What happened
On August 25, 1999, an Enstrom 280C helicopter, registration HB-XNR, was conducting a private sightseeing flight near Uetliberg, Switzerland. The flight, which included a pilot and one passenger, was intended to include stops at Uto Kulm and Wangen-Lachen.
During the approach to the Uto Kulm landing platform, the pilot attempted a landing out of ground effect (HOGE). As the aircraft transitioned into a hover, the airspeed decreased significantly. Approximately 50 meters from the platform, the helicopter began to yaw to the right, accompanied by a continuous loss of engine RPM. The pilot attempted to maneuver the aircraft toward level terrain, but the loss of power prevented a successful recovery. The helicopter struck the steep, wooded eastern slope of the Uptliberg, resulting in two serious injuries and the destruction of the aircraft.
The investigation
Investigators examined the aircraft's weight and balance calculations and the pilot's flight preparation. While the pilot had used a detailed spreadsheet to calculate fuel, load, and performance, the investigation revealed that the calculations were based on incorrect data. Specifically, the pilot used an empty weight figure from a training manual that was approximately 200 lbs lower than the actual empty weight of HB-XNR.
Technical inspections of the helicopter found no mechanical defects or pre-existing failures. Video footage from the passenger confirmed that the engine was effectively stalled due to the excessive load relative to the prevailing density altitude. The investigation also noted that the aircraft's official flight manual was not on board during the flight.
Findings
- The helicopter was overloaded by approximately 106 lbs for the prevailing density altitude conditions.
- The pilot utilized an incorrect empty weight for the aircraft, leading to an inaccurate mass calculation.
- The pilot's approach profile left insufficient altitude to recover speed or descend to level ground once the power loss began.
- The pilot's training level did not include specific instruction for complex high-altitude or HOGE landing maneuvers.
- The lack of the correct empty weight information in the cockpit contributed to the calculation error.