or from the rear 

of the crankshaft, but both of these methods robbed the engine 

of horsepower it could deliver to the propeller. Although super-

chargers for many years have been driven mechanically off the 

crankshaft, our engineer realized he had not reached the ultimate 

in the “state of the art” of supercharging, so he began looking for 

another means of driving his air pump.

Our hypothetical engineer realized that the largest percent of 

energy released by burning the fuel/air mixture was going out 

of the exhaust pipe in the form of heat. Realizing if he could in 

some way harness this wasted energy to drive his air pump, the 

horsepower normally robbed from the engine to drive the impeller 

could be used by the propeller.

We have all seen a windmill turning in the breeze, so our engineer 

rightfully reasoned if he put a turbine wheel in the exhaust stream, 

he could take the hot exhaust gas under pressure and expand 

it as it passed through the wheel to extract energy. He took an 

impeller, connected it by a common shaft to the turbine, and he 

had a means of driving his air pump by energy which was for-

merly going to waste. Supercharging by means of using exhaust 

gases to drive the air pump is called turbocharging. Now our 

engineer had progressed to the point where he required a means 

of controlling his turbocharger. As he climbed in altitude the 

pump must constantly put out a higher pressure ratio in order to 

maintain sea level conditions. He reasoned that if he can dump 

the exhaust gas at sea level through a butterfly valve in a leg off 

the exhaust pipe and ahead of the turbine wheel, he will be able 

to control the amount of energy being used to drive the turbine 

and thus control the speed of the compressor.

Lycoming Flyer  9

The butterfly valve (wastegate) can be positioned by means of 

mechanical linkage, but the disadvantage in this system is that 

the engine can be overboosted, causing detonation and severe 

engine damage if someone “forgets” and leaves the wastegate 

in the closed position. So our engineer was looking for an auto-

matic means for control which would eliminate someone putting 

“Murphy’s  Law” into practice. (Murphy’s Law states that if 

something can be done incorrectly, someone is bound to do it.) 

So he came up with an automatic system that sensed compressor 

discharge pressure and positioned the wastegate to maintain the 

correct manifold pressure.

The system contains a controller, which senses the compressor 

discharge pressure and regulates engine oil pressure used as the 

muscle for the actuator on the wastegate. When the controller calls 

for more compressor discharge pressure, it closes the oil bleed line 

from the wastegate so the wastegate actuator sees higher engine 

oil pressure and thus closes the butterfly. When the compressor 

discharge pressure comes up to the desired control pressure, the 

controller will bleed oil from the wastegate to maintain the cor-

rect butterfly position, which in turn will maintain the correct 

compressor discharge pressure. Now the system is complete and 

automatic and except in cases of poor or abrupt throttle manage-

ment, it does not overboost.

The automatic control system just described is basic, but it is 

the basis for most control systems used on Lycoming engines. 

In another article, we will also talk about the changes required 

in an engine to make it suitable for turbocharging, and the dif-

ference between an engine designed for turbocharging and the 

one that has just had a turbocharger added. We will also discuss 

turbocharging to increase power at sea level instead of only using 

it to maintain sea level pressure at altitude. (See “The Pilot and 

Turbocharging” in the Operation section.)

 

Although Lycoming publication SSP-885-2 covers the latest  

information concerning full-flow oil filters for our engines, we 

feel it is also important to emphasize and explain key aspects of 

the publication to help people in the field. SSP-885-2 is concerned 

with full-flow oil filters in our direct-drive engines, but does 

include one exception, the geared TIGO-541 which powers the 

Piper Pressurized Navajo.

Operators  and  mechanics  must  carefully  read  SSP-885-2  

before handling the various types of Lycoming-approved filters. 

Special note should be made of the differences of installing the 

canister-type vs. spin-on filters. The canister-type is installed 

with the housing not turned, but with an attaching bolt through 

the center of the housing, torqued to 20-25 foot pounds.

The spin-on filter calls for a different installation in that the 

filter housing itself is turned to a torque of 18-20 foot pounds. 

Never exceed the maximum torque limit. Maintenance people in 

the field using both types of filters must be very careful during 

installation of this part.

SSP-885-2 data includes the full-flow spin-on filter which is 

installed as optional equipment on all direct-drive Lycoming 

aircraft engines. Advantages of the spin-on filter include a resin 

impregnated paper that constitutes the filter element, which is  

heat-cured, acid resistant and capable of removing contaminants 

that would be injurious to the engine. The spin-on was designed 

to save weight and also shorten maintenance time, and is avail-

able in long and short sizes. There is no need to replace elements, 

O-rings, and various nuts and bolts and washers, or to clean the 

filtering units.

All models of Lycoming direct-drive engines can be converted 

to use  the  Lycoming-approved full-flow  filter element  or  

full-flow spin-on oil filter; however, before installing, check 

the distance between the firewall and the mounting pad on the 

accessory housing. Do not over-torque the filter at installation. 

After installation of the full-flow filter, always ground run the 

engine before flight and get oil temperature into the bottom of the 

green arc on the gage. After a good runup, shut engine down, and 

inspect the filter area for oil leaks. Also check engine oil level; 

addition of the filter assembly will require adding approximately 

one quart of oil.

Champion and Airwolf both offer a “can opener” service tool for 

the aviation mechanic. The tool easily cuts open the filter without 

contaminating it, so the element can be examined for any signs 

of metal chips indicating engine deterioration.

Before  discarding,  the  filter  elements  should  be  examined 

by unfolding  the pleated  element  and examining the  mate-

rial trapped for evidence of internal engine damage. In new or  

newly overhauled engines,  some small  particles of  metallic 

shaving might be found; these are generally caused during manu-

facture and should not be cause for alarm. However, positive 

evidence of internal engine damage found in the filter element 

justifies further examination to determine the cause.

To examine the cartridge-type filter element, remove the outer per-

forated paper cover, and using a sharp knife, cut through the folds 

of the element at both ends close to the metal caps. For examination 

of the spin-on filter, Champion Tool CT-470 or Airwolf AFC-470 

must be used to cut the top of the can.

Clean engine oil is essential to long engine life. Consequently, 

the quest for better ways to keep the lubricating oil free from 

contaminants is endless.

 

Although knowledge of detonation and preignition may be “old 

hat” to the old timers in aviation, lots of people in our industry 

are still somewhat confused over the difference between the 

two, and what causes either of them.

DETONATION

There is a limit to the amount of compression and the degree of 

temperature rise that can be tolerated within an engine cylinder 

and still permit normal combustion. When this limit is exceeded, 

detonation can take place. Piston engines are vulnerable to  

 0 L y c o m i n g F l y e r

D e t o n a t i o n a n d P r e i g n i -

t i o n

detonation at high power output because combustion temperature 

and pressure are, of course, higher than they are at low or medium 

powers. Leaning the mixture at high power can cause it.

Unless detonation is heavy, there is no cockpit evidence of its 

presence. Light to medium detonation may not cause noticeable 

roughness, observable cylinder head or oil temperature increase, 

or loss of power. However, when an engine has experienced 

detonation, we see evidence of it at teardown as indicated by 

dished piston heads, collapsed valve heads, broken ring lands 

or eroded portions of valves, pistons and cylinder heads. Severe 

detonation can cause a rough-running engine and high cylinder 

head temperature.

PREIGNITION

Preignition, as the name implies, means that combustion takes 

place within the cylinder before the timed spark jumps across 

the spark plug terminals. This condition can often be traced to 

excessive combustion deposits or other deposits (such as lead) 

which cause local hot spots. Detonation often leads to preigni-

tion. However, preignition may also be caused by high power 

operation at excessively leaned mixtures. Preignition is usually 

indicated in the cockpit by engine roughness, backfiring, and 

by a sudden increase in cylinder head temperature. It may also 

be caused by a cracked valve or piston, or a broken spark plug 

insulator which creates a hot point and serves as a glow spot. 

Specifically, preignition is a condition similar to early timing of 

the spark. Preignition is a serious condition in the combustion  

chamber and will cause burnt pistons and tuliped intake valves.

The best temporary in-flight methods for correcting preignition 

and detonation are to reduce the cylinder temperature by retarding 

the throttle, enriching the mixture, opening cowl flaps if avail-

able, or a combination of all of these.

Lycoming Engineering, in their continuing effort to improve our 

engines, developed a better method of manufacturing hardened 

alloy  steel cylinder barrels a number of years  ago by  means  

of a method known as nitriding. Simply described, the addition  

of nitrogen to the surface of an alloy steel produced a hard, wear-

resistant surface. Commercially, the introduction of nitrogen into 

the surface layers of alloy steel is brought about by subjecting 

the practically finished parts to an atmosphere of ammonia gas. 

