What happened
On July 22, 1961, during a glider training camp at the Bex Aerodrome, a Piper PA-16 Super Cub, registration HB-OEE, was performing a reconnaissance flight while towing a HB-657 Bocian glider. The pilot of the tow plane, who was performing his first mountain towing operation, identified an updraft near the Val-d'Illiez. Following the terrain, the aircraft flew east along the contour lines of the Giètes ridge.
As the aircraft crossed the ridge onto the eastern slope, it encountered sudden downdrafts. The pilot and the glider pilot both realized the danger and released the tow cable almost simultaneously. While the glider pilot was able to maneuver away from the slope, the tow plane struck two trees, which were decapitated by the impact, before crashing into a clearing approximately 180 meters away. The pilot of the HB-OEE sustained minor injuries, but the aircraft was destroyed.
The investigation
The investigation examined the flight path, the meteorological conditions, and the experience levels of both pilots. It was noted that the tow plane was flying at a low altitude, between 80 and 100 meters above the ground, and was operating near its minimum sustentation speed. The investigation also reviewed the instructions provided by the glider pilot, who was the camp leader, to the tow plane pilot regarding the flight objectives in the mountainous terrain.
Findings
- The primary cause was that the HB-OEE entered a zone of downdrafts while flying at an insufficient altitude above the ground.
- The pilot was flying near the minimum speed required for flight, which left him with insufficient maneuverability to avoid the terrain once the downdraft occurred.
- The pilot's flight technique was deemed inadequate, as he transitioned from a known updraft to a potentially dangerous lee side of the ridge at a very low altitude.
- The pilot lacked previous experience with glider towing in mountainous environments.
- The glider pilot provided only vague instructions regarding the flight path and failed to warn the tow plane pilot about the specific risks of the descending air on the eastern slope.