What happened
On May 6, 2012, at 16:45 local time, a Grob G103 Twin Astir, registration F-CFKT, was performing a winch-assisted instructional flight at the Lyon Corbas aerodrome. The flight was a dual-control session involving an instructor and a student pilot.
As the glider reached an altitude of approximately one meter, the aircraft experienced a sudden and violent left yaw. The instructor was unable to counteract this excursion. During the rotation, the aircraft turned 90 degrees, at which point the safety release functioned correctly, and the winch cable disconnected automatically. The glider continued to rotate left until it reached a heading of 160 degrees. The nose of the aircraft struck the ground with significant force, and the tail boom subsequently fractured when the glider impacted the ground on its rear section. The aircraft was destroyed in the accident.
The investigation
The investigation focused on the mechanical state of the aircraft and the condition of the takeoff surface. Examination of the wreckage revealed no mechanical malfunctions that contributed to the event. Traces of grass were found on the upper surfaces of the wings, several dozen centimeters behind the leading edge. Because the wings had been removed prior to the wreckage examination, investigators were unable to verify the status of the airbrake locks.
Investigators noted that the unpaved runway had not been mowed, with grass reaching a height of approximately 50 centimeters. Records indicated that the flying club had made multiple requests to the maintenance organization to mow the runway, but the task had not been completed; the grass was only mowed following the accident.