The process requires special heat treating furnaces which are 

air tight and capable of holding the parts at a high temperature. 

At this heat level (975oF), the ammonia gas flowing into the 

furnace  is  broken  down  into  its  elements  of  hydrogen and 

nitrogen, and this is the source of the nitrogen which penetrates 

the surfaces of the steel. In order  to produce  a  satisfactory  

nitrided surface, the process must be operated for an extended 

period  of time,  generally  from  25  to  80  hours.  Along  with 

cylinder barrels, Lycoming nitrides all  its  crankshafts  and 

some gears.

The nitriding process applied to cylinder barrels has been thor-

oughly service tested in military as well as commercial service. 

After having been  FAA type  tested,  we began production of 

engines using nitrided cylinders in 1960. The service record of 

these cylinders has been excellent. In fact, our management was 

so impressed with the favorable service record of the nitrided 

cylinder that they changed from chrome to nitrided cylinders 

in  all  our  higher  powered  turbocharged  and  supercharged  

engines. Some favorable characteristics of nitrided barrels are  

as follows:

1. Reduced cylinder wall wear - the harder a surface the more 

difficult to wear down.

2. Natural choked barrels provide improved piston ring life due to 

a resulting straight cylinder wall when engine is hot or operating, 

and a better job of sealing.

3. Nitriding permits use of chrome plated piston rings, which are 

more wear resistant and quite compatible with hardened steel.

4. Nitriding  provides  a hardened surface  with an increased  

fatigue strength.

5. It also has the ability to resist softening when excessively heated 

during engine operation.

These worthwhile features in the power plants have meant even 

longer operating life than standard steel barrels, and they mean 

dependability and economy.

Now chrome plated cylinders cannot be obtained from the factory, 

but the identification specification for them has been an orange 

band around the cylinder base, or the equivalent color on the 

edges of the top cylinder head fins between the two valve push 

rod shroud tubes. The color coding for the nitrided cylinders is 

azure blue and will appear in either of the two locations indicated 

above. The band around the cylinder base is used when cylinders 

are painted black as a separate operation prior to engine assembly. 

The color coding on the top edges of the cylinder head fins has 

been used on engines painted all gray after assembly.

From the service standpoint on nitrided cylinders, there

are three methods of handling an engine at overhaul.

1. The barrels can simply be reworked and returned to

service if they are not beyond service limits.

2. The barrel can be reground and then chrome plated, but

the factory has achieved better results with nitriding and

uses method three.

3. The old cylinder may be discarded and replaced with

a new one as in the Lycoming factory overhaul and

rebuild programs.

CHROME VS. NITRIDING

Either method provides a satisfactory hard-wearing surface, but 

application of the chrome plate is a critical operation. Engines 

with chrome or nitride-hardened barrels have a good wear charac-

teristic, and are more rugged than standard steel barrels. In spite 

of hardened barrels, a good, properly maintained air filter is still 

a must. Pistons are moving up and down at a rate of more than 

two thousand times per minute, and when dirt or any abrasive 

is introduced into the combustion chambers, it causes a lapping 

process which rapidly wears metal away. No engine can digest 

dirt and give a satisfactory service life.

Lycoming Flyer  

 

We aren’t attempting to make mechanics out of pilots by writing 

about compression ratio in aircraft engines, but we desire to help 

both groups by providing a simplified description as it relates to 

our engines.

In order to gain a reasonable amount of work from an internal 

combustion engine, we must compress the fuel/air mixture during 

each power stroke. The fuel/air charge in the cylinder can be com-

pared to a coil spring in that the more it is compressed, (within 

limits), the more work it is potentially capable of doing.

Engineering tells us that the compression ratio of an engine is a 

comparison of the volume of space in a cylinder when the piston 

is at the bottom of the stroke to the volume of space when the 

piston is at the top of the stroke. For example, if there are 140 

cubic inches of space in the cylinder when the piston is at the 

bottom and 20 cubic inches of space when the piston is at the top 

of the stroke, the compression ratio would be 140 to 20 or usually 

represented at 7:1. 

Although we can create a more efficient engine by increasing the 

compression ratio, there are limits and a compromise is needed. 

If the pressure is too high, premature ignition will occur and 

produce overheating. Compression ratio is a controlling factor 

in the maximum horsepower developed by an engine, but it is 

limited by present-day fuel grades and the high engine speeds 

and manifold pressures required for takeoff.

Our normally aspirated engines are generally categorized as  

either low-compression or high-compression power plants. In 

surveying the complete range of all Lycoming engine models, 

we note that compression ratios vary all the way from a low 

6.5:1 to a high of 10:1. Engineering has generally established the  

low-compression group as those with a compression ratio of 6.5:1 

to 7.9:1; and the high-compression group from 8:1 and higher.

All  Lycoming  engines  in  the  high-compression  category  

require a minimum of Grade 100LL (blue) or 100/130 (green) 

octane, FAA-approved aviation fuel, and nothing less. With  

high-compression engines we  must stress  the importance  of 

the manufacturer’s recommendations as outlined in the Engine 

Operator’s Manual or in the Pilot’s Operating Handbook. These 

engines require not only the correct fuel, but the proper oil, pre-

cise timing and a good air filter. All are very important in order 

to protect this high-performance power plant.

 

By F. F. Rohm, Chief Qualification Engineer (Ret.)

James Watt,  Scottish physicist, had an engine  problem even  

in 1769. Although steam engines had been invented before he 

was born, they were crude, inefficient machines and only a few 

were in use. So he had, after much experimental work, developed 

a relatively efficient condensing steam engine, the forerunner of 

the present-day type.

Being  a  good  businessman,  Watt  tried  to  sell  his  engine  

to coal mine operators who were then using draft horses to supply 

power to drive the pumps which kept the mines free of water.  

But  the  mine  owners  had  sales  resistance!  They  insisted  

on  knowing  exactly  how  many  horses  each  engine  would 

replace, or, in other words, the horsepower of the engine. How  

much work would his steam engine do? This, then, was James 

Watt’s problem.

Although simple machines such as sailing vessels, windmills and 

waterwheels had been used for centuries, Watt realized that for 

the most part, the majority of work in the world had been done 

by man and his domesticated animals. Work was measured and 

paid for by the day, from “sun to sun.” With the advent of reli-

able clocks, work was then accounted and paid for by the hour. 

Evaluating work by this time method, it was assumed that all men 

and animals could and did perform the same amount of work. 

This was far from being true.

Watt  realized  that  in  order  to  have  his  steam  engine  used 

by  the  coal  mine  operators,  he  would  have  to  answer  their  

questions — “how much work will it do, and how many men and 

horses will it replace?” Since the “power” of one horse was a gen-

erally known and a constant quantity, he would have to determine 

the “power” of his engine in order to compare it with the horses 

which it was to replace. His problem then was to define “power.” 

Power did not mean force. The mine owners cared nothing about 

the force Watt’s engine might exert. They wanted to know how 

fast the engine would pump water out of the mine; in other words, 

how fast will the engine do the work? Simply, that was the defini-

tion of “power.”

The methodical physicist experimenting with draft horses used 

to operate mine pumps found that, on an average, a horse pulling 

with a force equal to a weight of 150 pounds walked 2-½ miles 

LOW COMPRESSION     HIGH COMPRESSION

This interesting article was written for us by Fred Rohm,

who was our Chief Qualification Engineer when he retired.

Fred had a career of 44 years in the industry, with a ma-

jority of those years spent at Lycoming. Most of his career

at Lycoming was as Chief Experimental Engineer, which

establishes his qualifications to author this kind of article.

  L y c o m i n g F l y e r

per hour. Since work is force exerted through a given distance, 

it is measured in terms of feet pounds. Thus, on an average, one 

horse could do work at the rate of 33,000 feet pounds per minute 

or 550 feet pounds per second.

 Watt’s definition for one horsepower, which has now become 

universal, was, therefore, the doing of work at the rate of 33,000 

feet pounds per minute. Today, all conventional power producing 

units are rated on this basis.

The 250-horsepower engine in the modern light plane is capable 

of doing work at the same rate as that of 250 average horses. From 

an interested engineer’s point of view, it is capable of moving 

137,500 pounds of weight one foot in one second. Yet, what a dif-

ference there is in its size and weight (approximately 400 pounds) 

when compared to the horses it replaces!

 

The effect the propeller has on engine operation and on aircraft 

performance is quite significant. Based on questions which have 

been asked by aircraft owners and from experience, there are 

several  areas  of propeller-related information which may be  

of interest.

