What happened
On January 28, 1968, a Hughes 300 helicopter, registration HB-XCC, was conducting a training flight over the Jura mountains near La Dôle, Switzerland. The flight, which included the pilot and one passenger, departed from Geneva airport at approximately 16:15 local time.
While flying near the summit of La Dôle, the pilot performed a left turn and then descended slightly while turning. As the aircraft approached the mountain crest, it suddenly began to spin clockwise. The pilot was unable to arrest this rotation, and the helicopter descended into the snow on the western slope of the mountain. Despite the impact and the aircraft sustaining heavy damage, both occupants escaped without injury.
The investigation
Investigators examined the wreckage and the pilot's flight history. The aircraft's maintenance records showed it had been properly serviced, with the last inspection occurring just ten days prior to the accident. The investigation also looked into the mechanical integrity of the tail rotor system, as witnesses reported seeing the tail rotor stop spinning during the descent.
Technical analysis of the tail rotor drive shaft revealed that the shaft had snapped due to abnormal stresses. However, investigators ruled out a mechanical failure of the transmission or the tail rotor blades as the primary cause. Instead, the breakage was determined to have occurred as a result of the impact with the ground. Furthermore, the investigation noted that the aircraft was equipped with an older, less efficient tail rotor model, as the operator had opted not to upgrade to the newer, more effective version provided by the manufacturer.
Findings
- The pilot's flight experience was relatively limited, with fewer than 30 hours of total rotorcraft flight time.
- The aircraft entered a state of uncontrolled rotation due to an inadequate flight maneuver or improper use of controls.
- The pilot likely encountered a situation where the torque from the main rotor could no longer be compensated for by the tail rotor, potentially due to low airspeed or operating at an altitude where the tail rotor's effectiveness was reduced.
- The loss of control occurred at an altitude too low to allow for recovery through autorotation.