What happened
On May 24, 1969, at approximately 14:35, a mid-air collision involving a tow cable occurred near the Birrfeld airfield. A De Havilland Chipmunk HB-TUA, acting as a tug aircraft, was returning from a flight after releasing a glider. During the approach, the hemp tow cable attached to the HB-TUA struck the left wing of a K 8 B glider, registration HB-671, which was performing its landing pattern.
The impact caused approximately two meters of the HB-671 wing tip and part of the aileron to be torn away. Despite the structural damage, the pilot of the HB-671 managed to maintain control, leveling the aircraft just five meters above the ground before completing a successful landing near the runway threshold. The pilot of the HB-TUA was able to release the cable and land normally without further incident.
The investigation
The investigation examined the flight paths of three aircraft involved in the vicinity: the towing Chipmunk, the damaged K 8 B glider, and a Standard Austria S, registration HB-729, which was circling in the approach area. Investigators reviewed the flight logs, pilot experience, and local airfield regulations for Birrfeld.
Witnesses and pilots confirmed that the HB-729 was circling at an altitude of approximately 120 meters over the BBC factory buildings, a location not permitted for circling under existing airfield rules. The investigation focused on how the presence of this third aircraft influenced the situational awareness of both the tug pilot and the glider student.
Findings
- The primary cause of the collision was that the distraction caused by an aircraft circling in the approach zone led both the tug pilot and the glider student to neglect essential airspace monitoring.
- The pilot of the HB-TUA was heavily focused on the HB-729 as it moved through the landing pattern, which prevented him from noticing the HB-671 until the collision was imminent.
- Similarly, the student pilot of the HB-671 became preoccupied with the nearby HB-729, failing to recognize that his flight path was intersecting with the incoming tow plane.
- The presence of the HB-729 in an unauthorized area of the approach airspace acted as the catalyst for the loss of situational awareness.