Papillon Airways Eurocopter AS 350 B2 incident

The pilot heard a loud pop while descending into a canyon and had a loss of main rotor rpm. The pilot entered an autorotation to an open area and landed successfully. Examination of the engine revealed that it was intact and exhibited minimal impact related damage. Disassembly of the engine revealed that the rear bearing support housing was intact and undamaged. The number three bearing was intact, however, did not rotate freely. The inner race and ball bearings exhibited heavy rub marks and thermal damage and the outer race exhibited thermal damage. No oil residue was observed on the bearing components. The damage to the number three bearing was consistent with oil starvation. Debris was observed within the oil scavenge passage of the rear bearing support housing near the oil feed ring assembly. The rear bearing support housing (RBSH) was removed and placed on a flow test bench. The RBSH assembly flowed approximately one-half of the normal oil flow rate and irregular flow patterns were observed. Examination of the RBSH revealed no significant damage; however, the oil passages that feed oil to the number three bearing assembly contained deposits that most likely obstructed oil flow to the bearing. Analysis of the deposited material found that it was composed of coked synthetic oil. Prior to the incident, the engine manufacturer issued an Operating Information Letter that outlined recommended engine shutdown procedures. The two-step process consisted of a two minute idle period prior to shutdown followed after shutdown by motoring the engine with the starter motor for 10 seconds. The Operating Information Letter indicated that not following the procedures could result in coke build up in the number 2 and number 3 bearing oil jets and sump area. The operator reported that company pilots were instructed to comply with the recommended engine idle cool down procedure; however, company policy did not direct pilots to comply with the recommended post shutdown motoring after engine shutdown procedure due to concerns about depletion of oil in the engine oil reservoir. It was not determined if the bearing failure was a direct result of the operator not complying with the complete recommended shutdown procedure outlined by the engine manufacturer.

Verbatim NTSB analysis and probable cause from the NTSB dataset