Incident Overview
A helicopter conducting autorotation training experienced an engine overspeed and subsequent engine fire during the recovery phase of a practice autorotation. The crew executed a forced landing on the runway, egressed safely, and fire crews extinguished the fire.
Sequence of Events
At an altitude of 600 feet and an airspeed of 80 knots, the pilot under instruction (PUI) entered a practice autorotation after the certified flight instructor (CFI) rolled the collective twist grip to the idle position to simulate an engine failure. The PUI maneuvered the helicopter toward the runway. During the recovery sequence, as the PUI adjusted the collective for a power recovery, he inadvertently and unknowingly engaged the manual fuel control slide lock on his collective. This action allowed the CFI to rotate the twist grip past the "VOL" (flight) position.
System Description
The collective twist grip has two normal operating positions: "MIN" (idle) and "VOL" (flight). Rotating the grip counterclockwise to the "VOL" stop accelerates the engine to flight RPM. At the "VOL" position, the Digital Engine Control Unit (DECU) manages engine power to maintain RPM as the pilot moves the collective. A slide lock device prevents the twist grip from moving beyond "VOL". However, moving the slide lock lever forward approximately 5 mm latches it open, allowing the twist grip to rotate past "VOL". Once beyond the detent, pilot input overrides the DECU, enabling manual fuel metering that can exceed DECU-delivered fuel.
Engine Overspeed and Fire
With the twist grip moved past "VOL", excessive fuel flow to the engine caused engine and rotor RPM overspeed, leading to a catastrophic failure of the turbine section. The airport tower notified the crew that the helicopter was on fire. The crew performed a forced landing on the runway and evacuated, after which fire crews arrived and extinguished the fire.
Post-Accident Findings
Post-accident examination and testing of the engine fuel control unit and the manual slide lock mechanism revealed no preimpact mechanical malfunctions or failures. The helicopter manufacturer reported four prior inadvertent manual throttle activations that resulted in engine overspeed conditions and damage to engines and airframe structures.
Manufacturer Actions
In response, the manufacturer discontinued installing the twist grip mechanical flight stop device on all production AS350-B3 helicopters and replaced it with an electrical solenoid configuration. This new configuration is also available as a hardware modification for current owners and operators.