Accident Narrative
The airline transport rated pilot, with 1,500 hours of experience, had just departed a 6,305-foot-long runway. Security camera footage captured a puff of white smoke emanating from the back of the right engine shortly after the airplane became airborne. The airplane then initiated a right turn and proceeded on a westerly heading at low altitude before disappearing behind a tree line. A witness observed the airplane in a right turn toward the north. The witness believed the airplane was going to land on a closed highway, but instead the nose dropped and the airplane cartwheeled on the grass median west of the road.
Engine and Component Examination
Examination of the right engine revealed that the crankshaft gear had failed due to a fatigue fracture in one of its teeth. The fatigue crack originated from the pressure face of the tooth and intersected another crack emanating from the non-pressure face, resulting in separation of the tooth from the rim. Post-fracture damage near the origin prevented determination of whether the crack initiated in fatigue or if a crack propagated in overstress through the case and then continued through the core in fatigue. The microstructure of the gear was found to be inadequate, indicating a need for further research into the manufacturing process.
Manufacturing Process Review
A review of manufacturing records indicated that approximately 2,400 crankshaft gears were produced from the same batch of material as the accident gear and were heat-treated in two groups. Heat treatment records confirmed that furnace temperatures during the hardening step in each heat-treating process were not high enough to fully austenitize the material. Additionally, the quantity and orientation of gears loaded in the furnace were greater compared to other jobs completed for TCM. Initial data from a test load similar to that used in the heat-treating process for the accident gear suggested that under these conditions, the gears might not have been fully equilibrated at temperature during the hardening step. TCM manufactured several other gears at different hardness levels and conducted failure testing to see if a similar failure could be reproduced. Despite some hardness levels being well below TCM standards, the testing did not produce a similar failure. Evidence also suggested that the gear teeth may have been exposed to an excessive load at some point during operation.
Conclusion
The cause of the gear tooth failure could not be determined.