What happened
On June 28, 1975, an American AA-1A, registration HB-UBM, was participating in the Swiss Motor Gliding Championships at Langenthal airfield. Following a landing, the pilot inspected the grass runway and noted that recent rainfall had left the surface soft and covered with several puddles. Despite these conditions, the pilot proceeded with a takeoff attempt.
During the takeoff roll, the pilot instructed a passenger to manually extend the flaps in stages. As the aircraft reached approximately two-thirds of the runway length, it encountered a large puddle that caused significant deceleration. Fearing that a continued takeoff was too risky, the pilot reduced power to abort the attempt. However, the aircraft was positioned near the end of the runway, adjacent to a field of tall wheat.
As the aircraft decelerated, the main landing gear remained only 20-30 cm above the ground. The aircraft struck the tall wheat, which caused the right wing to drag. This drag induced a 1/8-turn yaw, leading the aircraft to nose over and flip onto its back. The American AA-1A came to rest upside down approximately 28 meters beyond the runway end. The accident resulted in one fatality and one injury among the occupants.
The investigation
The investigation examined the aircraft's maintenance history, the pilot's credentials, and the specific takeoff techniques employed. The aircraft was found to be airworthy, with weight and balance within limits. The investigation also reviewed the manufacturer's manual regarding "soft field take-off" procedures, which mandate using full flaps and lifting the nose wheel to reduce drag.
Witness testimony from experts indicated that the pilot failed to lighten the load on the nose wheel during the takeoff roll. Furthermore, the investigation looked into the environmental conditions, confirming the runway was in poor condition due to previous rain.
Findings
- The pilot chose to proceed with the takeoff despite recognizing the poor condition of the runway during the pre-takeoff inspection.
- The manufacturer-specified takeoff procedures for soft fields were not followed, specifically regarding the use of full flaps from the start and the failure to timely relieve pressure on the nose wheel.
- The aircraft's high takeoff weight, combined with high wing loading and relatively low engine power, negatively impacted the takeoff performance.
- The accident was characterized not as a late aborted takeoff, but as a failed takeoff attempt caused by the interaction between the aircraft and the runway surface.