What happened
During a scheduled maintenance check flight intended to test the engine and perform rotor track and balance, the crew of a helicopter experienced a critical mechanical failure. While descending from approximately 1,200 feet above ground level (agl) with plans to level off at 700 feet agl, the pilot attempted to pull up on the collective. At this moment, the collective would not move because it had become latched by the collective down lock.
The pilot and a mechanic attempted to unlatch the collective, but they were unable to do so before needing to flare for landing. Because the collective remained stuck in flat pitch, the aircraft performed a hard landing with forward speed. Following the collision with terrain, the flight crew evacuated the helicopter once it came to rest. An ensuing fire destroyed the aircraft. There were no fatalities reported among the two occupants.
The investigation
The investigation focused on an aftermarket avionics control panel that had been recently installed in the aircraft. The collective down lock is secured to the lower section of this panel. In a factory installation, there is a clearance of 0.3 inches between the collective and the down lock; however, at the time of the accident, the clearance was approximately 0.16 inches.
The down lock consists of a flexible plate designed to vibrate with the helicopter's normal rhythms. The manufacturer's installation instructions for the aftermarket panel required maintenance personnel to bend the locking plate so that it would spring away from the collective lock button. Examination of the aircraft's locking plate indicated that the locking plate bend was likely reversed, which caused the plate to spring toward the lock button rather than away from it.