What happened
On 12 December 2010, a Eurocopter AS 350 B2 helicopter, registered C–FORS, was performing cargo slinging operations near the Pickle Lake Airport in Ontario. The aircraft, operated by Forest Helicopters Inc., was in the process of moving fuel barrels using a 100-foot longline. After lifting the load above nearby trees, the pilot began transitioning into forward flight at approximately 250 feet above ground level.
During this transition, the engine suddenly lost power. The pilot reacted by lowering the collective, applying forward cyclic to maintain airspeed, and jettisoning the fuel barrels. The pilot attempted an autorotative landing, but the low altitude left insufficient margin for a safe recovery. The helicopter struck the ground in a level attitude, causing a main rotor blade to sever the tail boom. The aircraft slid approximately 6 and 0 feet before coming to a stop. The pilot escaped the wreckage uninjured.
The investigation
Investigators examined the engine and fuel system to determine the cause of the power loss. While no mechanical failures or blockages were found, testing revealed that air could become trapped within the fuel system. The investigation focused on how air might have entered the system through a leaking FCU NTL or Ng drive fuel-pump seal, which had been identified during troubleshooting ten hours prior to the flight.
Further examination looked into maintenance practices, specifically the draining of the Le Bozec airframe filter. It was noted that draining this filter with the boost pumps turned off could inadvertently introduce air into the fuel system. Additionally, the investigation noted that the Arriel 1D1 engine lacks an auto-ignition system, which limits the window for attempting an engine relight following a flameout.
Findings
- Air entrapment in the fuel system caused an engine flameout due to an interruption in fuel flow.
- A leaking FCU NTL or Ng drive fuel-pump seal, combined with specific boost pump check valve designs, likely allowed air to enter the fuel system.
- The engine power loss occurred at an altitude that did not allow for a safe landing.
- The practice of draining the airframe filter with boost pumps inactive increases the risk of introducing air into the system.
- The lack of an auto-ignition system on the Arriel 1D1 engine provides insufficient time for an engine relight after a flameout.