Frequency of Flight and its Effect on the Engine

We have firm evidence that engines not flown frequently may not achieve the standard expected overhaul life. Engines that are flown occasionally deteriorate much more rapidly than those that fly consistently. Pilots have asked, “What really happens to an engine when it’s flown only one or two times per month?” An aircraft engine flown this infrequently usually accumulates rust and corrosion internally. This rust and corrosion are often found when an engine is torn down.

Some operators are running the engines on the ground in an attempt to prevent rust between infrequent flights. This may harm rather than help the engine if the oil temperature is not brought up to approximately 165˚ F because water and acids from combustion will accumulate in the engine oil. The one best way to get oil temperature to 165˚ F is to fly the aircraft. During the flight, the oil normally gets hot enough to vaporize the water and most acids and eliminate them from the oil. If the engine is merely ground run, the water accumulated in the oil will gradually turn to acid, which is also undesirable. Prolonged ground running in an attempt to bring oil temperature up is not recommended because of inadequate cooling that may result in hot spots in the cylinders, baked and deteriorated ignition harness and brittle oil seals which cause oil leaks.

Pulling on the engine through by hand if it has not been run for a week or more is NOT recommended, and can result in increased wear. Refer to Lycoming Service Letter L180. If the engine is flown so infrequently that it does not accumulate the operating hours recommended for an oil change (25 hours for a pressure screen system and 50 hours for a full-flow filter system), then the oil should be changed at four-month intervals to eliminate water and acids.