What happened
On April 26, 2009, at approximately 13:45 local time, two gliders were operating in the vicinity of Terlet airfield (EHTL) in the Netherlands. The first aircraft, a Schleicher ASK-21 with registration PH-1292, was conducting a check flight with two occupants. The second aircraft, an RSchneider LS 3-a with registration PH-871, was performing a local flight with a single pilot.
As the aircraft approached the airfield circuit, the pilot of PH-1 .292 initiated a shallow left turn to lose altitude after approaching the circuit join point too high. Simultaneously, the pilot of PH-871 was flying straight toward the same area. The two gliders were on a nearly head-on collision course. Although both pilots spotted the other aircraft and attempted sharp left turns to avoid the impact, the maneuver was insufficient. The right wings of the two aircraft collided, causing significant structural damage to both. The Schleicher ASK-21 suffered a crushed wing leading edge, while approximately 80 centimeters of the RSchneider LS 3-a's right wing was torn off.
Following the impact, the instructor on board PH-1292 took control and performed an emergency landing at Terlet. The pilot of PH-871 performed an emergency landing in a nearby field, during which the aircraft's wing struck the ground during the rollout. There were no fatalities and no injuries reported.
The investigation
The investigation examined the visibility conditions, the equipment on board, and the flight paths of both aircraft. It was noted that while visibility was sufficient for VFR flight, the air was hazy, which reduced the contrast of the white gliders against the sky. The investigation also reviewed the use of the 'see-and-avoid' principle and the presence of electronic collision avoidance technology.
Findings
- The primary cause of the accident was the failure of the 'see-and-avoid' principle, as the crews did not detect each other in time to execute a successful avoidance maneuver.
- The head-on nature of the approach resulted in a high closing speed, which further reduced the time available for detection and reaction.
- The lack of Flarm equipment on PH-1292 meant that no electronic collision warning was generated for the pilot of PH-871.
- Hazy conditions and the white color of the gliders made them difficult to distinguish against the horizon and the hazy sky.
- The area near the circuit join point is a known high-traffic zone where aircraft often maneuver to lose altitude, increasing the risk of concentrated traffic.