Accident Summary
On the day of the accident, a Sikorsky S-61N helicopter (registration C-FHFS, serial number 61702) was climbing to pick up logs on a hillside. The helicopter was operating at approximately 85% torque from each engine and a rate of climb of about 2500 feet per minute, with the nose pointed toward the hill about 200 feet away. The two pilots heard a loud bang, and rotor rpm began to decay. The pilot flying turned away from the hill, but the helicopter descended into trees and came to rest nose-down and left side down on a steep hillside. Fuel tanks ruptured, spilling a large quantity of fuel, but no fire occurred. Both pilots suffered serious but non-life-threatening injuries.
Sequence of Events
After the loud bang and rotor rpm decay, the pilot flying initiated a turn away from the hill. Despite this, the helicopter lost altitude and impacted terrain. The pilot flying's station was encroached by impact damage. The seat and restraint system of the pilot not flying broke free from its mountings; that pilot was not wearing protective headgear.
Investigation Findings
Post-accident teardown inspections revealed that both engines had damage signatures consistent with not operating at impact. No engine malfunction that would cause shutdown was apparent, but the engines were equipped with overspeed governors that would shut them down in the event of an overspeed; these governors leave no indication of such shutdown. Main and tail-rotor blade damage was consistent with little or no rotor rpm at impact.
The main-rotor transmission, which had accumulated about 710 hours since last overhaul, underwent teardown inspection. Extensive wear and damage were found on both input freewheel units (IFWUs). These IFWUs had been replaced approximately 500 hours after the last transmission overhaul as a matter of practice; the removed units showed normal wear. The replacement IFWUs (part number 61047-35000-060) had about 210 flight hours and 70 engagements, with new cams and rollers. During routine maintenance before the accident, no significant metal was found in transmission oil filters, and no chip lights illuminated. After the accident, metal particles, including numerous bronze flakes, were found in the transmission, but chip sensors had not activated. Oilite bushings (part number S6135-20459-101) on which the roller retainer rests showed damage consistent with material unable to withstand normal forces or exposed to abnormally high vibrations.
Other IFWU components exhibited instability, slippage, and skidding: camshaft flats dented by rollers; rollers with flat spots and bronze contamination; input gear housing contaminated with bronze and heavily damaged from slippage; roller retainers with dents, scores, grooves, and uneven wear; and Oilite bushings extremely worn, cracked, and bent.
Information received about three years after the accident indicated that the Oilite bushings supplied by Sikorsky were not manufactured properly, having areas of large pores and incomplete sintering. These bushings were subject to premature wear and damage from normal use, causing roller instability. The instability, increased pinch angle, and bronze contamination caused rollers to slip or be ejected, disengaging one IFWU. The other IFWU, also worn and contaminated, failed shortly thereafter, resulting in total loss of drive to the main rotor. With the load of driving the main rotor lost, the engines oversped and shut down. Rotor rpm decayed, and control was lost.
The spiral bevel pinion and main spiral bevel gears showed wear patterns consistent with improper alignment. Measurements indicated a difference of 0.029 inch between surfaces, leading to the conclusion that these gears were misaligned during the last transmission overhaul. This misalignment likely caused torsional oscillations or vibrations. Fretting under bearing races and loose dowel pins indicated abnormally high vibrations during operation. The only anomalies identified that could cause such vibrations were the misalignment and subsequent movement of bearings and gears. The gear backlash likely went out of specification, increasing vibrations and movement.
Because neither engine could produce maximum 123% torque for the IFWUs, overtorque/overloading was not likely a cause.
Causes
The investigation determined that an Oilite bushing broke down due to manufacturing defects (large pores and incomplete sintering). This created roller instability, leading to IFWU disengagement. When the first IFWU disengaged, all power was transmitted to the remaining worn and contaminated IFWU, which quickly failed. Total loss of rotor drive resulted in loss of control.