What happened
On Easter Sunday, March 29, 1959, a de Havilland Chipmunk Mk. 22 (registration HB-TUD) was performing a routine aero tow at Grenchen airfield. The aircraft was towing a Rhönlerche glider, registration HB-590, for a training flight intended to reach an altitude of approximately 600 meters.
During the initial climb, the aircraft reached a height of about 90 meters when it began to tilt slightly to the left. In response to this instability, the pilot of the HB-TUD released the towline. Immediately following the release, the aircraft tilted to the right and entered a right-hand spin. The aircraft struck the ground, resulting in the death of the pilot. The glider, which remained undamaged, was able to return to the airfield safely.
The investigation
An administrative investigation conducted by the Federal Office of Civil Aviation examined the flight parameters and the aircraft's condition. The investigation established that the HB-TUD was operating at a weight of approximately 800 kg, within its permitted limits. While the technical records were not perfectly maintained, no mechanical defects were found in the aircraft or the release mechanism.
The investigation also looked into the pilot's flight history, noting that while well-qualified, the pilot had a known tendency to fly below the required minimum speed during takeoff climbs. Furthermore, investigators noted that the pilot was found with unsecured seatbelts, though it was unclear if this occurred during the flight or was an oversight prior to takeoff.
Findings
The investigation concluded that the accident was caused by a loss of control during the takeoff climb. Several contributing factors were identified:
- The aircraft was likely flying at or below the safe minimum speed required for towing.
- The sudden change in load caused by the release of the towline triggered the aircraft's instability.
- The pilot's decision to release the cable in response to the initial leftward tilt exacerbated the situation, leading to the right-hand spin.
- The altitude of 90 meters provided insufficient margin to recover the aircraft once the spin had developed.