What happened
On June 24, 2009, a training flight was underway at the Yverdon-les-Bains aerodrome (LSGY) as part of a winch launch training camp. The aircraft, a Grob G 103 A Twin II Acro with registration HB-1726, was being operated by the Air-Club d’Yverden-les-Bains. The flight crew consisted of a student pilot and an instructor.
During the takeoff roll, the glider accelerated rapidly via a winch system. However, as the aircraft gained speed, the right wing tip suddenly dipped into thick grass located alongside the mown portion of the runway. The instructor noted that a 15-knot wind was present at the time. In response to the sudden movement, the student pilot activated the cable release mechanism.
Despite the release of the winch cable, the aircraft could not be stabilized. The glider veered sharply to the right and entered a rapid pitch-up maneuver, rotating around its yaw axis until it reached a vertical position with the nose pointing toward the ground. The aircraft struck the earth with its left wing and nose, creating a 25 cm deep crater before coming to rest on its back. The impact caused severe damage to the fuselage and wings, and destroyed both canopies. The student pilot sustained two serious injuries, including pelvic fractures, while the instructor suffered a minor hand injury.
The investigation
The investigation conducted by the Swiss authorities examined the technical state of the Grob G 103 A Twin II Acro and the winch equipment. No mechanical failures were found in either the glider or the winch system. The aircraft's weight and center of gravity were within prescribed limits, and the cable's breaking fuse was functional, though its rated strength exceeded the manufacturer's recommendation.
The investigation also focused on the airfield conditions. While the runway had a mown strip, the surrounding grass was between 30 and 40 cm high. Investigators noted that the danger of high grass had been a known issue at the airfield in previous years. The investigation also reviewed the pre-flight briefings, noting that while the instructor had briefed the students on emergency cable-release procedures, the specific hazard posed by the current grass height was not addressed.
Findings
- The primary cause of the accident was a loss of control resulting from the right wing catching in the tall grass during the ground acceleration phase of the winch launch.
- The decision to proceed with the launch despite the high grass contributed to the accident, as the hazard of the vegetation height was not sufficiently recognized.
- The high grass created a significant lever arm that made recovery nearly impossible once the wing tip made contact with the ground.