Incident Summary
The pilot reported that while cruising at 700 feet above ground level, the helicopter unexpectedly yawed to the left. Immediately afterward, a low rotor caution light illuminated in the cockpit. The pilot initiated an autorotation descent to land in a flat, wet, and muddy area under dark night conditions. During the landing, the helicopter's skids began to dig into the soft surface, causing the helicopter to nose over. In an effort to prevent the nose-over, the pilot applied aft cyclic control input. However, the rotor blades then struck and severed the tailboom.
Post-Crash Examination
Following the accident, examination of the helicopter revealed that it still contained 51 gallons of fuel. The skids showed no signs of spreading deformation, indicating a relatively gentle touchdown despite the muddy terrain. Further inspection focused on the engine control system. Investigators found that the 'B' nut on the PC (power control) air line, which connects the power turbine governor to the fuel control unit, was backed off by approximately 2 to 3 flats (turns).
Engine Testing
The engine was removed and successfully operated in a test cell in its 'as received' condition, demonstrating that it could run normally. During subsequent tests, the 'B' nut was alternately tightened and loosened by one flat, but no variation in engine performance was observed. When the nut was completely removed, the engine experienced a stagnant start, failing to accelerate properly. According to the manufacturer, a breach in the PC air line allows air pressure to bleed out, which causes the fuel control unit to automatically shift to a minimum flow condition—below idle power. However, the test cell conditions may not have replicated the vibratory environment present in the aircraft's airframe at the time of the power loss.