What happened
On December 14, 2002, an SA315B Lama, registration HB-XTY, operated by Air Zermatt, was conducting a passenger flight near the Breitholog Glacier in the Zermatt region. During a routine flight at approximately 10:15 h, while flying at an altitude of 3,700 meters AMSL, the pilot heard a loud detonation accompanied by intense vibrations. The aircraft immediately began to descend.
Recognizing the emergency, the pilot attempted to manage the descent by adjusting the collective pitch. However, the pilot found that the collective pitch control was no longer responding effectively. While the cyclic and pedals remained functional, the aircraft's ability to control the rotor RPM and vertical descent rate was severely compromised. The pilot focused on finding a safe landing spot on the glacier, avoiding crevasses, and broadcast a "mayday" signal.
As the helicopter approached the ground at approximately 20 to 30 meters, the pilot attempted a flare to reduce the descent rate, but the pitch control failed to respond. The aircraft struck a snowdrift on the glacier, causing the main rotor to detach and the airframe to sustain heavy damage. Despite the violence of the impact, all three occupants—the pilot and two passengers—sustained only minor injuries or none at all.
The investigation
The investigation focused on the mechanical integrity of the flight controls. Investigators examined the collective pitch combiner, specifically looking for cracks or structural failures. The examination revealed that a mounting bracket (patte de fixation) had suffered a fatigue fracture.
Detailed analysis of the hollow axis within the mechanism showed evidence of improper assembly. Wear patterns and the condition of the passivation layer indicated that the axis had been positioned off-center for a significant period. Specifically, the investigation found that at least one of the two circlips intended to secure the axis in place was missing or improperly installed, allowing the axis to slide laterally.
Findings
- The primary cause of the accident was a collision with the terrain resulting from the loss of control due to mechanical failure.
- The mechanical failure was triggered by the disengagement of the articulation axis, which allowed it to shift out of position.
- This displacement caused the remaining mounting bracket to bear the full flight loads, leading to a fatigue fracture.
- The failure of the circlip prevented the axis from being properly secured, rendering the collective pitch control partially inoperative.