What happened
On August 12, 2019, a De Havilland DH8D-400, registration YL-BAH, was cruising at FL 240 within the Warsaw TMA2. Simultaneously, a Boeing 737-800, registration SP-LWA, was departing from Warsaw Chopin Airport (EPWA) and climbing through the airspace.
The crew of the Boeing 737-800 was initially cleared to climb to FL 230, which provided vertical separation from the De Havilland DH8D-400 while the aircraft worked toward achieving lateral separation. After crossing the flight path of the YL-BAH, the controller cleared the Boeing 737-800 to climb further to FL 340. However, at the time this clearance was issued, the lateral distance between the two aircraft was less than the required 5 NM.
As the Boeing 737-800 passed the DIBLO waypoint and began a left turn toward INDIG, the flight paths remained in a conflict state. At approximately 18:25 local time, as the Boeing 737-800 crossed FL 230, the distance between the aircraft had decreased to 2.62 NM, triggering a Short Term Collision Alert (STCA) via the PEGASUS_21 system.
Upon detecting the conflict, the controller immediately intervened by instructing the Boeing 737-800 to turn right to a heading of 295°. This maneuver increased the separation between the aircraft. The distance eventually grew to 3.93 NM with a vertical separation of 700 ft as they passed each other at FL 240. Once the required 5 NM lateral and 1000 ft vertical separation was established, the Boeing 737-800 was cleared back to the INDIG waypoint and transferred to ACC control.
The investigation
The investigation focused on the sequence of clearances issued by Warsaw Approach (APP) and the coordination between the Approach and Area Control Centre (ACC) sectors. The inquiry examined the timing of the climb clearance and the controller's assessment of the aircraft's planned trajectory.
Findings
- The primary cause of the incident was the premature issuance of the clearance to climb for the Boeing 737-800.
- The controller failed to account for the planned flight path of the Boeing 737-800 when issuing the climb clearance.
- The transfer of the De Havilland DH8D-400 to ACC control occurred too early, contributing to the loss of separation risk.