What happened
On April 17, 1970, at approximately 08:49, a Rhönlerche II glider, registration HB-589, was performing a winch launch at Grenchen Airport. During the flight, the instructor initiated a rope-release drill at an altitude of roughly 70 meters to test the student's reactions. Following the release, the student responded sluggishly to the maneuver, applying elevator pressure with delay.
Upon receiving instructions to bank left, the student attempted the turn using only the rudder while the aircraft was at an insufficient airspeed. This caused a rapid loss of altitude. In an attempt to prevent the aircraft from drifting off the airfield, the instructor took control, initially applying heavy elevator input to regain speed before initiating a right-hand reverse turn at tree-top height. During this low-altitude maneuver, the aircraft was flown at a high bank angle. As the instructor attempted to level the wings near the ground, the aircraft failed to respond effectively to control inputs. The right wingtip subsequently struck the ground, leading to a hard impact and a 180-degree rotation around the vertical axis. The two occupants, an instructor and a student, sustained two light injuries.
The investigation
The investigation examined the aircraft's maintenance status, the crew's qualifications, and the aircraft's center of gravity. The Rhönlerche II was found to be properly maintained and airworthy. Both occupants held the necessary licenses, and there was no evidence of medical impairment. Meteorological conditions were determined to be irrelevant to the accident.
Investigators also analyzed the aircraft's weight and balance. At the time of the accident, the center of gravity was located at 0.530 meters behind the reference plane. While this was within the permissible range of 0.365 to 0.540 meters, it was positioned near the extreme rear limit.
Findings
- The primary cause of the accident was the aircraft entering an aerodynamic stall during a low-altitude reverse turn.
- The aircraft's center of gravity was positioned far toward the rear limit, which likely contributed to a tendency to pitch up, exacerbating the stalled condition.
- The student's delayed response and the use of rudder-only inputs at low airspeed contributed to the initial loss of altitude.