Aircraft Maintenance: A modern approach to compression testing
Previously in this series about monitoring engine health, we have focused on oil filters and analysis since oil changes occur throughout the year, rather than only at the annual inspection. However, when annual inspection time arrives, there is one test that aircraft owners seem to fixate on above all else: the compression test.
Cylinder compressions are the ancient measuring stick of the aircraft maintenance world. For decades, mechanics were taught to use go/no-go compression numbers such as 60/80 as a guide to pull a cylinder for repair or replacement. Without much thought about where or why “failing” cylinders were leaking air during the testing, it’s likely that thousands of perfectly good cylinders were banished to the scrap yard needlessly. Even today, many aircraft for-sale ads list a series of compressions as a badge of honor that supposedly proves the health of the engine in just a few numbers. Buyer beware to those who subscribe to that belief.
During a compression test, air is fed into a cylinder with the intake and exhaust valves closed and the piston at top dead center. The air pressure in the cylinder is measured against the air pressure being fed into it, producing a number that essentially represents a rate of leakage from the cylinder in this condition. Using 80 psi as the most common input pressure, a cylinder compression reading would be some lesser pressure that the cylinder will hold, over the input pressure. For example: 75/80 would mean that with 80 psi on the input reference side of the gauge, the cylinder can maintain 75 psi as it leaks out past the rings and valves.
This “cylinder compression” number is of limited value by itself. To put at least some value on it, we need to have a reference number for comparison. Using common numbers such as 60/80 or 40/80 as minimums for a healthy cylinder is relatively meaningless. The FAA’s Advisory Circular AC 43.13-1B calls out these minimums, but it also says that this guideline only applies in lieu of the manufacturer’s maintenance instructions (which exist). Engine manufacturers provide a device known as a “calibrated orifice” with a known rate of leakage that is the benchmark for a compression test. Equipped with this tool, you “test your tester” first and note the measurement that is designated “no-go”. Note this calibrated orifice reference number (as read on the gauge) in the logbook entry. This is important because without it, you have no true reference for the compression numbers recorded on each cylinder.
While some may disagree, I would argue that compressions are of more value if recorded against the calibrated orifice reading, rather than the input pressure. Let’s say that you use 80 psi as input on your gauge, your calibrated orifice reads 45 psi on the pre-test calibration, and your cylinder test reads 58 psi. The numbers that matter are the 58 psi you measured on the cylinder, over the 45 psi from the calibrated orifice on the test. To me, 58/45 is a far more relevant number than 58/80. This is especially true since the 45 psi orifice reading may change slightly next year depending on conditions and equipment, as would your cylinder compression number along with it (even if nothing changed with the cylinder).
Even a “calibrated” cylinder compression number is of little value other than for a manufacturer’s go/no-go number. What matters most is where the leakage is occurring within the cylinder. The most valuable tool you have at your disposal during a compression test isn’t your gauge—it’s your ears. Listening for the source of the air leakage is critical to interpreting the results.
During the compression test for each cylinder, listen at the oil cap, intake filter, and exhaust for hissing air. Air noise at the air filter would indicate leakage past the intake valves, noise at the tailpipe points toward leakage past the exhaust valves, and noise from the oil cap indicates leakage past the piston rings.
Armed with the severity of the leakage (from the compression test reading) and the source of the leakage (from listening), you can make a plan of action for the next steps in evaluating your engine health and remedying any issues. Leakage past the rings is the lowest risk and has few catastrophic consequences. In fact, with the exception of extremely low compressions, there is no real link between output horsepower and compression. Leakage past the intake and exhaust valves, however, is extremely serious and should be addressed immediately.
A modern approach to compression testing gives numbers that actually mean something, and the information we need in order to proceed with more detailed investigation through borescope inspection. We will talk more about borescope inspections next time. Until then, I hope you and your families remain safe and healthy, and I wish you blue skies.