Aircraft equipped with a fixed-pitch propeller will usually have 

static RPM (full throttle with aircraft standing still) limitations 

and full power in-flight RPM limitations spelled out in the Pilot’s 

Operating Handbook. If static RPM is below the minimum speci-

fied, the engine could be low in power. However, experience has 

shown that this is not always true. Faulty induction air systems 

and/or faulty exhaust systems have been shown to contribute to 

indications of low power. A propeller which is ever so slightly 

less than perfect may cause the static RPM to be outside the des-

ignated full throttle static RPM zone. In addition to these other 

factors, it is not unusual to find a tachometer which is inaccurate. 

If an incorrect static RPM reading is observed during the engine 

check, any one or all of these components could be at fault. 

The tachometer may be the easiest to check as there are hand-held 

devices that quickly give an RPM reading that will verify the 

accuracy of the standard aircraft instrument. Knowing the accu-

racy limits of the aircraft tachometer may eliminate the need for 

further examination of the engine and propeller, or it may confirm 

the need for further troubleshooting. In any case, consider each 

component of the system before blaming low-static RPM reading 

on one of them.

Another aspect of operation with a fixed-pitch propeller came in 

the form of a question from a Lycoming engine owner. He indi-

cated that the propeller provided by the airframe manufacturer 

had been exchanged for a cruise propeller. (This exchange should 

only be done with FAA approval.) With the new cruise propeller in 

use, an increase in fuel usage was soon apparent. Operating costs 

increased, and an explanation was requested.

It is well known that the amount of horsepower taken from an 

engine will have a direct relationship to the amount of fuel used. 

Therefore, it can be deduced that use of the cruise propeller 

increased the horsepower requirement. This deduction deserves 

some additional explanation.

As an example, the standard propeller supplied with an aircraft 

may allow the engine to develop 180 horsepower at 2700 RPM at 

full throttle, in flight at sea level, with a standard temperature. The 

Lycoming O-360-A Series normally aspirated engine illustrates 

this example.

Next,  let  us  assume  that  this  same  engine/propeller  com-

bination  is  operated  at  75%  power  with  a  “best  economy” 

fuel/air  mixture  setting. Again,  assume  sea level  and  stan-

dard temperature  to simplify and standardize the discussion.  

75% power will  require  about  2450 RPM  with a  brake-spe-

cific fuel consumption of .435 pounds per brake horsepower 

hour.  Also,  75%  of  the  180  rated  horsepower  is  equal  to 

135  horsepower.  Fuel  usage  at  this  power  and  mixture  set-

ting  will  be  58.7  pounds  per  hour  or  9.8  gallons  per  hour. 

CUTAWAY OF A FOUR CYLINDER POWER PLAN

Lycoming Flyer  

The  mathematics  to  arrive  at  this  fuel  usage  are  simple: 

 

180 HP X 75% of power = 135 HP

135 HP X .435 BSFC = 58.7 lbs. of fuel

58.7 lbs. of fuel 6 lbs. per gal. = 9.8 gal. per hour

Having made some assessments about what can happen with a 

standard propeller, now we will try to see what happens when 

a cruise propeller is installed in place of the original. The first 

thing we must know about the cruise propeller is that it has more 

pitch than the standard propeller. This means it will take a bigger 

“bite” of air than the original propeller with each revolution. This 

bigger bite of air will have an effect on aircraft performance and 

on how the engine may be operated.

Taking a bigger bite of air increases the resistance to the turning 

propeller. Perhaps it may be easiest to imagine what happens 

by considering your hand when held in the airstream outside 

a moving automobile with  the palm forward as compared to 

having the side of the hand forward. Because of this increased 

resistance, the static RPM will be lower than with the original 

propeller. The same thing will be true when full throttle, in-

flight RPM, is compared to that of the standard propeller at a 

similar altitude and temperature. This will reduce takeoff per-

formance of any aircraft. Using the earlier example, the engine 

was rated at 180 horsepower at full throttle and 2700 RPM. 

Now, in spite of applying full throttle, the increased resistance 

reduces the maximum attainable RPM to something less than 

2700. As a result of not developing the rated 2700 RPM, the 

engine also will not develop the power for which it was rated. 

Since maximum power is less than full rated, aircraft perfor-

mance will suffer. This should be considered before a fixed-

pitch propeller is chosen or exchanged for a different model. 

At this  point, we must return to  the original question. Why 

does the engine require more fuel with the cruise propeller? 

It is an accepted fact that the cruise propeller is more efficient 

for cruise operation, so it would not be unusual to follow this 

line of  thinking. Seventy-five  percent  of  rated  power, using 

the original propeller at sea level and standard temperature, 

required a throttle setting to achieve 2450 RPM. Therefore, 

without more thoughtful consideration, it seems logical  that 

the  cruise  propeller might also be set for  2450  RPM when 

75% power is desired. Of course, there is an increase in per-

formance,  but  this  can  be  attributed  to  the  more  efficient 

cruise propeller. Next comes the realization that the improved 

cruise performance isn’t all efficiency. Instead of 9.8 gallons 

of fuel, the engine is now using a greater amount of fuel per 

hour. For purposes of this illustration, let us assume that the 

number is 11 GPH. By reversing the mathematics used earlier, 

it is  possible to estimate the horsepower  and percentage of 

power actually being used as a result of operating the cruise 

prop  at  2450  RPM  with  a  best  economy  fuel/air  mixture. 

11 GPH X 6 lbs. per gallon = 66 pounds

66 pounds .435 BSFC = 151.7 horsepower

151.7 HP 180 rated HP = 84.3% of power

Assuming a fuel usage of 11 gallons per hour for this problem 

provides a reasonably realistic example of the change that a 

different fixed-pitch propeller might create. It also illustrates 

the need for pilots to change their habits when a propeller is 

changed.  In addition  to the change  of habits, the  discussion 

shows a real need to reevaluate the takeoff, climb and cruise 

performance  of  an  aircraft  if  the  fixed-pitch  propeller  is 

changed for a different model.

Another very important point concerns leaning. Remember that 

Lycoming recommends leaning to best economy only at 75% of 

rated horsepower or less. It is very possible that leaning to rough-

ness or to peak on the EGT gage could cause serious damage if the 

engine is actually producing more than 75% of rated horsepower 

as shown in this illustration.

With this information as background, it is easy to see that set-

ting a desired power with a fixed-pitch propeller can only be 

accomplished if the pilot has a chart that applies to the specific 

aircraft/engine/propeller combination. Although the power chart 

for a new aircraft may come from data obtained by test flying 

with a calibrated torque meter, a fairly accurate chart can be 

derived for any fixed-pitch propeller and engine combination. 

Briefly, this is done by finding the maximum available RPM 

at any particular altitude and applying data from the propeller 

load curve.

To conclude, the purpose of this article is to make readers more 

aware of some operational aspects of the fixed-pitch propeller. 

Usually, it is only necessary to accept the material provided 

by the airframe manufacturer and to use the engine/propeller 

as directed.  If  a propeller change is made,  or on those rare 

occasions  when we question  the power available to the pro-

peller, the material presented here could prove to be helpful. 

From time to time, a field-service report states that an engine has 

damage. After further examination of the engine, this damage 

may be classified as “induced damage.” To clarify what is meant 

by this term, induced engine damage is a failure or unsatisfactory 

condition which results from operational or maintenance practices 

employed after the engine is placed in service. Although there are 

a variety of conditions which may fall into the induced damage 

category, this article will discuss two particular types of failure 

and the circumstances which can induce them.

Examination of an engine that is reported to have low compres-

sion, loss of power, erratic operation, metal contamination or 

even complete engine stoppage may result in a determination 

that pistons are burned or valves stretched. (Stretched valves are 

sometimes said to be tuliped.) These two types of damage can 

be initiated in a number of ways, but the chain of events is often 

the same; detonation is followed by preignition and the engine 

damage has begun. To prevent burned pistons and tuliped (or 

stretched) valves, action must be taken to eliminate the possibility 

of detonation and preignition.

  L y c o m i n g F l y e r

Detonation is a phenomenon which can occur in any internal 

combustion  engine.  The  possibility  of  detonation  cannot  

be completely eliminated. By definition, detonation is a vio-

lent  explosion.  When  used  with  reference  to  a  spark  igni-

tion internal combustion  engine like the Lycoming  aircraft 

piston  engines,  detonation  indicates  abnormal  combustion. 

Essentially,  detonation  is  an  uncontrolled  explosion  of  the 

unburned  gases  in  the  engine  combustion  chamber.  Some 

engines  are  more susceptible to  detonation  than others. For 

example, turbocharged engines are more susceptible than similar  

non-turbocharged models and engines with higher compression 

ratios are more likely to exhibit detonation than engines with 

lower compression ratios.

