What happened
On September 4, 2022, a Bell 407 helicopter, registration N687AM, was involved in an accident near Mount Baldy, California. The aircraft was being operated by Air Methods Corp to transport people, water, and equipment between a lower landing zone at approximately 4,300 feet mean sea level and the summit of Mount Baldy at 10,064 feet mean sea level.
While returning to the lower landing zone, the pilot was slowing the aircraft from 60 knots to 40 knots and turning onto final approach when a low rotor RPM warning light and horn activated. The pilot noted the rotor RPM had dropped to approximately 95% and responded by lowering the collective, which increased the RPM back to 100%. The pilot then issued a mayday call to ground personnel, reporting a partial engine failure and searching for a place to land.
Upon attempting to raise the collective again, the low rotor RPM warning recurred. The pilot decided to land in a nearby clearing rather than attempting an autorotation. During the landing attempt, the pilot overshot the clearing and struck trees, causing the helicopter to roll onto its right side. The accident resulted in 1 serious injury and 0 fatalities.
The investigation
Investigators examined the engine control unit (ECU) and incident recorder (IR) data. The data showed that in the 12 seconds preceding the activation of the IR, the power lever angle (PLA) values were between 48° and 50°. This is below the normal 70° required for the "FLY" position, and below the 62° threshold required for full engine governing.
Post-accident examination of the throttle assembly revealed that the force required to move the ball plunger out of the "FLY" detent was only 4 lb. This significantly deviates from the manufacturer's maintenance manual, which specifies that 14–15 lb of force should be required to move the throttle through the detent groove. Disassembly of the collective assembly showed wear on the bracket where the ball plunger engages the detent, which likely allowed the throttle to move toward the idle position with minimal resistance.
Findings
- The pilot did not ensure the throttle was properly positioned in the "FLY" detent before beginning the approach to land.
- The worn throttle detent bracket likely contributed to the pilot unknowingly rolling the throttle toward the idle position.
- The engine was performing normally at the lower PLA settings recorded prior to the impact.