What happened
On April 11, 1971, a Piper Super Cub, registration HB-OPV, departed Lausanne airfield towing two gliders toward Bex. The formation consisted of a Skylark IV, registration HB-767, on a short tow rope, and a Ka 6, registration HB-614, on a long tow rope.
While flying near Vevey at approximately 600 meters, the tow pilot observed deteriorating visibility and attempted to climb through a gap in the clouds to reach 1600 meters. During the transition from climbing to descending, the tow aircraft's speed dropped significantly below 100 km/h. To avoid a stall, the pilot of the Skylark IV released the tow rope. During the subsequent maneuvers, the tow rope from the Skylark IV draped over the wing of the second glider, forcing its pilot to also release the tow.
Unable to reach the Rhône plain due to the lack of rising air, the pilot of the HB-767 sought an alternative landing site near Blonay. While attempting an approach into a field, the pilot maintained an excessive approach speed of approximately 100 km/h on rising terrain. The glider touched down in the final third of the available space and collided with a garden fence, resulting in heavy damage to the aircraft. The pilot escaped without injury.
The investigation
The investigation focused on the circumstances leading to the premature release of the gliders and the subsequent emergency landing. Investigators examined the flight logs, weather conditions, and the technical specifications of the aircraft involved. A critical aspect of the inquiry was the assessment of the total weight of the glider formation relative to the towing aircraft's certified limits.
Findings
- The total weight of the two gliders combined was 634 kg, which exceeded the HB-OPV's certified towing limit of 540 kg by approximately 17% (a surcharge of 94 kg).
- This excessive weight caused the tow aircraft to struggle during the climb and contributed to the significant loss of airspeed during the transition to descent.
- The loss of airspeed created a critical situation that forced the pilots to release the tow ropes to prevent a stall.
- The pilots were forced to execute an emergency landing in a region with very limited landing options due to the low visibility and the necessity of the release.
- The approach speed during the final landing of the HB-767 was too high given the rising terrain and the slight tailwind present at the time.