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.
