What happened
On June 30, 2017, at the Château-Arnoux Saint-Auban airfield, a Dyn’Aero MCR R-180, registration F-PFVV, was performing a glider tow operation. The aircraft was towing a Schempp Hirth Duo Discus, registration F-CBEC, with an instructor and a student on board the glider.
During the initial climb, at an altitude of approximately 40 meters and with takeoff flaps extended, the tow plane suddenly rolled to the right. The pilot of the tow plane immediately corrected the movement and leveled the wings, though the aircraft lost about 10 meters of altitude during the maneuver. Sensing an abnormal situation, the glider instructor immediately released the tow cable. The glider, with its landing gear extended, performed a forced landing on the banks of the Durance river, sliding approximately 100 meters across a bed of pebbles. The tow plane pilot, after regaining control, noted the cable release and continued the climb to locate the glider before landing.
The investigation
The BEA examined the meteorological conditions, the aircraft performance, and the pilot's decision-making process. Investigators analyzed wind data from the airfield's weather station, which showed fluctuating winds and gusts between 10 and 11 knots. The investigation also reviewed the aircraft's flight data from the onboard FLARM system, which revealed variations in horizontal and vertical speeds just prior to the roll and altitude loss.
Furthermore, the investigators looked into the pilot's reliance on the stall warning system. The pilot noted that because the stall warning often triggered in turbulent conditions, he had become accustomed to disregarding it. The investigation also considered the operational environment, noting that the chief pilot had recently authorized the resumption of flight activities after a period of wind-related suspension.
Findings
- The loss of control in the Dyn’Aero MCR R-180 was likely caused by a stall induced by wind shear.
- At the time of takeoff, the wind direction and strength were unstable; the steady southern valley breeze had not yet established itself.
- The pilot's decision to proceed with the takeoff was likely influenced by the chief pilot's authorization to resume activities and a desire to maintain operational flow.
- The pilot did not increase the recommended takeoff climb speed to account for the turbulent conditions, which resulted in an insufficient safety margin relative to the stall speed.