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 Kitfox, however, the propeller continued to rotate and the engine appeared to be operating normally.
He 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 at the airport in Escanaba, Michigan, and touched down on the parking apron, then continued into a ditch, where it nosed over and came to rest inverted. The pilot sustained minor injuries.
A post-accident 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.
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 Identification: CEN16LA035
This November 2015 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.
This is an older ‘B ‘ drive and can have the engine/gearbox adapter loosen, resulting in oil loss, or excess internal pressure also causing oil loss.
scroll down and select ‘gear drive’….
I’m glad that the pilot was ok.!
The gear oil doesn’t simply disappear out of those reduction drives. It must have had a seal leak and the owner didn’t address it or the owner didn’t know how to correctly fill the unit.
Wylbur Wrong says
The NTSB did not fault the pilot flying the plane (who was taking a test flight as part of the purchase process….). The current owner was also an A&P and I would imagine that an A&P would know something about putting oil in a PSRU.
What bothers me is that there was no evidence of a leak. Yet when that PSRU was torn down, there was no evidence of leakage — and this was a “sealed” unit. This would indicate that it never had any oil in it!! I seriously doubt that this was the case.
So one has to ask, where did the oil go? If this used the type of oil I’m familiar with for an auto differential or manual transmission, it would be some pretty thick oil (80-110 weight).
I’m starting to think that the oil was drained and not replaced. Possibly during the last condition inspection. And why would that have happened? I can only offer conjecture: The A&P didn’t regularly work with aircraft having a geared prop (PSRU) that had its own oil supply, and so this was missed.
I tend to agree with your conclusion. Mistakes happen to the best of us sometimes. If the mechanic wasn’t familiar with geared prop engines he may have drained it when he did an oil change thinking the oil would be fed from the engine oil, like an old Continental in a Cessna 175.
FYI…it’s a 2-stroker…..the oil is mixed with the fuel and burned.
I fly a C175,B and love it.! great climb, airspeed and endurance [ for a Cessna ].