Accident Details
A helicopter experienced a total loss of engine power while maneuvering for a forced landing on a ridge line in mountainous terrain, resulting in a collision with trees and subsequent fall to the ground. The pilot reported that approximately 25 minutes into the flight, the engine began running erratically and a chip detector light illuminated. While only a few feet above the trees, the engine stopped producing power. A quick scan of the instruments showed no oil pressure or torque.
Engine Examination
Examination of the engine revealed dark, cloudy oil in the gear case sump. The upper chip detector contained debris. The No. 8 bearing cavity was dry, the shaft end nut was split and off the shaft, and the bearing was destroyed. The rotor shaft exhibited scoring at the bearing race and was dark brown in color. The No. 6/7 bearing had a similar appearance. The No. 2 shaft separated forward of the No. 6/7 bearing and exhibited a dry, deep red appearance. The Nos. 6/7 and 8 bearing oil delivery tubes were clear and dry. The oil supply line to the oil delivery tubes contained no residual oil. The oil screen in the Nos. 6/7 and 8 bearing oil supply T-fitting was clear and dry.
Oil System Analysis
Oil analysis discovered dark oxides indicative of engine lubricant starvation and friction polymers indicating excessive internal engine stress loads. The 3-stage oil pump was disassembled: the first (pressure) and second (gear case and No. 1 scavenge) stages were intact, visibly undamaged, turned freely, and the pump drive shaft through those two stages was intact. The pump drive shaft to the third stage was sheared. An area between the second and third (6/7 and 8 bearing scavenge) stage of the pump and the third stage gear cavity contained small chunks of metallic debris determined to be bearing material. The third stage gear assembly bound when turned by finger rotation.
Other Components
Oil supply distribution passages in the gear case casting were clear. The check valve in the Nos. 6/7 and 8 bearing oil supply line was properly assembled and pressurized air flowed through the valve in the direction of the flow arrow, with flow checked in the reverse direction. The oil pressure relief valve was also properly assembled; the relief valve spring and poppet were visibly undamaged, and the poppet seat was smooth to the fingertip.
Metallurgical Analysis
A metallurgical analysis of the oil pump gear shaft assembly found that it conformed to the requirements and specifications of the engineering drawing and that it failed in torsional overload.