Accident Sequence
The pilot was conducting an agricultural spraying operation. While executing a reversal turn to begin another swath, the turbine engine suddenly stopped and the propeller feathered. The pilot attempted to land on a road near the edge of the field being sprayed. However, a tractor was present on the road, forcing the pilot to land in a crop of tall wheat. Just before touchdown, one of the aircraft's wings impacted the terrain. During the landing roll on the soft terrain, the aircraft nosed over.
Post-Accident Investigation
A teardown inspection of the engine was performed. It was determined that the torque sensor idler gear duplex bearing (part number 3103035-1) had failed. This bearing failure was a condition that would normally result in an uncommanded in-flight shutdown of the engine. Further examination of the bearing revealed only minimal heat stress discoloration and no clear lack of lubrication. However, the bearing balls had been worn undersize, and the bearing cage had fractured into pieces.
The bearing was sent to Honeywell Mechanical Systems for additional inspection. Their analysis determined that the bearing condition was consistent with advanced rolling contact fatigue failure.
As part of the investigation, an FAA Airworthiness Inspector reviewed the maintenance history and determined that "the installation of the subject matched bearing set was performed properly by the last overhaul shop in accordance with the Allied-Signal (Honeywell) Overhaul Specification No. 72-00-17, revised July 20, 1992."
Despite the findings, the reason for the unexpected failure of this bearing after approximately 100 hours of operation could not be determined. It was noted that at the last overhaul, the torque sensor was not upgraded to the available -3 bearing, which uses a larger bearing assembly and a different gear set.