What happened
On July 15, 1978, a mid-air collision occurred approximately three kilometers north-northwest of the Sanetsch Pass in the Swiss Alps. The accident involved a Bell 47 G3 helicopter, registration HB-XFA, and a Pilatus B-4 glider, registration HB-1259.
The glider had departed from Saanen earlier that morning via aero-tow, separating from its tug near Gstaad. The helicopter had departed from Gruyères for a private alpine flight carrying one pilot and one passenger. At approximately 12:25, the two aircraft collided head-on at an altitude of roughly 2,550 meters. The impact was so severe that the helicopter's main rotor struck and severed the glider's fuselage. Both aircraft subsequently crashed onto a snow-covered debris field known as Lapi di Bou. All occupants of both aircraft were killed.
The investigation
Investigators examined the wreckage and conducted flight tests to understand the visibility and audibility between the two types of aircraft. The investigation established that the helicopter's main rotor was responsible for the structural failure of the glider.
Experimental flights conducted months later revealed that the helicopter is inaudible to a pilot in a glider during a head-on approach. Furthermore, the glider is extremely difficult to spot from the helicopter when approaching from the front or rear, as its white color and narrow silhouette blend into the background of cumulus clouds.
Findings
Several factors contributed to the failure of the pilots to avoid the collision:
- The pilots failed to detect the oncoming aircraft in time.
- For the helicopter pilot, visibility was hindered by haze in the Mittelland and the difficulty of spotting the small, white silhouette of the glider against the clouds.
- For the glider pilot, the sun's position at midday caused glare, and the helicopter was difficult to see against the bright, snow-covered terrain.
- The high closing speed, estimated at approximately 60 m/s, left very little time for evasive maneuvers once the aircraft were identified.
- The area was a published glider zone, which increased the likelihood of encountering other aircraft in the vicinity.