Detonation may occur in an aircraft engine as a result of main-

taining a manifold pressure that is too high for the specific  

engine speed and mixture setting being used. The engine power 

(i.e., speed and manifold pressure) and mixture settings recom-

mended in the Pilot’s  Operating Handbook  (POH)  for a  par-

ticular aircraft model  have been determined by a detonation 

survey. These  surveys use  special instrumentation to detect 

and record detonation as it occurs. Based on these surveys, the  

detonation-limiting conditions are defined. Data from the sur-

veys indicate that detonation occurs in varying degrees; it is 

sometimes  possible  to operate  an  engine for relatively  long 

periods in the first minor phase of detonation without inducing  

damage. Lycoming  does not recommend or  condone engine 

operation which even approaches conditions which might cause 

detonation.  The  laboratory  quality  equipment  used  for  the  

detonation survey is not practical for use in an aircraft engaged 

in normal flight operations. Without this equipment, the pilot 

may not know that detonation is occurring, and it is impossible to 

establish the fine line between the first phase of minor detonation 

and the detonation magnitude which induces preignition and/or 

engine damage. For this reason, it is imperative that power and 

mixture recommendations of the POH be carefully observed.

Preignition  is  a circumstance that causes destructive engine 

damage and  will be  examined  here  briefly. Most Lycoming 

engines are  designed  for ignition  of  the fuel/air mixture  at 

20 crankshaft angle degrees (CAD) before the piston reaches 

top dead center during the compression stroke. Some engine  

models specify ignition at 18, 23, or 25 CAD before top dead 

center.  If ignition of the  fuel/air mixture occurs before the 

scheduled point in the operational sequence of events, preigni-

tion exists and the compression stroke continues as the burning 

fuel/air mixture is trying to expand. This subjects the combus-

tion chamber and pistons to temperatures and pressures far in 

excess of those experienced during normal combustion. These 

excessive temperatures and pressures cause damage to pistons 

and valves. In some cases, both burned pistons and stretched 

valves will be found  in an engine which has  been subjected  

to preignition.

Considering  the  millions  of  hours  f lown  each  year  in  

piston-powered aircraft, engine damage from detonation and 

preignition is  quite  rare. The  infrequency of this happening 

means little if your engine is the one affected. Therefore, it seems 

appropriate to look more closely at some of the factors which lead 

to detonation and preignition.

The  possibility  of  overboost  is  a  characteristic  of  all  super-

charged  and  turbocharged  engines.  Generally,  overboost 

means the application of manifold pressure which exceeds the 

limit  specified  by  the  manufacturer.  Early  versions  of  the  

manually controlled turbocharger allowed quite a few pilots to 

inadvertently induce damage by overboost. With this system, the  

turbocharger  wastegate  was  normally  left  full  open  for  

takeoff; full throttle would produce 28 to 30" of manifold pres-

sure.  After  takeoff  at  full  throttle,  gradual  closing  of  the  

wastegate would slowly increase turbocharger speed and mani-

fold  pressure  to  maintain  climb  power  to  cruise  altitude  or  

to the critical altitude of the engine. The system worked fine until 

the wastegate was inadvertently left in the closed position. If the pilot 

then applied full throttle for takeoff or a go-round, it could produce 

60" or more of manifold pressure and failure of the engine.

More recent turbocharger installations usually include a pres-

sure relief valve and/or an automatic wastegate control which  

helps to avoid the possibility of overboost. Even with these pro-

tective devices, it is still possible to overboost by rapid throttle 

operation and/or inattention to limiting  manifold pressures  

at low engine speeds.

Automatic controllers may not be capable of preventing over-

boost if full throttle operation is attempted before engine oil 

is warmed up sufficiently. Lycoming Service Bulletin 369F  

addresses the problem of overboost and recommends, depending  

on the severity and duration of the overboost, a log-book entry, 

engine inspection or complete engine overhaul including replace-

ment of the crankshaft.

As  stated earlier, ignition of  the  fuel/air mixture must  take 

place at precisely the right time. A spark plug which has been 

dropped, or damaged in some other way, may induce preigni-

tion by causing a “hot spot” in the combustion chamber which 

self-ignites the fuel/air  mixture. This could also occur from 

use of unapproved spark plugs. Flight with defective magnetos  

or  flight  in  excess  of  certified  aircraft  limits  may  allow  

cross-firing within the magneto, improperly sequenced ignition 

of  the fuel/air  mixture and engine damage. Proper magneto  

engine timing is also an important factor. The timing is affected 

by wear and therefore should be checked and reset at specified 

intervals. Regular, meticulous spark plug and magneto mainte-

nance will help to avoid preignition and possible engine damage 

from these sources.

Although overboost  and incorrect ignition timing  are causes  

of  induced engine damage, this damage can often be attrib-

uted to fuel and the fuel/air mixture. The first problem related 

to fuel  is  simply having  improper fuel  in the aircraft tanks.  

A piston-powered aircraft refueled with jet fuel would have a fuel 

blend with greatly reduced octane level. A piston engine should 

not be started when even small amounts of jet fuel have been 

added to aviation gasoline because engine contamination and 

detonation are likely; attempted flight under these conditions will 

certainly result in destructive detonation and preignition. The use 

of 80 octane aviation fuel in an engine certified for 100 octane 

aviation fuel will produce similar results.

The lubricating oil may be a source of octane reducing fuel con-

tamination. Excessively worn piston rings may allow enough oil into 

Lycoming Flyer  

In some cases, it is possible to repair the engine by removing the 

metal contamination from the engine and oil system, including 

the oil cooler, and by replacing all damaged parts, but often it 

is necessary to replace the entire engine. If an engine is to be 

repaired, it must be remembered that repairing the damage is not 

enough; the cause of the malfunction which induced detonation 

and preignition must also be found and corrected. Did a magneto 

malfunction produce ignition outside the normal firing sequence? 

Were manufacturer-approved spark plugs installed in the engine? 

Did a cracked spark plug induce preignition? Was an approved 

fuel used, and if so, is there evidence of fuel contamination? 

Whatever the malfunction, it must be corrected along with the 

damage or the same problem could reoccur.

To conclude, induced damage in the form of tuliped valves and 

burned pistons can usually be avoided by understanding the 

sequence of events which lead to this form of engine damage. 

Careful attention to detail is required of pilots and maintenance 

personnel. Compared to the expense of repairing or replacing a 

damaged engine, it is worth the time and effort necessary to avoid 

induced engine damage.

We often tend to believe what we know, everyone knows. While 

participating in a flight instructor refresher recently, a young lady 

from Maine provided a reminder that this is often not the case.

This lady and her husband fly in Maine throughout the year. 

During the winter, they and their aircraft are frequently exposed 

to extremely cold temperatures. During the past winter, they had 

an unfortunate experience. The end of the engine breather tube 

froze over, a pressure buildup occurred in the crankcase, and the 

crankshaft nose seal ruptured. The oil leak that resulted covered 

the aircraft with oil from nose to tail. Fortunately, a safe landing 

was made before all oil was lost.

As she related her story, another flight instructor quickly indi-

cated that he had also experienced the same problem several years 

earlier. The safe landings in both cases are good news. The bad 

news is the expense incurred to repair the engine.

An incident like this is preventable, and for that reason, it is 

important that we repeat ourselves from time to time. We should 

not  assume that  everyone knows about the  “whistle slot”  or  

other methods of ensuring adequate crankcase venting.

First, the cause of this incident. Moisture is expelled from the 

engine crankcase through the breather tube which often extends 

through the bottom of the engine cowling into the airstream. 

Under very cold conditions, this moisture may freeze and continue 

a buildup of ice until the tube is completely blocked.

It is normal practice for the airframe manufacturer to provide 

some means of preventing freeze-up of the crankcase breather 

tube. The breather tube may be insulated, it may be designed so 

the end is located in a hot area, it may be equipped with an electric 

heater, or it may incorporate a hole, notch or slot which is often 

the combustion chamber to dilute the fuel/air mixture. The dilution 

will reduce the octane rating of the fuel and can lead to detonation 

and engine damage. While this scenario is not entirely typical of 

the engine that uses large amounts of oil because of worn or broken 

piston rings, it is possible for this situation to occur.

Even the use of 100 octane fuel in an engine in good mechanical 

condition does not  eliminate all  the possibilities of induced 

engine damage. Most engines operated at takeoff power or at  

a power setting in the high cruise range need a relatively rich 

fuel/air mixture to help cool the engine and reduce possibilities  

of detonation. Since lean fuel/air mixtures and high power set-

tings promote detonation, it is recommended that Lycoming 

engines not be leaned at power settings which produce more than 

75% of rated engine power unless this operation is approved in 

the POH. The pilot, by simply leaning the mixture excessively at 

power settings above the cruise ranges, may be responsible for 

inducing the detonation and preignition which leads to tuliped 

valves and burned pistons.

