What happened
On 21 March 2022, Saudi Airline flight SVA724, a Boeing 777-368(ER) registered as HZ-AK11, was operating a scheduled passenger service from Riyadh to Islamabad International Airport. While being vectored by Cherat Approach South, the aircraft was cleared to descend to Flight Level (FL) 170 via the KALMI 1A arrival.
During the descent, the controller provided traffic information regarding a military Beechcraft-350, callsign SAPHIRE-24, which was holding over position KALMI at FL160. As SVA724 passed approximately 3 NM west of the KALMI waypoint, the crew reported the activation of a TCAS Resolution Advisory (RA). At the moment of the alert, the Boeing 7/7 was descending through FL177 toward its assigned altitude.
In response to the RA, the crew initiated a climb up to FL181. Once the conflict was resolved and the aircraft was clear of the military traffic, SVA724 resumed its descent to FL170 and eventually continued to Islamabad for landing without further incident. No injuries were reported among the 370 persons on board, and no damage occurred to either aircraft.
The investigation
The Aircraft Accident Investigation Board (AAIB) of Pakistan examined the flight data and communications. The investigation established that while the controller had provided traffic information regarding SAPHIRE-24, this information was not acknowledged by the crew of SVA724.
Investigators also noted a discrepancy between the captain's statement and the recorded flight data. While the captain stated the aircraft was already level at FL170 when the alert occurred, the investigation confirmed the aircraft was actually in an active descent phase through FL177.
Findings
- The primary cause of the TCAS RA activation was the high rate of descent maintained by SVA724.
- The aircraft was descending at 1,800 feet per minute, which exceeded the maximum recommended rate of 1,500 feet per minute during the final stages of descent as specified in ICAO Doc 8168.
- The crew failed to properly acknowledge the traffic information provided by the Cherat Approach Controller regarding the military aircraft holding at KALMI.
- At the time of the RA, a vertical separation of 1,700 feet and a lateral separation of 3 NM existed between the two aircraft; standard separation minima were not breached.