What happened
On June 28, 2006, a separation infringement occurred in the Prague Flight Information Region (FIR) between a Scandinavian Airlines System (SAS) MD-82, registration OY-KHE, and a Germanwings A3/319, registration D-AILK.
At approximately 12:01 UTC, the SAS flight, operating from Munich to Copenhagen, entered the Prague FIR at FL 280. Due to significant thunderstorm activity, the crew requested a heading change to avoid convective clouds. The Prague Area Control Center (ACC) subsequently instructed the aircraft to climb to FL 310, and later to continue climbing to FL 340 with a vertical rate of at least 1500 ft/min.
Simultaneously, the Germanwings flight, traveling from Warsaw to Stuttgart, was instructed to descend from FL 340 to FL 320, and eventually to FL 300 at maximum vertical speed.
At 12:05 UTC, the SAS crew abruptly ceased their climb at FL 322 without notifying Air Traffic Control (ATC). The crew reported difficulty maintaining the required vertical rate, which they attributed to suspected engine icing caused by cold downdrafts near a thunderstorm. During the ensuing conflict resolution, the SAS crew also performed a TCAS Resolution Advisory (RA) maneuver without informing the controller. Radar analysis showed that the vertical separation between the two aircraft dropped to approximately 800 ft, with a horizontal distance of 4.26 NM, violating the established RVSM separation minima.
The investigation
The ÚZPLN investigation examined radar records, radio communications, and the actions of both flight crews and the Prague ACC. The investigation focused on why the commanded rate of climb was not maintained and why the change in flight profile was not communicated to the controller. The investigation also reviewed the impact of thunderstorm activity and the effectiveness of the Short Term Conflict Alert (STCA) system.
Findings
- The primary cause of the incident was the failure to maintain the commanded rate of climb by the SAS crew.
- The SAS crew failed to notify the Prague ACC when they ceased their climb at FL 322.
- The SAS crew did not report the execution of a TCAS RA maneuver to the controller.
- The loss of vertical separation was exacerbated by the crew's decision to initiate engine anti-icing procedures due to suspected icing, which impacted the aircraft's performance.
- The Prague ACC responded correctly to the STCA alert and issued instructions to both aircraft to maintain separation.
- Severe thunderstorm activity and associated cold downdrafts contributed to the unexpected change in the aircraft's climb performance.