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The Boeing 747-400 (B747) was tracking northwest on air route R340 at flight level (FL) 330 and estimated TASHA, a position 61 NM northeast of Mount Isa, at 1530 Eastern Standard Time. The Boeing 737-400 (B737) was tracking northeast on air route J64 at FL330 and estimated TASHA at 1531. The Isa sector controller, located in the Brisbane Air Traffic Control Centre, identified the potential conflict between the aircraft and offered the B747 crew a change of level to FL350. The crew preferred a lower level due to the ability to maintain a greater ground speed; the flight had departed later than scheduled and the crew were endeavouring to make up time en route.

At 1501, the controller instructed the B747 crew to descend, when ready, to FL310 with a requirement to reach that level by 31 NM southeast of TASHA. That position was the lateral separation point between the air routes and the controller required the 2,000 ft vertical separation standard to be established between the aircraft before they entered the area of conflict. The pilot in command (PIC) readback the amended clearance in accordance with Aeronautical Information Publication (AIP) procedures. Subsequently, the crew did not descend the aircraft in accordance with the clearance and it entered the area of conflict at FL330. There was an infringement of separation standards as the required vertical standard was not achieved before the aircraft entered the area of conflict.

At about 1508, a change of controllers occurred at the Isa sector position. The new controller instructed the B747 crew to change frequency at 1512. The crew contacted the Isa controller on the new frequency and reported maintaining FL330. The AIP required a crew operating in controlled airspace to report, after any en route frequency change, the last assigned level and whether the aircraft was on climb, in the cruise, or on descent. The crew did not report the assigned level of FL310 and the controller did not query the crew regarding that report.

The controller became concerned, as the B747 approached the lateral separation point, by the lack of a report indicating that the aircraft was on descent to the amended level. The AIP required a report from a crew when an aircraft had left a level at which level flight had been conducted in the course of a climb, cruise, or descent.

The controller conducted a single interrogation (one shot) of the aircraft's automatic dependant surveillance (ADS) system and attempted to contact the crew by very high frequency (VHF) radio. Automatic Dependant Surveillance was a system dependant on a datalink and a series of reporting `contracts' (a rate of reporting) established between an aircraft and a ground system. The Australian Advanced Air Traffic Control system (TAAATS) automatically initiated contracts and specified the type of report, the content of a report, and the reporting frequency required. As the contracted reporting occurred automatically, it required no flight crew action. There was no cockpit indication that a "one shot" request had been actioned by an aircraft's ADS system.

The ADS response from the aircraft at 1527 indicated that it was maintaining FL330 and was 26 NM from TASHA, within the area of conflict. The controller again attempted to contact the crew by radio and also by the controller pilot datalink (CPDLC) facility. The controller made another "one shot" interrogation of the aircraft's ADS. That ADS response at 1529 indicated that the aircraft was at FL329 and 14 NM from TASHA.

Flight crews were required to maintain continuous communications with air traffic control while in controlled airspace and within VHF radio coverage. Crews of ADS equipped aircraft were able to report to ATC using that facility; however, they were required to communicate using VHF radio when operating within radio coverage. The B747 was operating in non-radar airspace and the crew reported their position via ADS. The route was within VHF radio coverage for the sector.

The controller was about to instruct the B737 crew to climb to FL340 when an ADS altitude report of FL312 was received from the B747. That report established that the 2,000 ft separation standard had been achieved as it was within 200 ft of the assigned level.

At 1529:43, after five unsuccessful attempts to contact the B747 crew on VHF radio, the controller asked the crew of the B737 (on the crossing route) to contact the B747 crew and have them call on 125.2 Mhz. At 1531, the B747 crew contacted the controller on the VHF radio and reported maintaining FL310. At the same time the B747's ADS issued a Waypoint Report for TASHA which reported the aircraft's level as FL309. Later analysis of the ADS reports indicated that the B747 had descended 1,700 ft in about 28 seconds.

The automatic reporting rate for ADS tracks was set by TAAATS. The flight information region was divided into cells that were allocated a reporting rate for a Basic Report. The rate was normally 30 minutes (or 40 minutes for oceanic areas). A controller with the jurisdiction of an aircraft on an ADS track can manually amend the rate as required. Also, when the aircraft passed a designated waypoint the system automatically generated a Waypoint Change Event report that was appended to a Basic Report. Furthermore, an Altitude Range Event report was automatically generated when an aircraft left a contracted vertical range. When in the cruise, that vertical range was plus or minus 200 ft of the cleared flight level (CFL). Assignment of an amended level reset the range. When on climb, the reset range was the present level minus 200 ft to CFL minus 200 ft, with the reverse range for aircraft on descent. At that time, for the B747 maintaining FL330 and then assigned FL310, the vertical range would change from FL332 - FL328 to FL332 - FL312. As the aircraft descended through FL312 the Altitude Range Event report was generated and the contract reset to FL312 - FL308; to monitor the amended CFL (FL310).

The B747 PIC later reported that they had endeavoured to remain at FL330 for as long as possible due to turbulence at FL310 that would have likely required a speed reduction, which in turn, would have constrained their efforts to make up time during the flight. At the time of the issue of the amended clearance, the PIC was the pilot flying the aircraft and the first officer had left the flight deck shortly before to take a break. On the return of the first officer, the PIC briefed him on the clearance as per company procedures and the FO wrote the clearance on the flight plan. The incident report submitted by the crew stated that the annotations used by the FO on the flight plan indicated that descent should commence at 31 NM from TASHA, instead of the requirement to be at FL310 at that point.

The crew then became involved in troubleshooting a problem with balancing the main fuel tanks. The PIC stated that they had been distracted and forgot about the requirement to descend to FL310 by 31 NM southeast of TASHA. The PIC reported that he was aware of the B737 on the crossing track as he had heard the controller request the B737 crew to contact them (the B747 crew).

The South Pacific Air Traffic Services Coordinating Group's Southern Pacific Operations Manual (SPOM) V3.1 set the standard for ADS operations for air traffic control service providers and operators. The system status and serviceability was checked following the occurrence. Between 1332 and 1719 there were 31 downlink messages of which the minimum transit time was 6 seconds and the maximum transit time was 24 seconds. Those times were within the required SPOM system performance parameters. During that period there were no reported failures of the communication or TAAATS systems.

The investigation did not establish why the B747 crew did not respond to the controller's radio calls.

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