What happened
While performing heli-skiing operations near Blue River, British Columbia, the pilot of a Bell 212 HP helicopter, registration C-FWDV, attempted to land at a pick-up area near a glacier. During the final approach, at approximately 150 feet above the ground, the pilot increased the collective pitch to slow the descent. However, the engines failed to respond to the input, causing the rotor RPM to drop and a warning to sound.
Unable to arrest the sink rate due to the lack of engine response, the pilot directed the aircraft toward a frozen lake. The helicopter struck the snow-covered surface with significant force, causing the aircraft to yaw 9/0 degrees. While the deep snow mitigated some impact, the helicopter sustained substantial damage. The pilot, who was the sole occupant, escaped without injury.
The investigation
Investigators examined the engine controls and the Torque Control Unit (TCU) of the Twin-Pac system. Testing revealed that when the engines were operated in a manual mode, they functioned normally; however, in automatic governor mode, the engines experienced oscillations and failed to accelerate on demand.
Further inspection of the Nf governors showed they had been rigged at non-standard positions of 74° and 73°, rather than the standard 85° to 90°. This abnormal rigging was a result of a previously installed, non-approved TCU. This specific TCU had its pressure air ports blocked, necessitating the adjustment of the control arms to maintain the correct RPM range. While this modification had been in use for 150 flight hours, it was not documented in approved maintenance manuals.
Bench testing of the governors revealed excessive wear on the bushings and uneven seating of the port valves. Although the governors had only 823 hours of service—well below their 4500-hour overhaul limit—the abnormal rigging had amplified the effects of standard wear, leading to the failure.
Findings
- The installation of a non-standard TCU necessitated abnormal rigging of the engine Nf governors.
- This unapproved rigging amplified the impact of normal component wear, causing the governors to malfunction and preventing adequate power delivery.
- RPM and torque oscillations between the two engines likely exacerbated the existing wear in the governors, leading to simultaneous malfunctions.
- The loss of engine power occurred during a critical phase of flight, leaving the pilot with insufficient altitude or airspeed to implement manual recovery procedures.