What happened
On June 8, 1968, a pilot was conducting unauthorized simulated stall maneuvers, known as "supponierte Seilabwürfe," at the Spreitenbach airfield. The pilot was attempting to prepare himself for future glider towing duties, despite lacking the required flight hours on this specific aircraft type and having no instruction from a flight instructor to perform these maneuvers.
During the final maneuver, the pilot was flying a very tight circuit at low altitude. Simultaneously, a Piper aircraft was on approach to the same airfield. To avoid a potential mid-air collision with the approaching Piper, the pilot tightened his turn even further. As the aircraft approached the runway, it entered an uncontrolled flight state. The De Havilland Chipmunk 22, registration HB-TUC, struck the ground approximately 250 meters before the runway threshold, hitting a field with its left wheel before spinning and sustaining heavy damage. The pilot escaped the accident without injury.
The investigation
An investigation established that the pilot's motor flight experience was quite limited, totaling only about 28 hours at the time of the accident. Furthermore, this was the pilot's very first flight in the specific aircraft involved. The investigation examined the flight paths of both the Chipmunk and the approaching Piper, as well as the pilot's maneuvers and the aircraft's mechanical condition. Investigators also reviewed the flight instructor's observations, noting that the pilot's turns were exceptionally tight and performed at low altitudes.
Findings
- The pilot was performing unauthorized maneuvers without the necessary flight hours or instructor supervision.
- The pilot's attempt to maintain visual contact with the approaching Piper caused him to neglect his altitude and bank angle.
- The pilot likely applied abrupt control inputs, such as pulling back on the controls, to avoid the other aircraft, which led to a stall at a critical low altitude.
- The pilot's limited flight experience and lack of familiarity with this specific aircraft's handling characteristics contributed to the loss of control.