What happened
During a heli-logging operation near Wendle Creek, British Columbia, a Sikorsky S-61L helicopter, registration N346AA, experienced a sudden loss of engine power while lifting a load of logs. The aircraft was performing external lift duties at approximately 4200 feet above sea level when the engine noise ceased. Following the loss of power, the pilot released the 8250-pound load. As the helicopter descended, white smoke was observed from the exhaust, and the main rotor speed began to decay. The aircraft struck trees and the ground at 3700 feet, resulting in the destruction of the helicopter and 2 fatalities.
The investigation
Investigators examined the engines, the main gearbox, and the input freewheel units (IFWUs). While the engines showed no mechanical malfunctions that would prevent operation, the presence of white smoke suggested an overspeed condition occurred. The investigation focused on the IFWUs, which act as one-way clutches. Analysis of these components revealed that the rollers were not properly through-hardened during manufacturing. Furthermore, the oilite bushings within the units had deteriorated, leading to bronze contamination and instability in the roller path. This degradation caused the rollers to 'spit out'—a forceful and complete disengagement. The investigation determined that the failure of the first IFWU likely triggered an engine overspeed and shutdown, which then placed an immediate, overwhelming load on the second IFWU, causing it to fail almost simultaneously.
Findings
- The deterioration of oilite bushings caused roller instability and bronze contamination, hindering the IFWUs' ability to maintain engagement.
- Both IFWUs malfunctioned in rapid sequence, inducing engine overspeeds that led to a total loss of power.
- The loss of engine power resulted in a loss of drive to the main rotor, causing rotor RPM to decay and making the aircraft uncontrollable.
- The rollers in the IFWU components were not manufactured to the required hardness specifications.