Incident Description
On 2 August 2013, an Airbus A320 and a Boeing 737-800 were both descending along the same route to land at Warsaw Chopin Airport (EPWA). The aircraft were under the control of the Area Control Centre (ACC) before being transferred to the Approach Control (APP). The ACC controller instructed both crews to maintain a rate of descent (RoD) of 1500 ft/min. The A320 crew was instructed to descend to FL150 with a RoD of 1500 ft/min or greater, while the B738 was instructed to descend to FL170 with a RoD of exactly 1500 ft/min.
Chronology
At 10:18:50 UTC, the B738 crew contacted ACC at FL390. At 10:19:05, the ACC coordinator informed the APP assistant that the B738 would descend to the last flight level released by the A320. At 10:19:25, the A320 began descending from FL370. At 10:21:10, the B738 began descending from FL390. At 10:28:13 and 10:28:23, the A320 and B738 were instructed to contact APP. At 10:28:44, a Collision Alert Warning (CAW) activated. At 10:29:08, a Collision Alert (CA) activated. At 10:29:11, the A320 was instructed to turn 10 degrees right and informed of conflicting traffic. At 10:29:28, the B738 reported descending at 1500 ft/min, but the APP controller observed a higher RoD and instructed the crew to stop descent. At 10:29:40, the B738 was instructed to turn left 40 degrees. The minimum horizontal separation was 2 NM at FL220, with the required minimum being 7 NM. Both aircraft then diverged.
Findings
The investigation found that the two aircraft had similar estimated times over the BIMPA waypoint, so the ACC sector planned to transfer them to APP with vertical separation, as per the Letter of Agreement (LoA) between ACC and APP. The B738 crew significantly increased their RoD (to over 3000 ft/min) before contacting APP, reducing vertical separation. The ACC coordinator provided information about the descent rates to the APP assistant, but it was not confirmed that this information reached the APP controller. The coordination between ACC and APP was performed through the assistant rather than directly between controllers, which was not in accordance with ICAO Doc 4444. The B738 crew may have misinterpreted the descent rate instruction, as they believed it was "1500 ft/min or greater" rather than exactly 1500 ft/min.
Probable Cause
The probable cause of the incident was errors in air traffic management (ATM).