What happened
During the first flight of the day, the pilot was performing a hydraulic actuator check. The collective had been placed in the down position and the collective lock was engaged. After the rotor powered up to 100 percent flight idle, the pilot depressed the hydraulic test switch and moved the cyclic fore and aft to confirm remaining pressure in the accumulators.
During this process, the collective rose uncommanded, causing the helicopter to move forward in a nose-down attitude. The main rotor struck the ground, and the aircraft completed two revolutions before rolling onto its side. The impact resulted in 0 fatalities, though the accident caused the destruction of both the main and tail rotor systems. A small fire started in the exhaust area but was quickly extinguished.
The investigation
Investigators examined the collective locking mechanism (P/N 350A-27 3455-20 and P/N 350A-27 3107-26) and found that it failed to hold the collective in the full down position because of wear on the locking tab. Additionally, during servo accumulator checks, moving the cyclic control after the depletion of the accumulators caused the collective to move due to feedback within the control system. This movement caused the locking device to disengage.