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.