What happened
On May 28, 1999, a Bell 206B Jet Ranger, registration F-GFDN, was conducting a VFR ferry flight near Robertval, France. Approximately ten minutes after departing Grand-Fresnoy, the aircraft's turbine abruptly dropped to idle power. This was accompanied by an engine failure warning and a low rotor RPM alarm.
As the pilot was flying over a wooded area, they initiated a left turn to reach a suitable landing site, eventually attempting an autorotation into a wheat field. During the descent, the helicopter's skids sank into the crops, creating a tendency for the aircraft to pitch forward. In an instinctive attempt to prevent this forward tilt, the pilot applied significant aft cyclic input. This movement caused the main rotor blades to strike the tail boom, severing it and destroying the rear stabilizer. The impact caused the aircraft to pivot 180 degrees on its vertical axis before coming to a halt on its skids.
The investigation
The investigation focused on the sudden loss of engine power and the subsequent structural failure. Investigators examined the turbine and found no internal damage or lubricant leaks. However, they identified a rupture in the Compressor Discharge Air Pressure (Pc) piping at the connection to the fuel regulator.
Metallurgical analysis of the Pc piping revealed that the failure was caused by fatigue. The investigation also found that the piping lacked the required anti-vibration clamps. The examination of the tubing showed abnormal deformation and evidence of friction against the mounting nut, which had led to the fatigue crack.
Findings
- The engine power loss was caused by the rupture of the Pc piping, which prevented the compressor discharge pressure information from reaching the fuel regulator.
- The absence of anti-vibration clamps on the Pc piping allowed for excessive movement and vibration.
- This lack of support led to high-stress levels and friction between the tubing and the mounting hardware, resulting in a fatigue failure.
- The fatigue crack had likely been progressing over several previous flights.
- The structural failure of the tail boom was caused by the pilot's excessive aft cyclic input during the landing attempt, which moved the rotor blades into the path of the tail boom.