LAX00FA123 · Boeing 369D

The helicopter collided with trees in a mountainous area during an attempted forced landing on a ridge line following a total loss of engine power. The pilot stated that he had been airborne approximately 25 minutes when the engine began to run erratically, and a chip detector light illuminated. He was only a few feet above the trees as he maneuvered to land when the engine stopped producing power. A quick scan of the instruments revealed that there was no oil pressure or torque. The helicopter struck a tree, and fell to the ground. 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 exhibited 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 - 8 bearing oil supply T-fitting was clear and dry. Oil analysis discovered dark oxides that were indicative of engine lubricant starvation, and friction polymers that indicated 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 the two stages was intact. The pump drive shaft to the third stage was sheared. An area between the second and the third (6/7 and 8 bearing scavenge) stage of the pump and the third stage gear cavity contained small chunks of metallic debris that was determined to be bearing material. The third stage gear assembly bound when turned by finger rotation. Oil supply distribution passages in the gear case casting were clear. The check valve in the Nos. 6/7 - 8 bearing oil supply line was properly assembled and pressurized air flowed through the valve in the direction of the flow arrow on the body, and the 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. 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.

Verbatim NTSB analysis and probable cause from the NTSB dataset