Incident Overview
On 22 July 2015, an incident occurred in the Warsaw Flight Information Region (FIR EPWW) involving a Boeing 737-800 (B738) and a Boeing 777 (B77L). The B738, climbing to flight level (FL) 330, was in communication with Approach Control (KRL APP) for sequencing to an airport. The B77L was enroute east to west at FL340.
Sequence of Events
At 10:32:30 UTC, the B738 crew, approaching waypoint ATMIR and climbing to FL330, contacted KRL APP. The controller acknowledged radar contact at 10:32:44 and cleared the B738 for the IRLUN1T approach with descent to FL170. The crew acknowledged and requested a visual approach. At 10:33:17, the controller informed them they were second in sequence and initially cleared them toward waypoint GD908.
Meanwhile, the B77L, heading toward waypoint MAGAN near ATMIR, contacted Area Control (KRL ACC) at 10:33:29, reporting level at FL340.
At 10:33:53, the B738 crew reported that the Airborne Collision Avoidance System (ACAS) had issued a Resolution Advisory (RA) commanding a descent. The crew executed the RA. At 10:34:47, they reported being clear of conflict and continued descending to FL170. The controller later confirmed at 10:35:23 that his radar showed traffic above them. The B738 crew stated at 10:35:49 that the other aircraft was 1500 feet above, but ACAS had generated a descent RA. At 10:36:12, the controller acknowledged. At 10:36:50, KRL ACC asked the B77L crew if they had received an ACAS advisory; they reported a Traffic Advisory (TA) four minutes earlier.
Findings
The B738 had been climbing to FL330 when cleared to descend to FL170. At FL325, ACAS generated an RA. The minimum separation achieved was 1 NM laterally and 1500 feet vertically. An instructor from the ATS personnel training center, specializing in ACAS, determined that the B738's climb speed was excessive. This high rate of climb caused the ACAS system to project that the aircraft would overshoot FL330 and dangerously approach the B77L, potentially leading to a collision if uncorrected. The B77L only received a TA, consistent with this finding, as at those levels the RA warning delay for level aircraft is 10 seconds.
Probable Cause
The cause of the incident was excessive climb speed, which triggered an ACAS Resolution Advisory.
Safety Recommendations
No safety recommendations were formulated.