What happened
On 13 May 2009, a Boeing B747-400, registration 9V-SPI, was performing an approach to Changi Airport Runway 20R. Simultaneously, an Airbus A320, registration 9V-JSD, was being directed by air traffic control to descend and turn within the terminal area.
During the descent, the B747 flight crew received a TCAS Resolution Advisory (RA) instructing them to descend. At the same time, the A320 crew received a Traffic Advisory (TA). The two aircraft eventually reached a point where they were separated by only 1.5 nautical miles laterally and 500 feet vertically, failing to meet the required minimum separation of 3 nautical miles or 1,000 feet of vertical distance.
The investigation
The investigation focused on the actions of the Arrival Controller and the communication between the flight crews and air traffic services. Investigators examined why the A320 was cleared to descend below the Minimum Sector Altitude (MSA) of 3,400 feet and why the controller's corrective maneuvers failed to prevent the conflict.
The inquiry also looked into the reporting of the TCAS event. While the B747 crew notified the controller that they were "cleared of conflict," they did not explicitly state that a TCAS RA had occurred. Furthermore, the investigation reviewed the interaction between the aircraft's First Officer and the air traffic control Watch Managers following the landing.