Boeing Co 737-476, VH-TJO

As a result of the investigation, the company operating the A320 has amended its flight-planning process by making a modification to the flight-planning system. In the 6 months following the amendment's implementation, manoeuvring times remained within the working tolerances of position reporting.

As a result of this occurrence, the Bureau of Air Safety Investigation is currently investigating a perceived safety deficiency. The deficiency identified relates to the validation of air traffic services' flight progress strip data for aircraft operating outside radar coverage.

Any safety output issued as a result of this analysis will be published in the Bureau's Quarterly Safety Deficiency Report.

1. The flight plan for the A320 contained a manoeuvring time for the aircraft prior to setting course.
2. The air traffic control strip printing system was unable to allow for a discrete manoeuvring time in the strip preparation.
3. The Melbourne Sector 4 controller did not conduct a crosscheck calculation on the flight progress strip notation for the A320's estimated time of arrival at SUBUM.
4. The Adelaide Sector 4 and Melbourne Sector 1 controllers did not initiate any check action for the observed significant difference in the time interval for the A320 between Portland and SUBUM.
5. Air traffic control procedures were such that there was no assurrance that the flight crew's estimated times were the same as those being used by ATC to provide procedural separation.

Flight planning

The air traffic control strip printing system's interpretations of the A320's flight plan led to a latent error in the flight progress strips for the A320 that was not present in the B737 strips. The system defence to negate this error was removed when the Melbourne Sector 4 controller did not update the flight progress strip estimated time for the SUBUM position.

Air traffic control procedures

Once the Melbourne Sector 4 controller had read the flight plan estimated time for the A320 at SUBUM, the coordination procedures were such that this incorrect estimate was passed to Adelaide Sector 4 and then from Adelaide Sector 4 to Melbourne Sector 1 without further check. Consequently, when coordination was required to allow the climb of the B737, the two controllers concerned had incorrect information. They did not question the information because it had been based on a radar observation.

Air traffic controllers

Although the flight progress strips showed that a longitudinal separation standard existed, the Melbourne Sector 4 controller did not review the situation before transferring the aircraft to Perth International HF. However, a scan of the strips should have revealed the error because the flight-planned time for the route segment was 45 minutes which, when added to the known time over Portland, would have given an estimate for SUBUM of 1447 rather than 1500.

The decision by the oncoming Adelaide Sector 4 controller to check his longer range radar display, detected the actual position of the A320 on air route T134 and enabled him to implement remedial action.

Pilot reporting

The aircraft passed over the last radar-observed position and the crews would have made estimates for SUBUM, but were not required to report those estimates to air traffic control. The controller also made estimates for SUBUM and based a procedural separation standard on those estimates. He had no need to check those estimates with the pilots or any other controller. Consequently, when the error was made, there was no crosscheck with which to provide a safety net.

Both flight crews were operating within 2 minutes of their corrected estimates, based on their actual times at Portland, and therefore were not required to report any minor changes.

Consequently, the crews and the controllers were working a procedural standard from two different time bases, neither of which had been crosschecked with the other. In a procedural environment, pilots and controllers must have a single datum on which to base their reporting and separation.

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.