What happened
On August 11, 2020, at Kraków-Balice Airport (EPKK), a separation violation occurred between an Airbus A-3-21, registration HA-LXT, and a Piper PA-32, registration SP-NRS. The incident took place within the Kraków TMA during a period of low traffic intensity.
The Tower controller cleared the Piper PA-32 for takeoff at 13:05:00. Shortly thereafter, at 13:05:48, the controller issued a takeoff clearance to the Airbus A-321. During the takeoff roll of the Airbus A-321, the controller realized that the faster aircraft had been cleared to follow the slower aircraft too closely. To mitigate the risk, the controller contacted Kraków Approach (APP) to request a turn for the Piper PA-32 to move it away from the runway centerline. At 13:07:30, following the departure of the Airbus A-321, the vertical separation between the two aircraft dropped to 0 ft, with a horizontal separation of 2.8 NM, violating the required minimums of 1000 ft and 5 NM. This triggered a Short Term Collision Alert (STCA) on the radar system.
The investigation
The investigation examined the controller's actions and the operational environment. It was established that the Tower controller was one of the first personnel working with the newly implemented Electronic Flight Progress Strip (EFES) system, which had been operational for only 11 days. The investigation reviewed the coordination procedures between Kraków Tower and Kraków Approach, noting that while certain departures can occur without coordination, differences in aircraft performance require telephone coordination.
Findings
- The primary cause was the failure to account for the performance difference between the two aircraft, which resulted in a lack of necessary coordination.
- The controller relied on a "time counter" tool within the new EFES system, which counted down two minutes from the first aircraft's takeoff; the controller issued the second clearance mechanically based on this timer.
- Reduced traffic volume due to the COVID-19 pandemic contributed to a decrease in the controller's concentration.
- The controller's instruction to change the flight paths of the aircraft while they were below the minimum vectoring altitude violated standard vectoring procedures, although this specific action was intended to increase the actual distance between the aircraft.