What happened
On April 21, 2013, at 13:10 UTC, two gliders participating in the AZ CUP 2013 competition collided approximately 8 km east of Zbraslavice Airport (LKZB). The aircraft involved were an Alexander Schleicher ASW-15B, registration OK-1272, and a Standard Cirrus, registration OK-5678.
Both aircraft were flying as part of a large group of 57 crews participating in a speed task. The pilot of the ASW-15B had transitioned from a thermal into a glide phase, following a path through a group of other gliders. During the flight, the ASW-15B flew beneath the Standard Cirrus. Due to an excess of speed, the ASW-15B moved into a position where the Standard Cirrus was positioned directly above its flight path.
Between 13:13:05 and 13:13:11, the trajectories of the two aircraft intersected. The Standard Cirrus, which was descending at a rate of 1.2 m/s, struck the cockpit canopy of the ASW-15B. The impact shattered the canopy glazing. The pilot of the ASW-15B maintained control of the aircraft and performed an emergency landing in a field near Štipoklasy. The pilot of the Standard Cirrus continued the flight and landed safely at LKZB without further incident.
The investigation
The ÚZPLN investigation utilized GNSS flight recorder data from both aircraft to reconstruct the flight paths and relative positions of the gliders. The analysis focused on the vertical and horizontal separation between the two aircraft during the critical phase leading up to the collision. The investigation also examined the visibility conditions, noting that the pilot of the ASW-15B was wearing sunglasses, and assessed the impact of flying in close proximity to a large group of other competitors.
Findings
- Both pilots were properly licensed and the aircraft were airworthy.
- The meteorological conditions were favorable, with high visibility and clear to partly cloudy skies.
- The pilots were flying in a dense group of gliders, which increased the workload required to maintain safe separation.
- The pilot of the ASW-15B likely failed to recognize that the Standard Cirrus was descending at a higher rate.
- Neither pilot identified that their intersecting flight paths were creating a collision hazard in time to take evasive action.
- The pilot of the ASW-15B lost situational awareness regarding the vertical separation after flying beneath the other aircraft.