Introduction
On 13 June 1981 at 12:10 local time, an Enstrom F28A helicopter, registered as HB-XEB, was involved in an accident at Oberlunkhofen, Switzerland. The aircraft was owned by Flugschule Eichenberger AG and operated as a private flight. The pilot, a Swiss citizen born in 1946, held a valid private helicopter pilot license with an endorsement for the accident aircraft type. He had accumulated 75 total flight hours, with 10 hours in the previous 90 days.
Flight History
The pilot and a passenger departed from Buttwil airfield at 11:30 for a private sightseeing flight over Rotkreuz, Zugerberg, Aegeri, Zug, and the Reusstal valley. While flying north near Oberlunkhofen, the passenger, an architect, spotted two new buildings on a west-facing slope. The pilot flew a left turn at approximately 50 m above ground and crossed the buildings from east to west, reducing speed to near hover. As the helicopter began to rotate around its vertical axis, the pilot noticed a decrease in engine RPM of 500 min⁻¹. Despite fully opening the throttle and lowering the collective, the engine RPM did not recover quickly enough to allow safe flight. The helicopter settled onto the slope from a height of about 6 m.
Findings and Damage
The helicopter was destroyed. The passenger, seated on the right, sustained back injuries. The pilot was not injured. Post-accident examination revealed no pre-existing mechanical defects. The helicopter had a total airframe time of 8,018 hours, and the Lycoming HIO-360-C1A engine had 2,098 hours since new. The weight at the time of the accident was approximately 965 kg, within the maximum takeoff weight of 976 kg, and the center of gravity was within limits.
Meteorological Conditions
According to the Swiss Meteorological Institute, the general weather was dominated by a high-pressure system with a northeasterly wind (Bise). At the accident site, wind was from the northeast at 5-10 kt with gusts up to 15 kt. Clouds were 2-4/8 Stratus with base around 1000 m, and 6-8/8 Stratocumulus with base at 1400 m. Visibility was 5-8 km. Temperature was 19°C, dew point 11°C, and QNH 1028 mbar. The sun azimuth was 141° with elevation 61°.
Height-Velocity Diagram
The flight manual for the Enstrom F28A includes a height-velocity diagram indicating an unsafe flight region. At the moment of RPM drop, the helicopter was outside the safe flight area as defined by that diagram.