What happened
On June 1, 1963, a passenger flight was underway near the Spreitenbach airfield involving a Rhönschwalbe Ka-2b, registration HB-587, being towed by a De Havilland Chipmunk, registration HB-TUG. The takeoff proceeded normally at approximately 12:35 MEZ. However, shortly after passing the airfield boundary at an altitude of roughly 10 meters, the glider pilot reported hearing a cracking noise and feeling a vibration through the control stick.
Following this event, the glider began to experience significant altitude deviations relative to the tow aircraft. The glider climbed sharply and flew at an excessive pitch angle before subsequently dropping into an extreme low-altitude state. Due to these unstable flight characteristics, the tow aircraft was unable to maintain sufficient airspeed and altitude while flying toward Dietikon. The tow pilot initiated a right-hand turn to avoid residential areas, and during this maneuver, the glider pilot performed an emergency release from the tow cable at an altitude of approximately 30 meters.
After releasing, the pilot attempted to level the aircraft from its banked position. However, the glider lacked the necessary altitude and airspeed to establish a proper glide pattern. The aircraft entered a steep descent and struck an open field near a power line with significant vertical force. The pilot sustained minor injuries (lumbar contusions), while the passenger remained uninjured. The HB-587 sustained damage to the fuselage and control system.
The investigation
The investigation examined the flight sequence, the mechanical condition of both aircraft, and the environmental conditions. At the time of the accident, the area was experiencing unstable winds and light to moderate turbulence. Investigators specifically looked into the pilot's report of a mechanical noise and potential control surface malfunctions, noting that similar Schleicher aircraft had previously required modifications to their elevator coupling systems. However, records showed that all necessary updates had been completed during a major overhaul in late 1962/63, and the aircraft had passed recent inspections.
Findings
- The primary cause of the accident was the unstable altitude relationship between the glider and the tow aircraft, which prevented the tow plane from maintaining the energy required for a safe climb.
- The glider's sudden changes in pitch and altitude caused an excessive loss of energy in the tow aircraft, making continued climbing impossible.
- The pilot's decision to release the tow cable was a necessary action to prevent a greater danger to the occupants and people on the ground.
- No technical defects or mechanical failures were found in the glider's control systems or the tow cable.
- The pilot did not attempt to adjust the elevator trim, which had been set to a nose-heavy position prior to takeoff, though investigators concluded this would not have significantly altered the controllability of the aircraft.