History of the Flights
On 13 May 2009, a Boeing B747-400, registration 9V-SPI, was conducting an Instrument Landing System approach to Changi Airport Runway 20R. At about 1431 hours, the Arrival Controller cleared the aircraft to descend to 3,000 ft. The aircraft was approximately 20 nautical miles from touchdown and passing 5,200 ft. It then reported established on the localizer and was cleared to continue the approach. At about 1434 hours, while at 3,500 ft, the Arrival Controller instructed the B747 to descend to 2,000 ft.
At around the same time, an Airbus A320, registration 9V-JSD, was approaching the terminal area from the north. At 1432 hours, while at about 6,000 ft, the A320 crew contacted the Arrival Controller, who instructed them to descend to 2,500 ft and turn left to heading 360°. The A320 initiated the left turn. The crew later requested a heading change to 130°, but the controller did not approve and instead instructed a further left turn to heading 340°.
Air Traffic Control Actions
At about 1434 hours, the Arrival Controller assessed a potential conflict between the two aircraft. He instructed the A320, then at 3,800 ft, to stop its descent at 3,000 ft and turn left to heading 330°. Realizing this was insufficient, he further instructed the A320 to climb to 4,000 ft and directed the B747 to descend to 2,000 ft. The controller noted that the A320 had taken an unusually wide turn and was slow to execute it. He had earlier descended the A320 to 2,500 ft despite the minimum sector altitude of 3,400 ft, considering the aircraft would exit the sector before reaching that altitude.
TCAS Events
During the descent past 3,000 ft, the Traffic Alert and Collision Avoidance System (TCAS) on the B747 activated momentarily and issued a Resolution Advisory (RA) to descend. The flight crew complied. The A320's TCAS also activated momentarily and issued a Traffic Advisory (TA), which the crew noted. At the closest point, the aircraft were separated by 1.5 nautical miles laterally and 500 ft vertically, a loss of separation given the required minimum of 3 nm horizontal or 1,000 ft vertical.
Subsequent Communications
The B747's first officer, who was handling radio communications, did not inform the Arrival Controller of the TCAS RA during the flight, as he was busy preparing for landing. He later called the Singapore Air Traffic Control Centre about an hour after landing to report the incident. The A320 crew informed the controller that they had visual contact with the B747. Both aircraft continued their approaches and landed without further incident.
Investigation
The Air Accident Investigation Bureau of Singapore was informed of the incident on 3 June 2009. The investigation did not specify a probable cause.