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A Boeing 747 (B747) was on an international flight from Sydney to Los Angeles, maintaining flight level (FL) 290. The flight was operating in controlled airspace and was being managed by the Brisbane Sector 8 air traffic controller, using procedural control methods. The flight crew and controller were communicating via controller-pilot data link communications (CPDLC). The crew requested approval to climb to and operate between FL290 and FL330. The controller sent an approval at 0659 on the CPDLC for the crew to climb to the block level. Prior to changing levels, the crew received traffic information on their aircraft traffic alerting and collision avoidance system (TCAS) display. The traffic was approaching from the opposite direction, 2,000 ft above the B747. The B747 crew maintained their aircraft at FL290 and immediately reported via the CPDLC that they were unable to comply due to traffic. This message was sent at 0659 but was not received at the controller's terminal until 0706. The delay was believed to be due to network lag. The traffic was subsequently identified as a British Aerospace 146 (BAe 146), en route from Norfolk Island to Sydney at FL310.

The controller had assumed responsibility for the sector following a handover/takeover a few minutes after the request from the B747 crew, to operate in the block level, had been received. The previous sector controller had coordinated the use of the block level with adjacent sectors in anticipation of approving the request, after establishing that the two aircraft had passed each other. Neither controller had calculated an estimated time of passing (ETP) for the aircraft. An ETP of 0659 was subsequently calculated after the occurrence. Air traffic control standards require that aircraft travelling in the opposite direction on the same route cannot climb or descend through the level of the other aircraft during a specific period before and after the ETP. In this case, the appropriate time standard was 10 minutes. Consequently, the controller should only have approved the use of the block level after 0709.

The controller taking over responsibility for the sector stated that he was aware of the confliction and had prepared the message approving the use of the block level, with the intention of holding it in the CPDLC terminal until after the aircraft had passed each other. The CPDLC did not have a hold function in the "reply" mode in which it was operating. Consequently, the message was ready for dispatch. The "send" function on the terminal was actioned by selecting an icon and activating this via a computer "mouse" button. The controller's response to the initial request should have been either to acknowledge receipt with "standby" or "request deferred". This would have then enabled the controller to respond further to the message after establishing a passing standard. The controller awaited an audio response from the terminal to indicate that the message was being held. When this did not occur, he prepared and sent a message to the crew instructing them to maintain FL290. The crew responded to this instruction with "Wilco". This response was received at the controller's terminal at 0701. The controller could have also used Flight Service high frequency (HF) radio communications as an alternative to the CPDLC, in order to instruct the crew to maintain FL290. Delays of up to 5 minutes were common when using the data link system.

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