Aircraft Maintenance: Tackling top overhaul decisions

Owners often hold their breath for the compression test results during an engine inspection. This shouldn’t be surprising because it is one of the few parts of an annual inspection that result in simple numbers. And, those numbers carry enough weight to be included in the logbook entry for the annual inspection. We don’t record tire tread depth or brake disc thickness, but we always write the compression values in the logbook. So, it must be the most important inspection, right? Well, no.

While compression checks are a part of every annual inspection, they are merely a starting point in evaluating the health of the cylinders. This is especially true because the results can vary. If you have three different shops check your compressions on three different days, it’s highly unlikely that you’ll get exactly the same numbers. In fact, your results may vary considerably depending on the temperature of the engine, prop position, and calibration of the instruments used for the test. This is exactly why Continental Motors mandates the use of a calibrated orifice tool to determine a reference measurement before measuring a cylinder's compression (the company also requires borescope inspection as part of the evaluation process).

Marginal compression readings alone do not provide enough information to determine whether a cylinder should be removed. The purpose of the compression test is to identify leaks and the source of the leaks within the cylinder when the engine is not running. Identifying leaks is the key. Even if the compression values are within spec, it’s important to determine where air is escaping the cylinder because that is critical in determining whether the cylinder needs work. A proper compression test includes the calibrated differential pressure test, identification of the source of any leaks, and a borescope inspection of the inside of the cylinder.

In order to determine the source of the compression leak, the cylinder is pressurized with air, and the differential pressure is measured using a calibrated set of gauges. As important as the resulting measurement is, it’s just as important to identify where the air is escaping by listening alternatively at the exhaust pipe, crankcase breather, and air filter for the sound of escaping air. Air sounds in the exhaust pipe point to an exhaust valve leak, while air sounds at the crankcase breather or oil fill tube indicate leaking rings. Leaking intake valves would be heard at the air filter, but are less common.

Exhaust valve leaks are the most concerning source of low compression. This is because the exhaust valve is a highly stressed and vulnerable part of the cylinder assembly. A significantly worn or leaking exhaust valve is bound to deteriorate quickly. If you allow this process to continue, you’re risking failure of the valve and damage to the entire engine.

Piston ring leaks, on the other hand, can be caused by less severe circumstances such as aligning of the ring gaps or general ring/barrel wear. In these cases, the manufacturer’s guidelines recommend flying the aircraft at cruise power and re-checking the compression. The amount of time before the re-check depends on whether the compression results were within spec.

Much of the confusion and misconceptions about compression checks come directly from the FAA in the form of general guidance in Advisory Circular AC 43.13-1B, which includes the language: “If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in paragraphs 8-15b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected.” This would seem to be quite definitive if it were not for the fact that AC 43.13-1B specifically states that it is only a general guideline and that manufacturer’s data supersedes the information in the advisory circular. So, if you want to do it right, go straight to the engine manufacturer’s instructions.

Next time, we will talk about the importance of borescoping cylinders and a bit more about valves. Until then, happy flying!