In its simplest terms, differential compression testing is a tool that maintenance personnel use to check the health of an aircraft cylinder. This tool can be quite useful, but the results can be very misleading in the hands of someone who uses it incorrectly. The differential compression tester has also frequently been the subject of much controversy as there is not universal acceptance of minimum pressure readings or when to take corrective action. Adding to the controversy is the fact that the FAA publishes an advisory circular that gives cut-and-dried instructions for what to accept and what not to accept.
Because of these various approaches to the differential compression test, there is no way to write the definitive piece on how to perform it correctly. Instead, we will attempt to present you with all of the various interpretations of the test and the instructions presented by Continental, Lycoming, and the FAA. Additionally, we will give you a bit of food for thought on what to accept as the definitive method for compression testing, and provide you with some interesting details you might not know regarding the selection of compression testing equipment.
Teledyne Continental Motors recommended procedure
The following is TCM's procedures for differential compression testing. Although TCM's official document for this is TCM Service Bulletin M84-15, TCM offers the following as a summary of their Service Bulletin:
According to Teledyne Continental Motors, differential compression checks of piston engines have been used for many years to help evaluate the condition of engine cylinders. Done properly, the test can identify pressure leaks occurring through the intake or exhaust valves, by the rings, or through head or barrel cracks. The procedure involves applying a known pressure into the cylinder bore with the piston at Top Dead Center (TDC) and measuring the reduced pressure in the combustion chamber due to leakage. For TCM engines, the pressure is generally input through the top spark plug port and the pressure is measured with the differential compression tester.
Eighty pounds of pressure was established long ago in radial engine days as the input pressure. This is generally adequate to establish an accurate leakage reading, but not so high as to move the piston off of TDC. Likewise, what was generally regarded as the maximum permissible pressure drop of 25 percent, or an equivalent outlet reading of 60 psi, was established during the radial engine era. The procedure was documented in AC43.13-1b. This "bible" for compression testing is often still quoted, which as we will discuss, can lead to premature and possibly inaccurate conclusions concerning cylinder health. The correct document for TCM engines is TCM Service Bulletin M84-15. If your maintenance facility does not have this document and does not perform the test per the instruction enclosed, your test results will be suspect.
If your differential compression test results were less than 60/80 and you think it may be time for a top overhaul, some additional questions need to immediately be asked. The first thing to confirm is that the test was conducted using a TCM calibrated master orifice tool, P/N 646953A. In 1984, TCM determined that the calibration methodologies for various compression testers in the field were not consistent due to the lack of a uniform airflow requirement. To correct this, the master orifice tool was created to properly identify the pressure corresponding to 25 percent leakage. TCM's field experience has identified compression testing systems where the master orifice readings for 25 percent leakage are less than 50 psi. In simple terms, you probably wouldn't even look at these cylinders until the reading was less than 50, not 60. Make sure your facility has and is using this tool for your differential compression test.
Secondly, the determination must be made as to the source of the leakage: static or dynamic. The static leak sources are the valve seats, spark plug port seals, and cracks in the cylinder head or barrel or cylinder to barrel joint seal. Dynamic leakage occurs by the rings.
All static leaks are unacceptable and must be corrected. This, however, does not automatically translate to cylinder removal and repair. The key is to understand that the cylinder environment is a very dynamic one. It is not uncommon for small bits of combustion products to occasionally lodge under a valve seat. To examine this possibility, the cylinder should be "staked," a simple process of tapping the valve stem lightly with a plastic mallet while the cylinder is under pressure. The slight movement may dislodge the particle and allow a more effective seal to be established. You should verify that this test has been done, especially if you have not been noticing the engine roughness which is characteristic of a leaking valve seat.
Differential compression inspection checklist for Teledyne Continental Motors engines
Make sure you obtain possession of TCM Service Bulletin M84-15 which defines the procedures for obtaining accurate differential compression tests. This bulletin must be followed with discipline to obtain accurate test results.
Use the TCM Master Orifice Tool P/N 646953A and obtain the Master Orifice Reading. Test results are unreliable unless the Master Orifice Tool is utilized.
Properly identify any leakage. Is it static (valves, spark plug ports, cylinder head, or barrel leaks) or dynamic (ring bypass leakage)?
If the leakage is determined to be by the valve seats, verify that the cylinders were "staked" to examine the possibility that the valve seat was held open due to combustion products being lodged in the valve seat areas.
