Napa County Airport
Aircraft Accident/Incident Report

Napa, California 94558
Wednesday, April 30, 2003 19:20 PDT

The pilot made a forced landing after the helicopter experienced an engine over speed and subsequent engine fire when recovering from a practice autorotation. At 600 feet and 80 knots the pilot under instruction (PUI) entered the practice autorotation after the certified flight instructor (CFI) rolled the collective twist grip to idle to simulate an engine failure. The PUI executed the practice autorotation towards the runway. During the recovery sequence the PUI inadvertently and unknowingly engaged the manual fuel control slide lock on his collective as he adjusted the collective for a power recovery, allowing the CFI to twist the collective grip past the "VOL" (fly) position. The engine and rotor RPM oversped due to excessive fuel flow to the engine, which resulted in a catastrophic failure of the turbine section. The airport tower notified the crew that the helicopter was on fire. The crew made a force landing on the runway, egressed, and fire crews arrived to extinguish the fire. The collective twist grip has two normal operating positions. The "MIN" position sets the engine at idle, while turning the grip counter clockwise to the "VOL" (flight) position stop accelerates the engine to a flight rpm setting. At the "VOL" position the DECU (Digital Engine Control Unit) controls the engine power to maintain rpm as the pilot moves the collective. A slide lock device prevents the twist grip from moving beyond the "VOL" position. The manual fuel control slide lock is only on the right pilot seat collective. The twist grip can be moved beyond the "VOL" position by moving a slide lock lever on the collective twist grip forward and rotating the twist grip beyond the "VOL" position. Once the twist grip is out of the "VOL" detent position the pilot input has priority over the DECU and the pilot is manually metering fuel to the engine; this could allow the pilot to increase the amount of fuel delivered to the engine beyond what the DECU is delivering. The slide lock is spring loaded to its retracted locked position unless moved forward, usually by the pilots thumb, approximately 5 mm. At this point it is then "latched" open allowing the twist grip to be rotated beyond the "VOL" position. Post accident examination and testing of the engine fuel control unit and the manual slide lock mechanism found no preimpact mechanical malfunctions or failures. The helicopter manufacturer is aware of four prior inadvertent manual throttle activations that resulted in engine overspeed conditions and damage to both the engines and airframe structures. The manufacturer has discontinued installing the twist grip mechanical flight stop device and replaced it with an electrical solenoid type of configuration on all production AS350-B3 helicopters. This new configuration is also available to current owners and operators as a hardware modification.

the pilot under instruction's inadvertent activation of the collective manual fuel slide lock, which led to engine and main rotor overspeeds due to excessive fuel flow during power application. This resulted in failures of the gas generator turbine (N1) blades, power turbine (N2) blades, and created an external engine fire. A factor in the accident was the manufacturer's inadequate design of the twist grip slide lock, which had insufficient safeguards to preclude inadvertent activation.