What happened
On January 7, 2020, a Pakistan International Airlines (PIA) Boeing B77L, registration APBGY, was operating a flight from Islamabad to Karachi. While descending through the Nawabshah area, the aircraft was cleared by the Karachi Area Radar Controller East to descend from FL400 to FL180. This descent clearance was issued with the intent of providing separation from another aircraft, an ATR72, operating at FL170.
Simultaneously, an Emirates Airline Boeing B7 .77W, registration A6EPS, was transiting from Dubai to Delhi, maintaining FL350. The controller failed to identify the Emirates flight as a conflict during the descent clearance process for the PIA aircraft. As the B77L descended through the altitude of the reciprocal traffic, both aircraft triggered a Traffic Collision Avoidance System – Resolution Advisory (TCAS-RA). The crew of APBGY initiated a climb to FL369, while the crew of A6EPS initiated a descent to FL347. At the closest point of approach, the aircraft maintained a vertical separation of 1800 feet and a lateral separation of 0.54 NM.
The investigation
The Aircraft Accident Investigation Board (AAIB) Pakistan examined radar display data, audio recordings, and controller statements. The investigation focused on the sequence of descent clearances and the functionality of the Short Term Conflict Alert (STCA) system. Investigators reviewed the actions of both the Area Radar Controller East and the Area Procedure Controller East, who was supervising a trainee at the time.
Findings
- The primary cause was a lack of situational awareness by the Area Radar Controller East, who cleared the PIA flight to descend without accounting for the Emirates aircraft at FL350.
- The Area Procedure Controller East failed to provide necessary coordination and did not act upon an automated Flight Plan (FPL) alert that had been generated when the descent profile was modified.
- The STCA system failed to provide a 'Prediction Phase' warning due to specific user-defined parameters in the ATM database, which only triggered the 'Violation Phase' once the conflict was already imminent.
- The controller's attempt at an evasive maneuver via heading changes was too late to prevent the TCAS-RA activation.