The Boeing 737 (B737) had departed Sydney for Perth and the crew was maintaining the aircraft at flight level (FL) 280. The planned route was to track overhead Portland, Vic., then via air route T134 across the Great Australian Bight. The reporting point SUBUM was located on air route T134, approximately 220 NM south-south-west of Adelaide.
As the aircraft proceeded towards Portland, it was being radar monitored by the Melbourne Sector 4 air traffic controller. Flight crews were not required to report their positions while under radar observation. Accordingly, the time at which the aircraft was overhead Portland was recorded on the air traffic control flight progress strip by the sector controller, who then calculated an estimated time of passing SUBUM. This estimate was based on the flight-planned time interval from Portland to SUBUM and the actual time at which the controller saw the aircraft pass over Portland. The report was then coordinated to Adelaide Sector 4, which had jurisdiction for the route segment to SUBUM.
The Airbus A320 (A320) departed Melbourne for Perth and was tracking to intercept air route T134 via Portland. The crew was maintaining the aircraft at FL310.
The flight progress strips displayed information obtained from the respective company flight plans and showed that the B737 was estimating Portland at 1355 EST and SUBUM at 1444. They also showed that the A320 was estimating Portland at 1415 and SUBUM at 1500.
The Melbourne Sector 4 controller observed the B737 pass Portland at 1357 and, based on that observation, estimated that the aircraft would be approximately on time at SUBUM and elected not to change the estimated time of 1444. The position report was coordinated with Adelaide Sector 4. Air traffic control procedures allowed for a difference of up to 2 minutes between pilot and controller estimates without requiring a cross-check.
A short time later, the Melbourne Sector 4 controller assessed that the A320 passed over Portland at 1402 (13 minutes ahead of the estimate) but due to other duties, did not immediately notate the flight progress strip or coordinate this position with Adelaide Control.
At 1404, the Melbourne controller realised that he had not informed the Adelaide controller of the A320's Portland position report and commenced the coordination process. He reported the time at Portland as 1402 and the level as FL310. At this moment he realised that he had not made a calculation for the SUBUM estimate and used the estimated time of arrival as written on the flight progress strip as his revised estimate. This action resulted in his flight progress strip indicating that the A320 was going to be "on time" at SUBUM despite being 13 minutes early at Portland.
Communications for both aircraft crews were then transferred to Perth Flight Service on high frequency (HF) radio.
The Adelaide Sector 4 controller accepted the coordination from Melbourne Sector 4 on face value because the Melbourne controller was required to check the accuracy of data he was coordinating. The Adelaide Sector 4 controller checked his flight progress strips and noticed that the A320 was early at Portland but estimated to be "on time" at SUBUM. He considered that this discrepancy was probably due to a flight planning error that had been corrected by the Melbourne controller. His decision was influenced by the fact that both aircraft were estimated to be "on time" at SUBUM. He also considered that as they were vertically separated any error in the estimated times would not be significant, and chose not to pursue the matter any further.
The Adelaide Sector 4 controller then coordinated the Portland position details, including the estimated times of arrival at SUBUM, with Melbourne Sector 1, the control position for air route T134 from SUBUM. As a consequence, Melbourne Sector 1 had flight progress strips that indicated both aircraft being "on time" at SUBUM.
At 1444, the Perth Flight Service officer contacted Melbourne Sector 1 with the position report at SUBUM from the crew of the B737. This report included a request for a climb to FL310. As such a climb would negate vertical separation, the controller was required to establish a 10-minute longitudinal separation standard in order to approve the request. As the flight progress strips indicated estimates for SUBUM at 1444 and 1500, this standard appeared to have been achieved. However, because the B737 had only just entered his area of responsibility, he was required to check with the previous sector (Adelaide Sector 4) before authorising such a change. There had been a change of personnel at Adelaide Sector 4 and, as the oncoming controller also had the same time indications as the Melbourne controller, he agreed to the change and the crew of the B737 was instructed to climb to FL310.
After approving the climb, the Adelaide controller decided to check his radar display on the maximum range and saw that the A320 was only 30 seconds east of SUBUM. He immediately contacted the Melbourne Sector 1 controller to inform him of the confliction.
The Melbourne Sector 1 controller contacted Perth Flight Service and issued an instruction for the crew of the B737 to descend to FL290. Because of the amount of coordination required, it took almost 3 minutes to translate the Adelaide controller's observation into an acknowledged instruction for the B737 to descend. As the B737 had reached FL300 before the crew received the instruction to descend, and as the vertical separation standard was 2,000 ft, an infringement of the separation standards had occurred.