Accident Description
On 6 June 1981, at approximately 1735 local time, a glider of type Rhoenlerche II, registered HB-775, was engaged in a training flight under dual control at Bex aerodrome. The towing aircraft, a Piper PA-18-150 registered HB-PFK, began the takeoff on runway 33 with the glider in tow. Shortly after departure, the glider instructor felt a slight jolt, and the student pilot in the front seat announced that the tow rope had detached. Judging altitude and speed sufficient, the instructor initiated a right turn to return to the airfield. During the turn, the tow rope, still attached to the glider, became entangled in a barbed-wire fence, pulling it along for some distance. The glider pilot lost control, and the aircraft crashed in a right rotation into a field on the eastern edge of the aerodrome.
Occupants and Damage
Both occupants—the instructor and the student pilot—were uninjured. The glider was destroyed. The fence was damaged. The pilot of the tow plane, who was flying solo under the supervision of his instructor on his first towing flight, continued his flight and landed normally.
Investigation Findings
The investigation established that both crews held valid licenses and qualifications. Both aircraft had valid certificates of airworthiness and registration. The tow plane, a Piper PA-18-150, was equipped with a Farner-type tow release mechanism, cable-operated, and had accrued 179 flight hours. Examination of the release mechanism revealed no anomaly; however, the control system exhibited a lack of slack in the Bowden cable, which could have facilitated an inadvertent release of the tow rope.
Local weather conditions at the time of the accident were: 3/8 cumulus cloud at around 2000 m, light wind along the runway axis, and an air temperature of 23°C.
Analysis
The exact cause of the inadvertent opening of the tow release on the tow plane could not be determined with certainty. Two hypotheses were considered: accidental opening due to the lack of slack in the control cable, or inadvertent actuation by the pilot, possibly during a hand movement to locate the release ball. The accident occurred during the most critical phase of towing. The investigation noted that if the glider instructor had immediately released the glider's tow hook after the separation, the accident might have been avoided.
Probable Cause
The probable cause was the omission of a recommended manipulation during an emergency procedure.