What happened
On January 22, 1977, a Hughes 269C helicopter, registration HB-XPL, was performing a private flight from Schindellegi to the Lauberhorn race near Grindelwald. The flight included two passengers. The aircraft was flying in formation with another helicopter of the same type.
While traversing the Lütschental towards the Faulhorn pass, the trailing aircraft began to lose altitude. The pilot attempted to maneuver away from the terrain by turning right, but the aircraft entered a descent. In an attempt to regain airspeed, the pilot lowered the nose, which further increased the rate of descent. Facing an impassable ridge ahead, the pilot attempted an emergency landing on the sloping, snow-covered terrain. The helicopter struck the ground, sliding approximately 35 meters down the 19-degree slope before the fuselage overturned.
The investigation
SUST examined the mechanical condition of the Lycoming HIO-360-DIA engine and the fuel system. While the Fuel Control Unit showed a tendency to provide a lean mixture at high RPM, investigators found no evidence of a sudden power loss. The drop in manifold pressure reported by the pilot was attributed to the pilot's own control inputs rather than mechanical failure.
Environmental conditions at the time included moderate turbulence in the valley and potential Föhn wind gusts. The investigation also reviewed the aircraft's weight and balance, finding that the takeoff weight and center of gravity were within legal limits. However, the investigation noted that the aircraft's performance capabilities were insufficient for the altitude and temperature at the time of the incident.
Findings
- The pilot failed to maintain the legally required minimum flight altitude of 150 meters above ground level.
- The pilot utilized an inappropriate flight tactic by reducing airspeed to 30 mph, which was significantly below the manual's recommended 47 mph for the best rate of climb.
- The pilot did not sufficiently account for wind influences, specifically the potential for sudden gusts that could push the aircraft into a low-speed descent.
- The aircraft entered a state of high-angle rotor loading, potentially leading to a vortex ring state, which the pilot could not recover from due to insufficient altitude.