What happened
On March 2, 1972, a Hughes 500 helicopter, registration HB-XCW, was conducting mountain landing training on the Tsanfleuron Glacier in Switzerland. The flight, operated by the Geneva Civil Protection Service, began with an instructor and a pilot performing a series of landings. After several successful maneuvers, the instructor exited the aircraft to allow the pilot to practice solo landings.
During the second solo attempt, the pilot attempted to land near the previous landing site. As the aircraft approached the surface, the powerful downwash from the rotor blades kicked up a significant amount of loose snow. This cloud of snow completely surrounded the cockpit, stripping the pilot of all external visual references.
Convinced the aircraft was sliding forward due to sensory illusions, the pilot applied cyclic input to correct the perceived movement. This caused the tail rotor to dip into the snow, leading to a sudden loss of directional control and a spinning motion. Although the pilot managed to stabilize the aircraft momentarily, a subsequent loss of control caused the helicopter to pitch forward and crash into the glacier. The impact resulted in severe damage to the airframe, including the separation of the tail boom.
The investigation
Investigators examined the aircraft's mechanical condition and the environmental factors at the time of the accident. The investigation focused on the flight dynamics during the whiteout conditions and the structural failure of the tail rotor drive shaft. The inquiry also reviewed the lighting conditions on the glacier, noting that a passing cloud had moved the terrain into shadow, reducing contrast just as the solo maneuvers began.
Findings
- The primary cause of the accident was a loss of control near the ground due to a lack of visual references.
- The heavy snow kicked up by the rotor wash created a localized whiteout, isolating the pilot from the surrounding environment.
- The pilot experienced sensory illusions, perceiving a forward slide that was not actually occurring, leading to improper control inputs.
- The tail rotor drive shaft suffered a static torsional failure caused by the deceleration of the tail rotor when it made contact with the snow.
- While the instructor had identified sufficient landmarks for a safe landing, the sudden loss of light and the snow cloud made these references impossible to maintain.