Cylinder compressions are the old 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 for pulling a cylinder for repair or replacement. Without giving much thought to where or why the “failed” cylinders were leaking air during testing, thousands of perfectly good cylinders are likely to have been needlessly banished to the scrap yard. Even today, many airplane sales ads list a compression range as a badge of honor that supposedly demonstrates engine health in low numbers. Buyer beware of those who subscribe to this belief.
With the advent of tools like digital endoscopes, the industry has evolved to understand cylinder health at a much deeper level than a compression check can convey. This has led some to consider compressions almost meaningless. The truth is much more complex.
During a compression test, air is forced 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 pressure of air entering it, producing a number that essentially represents a cylinder leak rate at that condition. Using 80 psi as the most common inlet pressure, a cylinder compression reading would be a lower pressure that the cylinder will hold, above the inlet pressure. For example: 75/80 would mean that with 80 psi on the inlet reference side of the gauge, the cylinder can hold 75 psi as it leaks past the rings and valves.
This “cylinder compression” number has a limited value by itself. To put at least some value on it, we need to have a reference number for comparison. Using common numbers like 60/80 or 40/80 as minimums for a healthy cylinder is relatively meaningless. FAA Advisory Circular AC 43.13-1B calls for these minimums, but also says that this guideline not more it applies instead of the manufacturer’s maintenance instructions (which do exist). Engine manufacturers provide a device known as a “calibrated orifice” with a known leakage rate that is the reference for a compression test. Equipped with this tool, first “test your tester” and observe the measure designated as a “no-go”. Record this reference number of the calibrated orifice (as read on the indicator) in the log book entry. This is important because without it, you have no real reference to 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 inlet pressure. Let’s say you use 80 psi as an input to your gauge, the calibrated orifice reads 45 psi in the pre-test calibration, and your cylinder test reads 58 psi. The numbers that matter are the 58 psi you measured in the cylinder, above the 45 psi at the calibrated test port. For me, 58/45 is a much more relevant number than 58/80. This is especially true since the 45 psi port reading may change slightly next year depending on conditions and equipment, as will the cylinder compression number along with it (even if nothing has changed with the cylinder).
Even a “calibrated” cylinder compression number has little value other than a manufacturer’s go/no-go number. What matters most is where the leak occurs inside the cylinder. The most valuable tool at your disposal during a compression test is not your gauge, but your ears. Listening to the source of the air leak is critical to interpreting the results.
During the compression test for each cylinder, listen for the oil cap, intake filter and exhaust air whistling. Air noise in the air filter would indicate leaks past the intake valves, exhaust pipe noise points to leaks past the exhaust valves, and oil cap noise indicates a leak past the piston rings.
Armed with the severity of the leak (from the compression test reading) and the source of the leak (from listening), you can make an action plan for next steps to assess the health of your engine and solve any problem. Leaks beyond the rings are the lowest risk and have few catastrophic consequences. In fact, with the exception of extremely low compressions, there is no real link between power output and compression. Leaks past the intake and exhaust valves, however, are extremely serious and should be addressed immediately.
A modern approach to compression testing provides numbers that actually mean something and the information we need to proceed with further investigation through endoscope inspection. We’ll talk more about borescope inspections next time. Until then, I hope you and your families are safe and healthy, and I wish you blue skies.
