compression's on piston motors, lyc and cont

[climbhappy]

someone told me that compressions on cont motors are judged differently than ones on lyc.

example: on an io 470, that compr in 50s or 60s are okay while the 70s are the norm for lyc.

what's going on here?

 

[IFollowRoads]

Never heard that one. Ask the person for a reference. I always use PSI/80, and warn the owner things are gettin dicey below 65/80, and insist the jug come off below 60/80. I use this for any horizontally opposed engine.

If this guy has something solid that says different, well... the only constant is change.

 

[erj-145mech]

Compression checks are not the only indication of a weak cylinder. There are too many variables. If you perform a check and come up with say 71/80, I could come up right behind you, using the same set of gauges, and get a completely different reading.

In the maintenance manuals on my O-2A, there is a hard number of 38/80 where the cylinder must be removed. This is the only place in writing that I have seen where a hard number is noted. You won't find a hard number any where else.

If you take a Cessna 182 that hasn't flown in several months, do a ground run to get the engine to temp, and perform a comp check, you will end up with low readings. Now, go fly that engine for an hour or two, and the readings will be 25# higher. So, if you only run the first set of numbers, are you going to tell the owner that he has to buy six new cylinders? You certainly won't be in business long if you do.

 

[IFollowRoads]

So, if you only run the first set of numbers, are you going to tell the owner that he has to buy six new cylinders? You certainly won't be in business long if you do.

I'll send him to you.

 

[avbug]

Compression tests are one tool for sampling engine condition, but don't tell you anything by themselves. The same engine will give different results using the same compression tester at different temperatures, or with different technicians doing the sampling. Continental wants to see a calibrated orfice for the tester. 80 psi is a reference number that's commonly used, but there's nothing magic about that number.

Compression numbers go up and down with testing. Rocking the propeller can artificially change the numbers, as can a warm engine.

A compression test test doesn't tell you about cylinder wear...it tells you about an artificial condition which may revolve around valve wear or valve cleanliness, valve adjustment, cylinder wear, or a host of other things. It may also tell you nothing. Determining engine health based on a compression test is about as scientifically valid as looking at engine oil color. Compression tests are more of a layman's way of looking at the engine; the numbers are more meaningful to the layman than to the mechanic.

 

[IFollowRoads]

I have yet to see a jug that couldn't hold 60/80 that didn't need some luv.

Talk amongst yourselves.

 

[erj-145mech]

There's nothing unsafe or compromising safety about flying a cylinder that tested to 60/80. I've done it, and will continue to do so. That is providing that the engine does not have excessive oil consumption, oil fouling plugs, or other maladies.

 

[USMCmech]

Where this comes from is that AC43.13 states that a cylinder with compression below 60/80 is not acceptable.

Contenital published a service bulliten specifically stating that low compresion alone was OK as long as none of the leakage was from the valves. This SB superseeds the FAAs recomendation (as does all manufacturers instructions).

Lycoming has not issued a similar SB therfore you must revert to AC43.13, ergo 58/80 is no good.


In reality I'm sure a lycoming would run fine at 58/80, but it wouldn't be leagal. When I build my RV-7 with a Lyc, I'm not going to replace a otherwise healthy jug just because of compression.

 

[erj-145mech]

AC43.13 is an ADVISORY circular. It is not approved data. It is only acceptable data. It is a non binding guide line. That is the official word that I got from the Southwest Regions interpertation guru.

 

[Rally]

AC43.13 is an ADVISORY circular. It is not approved data. It is only acceptable data. It is a non binding guide line. That is the official word that I got from the Southwest Regions interpertation guru.

Not disagreeing with you but A) is he a lawyer B) did he put that in writing and sign it?

 

[USMCmech]

did he put that in writing and sign it?

If yes, can I get a copy?

 

[erj-145mech]

If yes, can I get a copy?

You most probably already have a copy. Look at paragraph 1. Purpose of AC 43.13-1B. It states that it is acceptable data. And do you know what AC stands for? ADVISORY Circular. You don't have to be a Philadelphia lawyer to understand what advisory means. Mr Webster has already done that too.

Unfortunately, the letter was destroyed in a hangar fire in 1994 in Conroe, TX, when my shop went up in flames. Basically, the letter re-iterated the first paragraph in the AC.

 

[bocefus]

erm,
14 CFR PART 43,
"a) Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator, except as noted in §43.16. He shall use the tools, equipment, and test apparatus necessary to assure completion of the work in accordance with accepted industry practices. If special equipment or test apparatus is recommended by the manufacturer involved, he must use that equipment or apparatus or its equivalent acceptable to the Administrator."

So it would seem that in some circumstances, like a Lycoming compression check for instance, you are held to "acceptable data". AC 43.13-1B is your guidance for this whether you believe it or not.

 

[erj-145mech]

Has anybody here looked at Lyc SI 1119A? Here is an exerpt from the Lycoming web site: Notice that it doesn't say to arbitrarily remove a cylinder when it reaches 60/80.

