What happened
On May 22, 1966, during active glider operations at the Amlikon airfield, a De Havilland Chipmunk, registration HB-TUB, became entangled with a heavy winch cable during its takeoff roll. The incident began following a disrupted flight schedule caused by a previous glider's failed launch. To avoid a collision with a glider being moved on the field, the HB-TUB pilot had to steer significantly to the right, which resulted in the aircraft rolling over the extended winch cable.
The pilot was unaware that the cable, approximately 1,000 meters long and weighing 80 kg, had become caught on the aircraft's tailwheel. While the glider pilot successfully unhooked the tow rope, the engine pilot continued the climb. As the aircraft attempted a right-hand turn at an altitude of only 40 meters, the trailing cable snagged on a group of trees, creating immense drag. This sudden increase in resistance caused the aircraft to lose airspeed rapidly, forcing the pilot to perform an emergency landing in an open field. The aircraft struck the ground, causing damage to the left wing and landing gear, though the pilot remained uninjured.
The investigation
The investigation examined the sequence of events leading to the entanglement and the effectiveness of the safety measures in place. Investigators looked into the coordination between the flight service leader, the ground crew, and the pilots. It was established that a student was instructed to stand on the end of the cable to provide a warning signal if the cable began to move. However, the investigation found that the signal to abort the takeoff did not reach the pilot due to engine noise and the pilot's intense focus on the takeoff procedure.
Findings
- The primary cause of the accident was the entanglement of the 80 kg winch cable with the tailwheel of the HB-TUB during takeoff.
- The flight service leader recognized the risk of the aircraft rolling over the cable but proceeded with the launch to maintain the flight schedule.
- The safety measure of using a ground observer to signal an abort was ineffective because the pilot was not alerted to the possibility of a sudden revocation of the takeoff clearance.
- The heavy drag created by the cable snagging on trees made the completion of the turning maneuver impossible, necessitating the emergency landing.