Amateur-built Kitfox 2-3 N93TG accident near Escanaba, Michigan

Probable cause

The airplane owner's inadequate maintenance and servicing of the propeller gearbox, which resulted in oil starvation, failure of the gearbox in flight, and a subsequent loss of propeller thrust. Contributing to the outcome was the airplane's low altitude at the time of failure, which precluded the airplane from reaching the runway.

NTSB analysis

While on the downwind leg of the traffic pattern for landing, the private pilot of the experimental amateur-built airplane heard a "clunk" sound from the front of the airplane; however, the propeller continued to rotate and the engine appeared to be operating normally. The pilot continued to the base and final legs of the traffic pattern and attempted to add engine power, but the engine "overreved." The airplane lost altitude as it neared the runway and touched down on the parking apron, then continued into a ditch, where it nosed over and came to rest inverted. A postaccident examination revealed that the propeller gearbox had failed in flight. All of the drive gear and propeller drive teeth were either worn or destroyed, and the gearbox drive gear displayed discoloration and heat signatures consistent with oil starvation. Additionally, there was no usable oil present in the gearbox, and no evidence of an oil leak. Although the airplane owner stated that he had added oil to the gearbox before the flight, it is likely that the flight departed with an insufficient oil supply in the propeller gearbox, which resulted in subsequent oil starvation. When the gearbox failed, the engine continued to operate; however, it ceased to drive the propeller, which resulted in a loss of thrust.

Text from the NTSB. Open the NTSB docket for CEN16LA035 for the full record.