Cylinder Differential Compression Testing: Subject to interpretation

Cylinder Differential Compression Testing Subject to Interpretation Teledyne Continental Motors May 2000 In its simplest terms, differential compression testing is a tool that maintenance personnel use to check the health of an aircraft...


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.

We Recommend

  • Article

    Compression Testing

    Compression testing is a way to determine engine health. We use differential compression testing in aviation because it is a more reliable method of testing engine compression than direct...

  • Article

    Engine Compression Testing: Back to basics

    Engine Compression Back to basics By Joe Escobar Compression testing can be an effective tool for monitoring engine condition. Despite the apparent simplicity of the test, it can be a...

  • Article

    Piston engine troubleshooting

    Piston Engine Troubleshooting By Thomas Ehresman March 1998 You work in a shop that specializes in turbine equipment. The few cabin class piston twins for which the FBO is responsible...

  • Product

    Cabin Pressurization Test Cart

    Tronair's portable cabin pressure tester can perform cabin pressurization tests on virtually any pressurized general aviation airplane. Utilizing an external compressed air source, such as shop air...