What happened
While climbing to collect logs on a hillside, a Sikovisky S-61N helicopter, registered as C-FHFS, experienced a sudden loss of rotor RPM. During the ascent, the crew heard a loud bang, after which the rotor speed began to decay rapidly. The pilot attempted to maneuver the aircraft away from the nearby hill, but the helicopter descended into trees and came to rest on a steep slope. The impact caused the fuel tanks to rupture, though no fire occurred. The two pilots sustained serious injuries, though they did not lose their lives. One pilot's station was damaged by the impact, and the seat of the other pilot broke free from its mounts.
The investigation
Investigators examined the engines and found no evidence of a malfunction that would cause an immediate shutdown, though the engines did overspeed and shut down following the loss of rotor drive. A teardown of the main-rotor transmission revealed significant damage to both input freewheel units (IFWUs). Specifically, the bronze Oilite bushings were found to be extremely worn, cracked, and bent. Further inspection showed that the camshafts were dented and the gear housing was contaminated with bronze particles.
Additionally, the investigation identified that the spiral bevel pinion and gears had been misaligned during the last overhaul. This misalignment caused high levels of vibration, evidenced by fretting and loose dowel pins within the transmission. While metal particles were present in the transmission oil filters, the chip sensors had not been activated, a condition attributed to the specific oil flow characteristics of the unit.
Findings
- The primary cause of the failure was a manufacturing defect in the Oilite bushings, which contained large pores and incomplete sintering. This defect caused instability in the rollers, leading them to slip or disengage the first IFWU.
- The sudden transfer of all rotor power to the second, already worn and contaminated IFWU caused it to fail almost immediately, resulting in a total loss of drive to the main rotor.
- The transmission was subjected to abnormal vibrations due to the misalignment of the spiral bevel pinion and gears during the previous overhaul process.
- The rapid decay of rotor RPM during a high rate of climb made it difficult to maintain control or enter an autorotative state.
- The pilot not flying was not wearing protective headgear at the time of the accident.