Accident
On May 16, 1965, during a public aviation event at Schänis airfield, a double tow of two gliders was attempted. The tow consisted of a Dewoitine D-26 (HB-RAE) towing two Ka-6 gliders, HB-683 and HB-703, in tandem. The accident occurred around 1340 hours.
Sequence of Events
The gliders were positioned for the double tow: HB-683 was ahead and staggered to the left relative to HB-703. The wingman of HB-683 gave signals to tighten the towrope and then to start. These signals were relayed to the tow pilot by a ground helper. The wingman of HB-703 gave no signals. After receiving the start signal, the tow pilot applied full throttle after a short roll. At that moment, the portion of the towrope connecting the tow plane to HB-703 was not fully taut. It suddenly became taut, causing the rope to break in the forward third. The pilot of HB-683 noticed the break, immediately released the towrope, and banked left to avoid entangling. This maneuver caused HB-683's tail skid to strike the left wingtip of HB-703, which had been lowered by its wingman. The impact broke the main spar of HB-703's left wing. The tow pilot observed the events via a rearview mirror and aborted the takeoff before the tow plane became airborne.
Investigation
The investigation was opened on May 27, 1965. The cantonal authorities did not conduct a separate investigation.
Aircraft and Pilots
All pilot licenses and aircraft permits were valid. The pilots had sufficient experience for a double tow. The tow pilot held a private pilot license with glider and tow endorsements. The pilot of HB-683 held a glider pilot license with instructor endorsements and a private pilot license. The pilot of HB-703 held a glider pilot license with aerobatics endorsement. No prior accidents or violations were recorded for any pilot.
Damage
No personal injuries occurred. HB-703 sustained a broken main spar in the left wing, with damage estimated at 1500–2000 CHF. HB-683 was undamaged. No third-party damage occurred.
Findings
Preparations for the double tow lacked specific instructions for the ground crew. The accident was caused by the start signal being given before the front glider's towrope was fully taut. When the rope suddenly became taut, it snapped, and the rear glider collided with the front glider despite the immediate release of the rear glider's towrope.