What happened
On August 2, 2013, an Airbus A320 and a Boeing 737-800 were both descending toward Warsaw Chopin Airport (EPWA) following the same route. During the transfer of control from the Area Control Center (ACC) to the Approach (APP) controller, the aircraft experienced a significant reduction in separation.
Both crews had been instructed by the ACC controller to maintain a specific rate of descent (RoD). The A3/20 crew was instructed to descend to FL150 at a rate of 1500 ft/min or greater. The B738 crew was subsequently instructed to descend to FL170 at 1500 ft/min. However, after establishing contact with the Approach controller, the aircrafts' vertical separation was compromised. While the A320 crew maintained a steady descent, the B738 crew significantly increased their rate of descent to over 3000 ft/min. This resulted in the aircraft reaching the same altitude, with the horizontal distance between them dropping to just 2NM, far below the required 7NM minimum.
Upon detecting the conflict, the Approach controller issued immediate corrective instructions, directing the A320 to turn right and increase its descent rate, while directing the B738 to turn left and level off its descent. These actions successfully restored separation.
The investigation
The investigation examined the communication between the ACC and APP sectors, the coordination between controllers, and the flight profiles of both aircraft. The investigation reviewed radio transcripts and radar data to determine how the vertical separation was lost during the handoff process. The inquiry also looked into the coordination protocols used between the ACC coordinator and the APP assistant.
Findings
- The primary cause of the incident was errors in Air Traffic Management (ATM) operations.
- The B738 crew exceeded the authorized rate of descent, descending at a rate exceeding 3000 ft/min despite the initial instruction.
- There was a lack of precise communication regarding the rate of descent; the investigation noted that the word "exactly" should have been used when specifying 1500 ft/min to prevent ambiguity.
- Coordination regarding vertical separation was performed between an ACC coordinator and an APP assistant rather than between two active air traffic controllers, which is inconsistent with required procedures.
- There was a breakdown in the transfer of information regarding the specific descent rates between the two controlling sectors.