A Boeing 747 (B747) was en route from Auckland to Melbourne, maintaining flight level (FL) 330. The aircraft was experiencing some turbulence. The crew were communicating with Auckland air traffic control via controller pilot data link communications (CPDLC) and were awaiting a response to a request to climb to FL350.
The crew had observed on their traffic alerting and collision avoidance system (TCAS) that another aircraft was crossing their route from the south-east to the north-west at FL350. They were aware that a clearance to climb to FL350 would not be available until that aircraft had passed. The other aircraft was a B737 en route from Wellington to Brisbane. The air routes intersected at the VIMAV reporting point. At 2002 the B737 crew reported at VIMAV maintaining FL350. At 2005, the B747 passed VIMAV.
At about 2010, the B747 crew received a conditional clearance to climb to FL350 at 2018. The delay in the climb approval was required to establish a 15-minute time standard between the two aircraft at the intersection of the air routes. The B747 crew immediately commenced climb to FL350 after the message was received. At 2012 they dispatched a message reporting they were maintaining FL350. When air traffic control notified the crew via high-frequency radio that they had not complied with the clearance, the crew descended the B747 to FL330. The B747 TCAS did not register an alert during the period that separation was infringed.
Normal company CPDLC practice for the B747 crew was for the non-flying pilot to read out the message on the screen, and for this to be confirmed by the flying pilot. Once the message and the subsequent response were confirmed by the two crew members the non-flying pilot dispatched the message to air traffic control.
The investigation found that the crew, for reasons unknown, had mis-interpreted the datalink message as an immediate clearance to climb. It is possible that, due to the turbulence being experienced, the crew acted hastily to acknowledge the message and climb the aircraft to reduce passenger discomfort. It is likely that either one or both of the crew were not as vigilant in their checking of the message as they would have been under less demanding circumstances.