How Common Are Oil Leaks in Training Airplanes?

Pilots need to be able to recognize the difference between a serious problem and not-so-serious mess.

Meg Godlewski

Wednesday, May 20, 2026 at 09:00 AM ET

Verified

Edited By:

Travis Tingle

Airplanes often have greasy bellies, but knowing the difference between a normal and abnormal amount of oil is critical for a pilot. [Credit: Meg Godlewski]

Key Takeaways:

The "greasy belly" often seen on training airplanes is typically due to an overfilled oil sump, causing excess oil to be blown out of the breather tube as a mist, rather than an excessive loss during flight.
This expelled oil mist attracts dirt, creating dark stains, and is further exacerbated by busy flight schools that may not clean their aircraft often.
True oil leaks or improperly secured oil caps would typically result in oil covering the engine compartment, distinguishing them from the more common overfilling issue.

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Question: Why is it that so many training airplanes seem to have perpetual oil leaks? Every airplane at the flight school I train at has a greasy belly. Does this mean they are losing an excessive amount of oil during the flight?

Answer: A greasy belly usually doesn’t mean excessive oil is being lost during flight. Rather, it could be that the oil sump was overfilled. For example, if the engine has a capacity, per the pilot’s operating handbook, of 8 quarts, and it is filled to 8 quarts, you can expect some of that oil to be blown out of the breather tube and down the belly of the aircraft.