What happened
On July 14, 2011, a GROB G 103 Twin Astir II, registration F-CFYK, was performing a towed takeoff for a local flight at the Arcachon-la-Teste-du-Buch aerodrome. The flight was an instructional session involving an instructor and a student pilot.
During the takeoff roll on the unpaved runway, the glider began to tilt to the left after approximately 100 meters of travel. Shortly thereafter, the aircraft's trajectory deviated sharply to the left, resulting in a ground loop. The instructor promptly released the tow cable to interrupt the takeoff. The glider eventually came to a stop on a heading of 140°. The impact caused a structural failure of the rear tail boom.
The investigation
The investigation focused on the sequence of events during the takeoff roll and the environmental conditions at the aerodrome. Investigators noted that recent precipitation had left the unpaved runway surface soft. At the time of the accident, the wind was reported from 280° at 6 to 8 knots, with visibility exceeding 10 km.
The instructor, who had significant experience on the aircraft type, noted that while he was monitoring the pitch axis, he was unable to correct the leftward tilt once it began. He suggested that the left wing skid might have prevented effective correction. The student pilot, who had limited flight time, reported being surprised by a sudden roll, which he attributed to a possible wind gust lifting the right wing.
Findings
- The primary cause of the accident was the instructor's insufficient division of attention during the takeoff phase; by focusing heavily on managing the pitch axis, the instructor failed to react quickly enough to the student's loss of roll control.
- The sinking of the left skid into the soft ground contributed to the loss of control.
- The structural failure of the aircraft's rear boom was a consequence of the ground loop.