And finally,  a small amount of  dirt in the  fuel system  may 

be responsible for clogging a fuel injector nozzle or nozzles.  

A  partially  clogged  fuel  injection  nozzle  will  reduce  fuel 

flow  to that cylinder and  will  cause a lean  fuel/air mixture.  

A nozzle which is partially clogged in an aircraft that has a pressure  

operated fuel flow indicator will cause that indicator to display 

a higher than normal fuel flow. Leaning in an attempt to correct 

the high indicated fuel flow will result in an even leaner mixture 

in the affected cylinder. Again, it is possible that a burned piston 

or tuliped valve will be the final result.

Understanding  and avoiding  those  factors  which  lead  to in- 

duced engine damage is certainly preferable to the discovery 

of  tuliped  valves  or  burned  pistons  in  your  engine.  This  

entire discussion is aimed at promoting an understanding which 

will allow pilots and maintenance personnel to direct their efforts  

to those elements which will reduce the possibility of induced engine 

damage. Observing the refueling of the aircraft and checking  

the  fuel  system  for  indications  of  contamination  are  

tasks expected of the pilot. Meticulous management of power 

and fuel/air mixture as recommended by the POH is also a pilot  

activity which will reduce the possibility of induced damage.

Maintenance  personnel  play  an  equally  important  role.  

Troubleshooting a fuel-injected engine for rough idle may lead to 

the cleaning or changing of partially clogged fuel injector nozzles. 

Damage could result if the engine were operated at takeoff or 

climb power with reduced fuel flow to one or more cylinders. A 

close check of magneto timing and magneto condition at regular 

inspection intervals will help to ensure the continued satisfactory 

operation of any engine.

There are some “after-the-damage” factors that maintenance 

personnel should consider. Suppose  that  a  power loss has 

been reported. A compression check reveals low compression; 

a stretched or tuliped valve may be found. This is an indica-

tion that the engine has experienced detonation and preignition. 

A borescope examination should be conducted to see if a piston 

has been burned. A burned piston often results in damage to cyl-

inder walls and piston skirts; it also may contaminate the engine 

with metal particles. There is no healing process for this damage. 

  L y c o m i n g F l y e r

called a “whistle slot.” The operator of any aircraft should know 

which method is used for preventing freezing of the breather 

tube, and should ensure that the configuration is maintained as 

specified by the airframe manufacturer.

Because  of  its  simplicity,  the  “whistle  slot”  is  often  used.  

Although the end of the tube may extend into the air stream, a 

notch or hole in the tube is located in a warm area near the engine 

where freezing is extremely unlikely. When a breather tube with 

whistle slot is changed, the new tube must be of the same design. 

Replacing a slotted tube with a non-slotted tube could result in an 

incident like the one described by the lady from Maine.

The Flyer may have carried information on this subject in the 

past, but the reminder from someone who had an unfortunate 

incident prompted this story. Preventing possible freezing of the 

crankcase breather tube by use of a whistle slot or other means is 

an important little detail which all of our readers should be aware 

of. Many may benefit from the knowledge.

If you are told that a cylinder head on your engine should be 

welded because of a crack, think long and hard about the step 

you are about to take. The argument is that it is less expensive to 

recondition a cylinder head than to buy a new one. This is true 

when the only consideration is getting your aircraft back in the 

air as cheaply as possible. For the long run, welding the aluminum 

parts of an aircraft engine to repair cracks may not be a permanent 

solution, and may cause you many headaches.

The recommendation to replace rather than weld is based on 

years  of  experience  and  thousands  of  examinations  in  the  

Lycoming Metallurgical Laboratory. Based on this experience, 

the vast knowledge of the individuals who work in this Met Lab 

should not be ignored. The brief explanation that follows will 

attempt to relate some of this experience and to explain why 

welding is not generally recommended.

Starting with two very basic but critical items in the welding 

process, we look at the material to be welded and the welder. 

The material is an aluminum alloy, and it takes more than just a 

very good welder to successfully weld aluminum. The experience 

in qualifying welders at the factory shows is that only the most 

outstanding can pass the annual FAA qualifying examination for 

Aircraft Certified Welder. Even a good welder may leave tungsten 

in the weld. This causes it to be unsatisfactory.

Even an expert welder with complete knowledge of the aluminum 

alloy material cannot assure a satisfactory weld in a cracked 

engine part. There are many inherent pitfalls over which the 

welder has no control. Cylinder heads are made of aluminum 

alloy. Cracks sometimes occur inside the dome area. The metal-

lurgists consider repair by welding to be absolutely foolish, and 

their experience provides them with good reasons. The surface 

area in the dome is affected by a thermochemical attack — to put 

it simply, corrosion. This corrosion is a form of oxidation that will 

not fuse properly during welding. It is not practical to remove all 

of the corrosion, because this would entail remachining the entire 

interior surface which would change the designed compression 

ratio of the engine. Those who attempt to weld this area make 

an effort to clean the crack thoroughly. Unfortunately, this is of 

little help because the area adjacent to the cleaned-out crack will 

still be affected by thermochemical attack. Because this corrosion 

will not allow the weld to fuse completely, new cracks are almost 

certain to occur.

Thermochemical attack is not something  we can see. To the 

novice, a welded crack in a cylinder head may look great. It 

may not even be possible  to identify the  weld. But when the 

part  is dissected by a metallurgist  for examination  under a  

high-powered microscope, the weld, the poor fusion and cracks 

around the weld are immediately identified.

Based on this explanation, readers should now understand why 

Lycoming does not recommend the welding of cracks in cylinder 

heads. These cracks usually occur as the result of fatigue over 

long hours of use, and the odds of achieving long-term satis-

faction by welding are extremely remote. If you are buying an 

aircraft, watch out for the engine with reconditioned cylinders 

that have been repaired by welding, and if you own an engine 

which has experienced cracks, remember that you probably are 

wasting money by having those cracks welded. Experience has 

shown that replacement of these parts is likely to be most eco-

nomical and is likely to cause fewer headaches over the long run.

After certifying  hundreds of  flat,  opposed cylinder  aircraft 

engines, and after building more than three hundred thousand 

engines for general aviation, Lycoming engineers have learned 

what it takes to produce properly balanced engines. Many engines 

have been tested over the years and a great deal of data is available 

to support the building of engines to factory specifications. To 

put it simply, the subject of engine balance is well understood by 

Lycoming and is a major consideration in the design and manu-

facturer of all Lycoming-Certified engines.

These statements are contrary to what has been implied by several 

advertisements and magazine articles. Some reports in several 

industry publications say your Lycoming-Certified engine would 

be much better if you would just take the time and spend the 

money to have it “custom balanced”. The implication is that the 

manufacturer knows nothing about the importance of balancing. 

This is absolutely false. Lycoming-Certified engines are very 

carefully balanced to the degree that is necessary. They are not 

balanced to a point of absolute perfection because they run at 

relatively low speeds (compared to some automobile engines) and 

therefore do not require the degree of balancing being advocated 

by shops that deal in this specialty.

To justify this excessive emphasis on balancing, the automobile 

racing engine is often cited as another place where balancing 

Lycoming Flyer  

is important. This is like comparing apples to oranges; the two 

types of engines are not the same. A typical Lycoming direct drive 

engine is red-lined at 2700 or 2800 RPM while an auto racing 

engine may operate at more than three times this speed.

In order to delve into the subject a little deeper, several ques-

tions  were  directed  to  the  Senior  Analytical  Engineer  at  

Lycoming. His responses may help those who have doubts about 

the quality of engine balancing done at the factory.

Several  Lycoming engine  components are dynamically bal-

anced. For non-engineers, that means that the proper balance is 

determined while the part is in motion; it is spinning as it would 

be during operation. These parts are the starter ring gear sup-

ports and the crankshaft. In addition, dynamic counterweights 

are statically balanced to control the location of the very critical 

center of gravity.

Balance also includes the control of weight for a  number of 

moving engine components. Items such as piston pins, piston 

plugs, and piston rings are 100% machined to close tolerances 

that provide consistent weights. These are three types of piston 

plug, and although they are interchangeable, each type must be 

used as a set because of their weight differences. The weight of 

dynamic counterweights is also carefully controlled.

A  second  aspect  of  engine  balancing  is  accomplished  by  

matching  some  engine  parts  by  weight.  Pistons  and  con-

necting  rods  are  in  this  category.  Both  of  these  parts  are 

organized in matched sets, by weight, before installation in a  

Lycoming engine. Should it be necessary to replace one of these  

matched  parts  during  the  service  life  of  the  engine,  there 

is a  system to  keep the balance within specified  tolerances.  

