Boeing Co 747-438, VH-OJP

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.