What happened
On May 30, 1977, a student pilot was conducting a solo navigation training flight in a Hughes 369 HS, registration HB-XFG, departing from Lausanne. The flight was part of a triangular route intended to cover the areas of Lausanne, Froideville, Môtiers, and Gruyères.
At approximately 18:05, witnesses near Petit-Pré Baillod observed the helicopter traveling at high speed. Suddenly, an explosion-like sound was heard, and the tail boom containing the anti-tail rotor detached from the main fuselage. Following the separation, the aircraft began to rotate clockwise and lost altitude, ultimately crashing into a forest at 18:06. The impact resulted in the death of the pilot and the total destruction of the aircraft. There were no other fatalities, though some damage was sustained to the forest area.
The investigation
Investigators examined the wreckage and the flight history of the aircraft. The engine and transmission components were found to be in good working order, with no evidence of mechanical failure being the primary cause. The investigation noted that the aircraft's weight and center of gravity were within prescribed limits.
Evidence from the wreckage showed that the main rotor blades had struck the tail boom, severing it. The investigation also reviewed the pilot's experience, noting that while he had significant training hours under supervision, he had very limited experience flying solo. Furthermore, the meteorological conditions at the time were challenging, characterized by stormy weather, high humidity, and moderate to strong turbulence near the mountain ridges.
Findings
- The primary cause of the accident was likely that the pilot was flying at or exceeding the maximum allowable speed of 240 km/h.
- In an attempt to reduce speed, the pilot likely performed a sudden "quick-stop" or flare maneuver.
- Due to the loss of control effectiveness at high speeds, the sudden cyclic input caused the main rotor to strike and sever the tail boom.
- Pilot fatigue contributed to the accident, as the pilot had already completed approximately five hours of flight that day.
- The lack of experience in navigating difficult terrain and adverse atmospheric conditions further contributed to the event.