(See “Notes on Replacing Rods or Pistons” in the Maintenance 

Section of this book.)

Perhaps the engineering answer to other questions may be enlight-

ening – or to the non-engineers, confusing. The point is, Lycoming 

engineers are well aware of what is needed to make a Lycoming-

Certified engine safe and reliable for the long TBO times that are 

recommended for these engines. Thousands of hours of engine 

testing followed by days and weeks of data analysis provide the 

basis for the Lycoming engine design.

The engineering answer as to why there is a need for matching 

and balancing of engine parts is this: “As a matter of sound design 

practice, matching and balancing components will load crank-

shaft bearings in a predictable manner and reduce the reaction 

loads at the engine mounts.” Just as we suspected – right!

Since those engineers did such a good job on that last question, 

another was  ventured, “Is  there any danger or  problem with 

additional balancing by non-factory activities?” Those of us who 

are pilots will understand some of the response, but it will take 

those who speak the language of engineering to comprehend 

the rest. Here it is: “There are occasions when dynamic balance 

of the prop/engine combination can provide reduced first order 

vibration, but additional internal balancing of Lycoming pro-

duction engines is not required nor recommended. The rotating 

and reciprocating masses of the six and eight cylinder opposed 

engines are inherently balanced. The rotating masses of the four 

cylinder opposed design are balanced. The rotating masses of 

the four cylinder opposed design are balanced. The reciprocating 

masses of the four-cylinder engine are not balanced as a vibratory 

inertia moment at second order exists in the plane of cylinder 

center lines. Matching the weights of components closer will 

not reduce the second order moment. A redesign incorporating 

counter rotating layshafts rotating at twice engine speed could 

be implemented.

“Additional internal balancing contributes little to engine

smoothness, and it may even be harmful when material is

removed from highly stressed parts of the engine.”

To summarize, these are the points we have attempted to com-

municate  by  providing  information  about  the  balancing  of  

Lycoming-Certified engines. First, Lycoming engineers have 

acquired vast amounts of data though years of engine testing. 

This  knowledge  is  used  to  insure  that  Lycoming  Certified  

engines are carefully balanced during manufacture. It is also 

used to formulate a system which allows satisfactory engine bal-

ance to be maintained when weight matched parts are replaced 

during the operational life of the engine. The engine balancing 

done by Lycoming  is part  of  an organized system which is  

intended to provide a high-quality product.

The second point is that additional internal custom balancing, 

by removing material from highly stressed parts is not recom-

mended by Lycoming. Lycoming does currently offer additional 

custom balancing for Non-Certified/Experimental Engines, sold 

through Thunderbolt Engines (Factory Custom-Build Shop), 

but this is accomplished via another method. Due to the sheer 

number of parts available at the factory, Lycoming is able to 

satisfy customers’ close to exact balancing demands by weighing 

and matching parts.

From time to time, there is a question about the advantages of a 

fuel injection system over a carburetor. That is probably the wrong 

way to approach the matter when there is a choice to be made. 

Each of these methods of fuel metering has its own unique set of 

characteristics. It may be helpful to consider the advantages or 

disadvantages of each system.

First, consider why we need a carburetor or fuel injector as a 

part of any engine. Both devices provide a means of delivering a 

metered amount of fuel to be mixed with a measured volume of 

air. This is necessary because combustion can only occur when 

the air/fuel mixture falls  within a given range. The  extreme  

outside limits of this range are approximately 20:1 at the lean end 

and 8:1 on the rich end. For practical purposes, the operational 

air/fuel mixture range for most air-cooled Lycoming engines will 

fall between 16:1 at lean and 10:1 when operating at full rich. 

Obviously, both the fuel injector and the carburetor are capable 

of metering within these limits.

The float type carburetor is a device which mixes fuel with air 

and has been used for many years. It has the advantage of being 

 8 L y c o m i n g F l y e r

relatively simple. There are no diaphragm or springs — in gen-

eral, very few moving parts. Installation on the engine is simple. 

All of this adds up to the significant advantage of being the least 

costly method of fuel metering. One additional item should also 

be considered. The fuel lines to a carburetor are large enough that 

there is little chance of them becoming clogged by the very tiny 

particles of foreign matter that may be found in the fuel.

Along  with  these  advantages,  the  disadvantage  frequently  

attributed  to  the  carburetor  is  its  inherent  capability  for  

developing ice in the vicinity of the throttle plate. For the pilot 

who understands and recognizes carburetor icing, this disadvan-

tage is easily managed since all certified aircraft are required 

to have a carburetor air-heating system which will prevent or 

eliminate icing.

Since  the fuel injector is more  complex  and  expensive  than  

a  carburetor, why  should  it be considered? Because the  fuel  

injector has its own set of advantages which in some cases are 

worth the additional cost.

First, the fuel injector causes air and fuel to be mixed at the 

cylinder intake port. Therefore, the refrigeration-type icing that 

occurs in a carburetor venturi when fuel vaporizes in moist air 

cannot happen when a fuel injector is used for fuel metering. 

Many pilots consider this to be a significant advantage.

The primary characteristic of the fuel injector is improved fuel 

distribution to each cylinder. This feature reduces the possibility 

of one cylinder operating at a very lean air/fuel mixture while 

another may be operating near the rich end of the mixture scale. 

The improved distribution allows leaning that results in slightly 

lower overall fuel consumption. This is of particular value in the 

higher horsepower engines where saving a small percentage of the 

fuel being burned may result in a significant dollar savings.

Finally, the fuel injector will meter fuel regardless of aircraft 

attitude while a float-type carburetor can only operate  in  an 

upright position. This advantage, of operating in any attitude, 

makes the fuel injector an ideal fuel-metering device for the  

engine that is designed for aerobatics.

Questions that  frequently are asked of Lycoming sales per-

sonnel, engineers  and  technical  representatives  indicate that 

among aircraft owners and aviation writers  there is a  myth 

regarding  Lycoming  piston engines.  Many of these individ-

uals assume each Lycoming engine in a series to be essentially 

the same. For example, some believe that all 360-cubic inch  

displacement engines are inherently the same except for dif-

ferences in fuel metering or turbocharging. The idea that these 

engines are the same is false. A few specific examples may help 

to put this myth to rest.

Lycoming builds O-320 engines that produce 150 HP or 160 HP. 

The 150 HP O-320-E series engines operate at a compression ratio 

of 7.0:1. The O-320-D series has high-compression pistons which 

raise the compression ratio to 8.5:1, and increase rated output to 

160 HP. Those who believe that the pistons are the only difference 

in these engines will be disappointed if they plan to upgrade their 

O-320-E to the higher horsepower by simply changing pistons. 

Many models in the O-320-E series were designed for economy. 

Thousands of these low-compression engines were built with plain 

steel cylinder barrels instead of the nitrided barrels used in the  

O-320-D series engines. They also  had two narrow bearings 

instead of one long front main bearing. The engines were certi-

fied at 150 HP and were not intended to withstand the additional 

stress of higher horsepower.

Because of the similarity in designation, it would be easy to 

believe that the O-360-AlA and the IO-360-A1A are the same 

engine  except that  the first engine has  a carburetor  and the 

second a fuel injection system. Here are some features of each 

engine for comparison. The O-360-AlA has a bottom-mounted 

updraft  carburetor,  parallel  valves,  8.5:1  compression  ratio 

and produces 180 HP. The IO-360-AlA features a  horizontal  

front-mounted fuel injector,  angle  valves, 8.7:1 compression  

ratio,  and  is  rated  at  200  HP.  The  IO-360-A1A  incorpo-

rates  additional design  items  which are not included in the  

O-360: piston cooling nozzles, stronger crankshaft, tongue and 

groove connecting rods with stretch bolts, tuned intake system 

and rotator type intake valves. There are actually few similarities 

except for the 360-cubic inch displacement.

There have been suggestions  that  by  putting 10:1  compres-

sion  ratio  pistons in  an IO-360 engine, it could be the same 

as  the HIO-360-D1A. These are some characteristics of  the 

HIO-360-D1A helicopter engine that  can be compared with 

the  data on  the IO-360 listed  in the previous  paragraph. To 

start, the HIO has conical rather than dynafocal mounts. The 

main bearing is a thick-wall bearing instead of the thin-wall,  

high-crush bearing used in the IO-360. Other differences include: 

crankshaft designed for small crankpins, high-speed camshaft, 

rear-mounted RSA7AA1 fuel injector, large intake valves and 

torsional vibration damper magneto drives.

