What happened
On September 4, 1999, an Agusta-Bell 206B helicopter, registration F-GFMX, was performing a series of private flight rotations near Vichères, France. During the final approach to a private helisurface at Le Petit Montaigu, the pilot heard a whistling sound from the engine, followed immediately by a loss of power and a low rotor RPM warning.
Believing the aircraft was too low to safely execute an autorotation, the pilot attempted to maintain a level attitude by pushing the cyclic forward to reach the intended landing zone. This maneuver caused the aircraft to lose altitude rapidly, leading to a violent impact with the ground. The crash resulted in one pilot and three passengers injured, with one passenger suffering serious injuries. The aircraft was destroyed.
The investigation
The investigation focused on the engine shutdown and the fuel management practices of the day. Technical examinations of the engine and fuel system showed that the turbine and fuel pumps were functional. However, investigators found fine black particles in the low-pressure fuel filter.
Analysis of the flight's fuel logs revealed that the pilot had been managing fuel levels very tightly to maintain weight limits for the various passenger loads. The investigation also reviewed video footage from a passenger, which confirmed the aircraft was flying at a height of approximately 30 meters and a speed of 30 knots at the time of the failure. This flight profile placed the helicopter in a vertical safety zone where an engine failure makes a successful autorotation nearly impossible.
Crucially, investigators examined the pilot's documentation. While the pilot had the flight manual, it lacked specific information regarding the minimum usable fuel quantity. The necessary limitation—stating that a minimum of 10 US gallons must be on board—was only present in the aircraft's airworthiness notes, which the pilot did not possess.
Findings
- The engine shutdown was caused by the ingestion of air into the fuel pump.
- As the pilot pitched the aircraft to decrease speed during final approach, the fuel surged to the rear of the tank, causing the forward-mounted boost pump to draw in air.
- The aircraft was operating with a fuel quantity below the manufacturer's minimum requirement.
- The pilot was unaware of the 10 US gallon minimum limit because it was not documented in the provided flight manual.
- The aircraft was not equipped with an optional low-fuel warning light that would have alerted the pilot when levels reached 20 US gallons.