What happened
On November 1, 2005, at 14:23 UTC, a loss of separation occurred in the SW BEPAS area within the Munich FIR. The incident involved a Boeing 737-500, operated by České Aerolinie (flight ČSA 689), and a De Havilland DHC8D, operated by Deutsche Lufthansa (flight DLH 2JC).
While flying at FL 320, the crew of ČSA 689 received an instruction from Munich ACC (EDDM) to descend to FL 270. However, the crew initiated a descent to FL 170, which was in violation of the cleared altitude. As the aircraft transitioned to Prague ACC (ACC Praha) frequencies, the crew reported their descent to FL 170, a detail that was not registered by the Prague controller.
At approximately 14:23 UTC, Munich ACC contacted Prague ACC to inquire about the progress of ČSA 689 and to provide traffic information regarding DLH 2JC, which was flying at FL 250 on a similar course. At the time of the inquiry, the ČSA 689 crew reported they were already at FL 256, having bypassed the cleared FL 270. This resulted in a vertical separation of only 500 feet, which is 50% of the required minimum.
The investigation
The ÚZPLN investigation examined the actions of both flight crews, the procedures and radio communications of the air traffic controllers at both Munich and Prague ACC, and the coordination between the two sectors. The investigation relied on radar data and radio communication recordings from both Munich and Prague control centers.
Findings
- The crew of ČSA 689 failed to adhere to the assigned flight clearance by descending to FL 170 instead of the authorized FL 270.
- The Prague ACC controller failed to register the crew's report regarding the descent to FL 170.
- The reduction in separation was caused by the crew's unauthorized descent and the controller's failure to note the discrepancy.
- At the time of the incident, the vertical separation was 500 feet and the horizontal distance was 4.31 NM.
- No TCAS activation occurred on either aircraft, and the aircraft were on parallel tracks, meaning there was no immediate threat to safety.