What happened
On April 13, 2013, a towing operation commenced at the Lager Hammelburg airfield (EDFJ) involving a Piper PA 18-135 and a Ka 6 glider. The takeoff from runway 28 initially proceeded normally. However, shortly after liftoff, the glider climbed above the tow plane. In an attempt to regain its position, the glider pilot pushed the control stick forward, causing the tow rope to slacken.
As the rope subsequently tightened, the glider was abruptly accelerated, climbing above the tow plane a second time and inducing a lateral pendulum motion. During this period, the glider pilot was unable to release the tow hook because both hands were occupied pushing the control stick forward. The weak link in the tow rope eventually ruptured. The glider pilot performed two 180-degree turns at low altitude and landed safely without injury.
Simultaneously, the tow plane pilot attempted to transition into a climb. At an altitude of approximately 30 to 40 meters, the aircraft's nose pitched downward into a descent. The pilot reported that the glider was no longer visible in the rearview mirror. As the pilot attempted to pull the tail up and released the tow rope, the aircraft struck the ground. The pilot sustained one serious injury, and the Piper PA 18-135 was destroyed.
The investigation
The BFU examined both aircraft and found no technical defects in the towing mechanisms. The forward release hook on the Piper PA 18-135 and the ground release hook on the Ka 6 were both found to be in working order. The aircraft's weights and centers of gravity were within permissible limits. The investigation also noted that the glider's use of a ground-mounted release hook contributed to the susceptibility to flight path oscillations.
Findings
- The primary cause of the accident was the glider climbing above the tow plane at a low altitude, which pulled the tail of the tow plane upward.
- The glider pilot failed to execute a timely release of the tow rope to abort the takeoff when the flight path oscillations began.
- The glider's configuration, specifically the use of a ground release hook, increased the tendency for the aircraft to overshoot the tow plane during the takeoff phase.