Flight History
On 28 June 1985 at 1815 local time, a flight instructor and a student pilot departed from Grenchen Airport in a Bücker 131/APM150, registration HB-UUL, for an aerobatic training flight, which also served as the student's conversion to the type. Over the Limpachtal valley south of the airport, the crew practiced spins and figures required for the aerobatic exam. After 15 to 20 minutes, the practice area was moved to the vicinity of the nearby highway. Later, the aircraft flew over Grenchen Airport. During this overflight, the student switched the fuel cock from the "Acrobatic" position to the "Cruise" position. Noticing at about 1200 m that the instructor intended to continue aerobatic training, the student returned the fuel cock to the "Acrobatic" position.
After the aerobatics, the aircraft descended toward the end of the downwind leg for runway 07. According to the student, he then switched the fuel cock back to "Cruise" and locked it in that position. Following a high approach with a left sideslip onto runway 07 (hard surface), the aircraft touched down shortly after midfield.
Accident Sequence
The instructor intended to perform several touch-and-go landings with the student. After landing, he set the engine to full power. According to the student (the instructor could not recall precisely due to retrograde amnesia), the engine began to sputter, and the instructor moved the throttle back and forth. The engine then ran normally again. The student stated that this sputtering during touch-and-go was a familiar occurrence for HB-UUL. The aircraft lifted off and entered a climb. The instructor was flying the aircraft; the student followed on the dual controls and throttle.
Approximately 10 to 15 seconds after takeoff, the engine stopped suddenly and without warning. The student reported that the instructor moved the throttle back and forth without effect, simultaneously lowering the nose and initiating a right turn-back. After completing the turn, the aircraft collided with a tree in a row of trees perpendicular to the approach direction, in the "Riedergraben" area, and crashed to the ground. The accident site was about 210 m beyond the end of runway 07 (hard surface) and about 165 m south of the runway centerline. Both pilots were trapped in the wreckage and seriously injured. They were freed by airport helpers and the fire brigade, which was alerted immediately.
Investigation Findings
Both pilots held valid and appropriate licenses and were authorized to conduct the flight. The aircraft was certified for flight; its weight and center of gravity were within limits. Before the flight, 65 to 70 liters of fuel were in the tank, filled to that quantity by another instructor and student, as was customary for aerobatic flights. Flight time until the accident was 40 minutes, including climbs, aerobatics, and descent. At an average consumption of 40 l/h, about 30 liters were used, leaving a remaining quantity of 35 to 40 liters at the time of the accident.
After the accident, the fuel cock (indicator next to the engine) was in the "Off" position. The fuel cock control lever in the rear seat was in the "Cruise" position; in the front seat, no determination was possible due to severe damage. The control linkage to the fuel cock was heavily deformed by the impact.
The engine and fuel injection system were bench-tested after the accident, revealing no evidence of pre-existing defects that could have caused or contributed to an engine malfunction. The pendulum hose for the "Acrobatic" fuel pickup met manufacturer specifications.
The terrain beyond runway 07 is flat and offers sufficient emergency landing opportunities, although some meadows had tall grass.
A ground test conducted on 20 July 1985 with a similar aircraft (HB-UUY) with the engine running and fuel cock closed yielded:
- 1.5 minutes idle, then full throttle: engine stopped after 3-5 seconds.
- 0.5 minute idle, then full throttle: engine stopped after 5 seconds.
- Full throttle (then fuel cock to 'Off'): engine stopped after 7 seconds.
Flight tests on 18 September 1985 at Grenchen on runway 07 with HB-UUY (two persons on board, same fueling, wind 110/4-6 kt) showed that reaching the accident site after a touch-and-go (landing at midfield) and normal climb rate, followed by a turn-back, was only possible if the engine stopped about 15 seconds after takeoff, when the aircraft was already at 100 m altitude.
Cause
The accident is due to a failed emergency landing following an engine failure shortly after takeoff for unknown, non-technical reasons, and a collision with trees after completing a turn-back.