Finally,  both  the  Navajo  engines  and  the  turbocharged  

Lycoming  used  in  the  Mooney  TLS  are equipped  with dif-

ferential  and  density  controllers  that  automatically  set  the 

maximum allowable horsepower when the throttle is advanced 

fully  for  takeoff.  Some  believe  that  the  TIO-540-AF1A 

which powers the Mooney TLS is  simply  a  derated  Navajo  

engine. This conclusion could hardly be more inaccurate. The 

most obvious difference, even to the complete novice can be seen 

by looking at the rocker box covers. The TIO-540-AF1A is rated 

at 270 HP and has parallel valve-down exhaust cylinders. The 

Navajo series has three engines at 310 HP, 325 HP and 350 HP. 

All have cylinders designed with up exhaust and angle valves. 

Other differences respectively in the 270 HP AF1A and  the 

Navajo series engines are: small main bearing instead of large 

main bearing, 8.0:1 compression ratio rather than 7.3:1, inter-

cooled and non-intercooled, pressurized Slick magnetos versus 

Bendix/TCM magnetos and an RSA5AD1 fuel injector in place 

of the RSA10AD1 injector. There are some other differences, but 

those comparisons listed should convince even the most skeptical 

that these engines are vastly different.

Lycoming Flyer  9

Comparing various parts and accessories used in engine models 

which some individuals have considered to be much the same, 

illustrates the differences. Although some Lycoming models 

are closely related, this cannot be  assumed. A review of  the 

engineering parts list for each engine model by a knowledgeable 

individual is the only sure way of establishing similarities and 

differences. Those who may have been taken in by the myth that 

all Lycoming engines of a particular displacement are very much 

the same are now armed with a better knowledge.

There are many who look  for an aircraft engine on  the open 

market. While there  is nothing wrong with this approach to 

acquiring  a  needed  power  plant, it sometimes  results  in an 

unfortunate choice. Perhaps a little information on the possible  

pitfalls may help to reduce the number of bad choices.

Individuals working on home-built aircraft may be particularly 

susceptible to this type of error. At Lycoming, there have been 

many calls from people who grabbed an engine that seemed to 

be an exceptionally good deal — only to find that this “engine of 

their dreams” would not fit into the aircraft they are building.

Consider the circumstances which lead to these problems. The 

person looking for an engine is usually building an aircraft from 

his own plans or from a kit supplied by a kit manufacturer.

As the airframe begins to take shape, obtaining a suitable engine 

may be reason for some concern and anxiety. When a Lycoming 

0-320, 0-360 or other engine with appropriate horsepower rating 

is found, there is a temptation to buy now and ask questions later. 

This could be a serious mistake.

The  article  “Low-time  Engine  May  Not  Mean  Quality  and  

Value” that appears next in this booklet explains that old engines 

with low time are frequently affected by internal rust and corro-

sion. Any engine that is not used frequently should be preserved. 

The condition of the engine is just one of the items to be consid-

ered when acquiring a power plant in the resale market.

Other mistakes often involve the engine model. Unfortunately, 

there are those  who believe that all  Lycoming 0-320  engine 

models are alike, and that all Lycoming O-360 engine models 

are also very similar. The Lycoming-certified aircraft engine 

list shows 58 O-320 models and 51 0-360 models. While these 

engines may be similar in many respects, it is the differences that 

are likely to cause installation problems. These differences should 

be well understood before an engine is purchased.

What are the differences that may cause installation problems? 

The engine-mounts should be considered. Older engine models 

were built with conical mounts that make installation somewhat 

easier, but which do not dampen engine vibration as well. With 

very few exceptions, engines certified during the l970s and 1980s 

have dynafocal mounts.

Although  the  type  of  engine  mount  is  not  likely  to  be  a 

serious problem, the shape of the sump, the location of the  

carburetor or an engine-mounted oil filter may result in airframe 

interference which makes installation of a particular engine 

model difficult or impossible. Some aircraft, for example, do not 

have enough space between the engine and the fire-wall for an  

engine-mounted oil  filter. In  the  case  of  an  engine  with  a  

single-unit dual magneto, there is nothing that can be done since 

the filter is a required part of the engine design. All Lycoming 

engines with two individual magnetos can be configured to 

operate without an oil filter. Should an oil filter and the space 

needed to remove it be the only problem in adapting this type of 

engine to an airframe, the filter and adapter can be removed and 

an oil pressure screen housing can be installed instead. Should this 

step be necessary, the recommended oil change interval is reduced 

to 25 hours. A second option would involve removing the filter 

from its standard location and mounting it remotely.

Engine to firewall is not the only area where space may be limited. 

The sump is often tailored in size and shape to meet the require-

ments for a particular airframe. For that reason, the home-builder 

may find that some engine models will not fit the plane being built 

because of interference. As if this were not enough to be con-

cerned about, the carburetor or fuel injector location must also be 

considered. These fuel-metering devices are frequently mounted 

under the engine in an updraft configuration, but there are also 

front- and rear-mounted configurations. Some engine models are 

equipped with horizontal carburetors. All of these variations in 

model, may have an effect on engine/airframe fit.

Another error in choice which occurs all too frequently is the 

purchase of an engine originally designed for a high-wing aircraft 

when the builder has a low-wing design under construction. The 

low wing needs a fuel pump, but the high wing usually delivers 

fuel to the carburetor by gravity. In most cases, a fuel pump cannot 

be added to the engine because the drive mechanism was not built 

in during engine manufacture, and the accessory housing was not 

machined to allow mounting of a fuel pump.

As a result of contacts with individuals who have made engine 

purchases for their aircraft, we know that the variations in 

engine configuration outlined in this article have resulted in 

problems. The purpose of bringing these issues to the atten-

tion of Flyer and Key Reprints readers is to help them avoid 

making the same mistakes others have made. If a particular 

engine model has been recommended by a kit manufacturer, it 

is best to search out that model. Although similar, other engine 

models may not meet your needs. 

Choosing  the  right  engine  is  often  a  difficult  decision  that  

ultimately could affect the success of the home-built aircraft. 

Finding a used engine is tricky and, as we have already cov-

ered, the builder has to keep a lot of factors in mind such as size  

and configuration.

Lycoming recognizes  that home-built aircraft builders are  

mechanically inclined and technically trained and are always 

striving for more options and new technologies. Therefore,  

Lycoming has recently launched several new product lines that 

offer builders the “Power of Choice.”

Lycoming  works  very  hard  with  Experimental  Aircraft 

Manufacturers to ensure that they have power plants for their 

customers. Lycoming currently offers fully assembled Certified 

 0 L y c o m i n g F l y e r

and Non-Certified Engines through most if not all Experimental 

OEMs. Since these manufacturers designed the aircraft, they are 

well equipped to handle your powerplant questions and needs.

Lycoming has recently launched Thunderbolt Engines. This is 

where technology and passion meet. Only the most premium 

engines carry the Thunderbolt Engine Medallion. These engines 

will be custom-built to your specifications from horsepower to 

engine color and everything in between at Lycoming’s perfor-

mance-proven facility in Williamsport, PA. It’s one-of-a-kind 

pairing of Lycoming reliability and cutting-edge technology for 

the kind of power and status only the most passionate ever attain. 

Please contact Thunderbolt Engines at 570-327-7115 to exercise 

your “Power of Choice.”

Lycoming has also launched an impressive lineup of engine kits 

that are available through an exclusive network of internation-

ally recognized shops. These engines will be assembled from 

100% Lycoming parts and tested before delivery. Through this 

exclusive network, Lycoming’s Kit Engine product line delivers 

the power plant solutions that experimental aircraft builders have 

been asking for.

For more facts on the power of making the right choice, please 

visit us at www.lycoming.textron.com. 

 

Reading the “Aircraft for Sale” advertisements can be interesting 

and misleading. As aviation-oriented people, we are conditioned 

to look for certain bits of information which we believe will 

allow us to evaluate the product offered for sale. In the case of 

airplanes, this information can generally be segregated into three 

categories — airframe, avionics and engine. For purposes of this 

article, you are on your own with respect to airframe and avionics. 

There does seem to be information on engines which cannot be 

emphasized too strongly.

Engine information is usually provided as hours of operation 

since new or from some major maintenance event. For example, 

700 TTSN  would indicate that this aircraft and engine have 

been flown for 700 hours since  new from the factory. Other, 

but not all, engine-related abbreviations include SMOH (hours 

since major overhaul, SPOH (hours since prop overhaul), STOH 

(hours  since  top  overhaul)  and  SFRM  (hours  since  factory 

remanufacture). Assuming that the recommended TBO of the  

engine being considered is 1800 or 2000 hours, it would appear 

that hours of use in the 400- to 800-hour range would automati-

cally make this engine a very valuable commodity. Unfortunately 

this is not always true, and therefore an advertisement like those 

discussed earlier may state numbers and facts which are abso-

lutely correct, but still misleading.

