On September 29, 2002, an Eurocopter AS350-B3 (registration N352SA) operated by US Dept OF Agriculture, US Forest Service was involved in an aviation accident near Bishop, CA. Investigators recorded the probable cause as: The collective locking mechanism failed to hold the collective in the full down position due to wear on the locking tab. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a pre-flight hydraulic actuator check, a helicopter's main rotor struck the ground, causing two revolutions and a rollover that destroyed both rotor systems. A small fire was extinguished. Investigation found a worn collective locking mechanism failed to hold the collective down.
Accident Details
The helicopter was performing a hydraulic actuator check for the first flight of the day. With the collective placed in the down position and the collective lock engaged, the rotor was powered up to 100 percent flight idle. After the pilot depressed the hydraulic test switch, he moved the cyclic fore and aft to confirm remaining pressure in the accumulators for a few control movements. The collective then 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.
Investigation Findings
Examination of the collective locking mechanism (part numbers 350A-27 3455-20 and 350A-27 3107-26) revealed that it failed to hold the collective in the full down position due to wear on the locking tab. During the servo accumulator checks, movement of the cyclic control after accumulator depletion caused the collective to move due to feedback in the control system. The locking device disengaged, and the collective increased, causing the aircraft to move.
Probable cause
The collective locking mechanism failed to hold the collective in the full down position due to wear on the locking tab.
What happened in the 29 Sept 2002 Eurocopter AS350-B3 accident near Bishop, CA?
During a pre-flight hydraulic actuator check, a helicopter's main rotor struck the ground, causing two revolutions and a rollover that destroyed both rotor systems. A small fire was extinguished. Investigation found a worn collective locking mechanism failed to hold the collective down.
What aircraft was involved and where did it happen?
The accident on 29 Sept 2002 involved an Eurocopter AS350-B3, registration N352SA, operated by US Dept OF Agriculture, US Forest Service, at Bishop, CA.
What was the probable cause of the accident?
The collective locking mechanism failed to hold the collective in the full down position due to wear on the locking tab.
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