What happened
On 29 November 2018, Saudi Arabian Airlines flight SVA9431, an Airbus A332 with registration TC-OCN, was descending from FL370 to FL350 while operating on a route from Jeddah to Multan. During the descent, Karachi ATC (ACC Radar North) instructed the crew to descend due to crossing traffic from an Oman Airline flight.
The controller subsequently issued repeated instructions to the crew to expedite their descent to reach FL350. While the aircraft's initial rate of descent was within recommended limits, the pressure to accelerate the descent caused the vertical rate to increase significantly, reaching over 3,000 feet per minute. This high rate of descent triggered a Traffic Collision Avoidance System Resolution Advisory (TCAS-RA) "Climb" command.
In response to the TCAS-RA, the aircraft climbed to FL358. During this maneuver, the aircraft's horizontal separation from a Pakistan International Airlines Boeing 777, flight PIA303, fell below the standard 15NM minimum. No injuries or aircraft damage were reported.
The investigation
The Safety Investigation Board (SIB) Pakistan examined radar tracings, ATC audio recordings, and video records of the incident. The investigation focused on the sequence of instructions provided by Karachi ACC Radar North and the movement of three other aircraft in the vicinity: an Oman Airline B738, an Air Blue A320 (VNA650), and a PIA Boeing 777 (APBMH).
Findings
- The primary cause of the TCAS-RA was the excessive rate of descent performed by SVA9431, which was driven by repeated instructions from ATC to expedite the descent.
- The controller failed to provide timely traffic information regarding the potential conflict with PIA303 while instructing the aircraft to descend.
- Inadequate situational awareness by the controller led to a loss of standard horizontal separation between SVA9431 and PIA303 during the period when vertical separation was not maintained.
- The investigation noted that the controller's repeated instructions to expedite were issued without explaining the underlying reason, which could have allowed the crew to better manage their flight profile.
- It was also discovered that certain ACC sector delineations had been implemented without formal approval or incorporation into the AIP-Pakistan.