Consider  a  situation  which  occurred  recently.  A  Lycoming  

IO-360 engine with less than 700 hours since new was reported to 

be using oil at the rate of two-thirds quart per hour and losing oil 

pressure during flight. On closer examination, it was determined 

that deterioration and wear had  caused metal contamination 

throughout the engine. An engine overhaul was necessary, and 

it included replacement of items such as the camshaft, oil pump 

gears and pistons. Why should an engine with less than 700 hours 

since new be in this sad state?

It should be apparent that the number of hours the engine has 

operated is only part of the story. We need to know all the facts 

if we are to understand what may have happened to this normally 

reliable engine, and also if we are to determine the value of a 

low-time engine in a preowned airplane.

The  engine  with  metal  contamination  and  less  than  700 

hours  of  operation  had  been  installed  brand  new  from  the  

factory — more than 12 years before. The engine  logbook 

shows that during the first 10 years of service, this engine had 

averaged less than four hours of flight time each month. Chances 

are excellent that there were some months when the engine was 

not flown at all.

Lycoming  Service  Instruction  No. 1009  states  that the rec-

ommended  TBO  is  based  on  the  use  of  genuine  Lycoming 

parts, average experience in operation and continuous service.  

Continuous service assumes that the aircraft will not be out of 

service for any extended period of time. If an engine is to be out 

of service for longer than 30 days, it should be preserved as speci-

fied in Lycoming Service Letter No. L180. Service Instruction 

No. 1009 also states that because of the variations in operation 

and maintenance, there can be no assurance that an individual 

operator will achieve the recommended TBO.

The point of this discussion is simple. A low-time engine may 

not add value to an aircraft, and the buyer should be aware of all 

factors which may affect the condition and value of the engine. An 

engine which is not flown frequently is subject to deterioration as 

a result inactivity. When the engine does not achieve flight oper-

ating temperatures on a regular basis, the moisture and acids that 

form as a result of combustion and condensation are not vaporized 

and eliminated through the exhaust and crankcase breather. As 

moisture and acids collect in the engine, they contribute to the 

formation of rust on the cylinder walls, camshaft and tappets.

As the engine is run after rust has formed, the rust becomes a 

very fine abrasive causing internal engine wear, particularly to 

the camshaft and tappets. As these components wear, they make 

more metal which attacks the softer metals in the engine. Piston 

pin plugs are examples of parts that may wear rapidly when rust 

becomes an abrasive inside the engine. This wear could eventu-

ally lead to failure.

The infrequently  flown engine is just one example of a low-

time engine not meeting the expectations of a buyer  or  new 

owner. The term zero SMOH is always enticing since it indicates 

the engine has been overhauled,  has zero  hours since  over-

haul and now may be expected to fly happily on through a full 

manufacturer-recommended  TBO. This will happen in some 

cases, but in others, there will not be a chance of this happening.  

It depends on the quality of the overhaul.

Lycoming Service Bulletin No. 240 recommends  parts to be 

replaced at overhaul regardless of the apparent condition of the 

old parts. The number of these new parts used in the engine at 

Lycoming Flyer  

overhaul will probably determine the possibilities of achieving 

a full TBO. Consider that most overhaulers install reconditioned 

cylinders on the engines they overhaul. These cylinders are not 

traceable. There is no requirement to maintain a record of their 

previous history. They may have only 2000 hours of operation, 

but they could just as easily have 5000, 7000 or more hours of 

operation. Those cylinders may have been cracked and repaired 

by welding — a procedure that Lycoming metallurgists do not 

recommend because the strength of a repaired cylinder head 

may be significantly less than that of a new head. There is no 

requirement to let a prospective engine buyer know if cylinders 

have been welded, and this cannot be determined even by close 

examination. The possibility of finding a reconditioned cylinder 

with cracks after a few hundred hours of operation is very real. 

Should this happen, it will be a costly experience.

The  lesson  to  be  learned  here  is  a  very  old  one  —  “Buyer  

Beware.” Whether you are looking at those “Aircraft for Sale” 

advertisements or looking for a replacement engine for an aircraft 

you already own, consider carefully what you are about to buy. 

What do you really know about the engine other than the low-time 

number? How much validity does that number really have? What 

questions can you ask which may help you ensure this engine will 

meet your expectations?

Perhaps simply rereading the paragraphs you have just read may 

help you to formulate questions you want answered before taking 

the plunge. In the case of a low-time engine with a history of 

infrequent flight, borescope examination of the cylinders and 

an inspection of cam and tappet surfaces by a competent and 

knowledgeable A & P mechanic would be a very wise move. 

Always remember that low numbers in the hours of operation 

records do not guarantee reaching TBO with many long hours of 

trouble-free operation. The buyer must investigate every detail 

of engine history as closely as possible, and be satisfied that the 

product does have the value which the low hours of operation 

number suggests.

Like ducts in a heating system, the baffles and seals of an 

engine compartment form a channel that’s designed to trans-

port air from one location to another along a prescribed route. 

In this case, the “duct” funnels ram air through the engine 

compartment and back out into the slipstream, cooling down 

heat-sensitive components in the process. Faulty or improperly 

performing baffles and seals, like a leaky duct, are inefficient 

and apt to cause damage to your assets. 

“Both are critical to cooling your engine,” says aviation col-

umnist and former  Shell  Oil chemist, Ben Visser. Common 

engine problems related to faulty baffles and seals include 

abnormally high cylinder head temperatures, sticking valves 

and spark plug overheating. 

To understand the importance of the function of baffles and 

seals, Visser says you first have to grasp the physical process 

of how an air-cooled engine is cooled. He explains that when 

an aircraft is flying, air enters the cowling and is slowed in the 

plenum formed by the cowling, engine, baffles and seals. The 

effect creates a static, or higher pressure area, above the engine. 

Since gasses move from high pressure to low pressure, the air 

then flows down through the cylinders and across the oil cooler 

to the low-pressure areas below and behind the engine. The air 

exits the cowling through cowl flaps or other flaring openings, 

carrying away excess heat.

If the baffles are broken or misshapen, Visser says the deformity 

can reduce the volume of air passing some or all of the cylinders, 

meaning less than expected cooling for the cylinders or for the 

oil cooler. Seals can create similar problems. Visser says if the 

seals are not in good condition or are not properly adjusted, air 

can “bleed up” and reduce the static pressure, slowing the flow 

of cooling air and increasing engine temperatures. 

Higher  engine  temperatures  can  foreshadow  trouble  to 

come.  Lycoming says that if  cooling air is  not “adequately  

contained and directed, hot spots which promote a lead or carbon 

buildup”  on  the  valve  guides  can  occur,  potentially  leading 

to valve sticking problems during startup. Paul McBride, aka  

“Mr. Lycoming,” says a stuck valve most of the time ends up 

bending a push rod and causing an oil leak, but can also cause a 

large reduction in engine power and very expensive damage to 

the crankcase. McBride, now an aviation columnist and lecturer, 

retired from Lycoming after a four-decade career.

Other problems with insufficient cooling include overheating 

the spark plug barrels, a problem that deteriorates ignition leads 

and boosts temperatures in the insulator tip high enough to cause 

preignition and piston distress. Lycoming points out that adequate 

air flow is particularly important during hot weather in order 

to provide proper cooling of the oil cooler; oil that runs too hot 

breaks down and causes more friction inside the engine.

Visser recommends having the baffles checked any time the 

engine is being serviced or before a new engine is installed. The 

seals, he says, should be checked during periodic inspections. 

While the first step in diagnosing abnormal engine temperatures 

in normal operations is making sure the temperature gauge is 

providing accurate readings — a problem that mechanics say 

accounts for most of the high temperature complaints — Visser 

says the next step is to check all the seals for fit and condition. “If 

the seals aren’t soft and pliable, replace them,” he says. 

One  way to observe how well the seals are  performing their 

stop-gap function is  to remove the  cowling  and  look  at the 

residues left where the cowling and seals rub together. Visser 

says having one continuous line of smudge means the seal is  

doing its job. If there are breaks in the line — which might show 

up as  unmarked  area where  the air was  rushing through the  

gap — that could mean leaks and lower static pressure above the 

engine. Visser also recommends inspecting cowl flaps or flaring 

openings at the rear of the cowling for excessive leakage, indi-

cated by discoloration. 

If high cylinder head temperatures continue to  be problem-

atic, Lycoming suggests having the ignition and fuel systems  

inspected for problems. 

  L y c o m i n g F l y e r

NOTES

Lycoming Flyer  

NOTES

  L y c o m i n g F l y e r

NOTES