The private pilot recently purchased the Ercoupe 415 and was returning it to his home airport. Before the flight, he added 1/2 quart of oil to the engine, bringing the oil level to between 3-1/2 and 4 quarts.
About 30 minutes after departure, he noticed that the oil pressure was dropping. He reduced engine power and turned the airplane toward the closest airport.
The oil pressure continued to decrease, even though the engine continued to produce power, and he chose to perform a precautionary landing on a road near Dawsonville, Georgia.
Upon touchdown, the plane skidded across the road until it hit a tree and came to rest in a ditch. The pilot sustained minor injuries in the crash.
Examination of the engine revealed a hole in the oil sump consistent with impact damage, and oil was noted coming from the crankcase breather tube. In addition, oil was noted on the bottom of the airplane from the nose to the empennage.
Even after the oil leaked from the broken oil sump and from the breather tube, the post-accident engine oil level was 3-1/2 quarts.
A subsequent engine run demonstrated that the engine would experience a momentary drop in oil pressure indication if the oil sump was overfilled. The engine and the oil pressure gauge otherwise operated normally.
Since the pilot lacked operational experience with the airplane, it is likely he unintentionally overfilled the oil sump, which resulted in an interaction between the oil in the sump and the rotating crankshaft. The interaction likely resulted in air bubbles becoming entrained in the oil and a subsequent oil pressure fluctuation.
Additionally, since he checked the oil level following a previous flight, it was likely that the oil had not fully drained back into the sump when the reading was taken. Considering that the oil pressure gauge operated normally after the accident, it is unlikely that the gauge was producing a faulty reading in flight.
Probable cause: A decrease in oil pressure due to the pilot’s unintentional overfilling of the engine oil sump, and the pilot’s subsequent decision to perform an off-airport precautionary landing, which resulted in hitting a tree.
NTSB Identification: ERA17LA109
This February 2017 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.