N452MCBell 206B · 1995

On March 27, 2012, about 0812 central daylight time, a Rotorcraft Development Corporation OH-58C, N452MC, registered to and operated by Custom Air LLC, was substantially damaged by a fire following an uneventful forced landing in a field following loss of engine power near McComb, Mississippi. Visual meteorological conditions prevailed in the area and no flight plan was filed for the 14 Code of Federal Regulations (CFR) Part 91 positioning flight from John E. Lewis Field Airport (MCB), McComb, Mississippi, to Woodville, Mississippi. The commercial pilot, the sole occupant, was not injured. The flight originated from MCB about 3 minutes earlier.

The pilot stated that approximately 3 minutes into the flight while flying at 200 feet above ground level and 100 MPH, the helicopter yawed to the left consistent with an engine failure. He lowered the collective and initiated a descent for an autorotative landing and reported the cabin filled with smoke. He landed uneventfully in a field, exited the helicopter and noticed a fire in the engine compartment area. He called his ground crew and alerted them of the forced landing.

Following recovery of the helicopter, the engine and portion of the airframe was inspected by a representative of the engine manufacturer with FAA oversight. The inspection revealed shrapnel type damage to the air discharge tubes, horizontal fire shield, exhaust stacks, engine decking and main rotor blades. The inspection also noted that the extensive post landing fire damage to the accessory gearbox. Following inspection of the engine, it was then removed from the helicopter and shipped to the manufacturer's facility for teardown examination with FAA oversight.

Teardown examination showed damage to the Nos. 5 and 8 bearings which was consistent with reduced oil supply to the bearings during engine operation. The third stage turbine wheel fractured and approximately 2/3's of the wheel exited the engine. Only one fragment was recovered, and metallurgical examination of the fracture by Rolls-Royce personnel revealed features were interdendritic and consistent with tensile overload. No fatigue-like fractures were observed on the fracture surfaces.

Examination further indicated that the turbine module was present from the No. 3 nozzle aft and remained mated to the outer combustion case. Inspection of the turbine section revealed the gas producer and power turbine supports remained mated together with the No. 3 nozzle exposed on the forward end of the turbine module. The gas producer support was normal in appearance with no visible damage, while the power turbine support exhibited rub damage on the aft side, in an around the bore area from contact with the No. 2 wheel. The forward side exhibited impact damage from the No. 3 wheel burst around the outer flange and on the forward face of the inner bore. The No. 3 nozzle exhibited damage on the trailing edge face of the nozzle around the inner bore area and a fracture of one hub tang. The power turbine inner shaft was fractured just aft of the clamp nut with a segment of the shaft being retained inside the No. 4 wheel bearing bore. Inspection of the No. 8 bearing oil delivery tube pressure line "B-nut" revealed it was finger tight with no evidence of leakage; the oil delivery tube and nozzle were free of obstructions. The No. 5 bearing which was determined to be a FAA PMA part was sent to the NTSB Materials Laboratory for further examination. A copy of the engine examination report from Rolls-Royce is contained in the NTSB public docket.

Examination of the No. 5 bearing was performed by the NTSB Materials Laboratory located in Washington, D.C. The results of the examination revealed the bearing cage showed evidence of severe deformation and damage. The pockets were warped into rounded T shapes with widths exceeding the maximum specified in the engineering drawing. The material on the outer face of the cage had been gouged out in a circumferential direction. Examination of the cage revealed the microhardness of the bearing cage was higher than the prescribed hardness. Further inspection of the cage revealed steel material consistent with that prescribed for the outer and inner race and balls of the bearing. Examination of the inner race of the bearing revealed the raceway showed material transfer in the form of galling and spalling, with folding and flattening of the transferred material. The inner race was lower than the prescribed hardness. Inspection of the outer race also revealed spalled areas with flattened galled material. The outer race hardness was lower than the prescribed hardness. Inspection of the bearing balls revealed only a few retained their spherical shape and original size, while the majority had deformed into prolate spheroids with deformed surfaces from spalled material. All of the balls also exhibited a surface roughness atypical for a low-friction thrust bearing, and the hardness was lower than specified. A copy of the report is contained in the NTSB public docket.