What happened
During a training flight, a pilot under instruction was performing a practice autorotation in an AS3CO B3 helicopter. At an altitude of 600 feet and a speed of 80 knots, the certified flight instructor simulated an engine failure by rolling the collective twist grip to idle. The student pilot executed the practice autorotation toward the runway.
During the recovery phase of the maneuver, the pilot inadvertently engaged the manual fuel control slide lock on the right-seat collective while adjusting the collective for power recovery. This action allowed the instructor to rotate the collective grip past the "VOL" (fly) position. The resulting excessive fuel flow caused both the engine and rotor RPM to overspeed, leading to a catastrophic failure of the turbine section. After the tower notified the crew that the helicopter was on fire, the crew performed a forced landing on the runway. All occupants were not injured, and they successfully egressed the aircraft before fire crews arrived to extinguish the flames.
The investigation
Investigators examined the engine fuel control unit and the manual slide lock mechanism. Post-accident testing revealed no mechanical malfunctions or failures occurred prior to the impact. The investigation noted that the twist grip features a slide lock device intended to prevent movement beyond the "VOL" position, but this can be bypassed by moving the lever forward. Once bypassed, pilot input takes priority over the Digital Engine Control Unit (DECU), allowing for manual fuel metering.
Findings
- The engine and rotor RPM overspeed was caused by excessive fuel flow to the engine.
- The pilot unintentionally moved the manual fuel control slide lock into a position that allowed the twist grip to be rotated beyond its normal flight limit.
- The manufacturer had previously identified four similar incidents involving inadvertent manual throttle activations resulting in engine overspeed and structural damage.
- As a result of these known risks, the manufacturer has replaced the mechanical flight stop device with an electrical solenoid configuration on all new AS350 B3 production models.