In General Aviation the compression check is used quite universally as a maintenance aid. It was also used for many years by the military and airline maintenance people on their reciprocating engines. Despite universal use, little, if anything, was available in writing as a reference concerning its application to General Aviation powerplants.
Although two methods of checking compression were used in the past, only one is in general use today. The direct compression (old automotive type) has given way almost completely to the differential compression test because the differential check is considered the best of the two. It is a much more precise method of locating specific areas of trouble and it is simple to use.
WHY A COMPRESSION CHECK?
A compression test can be made any time faulty compression is suspected, and should be made if the pilot notices a loss of power in flight, finds high oil consumption, or observes soft spots when hand pulling the prop. It is also considered part of the 100-hour engine inspection and the annual inspection. But most experienced maintenance personnel feel that the compression check is best used to chart a trend over a period of flight hours. A gradual deterioration of charted compression taken during routine maintenance checks would be a sound basis for further investigation and possible cylinder removal. This attempt to reduce the possibility of engine failure is generally called preventive maintenance.
Preventive maintenance in the form of cylinder removal should not be done on the basis of one reading. Mechanics make honest errors and equipment becomes inaccurate. Even a difference in engine temperature when the check is done can easily affect the accuracy of the reading.
Because the differential check is so widely used, several key points regarding this maintenance aid are listed here for the information of those not familiar with its use.
DIFFERENTIAL COMPRESSION TEST
We will attempt to repeat the operating instructions which accompany the equipment. This should be read and followed carefully for best results. The following recommendations will supplement the instructions accompanying the equipment:
1. A standard 80 lbs. of input air is recommended. More pressure makes it difficult to hold the prop.
2. A loss in excess of 25 percent of the 80 lbs., or a reading of 60/80 is the recommended maximum allowable loss.
3. The engine should have been run up to normal operating temperatures immediately preceding the compression check. In other words, we recommend a hot engine check.
4. The differential compression equipment must be kept clean and should be checked regularly for accuracy. Check equipment with the shutoff valve closed and regulated pressure at 80 psi (the cylinder pressure gage must indicate 80 psi plus or minus 2 psi) and hold this reading for at least 5 seconds. Home made equipment should be carefully calibrated.
5. Combustion chambers with five piston rings tend to seal better than 3 or 4 piston rings, with the result that the differential check does not consistently show excessive wear or breakage where 5 piston rings are involved.
6. If erratic readings are observed on the equipment, inspect compressor system for water or dirt.
7. If low readings result, do not remove the cylinders without a recheck after running up the engine at least three minutes, and refer to the cross checks listed later.
8. If valves show continual leakage after recheck, remove rocker box cover and place a fiber drift on the rocker arm immediately over the valve stem and tap the drift several times with a one or two-pound hammer. When tapping valves thusly, rotate the prop so that the piston will not be on top dead center. This is necessary in some engines to prevent the valve from striking the head of the piston. Then rotate engine with the starter and recheck compression.
9. Caution. Take all necessary precautions against accidental firing of the engines.
CROSS CHECKING IS IMPORTANT
Rather than rely on one source of information concerning the condition of the combustion chamber, it is wise to make cross checks, particularly when the compression readings are questionable. Therefore, we would like to recommend the following before removing a cylinder.
1. Remember that spark plugs tell a story. Carefully check the spark plugs removed from any cylinder with a low reading.
2. Use at least a goose-neck light or preferably a borescope and carefully check the top of the piston and cylinder walls.
3. Consider the health history of the engine. Has it had previous difficulty of this nature?
4. Has the pilot observed any loss of power in the engine during flight or during run up?
5. How has the engine been maintained and operated during its life? If the maintenance and care have been proper and consistent there is less likelihood of trouble.
6. The supervisor of maintenance should evaluate the known factors such as those discussed here and make a recommendation to the pilot.
CONCLUSION
Whatever your opinion of the compression check as a maintenance aid, it is probable that no pilot or mechanic would care to omit it during a 100-hour or annual inspection. On the other hand, since most everyone seems to use it on the flat opposed engines, we ought to share our experiences with its application to our powerplants. This has indicated that the differential is the best method of checking compression, and particularly so when the readings are charted as a trend over a number of routine inspections. It is a good tool for preventive maintenance and aids in avoiding in-flight failures. Cross checking is good procedure, rather than relying on one source of information concerning the condition of the combustion chamber.

 

[craftkeeper2001]

Lycoming has a manual called key reprints as well that a few here would benefit from and the AC 43.13 1B is approved data if no other exist from the manufacturer. NUFF SAID

 

[erj-145mech]

AC 43.13 1B is approved data if no other exist from the manufacturer. NUFF SAID

Horse Dukey. From the first page of AC34.3-1B:

1. PURPOSE. This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions.

Notice that it doesn't say approved, it only says that its acceptable. There is a difference.

NOW ITS NUFF SAID!

 

[bocefus]

whatever gave you the idea that you need approved data for a compression check, or approving an aircraft for return to service?