What happened
On July 6, 2015, a Piper PA25 tug, registered F-GHSQ, was performing a glider tow at the Buno-Bonnevaux airfield. The aircraft being towed was a Janus B glider, registered F-CFAO, carrying a pilot in the rear seat and a passenger in the front.
During the takeoff roll, the glider pilot attempted to adjust the wing camber flaps to +8° to optimize the wing profile for the increasing airspeed. However, the pilot was unable to lock the flap control in place and had to hold the lever manually with his left hand. Simultaneously, the glider's airbrakes, which had not been fully retracted during pre-flight checks, were partially pulled out of their housing due to aerodynamic suction.
As the aircraft climbed to approximately ten meters, the tug pilot received radio messages regarding the airbrake position. While attempting to retract the airbrakes, the glider pilot had to release the flap control, causing the camber flaps to shift unexpectedly to -4°. This sudden change in configuration created significant drag and destabilized the tow. The tug pilot reported a loss of airspeed and a loss of visual contact with the glider in his rearview mirror. To prevent a stall or a more serious accident, the tug pilot made the decision to cut the tow cable. The glider subsequently performed an emergency landing in a field, where its left wing struck an irrigation winder, resulting in damage to the aircraft.
The investigation
The investigation focused on the mechanical configuration of the Janus B and the sequence of pilot actions. Investigators examined the cockpit controls, noting that the camber flap locking mechanisms are only visible from the front seat, making it impossible for a rear-seated pilot to verify the lock. The investigation also reviewed the pilot's pre-flight checks, specifically the airbrake verification, and the aerodynamic effects of the unlatched flaps and airbrakes during the high-drag takeoff phase.