What happened
On the afternoon of 6 December 2007, a separation breakdown occurred between an Airbus A319, registration 9V-SBE, and a Boeing B737, registration PK-GGN, within the Changi Control Zone. The Airbus A319 had departed Singapore Changi Airport for Yangon, Myanmar, and was climbing through 8,000 ft when it was cleared to 10,000 ft. Simultaneously, the Boeing B73 37 was approaching from the south, level at 9,000 ft.
As the two aircraft converged, the vertical separation dropped to 800 ft, while the horizontal distance was only 0.67 NM. This violated the required minimum separation of either 1,000 ft vertically or 3 NM horizontally. While the Airbus A319 crew received a Traffic Advisory from their TCAS, they did not perceive the proximity as a major concern. The crew of the Boeing B737 also noted a Traffic Advisory, though they were operating in Instrument Meteorological Conditions and could not visually identify the other aircraft.
The investigation
The investigation focused on the actions of the Approach Controller at the Singapore Air Traffic Control Centre (SATCC). It was established that the controller had intended to delay the climb of the 9V-SBE until it had passed the PK-GGN. However, the controller issued the clearance to climb to 10,000 ft while the aircraft were still on converging paths.
Investigators examined the controller's working conditions, noting the incident occurred during the final hour of a three-day shift cycle. The controller reported feeling tired, and the investigation noted that the widespread heavy rain and cloud cover in the area had increased the complexity of the traffic situation, despite the number of aircraft under control being within normal limits.
Findings
- The primary cause of the separation loss was the erroneous clearance provided by the Approach Controller, instructing the Airbus A319 to climb through the altitude occupied by the Boeing B737 while the aircraft were converging.
- The controller was experiencing fatigue during the final hour of their scheduled shift.
- Adverse weather conditions, including heavy rain, increased the complexity of the air traffic workload.
- While the controller's recovery actions were timely, they failed to provide necessary traffic information to the pilots to assist in visual identification.
Safety action
Following the incident, the air traffic services provider implemented several safety measures, including instructing controllers to remain vigilant when issuing altitude and heading instructions and to confirm clearances if an error is suspected. Additionally, recommendations were made to review the long-standing controller shift system and to implement scientific tools for assessing controller fatigue.