What happened
On October 5, 1966, a Tipsy Nipper Mk II (registration HB-SPF) was conducting a flight from Bex toward Sion. While navigating the terrain near the Pas de Cheville, the pilot found the aircraft lacked sufficient altitude to clear the mountain pass. To regain height, the pilot attempted to fly along the valley floor, following the terrain to find rising air currents.
During a right-hand turn intended to reposition the aircraft toward the valley entrance, the left wing of the aircraft struck a long-span telephone cable. The impact caused the aircraft to lose stability and descend almost vertically into a wooded area. The pilot sustained injuries to the head and wrist, and the aircraft was severely damaged.
The investigation
The investigation examined the flight path, the aircraft's technical condition, and the environmental factors at the time of the accident. The Tipsy Nipper Mk II was a single-seat, light aircraft with approximately 36 total flight hours. Investigators found no evidence of mechanical failure or issues regarding the aircraft's weight and center of gravity.
Regarding the environment, the weather was sunny with light winds, though moderate turbulence was present near the slopes. The investigation specifically looked at the telephone line, which spanned 820 meters between two pylons. While the line was a significant obstacle, it did not meet the official criteria to be classified as a formal aerial obstacle at that time.
Findings
- The pilot attempted to use rising air currents to gain altitude but was caught by downdrafts (rabattants) of approximately 1.5 to 2 m/sec.
- The pilot was unaware of the presence of the telephone cable during the maneuver.
- The pilot flew at an altitude within the valley that did not provide sufficient margin for safe maneuvering or emergency clearance.