What happened
On June 2, 2016, during the third competition day of the VI Polish Glider Championships in the 15m class, a mid-air collision occurred in a thermal column between two gliders near the Strzysebice (EPLS) aerodrome. The incident took place at approximately 13:05 LMT in a heavily populated airspace containing numerous gliders waiting for the start of the flight task.
The pilot of an Arcus M (registration D-KKKO) had been circling in a thermal near the base of a cloud. To avoid flying into the cloud, the pilot deployed aerodynamic brakes to initiate a descent while remaining within the thermal. Simultaneously, a Jantar 2B (registration SP-3371) entered the same thermal from the west to begin centering.
As the Jantar 2B began its turn, the pilot heard a FLARM warning and observed the Arcus M approaching rapidly from the rear-left. In an attempt to avoid the collision, the pilot of the Jantar 2B pushed the control stick forward to dive. However, the Arcus M, descending at a high vertical speed of approximately 5 m/s and traveling at a forward speed of about 150 km/h, overtook the Jantar 2B. The left wing spar of the Arcus M struck the rear fuselage of the Jantar 2B near the tailwheel.
The impact was severe enough that the pilot of the Jantar 2B momentarily lost control and, fearing a crash, opened the canopy and unbuckled his harness to prepare for a parachute jump. Realizing the aircraft remained controllable, the pilot stabilized the flight and landed safely at the aerodrome. The Arcus M also remained controllable despite significant damage to its left wing spar and aerodynamic brakes, allowing the pilot to return to the airfield safely.
The investigation
The PKBWL investigation examined the flight paths of both aircraft using GPS data, which revealed that the two gliders were traveling at significantly different speeds at the moment of impact. The investigation also reviewed the maintenance records, pilot licenses, and the operational organization of the competition. The investigation confirmed that both pilots were fully licensed, medically fit, and were not under the influence of alcohol. The investigation also noted that the high density of glider traffic in the area required extreme vigilance from all participants.
Findings
- The primary cause of the collision was the Arcus M performing a high-speed descent within a thermal column that was already occupied by other aircraft.
- The Arcus M was descending with significant vertical speed using aerodynamic brakes to avoid cloud penetration.
- The Jantar 2B entered the same thermal and was in the process of centering, creating a high-speed closing rate between the two aircraft.
- The disparity in speeds between the descending Arcus M (approx. 150 km/h) and the maneuvering Jantar 2B made the collision difficult to avoid despite the use of the FLARM warning system and evasive maneuvers.