What happened
On May 13, 2014, at 14:36 UTC, a serious incident occurred within the Prague TMA involving two ATR72-500 aircraft, registrations OK-GFR and OK-GFS, operating for ČSA. Both aircraft were participating in a holding pattern over the RATEV waypoint at flight level 100.
The air traffic control environment was highly complex at the time. Controllers were managing an emergency situation involving an Airbus A320 (SVR740) that had diverted to Prague Ruzyně following a bird strike and engine failure. Additionally, significant thunderstorm activity in the Prague TMA and FIR required multiple aircraft to perform deviations, increasing the operational workload. To manage the traffic, the arrival and departure sectors had been merged.
During the sequencing process, the controller at the AEC position issued a descent clearance to OK-GFS to descend to FL100. However, OK-GFR was already established at that same altitude. This resulted in a vertical separation of only 475 ft and a horizontal separation of 1.05 NM. As the aircraft continued their maneuvers, the Short Term Conflict Alert (STCA) was triggered when the horizontal distance reached 3.9 NM. Both crews subsequently responded to TCAS RA alerts. Despite the rapid corrective actions taken by the controllers and the flight crews, the radar separation minima dropped to 1.83 NM, representing only 61% of the required separation.
The investigation
An investigation by the ÚZPLN examined radar records, audio recordings, and operational documentation. The investigation focused on the controller's actions and the accuracy of flight progress strips. Analysis of the cockpit/station background noise revealed that the controller was verbally confirming available altitudes to himself while formulating a plan. However, the investigation found that the controller either misspoken or was misled by incorrectly maintained flight progress strips. Specifically, the records for OK-GFR had been overwritten, making it difficult to verify the exact altitude change, which likely led to the erroneous clearance being issued to OK-GFS.
Findings
- The primary cause of the incident was the issuance of an incorrect descent clearance by the AEC controller, which placed both aircraft at the same flight level.
- High operational workload, caused by managing an engine-failure diversion and significant thunderstorm activity, contributed to the error.
- Inaccuracies in the flight progress strips for OK-GTF contributed to the controller's error.
- The STCA and TCAS RA systems functioned correctly and assisted in resolving the conflict.