What happened
On April 14, 2015, an airspace event occurred within the Warsaw Flight Information Region (FIR EPWW) near the SABAB navigational waypoint in the ACC T sector. The incident involved two aircraft: a Boeing 737-800 climbing from its departure route toward FL 360 and an Embraer 190 cruising at FL 350.
Initially, the crew of the Boeing 737-800 was cleared to climb to FL 340. After verifying separation from the Embraer 190, the air traffic controller authorized the Boeing 73 37-800 to continue its climb to FL 360. However, during the climb through FL 340, the Boeing 737-800 experienced a significant decrease in groundspeed, likely due to strong winds. This reduction in speed caused the distance between the two aircraft to decrease, triggering a Short Term Conflict Alert (STCA) red alert on the radar system.
The investigation
The investigation examined the air traffic controller's actions, the flight trajectories, and the radar data. The controller involved held all necessary valid licenses and medical certifications, having completed refresher training in late 2014. The investigation noted that the controller was on their second shift following two days of rest, and while the number of open sectors was appropriate, the traffic density was at the upper limits of manageable capacity.
Radar analysis and trajectory studies confirmed that the separation between the two aircraft was not actually lost; however, the investigation determined that the event should be classified as an occurrence (event) rather than a standard air traffic incident due to identified errors in Air Traffic Management (ATM) procedures.
Findings
- The primary factor in the generation of the STCA alert was a 30-knot reduction in the groundspeed of the Boeing 737-800 during its climb.
- The air traffic controller's clearance for the climb to FL 360 was issued based on a measured separation of 7.46 NM, but the subsequent change in speed reduced the margin of safety.
- The traffic volume in the sector was at the upper limits of the controller's workload capacity at the time of the event.