What happened
On December 20, 1964, during a local landing competition at Birrfeld Aerodrome, a Grunau-Baby II glider, registration HB-188, was being towed for a flight. During the takeoff roll, the aircraft's left wing made contact with the ground, causing the glider to veer to the left. Instead of releasing the tow cable, the pilot applied significant back pressure on the elevator. This action, combined with the aircraft's trim state, caused the glider to lift into an extremely steep, uncontrolled climb.
At an altitude of approximately 6 to 10 meters, the tow cable snapped. The sudden loss of tension caused the glider to lose airspeed and enter a slip, leading it to strike the roof of a hangar. The aircraft then became entangled in overhead tension cables for a workshop's power supply. The pilot escaped the aircraft without injury, though the glider sustained moderate damage to both wings.
The investigation
The investigation examined the aircraft's technical condition, the pilot's experience, and the environmental factors at the time of the accident. The HB-188 was found to be in airworthy condition, with no mechanical defects identified. Investigators also reviewed the pilot's flight history, noting that while experienced, the pilot had not flown this specific model for approximately three years.
Technical checks revealed that the aircraft was operating in a tail-heavy configuration. Because the pilot's weight (approximately 60 kg) was significantly below the required threshold, the center of gravity was 6 cm outside the permitted longitudinal range. Additionally, the investigation found that the tow cable's breaking strength was 540 kg, which exceeded the 350 kg limit specified in the aircraft's logbook.
Findings
- The aircraft was improperly trimmed; the pilot's low weight resulted in a tail-heavy center of gravity, making the glider prone to rapid, steep climbs.
- The pilot failed to release the tow cable immediately after the wing touched the ground, instead attempting to use the elevator to lift the aircraft.
- A crosswind and propeller wash from the towing aircraft likely contributed to the initial leftward veer and wing strike.
- The use of a breaking strength of 540 kg instead of the mandated 350 kg meant the cable did not break at the designated weak link, but rather snapped elsewhere in the line.