What happened
On July 2, 1999, at approximately 16:30 LT, two gliders were participating in an informal soaring flight near the Para mountain in the municipality of Château d’Oex. The aircraft involved were an ASW 19, registration HB-3172, and another glider, registration HB-1573.
Initially, the pilots were flying independently in favorable weather conditions with excellent visibility. Around 16:15, the two aircraft converged near the ridge connecting Pic Chaussy and La Para. The pilot of HB-15 and 73 observed HB-3172 approximately 150 meters above him performing a right-hand spiral. The pilot of HB-1573 subsequently entered a right-hand turn in a strong thermal. After a brief radio exchange to confirm positions, the two pilots lost visual contact with one another. While the pilot of HB-3172 had reached his altitude ceiling, the pilot of HB-1573 continued to climb. Shortly after their last radio contact, the left wing of HB-1573 struck HB-3172. Both pilots successfully evacuated the aircraft via parachute and landed safely on the northern slope of the mountain, remaining uninjured. Both gliders were destroyed upon impact with the rocky terrain.
The investigation
The investigation established that both pilots held valid, recently obtained glider licenses and were in good health. The aircraft were well-maintained, with all weight and balance parameters within limits. No mechanical failures or airworthiness issues were identified during the examination of the wreckage.
Investigators noted differences in the visibility of the aircraft. The wingtips of HB-1573 were painted bright red, and it featured a large Swiss flag on the tail. In contrast, the wingtips of HB-3172 were orange and faded, and it carried a smaller flag. Additionally, while the pilot of HB-1573 attempted to activate his ELT after landing, no signal was detected by monitoring stations; the pilot of HB-3172 had not armed his ELT for the flight.
Findings
- The collision occurred because the two aircraft were performing different orbital patterns that intersected.
- The pilot of the lower aircraft (HB-1573) was well-positioned to see the aircraft above him, but the pilot of the upper aircraft (HB-3172) faced significant blind spots due to the cockpit design.
- The pilot of HB-3172 was in a right-hand turn at the time of impact, which further restricted his view to the left.
- The primary cause of the accident was insufficient mutual surveillance by the pilots.