What happened
On August 13, 2020, a flight training session involving a glider tow at the Hessisch Lichtenau airfield resulted in a fatal accident. The tow formation consisted of an Avions Pierre Robin DR 400/180 R acting as the tug and an Alexander Schleicher K8B glider.
Following a successful first tow, the crew changed pilots for a second attempt. During the takeoff roll on runway 09, the aircraft reached a liftoff speed of approximately 110 km/h. As the tug aircraft climbed, its speed began to decrease, dropping to roughly 100 km/h within seconds. Simultaneously, the aircraft deviated from the takeoff centerline and entered a sharp left turn.
As the aircraft climbed, it developed a significant bank angle and began to lose altitude rapidly. The glider pilot, observing the unstable flight of the tug, immediately released the tow cable. The K8B glider subsequently landed safely on the airfield. However, the DR 400/180 R entered an uncontrolled descent, banking heavily to the left before striking the ground approximately one minute after takeoff. The pilot of the towing aircraft sustained fatal injuries, while the glider pilot remained uninjured.
The investigation
The BFU examined flight data from the FLARM GPS units of both aircraft, as well as ground station data. The investigation reviewed the flight history of both pilots, the maintenance records of the DR 400/180 R, and the environmental conditions at the time of the accident. Investigators also analyzed the aircraft's configuration, noting that the engine was functional and the stall warning system had likely been triggered during the climb, though the pilot did not take corrective action to increase speed or reduce the angle of attack.
Findings
- The primary cause of the accident was the tow aircraft entering an aerodynamically stalled state during the initial climb.
- A critical combination of low airspeed and an excessive bank angle at a low altitude left the pilot with insufficient altitude to recover.
- The pilot initiated a left turn before reaching a safe altitude (at least 200 m), contrary to standard safety procedures which recommend a straight climb.
- The airspeed of the tug decreased significantly during the climb, reducing the safety margins available to the pilot.
- The pilot's experience level in performing glider tow operations may have been insufficient for the complexities of this specific maneuver.