Accident Details
On 15 June 1966 at approximately 1634 hours local time, a Bücker-Lerche 131 R aircraft, registration HB-URN, crashed into a wooded ridge near Beromünster, Switzerland. The pilot, a 44-year-old male (born 1922), was killed, and the aircraft was completely destroyed. The accident occurred during aerobatic training in preparation for the national motor flight championships scheduled for 18–19 June 1966 at Beromünster.
Pilot Information
The pilot held a private pilot license issued on 29 August 1954, with aerobatic endorsement granted on 19 May 1961. He also held a limited commercial pilot license and a restricted radiotelephone operator certificate. His total flight experience was 651 hours and 3,060 landings, including approximately 40 hours of aerobatics and about 6 hours on the Bücker-Lerche type. Flight training began in August 1953, and aerobatic training started in March 1961. According to the investigation, no pre-existing health issues or flight incapacitation were found.
Aircraft Information
The Bücker-Lerche 131 R (a modified Bücker 131) was owned by the Motorfluggruppe Zürich of the Aero-Club of Switzerland. It had been converted in 1963 with a new Lycoming I0-360-B1B engine of 180 hp. On 31 May 1966, it received modified wings with a special profile for inverted flight, allowing a climb rate in inverted flight up to 7 m/s. The aircraft was single-seat, with a maximum takeoff weight of 610 kg, and all aerobatic maneuvers were permitted within load factors of +6 g and -3 g. The airframe had accumulated 1,758.08 hours total, with 219.33 hours since last overhaul. No technical defects were found in the wreckage examination.
Weather and Location
The accident occurred over the Chüewald ridge, a wooded hill rising about 150 m above the airfield, located 1.5 km east of Beromünster airfield. Coordinates: 659.310/226.820, elevation 800 m AMSL. Weather at the time was mostly cloudless with occasional high cloud, visibility over 30 km, light west wind, weak turbulence. QFE at Beromünster at 1630 was 936 mb. Switzerland was under a flat pressure pattern with persistent thunderstorm tendencies.
Sequence of Events
The pilot departed Beromünster at 1626 hours for aerobatic training intending to fly the compulsory program for the national championships. He began the first figure, a looping-avalanche, at about 1000 m above ground. He then performed a normal retourement and a "standing eight," during which he lost significant height due to overly wide circles. A subsequent inverted renversement was executed well but placed the pilot near the minimum altitude of 500 m above ground. He drifted laterally off the prescribed aerobatic axis (north-south) and crossed over the ridge east of the airfield. After a steep looping, the pilot transitioned to an inverted climb at 25–30°, rolled to normal flight, and continued climbing at the same angle. Observers believed he recognized his low altitude and would climb, but he instead initiated an inverted retourement from 200–250 m above the ridge (350–400 m above airfield). The low altitude and high entry speed led to impact with trees.
Investigation Findings
Examination of the wreckage revealed no pre-impact mechanical failure. The fuel tank was deformed but contained fuel; the fuel selector was in the "Akro" position; the throttle was found at full power and jammed; mixture was set to rich; propeller damage indicated high RPM; seat belts were fastened; altimeter in the rear seat was set to 925 mb; all other instruments were destroyed; trim was locked in a nose-down position. Weight and balance were within limits. No evidence of flight control jamming or malfunction was found.
The investigation noted that the pilot was generally hesitant toward high-g maneuvers and had difficulty maintaining position during aerobatics. The pressure of the last training opportunity before the championships may have led him to focus on figures and program flow at the expense of monitoring position and altitude. The shallow climb angle (25–30°) of the final figure also contributed to altitude loss. Whether a blackout effect played a role in the final phase could not be determined.
Probable Cause
The accident was caused by insufficient monitoring of space and altitude (ungenügende Raum- und Höhenüberwachung).