Amateur-built LANCAIR IV P N366TF incident near Fulton, New York

Probable cause

The loss of oil pressure and subsequent engine overspeed/surge during a zero g-force maneuver, which resulted in the separation of a crankshaft counterweight and loss of engine power.

NTSB analysis

The pilot and a flight instructor planned to conduct an inflight calibration of an angle of attack indicator. Following a zero g-force maneuver, engine oil pressure decreased to 3 psi and engine speed increased to 3,140 rpm. Immediately after the engine overspeed, a total loss of power occurred. The pilot performed a forced landing to the departure airport without further incident.

Examination of the engine revealed one of the two rear crankshaft counterweights (6th order) separated from the crankshaft, with a 6th order counterweight connecting pin lodged in the crankcase between cylinder Nos. 4 and 6. An impression in the No. 1 connecting rod matched the shape of a 6th order counterweight connecting pin. The oil sump contained significant debris, including four counterweight pin retaining snap rings, four counterweight pin retainer plates, and two 6th order counterweight connecting pins. Based on the examination, the lack of oil pressure and subsequent engine rpm overspeed/surge likely caused the separation of the crankshaft counterweight and engine failure.

Text from the NTSB. Open the NTSB docket for CEN17IA233 for the full record.