What happened
On October 16, 2018, at approximately 04:29 UTC, a serious loss of separation occurred near waypoint OLSAR within the Kolkata Flight Information Region (FIR). The incident involved two heavy aircraft: a Saudi Arabian Airlines B77W, registration HZAK14, flying from Riyadh to Manila, and a United Parcel Service Airlines B744, registration N578UP, traveling from Suvarnabhumi to Mumbai.
At the time of the encounter, the B77W was maintaining flight level 330 but had deviated left of its assigned track to avoid inclement weather. The B744 was operating on a reciprocal track at flight level 320. Initially, the Radar Controller denied a request from the B7 and 44 to climb to flight level 340, specifically citing the presence of the opposing B77W traffic. However, during a subsequent request for the same climb, the controller authorized the ascent.
As the B744 climbed through the level occupied by the B77W, a Short Term Conflict Alert (STCA) was triggered. The aircraft reached a minimum lateral separation of 2.5 NM and a vertical separation of only 300 feet. While the crew of the B77W reported receiving a Traffic Advisory (TA), neither aircraft reported receiving a Resolution Advisory (RA). The conflict ended after the B744 was instructed to descend back to its original level.
The investigation
The AAIB India investigation focused on the actions of the Upper Bhuvneshwar Sector (UBS) Radar Controller and the operational environment. The investigation established that the controller was managing 24 aircraft at the time of the incident, handling a high volume of traffic within a large scanning area that requires intense coordination with adjacent control units.
Investigators examined the communication transcripts, which showed that while the controller was initially aware of the conflicting traffic, the high traffic density and the need to manage multiple handovers contributed to a lapse in monitoring. The investigation also noted that the controller had mistakenly identified another aircraft, UTP9948, as the primary traffic concern, leading to the erroneous authorization of the climb.
Findings
- The primary cause of the incident was a momentary loss of situational awareness by the Radar Controller.
- The controller authorized a climb for the B744 through the flight level occupied by the B77W because they misjudged the proximity of the reciprocal traffic.
- The controller incorrectly focused on aircraft UTP9948, which was at a different location, rather than the immediate threat on the reciprocal track.
- High traffic density and a large sector scan area in the UBS sector increased the complexity of the controller's workload.
- The B77W was slightly offset from its track due to weather, which prevented an even closer encounter but did not mitigate the vertical conflict.