What happened
On July 17, 1968, a Piper Cherokee PA-28-180, registration HB-OZD, was conducting a training flight at Grenchen Airport when an unexpected loss of power occurred during a touch-and-go maneuver. The flight, which involved an instructor and a student pilot, was in the process of a go-around after a landing. As the student pilot reached for the flap lever, the instructor abruptly moved the throttle to idle.
Because the engine power was removed while the aircraft was still accelerating on the runway, the aircraft could not be stopped within the remaining paved surface. The aircraft overran the end of the runway, crossed a road running perpendicular to the runway axis, and came to rest in a cornfield. The aircraft sustained damage to the landing gear, wings, and fuselage, but there were no injuries to the occupants.
The investigation
The investigation examined the sequence of events during the training session and the actions of both crew members. It was established that the instructor had previously warned the student that he would reduce power to idle if certain flight principles—specifically regarding hand placement on the throttle—were not followed. During the go-around, the instructor's sudden reduction of power caught the student pilot by surprise, preventing the student from immediately applying full power to maintain the climb.
Investigators also looked at the braking capabilities of the aircraft and the surface conditions. The runway surface was wet with rain, which significantly reduced the effectiveness of the brakes. Furthermore, the investigation reviewed the student pilot's decision to maintain a straight course across the road rather than attempting a sharp maneuver (a "cheval de bois") suggested by the instructor, which the student feared would cause greater structural damage.
Findings
- The primary cause of the accident was the inappropriate intervention by the flight instructor during a go-around that he had himself commanded.
- The instructor's decision to cut power for pedagogical reasons failed to account for the critical timing, the aircraft's weight, the remaining runway length, and the reduced braking friction on the wet grass.
- The instructor failed to utilize the aircraft's handbrake to assist in stopping the aircraft when he noticed the student's delayed reaction.
- The student pilot's decision to proceed straight across the road was considered appropriate, as the alternative maneuver posed a high risk of severe structural damage.