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.
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.