What happened
On the morning of September 5, 2000, a SA315B helicopter, registration HB-XTW, departed from the Sex Rouge glacier for a cargo transport mission near Col du Pillon. The aircraft was operated by Air Glaciers S.A. with a single pilot on board. During the descent toward the destination, the engine power dropped significantly, followed by the appearance of white smoke from the turbine. The aircraft entered a left-hand rotation and lost altitude rapidly, crashing in a clearing near Lac Retaud at approximately 1,600 meters. The pilot sustained fatal injuries at the scene.
The investigation
Investigators examined the wreckage and the engine, finding no evidence of pre-existing mechanical failure or flight control issues. The investigation focused on the aircraft's storage conditions the previous day. Due to deteriorating weather, the helicopter had been parked on a platform tilted backward and covered with protective tarps. While the crew had removed covers and de-iced the rotor blades the following morning, they did not dismantle the air intake grilles.
Technical analysis of the engine and fuel revealed that while the fuel and oil were within normal parameters, the air intake design presented a specific vulnerability. The investigation also reviewed the cockpit instruments, noting that certain warning lights were active at the time of impact, though the pilot's ability to notice them was hindered by the flight phase and external focus.
Findings
- The aircraft was parked on an incline that prevented water from draining out of the air intakes.
- Because the air intake drains are located only at the front, water stagnated at the rear of the intakes due to the backward tilt of the aircraft.
- Overnight freezing temperatures caused this trapped water to form ice plates within the intake.
- During the descent, vibrations and heat from the engine caused these ice plates to detach and be ingested by the turbine.
- The ingestion of ice caused an undetected engine flameout.
- The pilot was likely unable to perceive the power loss immediately because the aircraft was in a low-power descent phase, and his attention was directed toward external landing site coordination.
- The helicopter lacked an acoustic alarm to alert the pilot to a drop in turbine or rotor RPM.
Safety action
- It is recommended that aviation authorities study modifications to the Artouste III engine air intakes to ensure proper drainage regardless of the aircraft's parking attitude.
- Authorities should consider the implementation of an acoustic alarm system to alert pilots to a loss of turbine or rotor RPM, particularly during high-workload external load operations.