What happened
On the afternoon of September 1, 1991, a Cessna F172N bearing registration HB-CNP departed from Birrfeld Airport with a private pilot and three passengers aboard. The aircraft was rented for a VFR flight and was significantly overloaded, with a takeoff weight of 1053 kg exceeding the maximum allowable limit of 1043 kg by 10 kg. The weather conditions were hot, with temperatures running 14°C above standard, which further degraded performance.
The pilot flew over Lake Lucerne and the Bürgenstock ridge before turning toward Weggis to locate his parents' home. Witnesses observed the aircraft performing a series of low-level maneuvers, including banking turns and wing-waving gestures directed at the ground. As the aircraft approached a steep, forested mountainside near Oberwilen, it was flying at low speed with reduced power. The pilot then applied full throttle and pulled back on the controls to gain altitude over a small treeline. This aggressive pitch-up maneuver caused the nose to rise sharply. The aircraft subsequently banked left toward the rock face and entered a spin, crashing into the terrain. All four occupants were killed instantly.
The investigation
SUST examined the wreckage, which was found in a meadow with the engine still delivering power at impact (approximately 2500 RPM). The propeller blades showed signs of rotation, confirming the engine was running. Medical examinations confirmed death by trauma and ruled out alcohol or medical incapacitation. Technical inspections revealed no pre-existing mechanical failures; the aircraft was airworthy except for the overload.
Findings
The accident resulted from a combination of factors:
- Pilot Inattention: The pilot focused on greeting family rather than monitoring flight parameters.
- Poor Tactical Decision: Approaching steep terrain at low speed and high angle of attack left no margin for error. The correct escape maneuver would have been a right turn toward the lake, not a left turn into the mountain.
- Performance Limitations: The aircraft was overloaded by 10 kg, and high temperatures reduced engine power and lift capability.
Safety action
The report highlights the dangers of "relative flights" where pilots fly low to greet acquaintances. It emphasizes that such maneuvers in mountainous terrain require constant attention and adequate speed to avoid stalls.