What happened
On April 21, 2007, at approximately 14:24 local time, two motor gliders collided in the vicinity of Piz Nuna, near Ardez, Switzerland. The first aircraft, a Schleier ASH 25 E with registration D-KZGO, was flying from Germany toward the Engadin region. The second aircraft, a Stemme S 10-V with registration HB-2187, was operating in the same area near Samedan.
Both aircraft were engaged in thermalling within a thermal updraft, performing clockwise circles. While the D-KZGO was circling in a tighter pattern with a higher climb rate, the HB-2187 was flying in a larger radius. Moments before the impact, the passenger in the D-KZGO spotted the HB-2187 approaching from the rear right and alerted the pilot, but it was too late to maneuver. The right wing of the HB-2187 struck the right wing of the D-KZGO, causing approximately half of the D-KZGO's wing to break off, followed by a secondary impact from the left wing of the HB-21 and 87 against the wing root of the D-KZGO.
The collision rendered both aircraft uncontrollable. The pilot of the D-KZGO exited the aircraft, though his parachute failed to deploy; however, the passenger successfully deployed his parachute and landed on a glacier. The pilot of the HB-2187 remained in the cockpit as the aircraft struck a steep, snow-covered slope. The collision resulted in two fatalities and one injury.
The investigation
Investigators examined flight data from onboard recorders, radio communications, and wreckage analysis. The investigation established that both aircraft were operating under Visual Flight Rules (VFR) and were in a gliding configuration with engines retracted.
Regarding the search and rescue efforts, the investigation found that the recovery of the occupants was delayed. Although both aircraft were equipped with emergency locator beacons (ELBA), the beacon in the D-KZDO was switched off, and the beacon in the HB-2187 was destroyed upon impact. The search was further complicated by a lack of information regarding the intended flight path of the HB-2187 and incorrect initial assumptions regarding the search area.
Findings
- The primary cause of the accident was that the two motor gliders approached each other on converging headings within a thermal updraft, a movement that was noticed by the crews too late.
- A contributing factor was that the traffic and collision warning system (FLARM) installed in the HB-2187 was not in an operational state because its UHF antenna was not mounted.
- The investigation noted that the pilot of the HB-2187 may have been experiencing reduced situational awareness, potentially due to hypoxia, as the aircraft was flying at altitudes between 3,000 and 3,850 meters without an oxygen system.
- The search and rescue operation was hindered by the fact that the D-KZGO's emergency beacon was manually turned off prior to the accident.