What happened
On September 13, 2006, a Hughes 269 C helicopter was conducting authorized autorotation training maneuvers south of the Waizenhofen landing site near Thalmässing. The flight involved two pilots, with the intention of practicing the procedure after a demonstration had been performed from a higher altitude.
During the maneuver, the pilot initiated an autorotation from approximately 300 ft above ground level. While the main rotor RPM remained within the green operating range during the initial descent, the pilot attempted to increase airspeed at approximately 150 ft above the ground. During this attempt, the main rotor RPM decayed into the red arc. As the pilot attempted to flare the aircraft to arrest the descent, the engine failed to respond sufficiently to the power demand to recover the rotor speed. Consequently, the helicopter struck the soft field with significant force. The forward momentum on the uneven soil caused the aircraft to trip, resulting in a rollover.
The investigation
The BFU examined the wreckage of the Hughes 269 C and documented all control settings and instrument indications. The investigation included a technical review of the HIO-360 engine, specifically inspecting the spark plugs from cylinders 1 and 3, which were found to be clean and light brown. The investigation also verified the functionality of the dual throttle controls, confirming that both operated the engine linkage, and checked the integrity of the flight controls, including the swashplate, cyclic, and pedals.
Findings
- The primary cause of the accident was the failure to maintain sufficient main rotor RPM during the transition from descent to flare.
- The pilot's attempt to increase airspeed at a low altitude led to a critical decay in rotor speed.
- The engine did not provide enough power to recover the RPM before ground impact.
- The soft nature of the field soil contributed to the sudden deceleration and subsequent rollover of the aircraft.
One pilot sustained light injuries, while the aircraft was heavily damaged.