Accident Overview
On March 27, 2012, about 0812 central daylight time, a Rotorcraft Development Corporation OH-58C helicopter, registration N452MC, operated by Custom Air LLC, sustained substantial damage from a fire following a forced landing in a field near McComb, Mississippi. The forced landing was necessitated by a loss of engine power. Visual meteorological conditions prevailed, and no flight plan was filed for the positioning flight under 14 CFR Part 91, which originated from John E. Lewis Field Airport (MCB) about three minutes earlier, destined for Woodville, Mississippi. The commercial pilot, the sole occupant, was not injured.
Pilot Actions
The pilot reported that approximately three minutes into the flight, while at 200 feet above ground level and 100 miles per hour, the helicopter yawed to the left, consistent with an engine failure. He lowered the collective and initiated a descent for an autorotative landing, during which the cabin filled with smoke. After an uneventful landing in a field, he exited the helicopter and observed a fire in the engine compartment area. He contacted his ground crew to alert them of the forced landing.
Engine and Airframe Inspection
Following recovery of the helicopter, an inspection of the engine and portions of the airframe was conducted by a representative of the engine manufacturer under FAA oversight. The inspection revealed shrapnel-type damage to the air discharge tubes, horizontal fire shield, exhaust stacks, engine decking, and main rotor blades. Extensive post-landing fire damage was noted to the accessory gearbox. The engine was subsequently removed and shipped to the manufacturer's facility for teardown examination with FAA oversight.
Teardown Examination
The teardown examination showed damage to the Nos. 5 and 8 bearings consistent with reduced oil supply during operation. The third-stage turbine wheel fractured, with approximately two-thirds of the wheel exiting the engine; only one fragment was recovered. Metallurgical examination of the fracture by Rolls-Royce personnel revealed features that were interdendritic and consistent with tensile overload; no fatigue-like fractures were observed. Further examination indicated that the turbine module was present from the No. 3 nozzle aft and remained mated to the outer combustion case. The gas producer support appeared normal with no visible damage, while the power turbine support exhibited rub damage on the aft side and impact damage on the forward side from the No. 3 wheel burst. The No. 3 nozzle showed damage to the trailing edge face and a fracture of one hub tang. The power turbine inner shaft was fractured just aft of the clamp nut. Inspection of the No. 8 bearing oil delivery tube pressure line B-nut revealed it was finger tight with no evidence of leakage, and the oil delivery tube and nozzle were free of obstructions. The No. 5 bearing, determined to be an FAA PMA part, was sent to the NTSB Materials Laboratory for further examination.
Bearing Examination
Examination of the No. 5 bearing by the NTSB Materials Laboratory in Washington, D.C., revealed severe deformation and damage to the bearing cage. The pockets were warped into rounded T shapes exceeding maximum specified dimensions, with material gouged out circumferentially. Microhardness of the cage was higher than prescribed. The inner race showed material transfer in the form of galling and spalling, with hardness lower than prescribed. The outer race also exhibited spalled areas with flattened galled material and hardness lower than specified. Most bearing balls had deformed into prolate spheroids with surface roughness atypical for a low-friction thrust bearing, and hardness was lower than specified. Only a few balls retained their spherical shape and original size.