What happened
On May 27, 2017, at Mollis Airport (LSMF), a towing operation was underway to transition a glider pilot to the ASW 19 B model. The tow aircraft, a Robin DR 400/180 R with registration HB-EZX, was pulling the glider, registration HB-1714, during a training flight.
Immediately after takeoff, the glider exhibited unstable flight characteristics, oscillating and momentarily climbing above the tow plane before landing hard on the runway. Attempting to abort the takeoff as previously planned, the glider pilot attempted to release the towline. However, the release failed to occur. The glider continued to climb, eventually rising approximately 15 meters above the tow plane. This height difference caused the tail of the HB-EZX to be pulled upward, leading to a total loss of control. The tow plane rolled onto its left wing and crashed, resulting in the destruction of the aircraft and serious injuries to the pilot. The glider landed hard on the grass before returning to the runway, sustaining minor damage, while the glider pilot remained uninjured.
The investigation
SUST examined the wreckage and flight data recorders from both aircraft. Investigators found no mechanical defects in the release mechanisms of either the HB-1714 or the HB-EZX. The investigation focused on the pilot's actions and the physical interaction between the two aircraft.
It was noted that the glider's release button was identical in size and shape to the adjacent pedal adjustment knob, which may have contributed to a manual error. Furthermore, the investigation revealed that the glider pilot had an undisclosed medical condition, specifically *retinopathia centralis serosa*, which could impact visual perception and flight control.
Findings
- The primary cause of the accident was the excessive height of the glider above the tow plane, which forced the tow plane's tail upward.
- This height difference was caused by a combination of the glider pilot's inadequate flight control, improper training, a failed release attempt, and reduced situational awareness.
- The glider pilot failed to realize the towline had not actually disconnected, focusing instead on the landing.
- The glider pilot's previous training had not sufficiently addressed longitudinal control difficulties, and recent training had been conducted via winch launch rather than towing, leaving skills unpracticed for this specific operation.