The pilot flying an Air New Zealand Boeing 777-300ER was under additional workload due to an error while inputting data into a key system when it descended below altitude limits during an approach to Brisbane airport.
The incident occurred on 8 November 2017, as the aircraft (registered ZK-OKN) was operating a flight from Auckland, with the captain acting as the pilot flying.
The Australian Transport Safety Bureau’s investigation found that the crew did not properly select the correct altitude in the aircraft’s flight mode annunciator (FMA). They also failed to check that the required flight mode change had occurred, which led to the pilot’s workload increasing substantially.
That subsequently caused the pilot to lose awareness of the aircraft’s altitude relative to the distance to landing, which combined with a preconception that it should not get too high while positioning for final approach, led it to descend below the 3,000ft altitude limit required on final approach.
“The flight crew corrected the error and levelled the aircraft at about 2,000ft shortly before being alerted of the altitude breach by ATC,” says the ATSB. It subsequently landed safely.
The investigator says that the incident highlights how an error can increase the workload on pilots, “particularly during a phase of flight that has an already high workload.” It also highlighted the importance of crew confirming mode changes in the FMA.