Accident Description
During a maintenance check flight, the helicopter experienced an uncontrolled descent and collision with terrain after the collective down lock engaged. The pilot and a mechanic were performing a maintenance flight check of the engine and a rotor track and balance. The pilot initiated a descent from approximately 1,200 feet above ground level (AGL) and prepared to level off at about 700 feet AGL. When he attempted to raise the collective, it would not move and was observed to be latched by the collective down lock. Both occupants attempted to unlatch the collective but did not have sufficient time before the pilot had to flare for landing. With the collective stuck in a flat-pitch position, the helicopter landed hard with forward speed. After coming to rest, the flight crew evacuated the helicopter. An ensuing post-accident fire destroyed the helicopter.
Maintenance Issues
A new aftermarket avionics control panel had been installed prior to the flight. The collective down lock, which is secured to this panel, was adjusted before the flight. The down lock attaches to the lower section of the avionics panel. When the collective is lowered to the lower pitch stop, the clearance between the collective and the down lock is approximately 0.16 inches. In the original factory installation, this clearance is 0.3 inches. The down lock is a flexible plate that is free to vibrate with the helicopter's normal rhythms. This is the second known accident where the collective lock has inadvertently engaged in flight with this particular aftermarket avionics panel installed. The manufacturer of the aftermarket avionics panel provided installation information indicating that maintenance personnel must bend the collective locking plate so that it springs away from the collective lock button. Examination of the accident aircraft's locking plate revealed that the bend was likely reversed, allowing the locking plate to spring toward the lock button.