If the leakage is dynamic, or by the rings, have the engine started or the aircraft flown for a short period of time to obtain a second compression reading. In general, do not react to a single low dynamic compression reading unless there are other signs of deteriorating engine condition.
Use your dynamic compression reading as only one element of your cylinder health evaluation. Consult the TCM service programs desk for further clarification.
If the leakage is dynamic, or is at the ring to barrel seal, several factors need to be considered and some technical background is again helpful in assessing the proper course of action. Cylinder differential compression tests are conducted using 80 psi test air. During actual engine operation, cylinder pressures up to 1,000 psi force the compression rings against the cylinder wall and the actual running dynamic seal can be quite different than the static measurement. That is one reason why it is not uncommon to have a normally running engine show a low differential compression leak check at scheduled inspection or maintenance.
Other less qualitative factors also influence the dynamic leakage evaluation. The amount of oil on the cylinder wall can have an influence, with more oil leading to an improved differential compression reading. On occasion, due to the rotation of the rings in the piston, the ring end gaps can align to produce an increased leak path and a degraded differential compression test result. Piston design, cylinder choke, and other factors may affect the test result you get but not necessarily engine performance. So what is the proper course of action with respect to dynamic leakage?
TCM's recommendation is to realize that the differential compression leak test is only one indicator of cylinder health, especially for dynamic leakage. If your oil utilization pattern is not changing dramatically and the other health monitoring items described in the TopCare Health Checklist are nominal, it may be best to simply continue to monitor the situation. In the absence of other indicators of poor cylinder health or changing engine characteristics, you should not react to a single low dynamic differential compression reading.
The best approach is often to start and run the engine for a period of time or fly it around the pattern and then test again. It is very common to conduct a second test of a low dynamic leakage reading and discover that the new value is substantially different. There are so many factors in play that these readings may vary significantly for your TCM engine. Look at the overall history and performance of your engine and ask for common sense explanations of the recommendations you receive.
In preparing the TCM TopCare Program, engine testing was conducted on a TSIO520UB engine that demonstrated that certification horsepower was delivered by the engine even when some cylinders were at or below the minimum allowable calibrated compression readings established by the master orifice tool. Figure 1 (page 58) provides a summary of the differential compression readings obtained on No. 2 cylinder during a 600-hour engine endurance test. As you can clearly see, wide fluctuations occurred throughout the testing.
As with many maintenance actions, technique is important. Assuring the piston is on top dead center and other tricks-of-the-trade can help improve the quality of the differential compression test results.
Want to bet your wallet on it?
Manufacturers' maintenance manuals quite clearly allow an engine to continue to operate despite dropping below the 60/80 pressure differential. Lycoming offers a bit of caution and recommends further investigation, while TCM encourages checking that the engine is making rated power and checking the cylinder against their Master Orifice tool. In fact, the manufacturers encourage owners of aircraft not to readily accept a below-60/80 reading and to investigate the cylinder further before rejecting it. Yet, signing off an engine as airworthy with a reading that is below 60/80 directly contradicts an FAA Advisory Circular 43.131A and the new 43.131B.
Now, it's clear that you are safe in terms of FAR compliance. An AC, by definition is only advisory material, and the regulations make it clear that you have to be in compliance with manufacturers' instructions for continued airworthiness. However, in speaking recently with an industry contact and "expert witness," more than one technician has been taken to task for defying the instructions in AC 43.13-1A. And it doesn't matter that the manufacturers allow it. The fact that the FAA tells you to reject a cylinder below 60/80 in black and white means that you may one day be defending yourself against this statement in court. The following reference from 43.13-1B has been used more than once in a law suit.
If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in paragraphs 815b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected.
In the end it's your choice, but you may want to cover yourself from future law suits by routinely inspecting or rejecting anything that falls below 60/80.
Note: Advisory Circular 43.13-1A will expire July 31, 2000. After July 31, you must reference 43.13-1B. 1A was originally scheduled to expire in April, but the date has been extended.
From Textron Lycoming's Key Reprints
According to Textron Lycoming, 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
Textron Lycoming 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 gauge must indicate 80 psi plus or minus 2 psi) and hold this reading for at least 5 seconds. Homemade equipment should be carefully calibrated.
5. Combustion chambers with five piston rings tend to seal better than three or four piston rings, with the result that the differential check does not consistently show excessive wear or breakage where five 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 1- or 2-lb. 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.
Compression testers
How they work — what to look for
