A Boeing 747-438 (north-east bound B747) was travelling on air
route B200 within the Tahiti flight information region (FIR) at
flight level (FL) 330 and was assigned FL350 by Tahiti air traffic
control. When the aircraft was at FL339, the crew reported that
they observed a traffic alert and collision avoidance system (TCAS)
indication of another aircraft. That aircraft was a Boeing 747-400
(south-west bound B747) that was travelling in the opposite
direction at FL340 also on air route B200, approximately 40NM
ahead. The crew of the north-east bound B747 immediately descended
to FL330. However, there was an infringement of separation
standards.
The air traffic controller had planned to assign FL350 to the
crew of the north-east bound B747 to maintain a separation standard
with a third B747 travelling on B200 at FL330 in the opposite
direction. However, FL350 was not available to the north-east bound
B747 crew until the controller in Tahiti could established a
separation standard with the south-west bound B747 travelling in
the opposite direction at FL340.
The controller had prepared a pre-formatted controller-pilot
data link communication (CPDLC) message for transmission to the
crew of the north-east bound B747. CPDLC is a `means of
communications between a controller and pilot using data link for
[Air Traffic Control] communication' (ICAO Doc 4444 ATM/501 1-5).
The message was a clearance that instructed the crew to `climb to
and maintain FL350'. The controller prepared the message in
advance. That was reported to be a common practice and assisted
with workload management. The controller intended to send the
message to the crew of the north-east bound B747 once they had
passed the south-west bound B747 and a separation standard had been
established. However, he unintentionally sent the message before
the two aircraft had passed. On receipt of the clearance to climb,
the north-east bound B747 commenced climb to FL350.
The controller immediately realised the error. He reported that
he had made seven unsuccessful attempts to contact the crew of the
north-east bound B747 using `selcal'. Selcal is a coded tone sent
to a specific aircraft that indicated to the crew that an ATC unit
was attempting to contact them via HF radio. The crew of the
north-east bound B747 reported that they did not receive an
indication that the controller was attempting to contact them via
selcal and did not reply. The controller eventually sent another
CPDLC message to the crew instructing them to 'maintain FL330 due
traffic'. The crew of the north-east bound B747 acknowledged
receipt of that CPDLC message. They reported however, that they had
already commenced descent to FL330 when they initially observed the
south-west bound B747 on the TCAS.
Air traffic controllers used automatic dependent surveillance
(ADS) to verify the position of appropriately equipped aircraft
operating within the Tahiti FIR in accordance with the South
Pacific Operations Manual (SPOM). The SPOM detailed the procedures
and requirements applicable in South Pacific FIR's for ADS approved
aircraft and applied within the Tahiti FIR. ADS provides data,
including position and altitude information, from navigation
equipment on-board an aircraft to air traffic control via a data
link. The information is updated at specified time intervals known
as the periodic reporting rate. The periodic reporting rate for ADS
reporting in the Tahiti FIR was 30 minutes.
Despite the unintentional clearance issue, the controller did
not realise that there had been an infringement of separation
standards. He had received an ADS report from the north-east bound
B747 that confirmed the aircraft was level at FL330, before he sent
the clearance to climb, and he received a report from that crew,
subsequent to the occurrence, confirming that they were level at
FL330. There were no ADS reports from the north-east bound B747 to
Tahiti ATC when the aircraft was changing levels. The controller
was not aware that the north-east bound B747 had left FL330 and,
therefore, was not aware there had been an infringement of
separation standards.
Preparation of the CPDLC message in advance may assist
controllers with workload management. However, controllers need to
exercise care and ensure that pending messages are not
unintentionally sent.
The aircraft operator could not determine why the crew of the
north-east bound B747 did not receive an indication that the
controller was attempting to contact them using selcal. The selcal
equipment on board the aircraft was operational before and after
the occurrence, although Tahiti air traffic control had reported
degraded HF communication on the night of the occurrence.