NTSB Factual Report

On September 29, 2002, about 0854 Pacific daylight time, an Eurocopter AS350-B3, N352SA, collided with terrain during a premature liftoff while engaged in a pre-departure hydraulic flight control check at the Bishop, California, airport. The U.S. Forest Service was operating the helicopter as a public-use flight under the provisions of 14 CFR Part 91. The commercial pilot and three passengers were not injured; the helicopter sustained substantial damage. The cross-country flight was departing the Bishop airport for Bridgeport, California. Visual meteorological conditions prevailed, and an agency flight plan had been filed.

Forest Service officials reported that the helicopter was performing a hydraulic actuator check for the first flight of the day. The collective had been placed in the down and locked position and the rotor powered up to 100 percent flight idle. After depressing the hydraulic test switch, the pilot moved the cyclic fore and aft (pumped) to confirm there was remaining pressure for a few control movements. The collective rose uncommanded and the helicopter moved forward in a nose down attitude. The main rotor struck the ground and the helicopter made two revolutions before rolling over onto its side, destroying both the main and tail rotor systems. A small fire ensued in the exhaust area, but was quickly extinguished.

Examination of the collective locking mechanism was accomplished by Forest Service investigators. According to their report, the collective control lock mechanism (P/N 350A-27 3455-20 and P/N 350A-27 3107-26) failed to hold the collective in the full down position (due to wear) while performing before takeoff checks. During servo accumulator checks, movement of the cyclic control after accumulator depletion caused the collectiove to move (feedback). The locking device disengaged and the collective increased causing the aircraft to move.

NTSB Probable Cause

the disengagement of the collective locking mechanism due to component wear.