Distracted Qantas pilots mucked up programming their Airbus A380 cockpit computers and only realised they did so as they hurtled down a Los Angeles airport runway, air safety investigators have found.
Interruptions to pre-flight procedures meant pilots had no readout of the target speeds they needed to reach in order to lift off — something the captain only noticed as the plane hit 100 knots (185km/h) accelerating along on the tarmac.
While one instrument gave them their instantaneous rolling speed, the pilots had to read from handwritten notes to recall the lift-off target speeds they had to reach, investigators said.
That pilots had handwritten notes as a back-up was "fortuitous", Fairfax Media has been told.
A chain of distractions involving changing wind strengths, a fuselage door malfunction and confusing runway lighting led to the slip-up, the air crew told investigators from the Australian Transport Safety Bureau.
An unfavourable tail wind caused a last-minute change of runway for departure from LA, bound for Melbourne on October 8 last year. That change required new take-off calculations.
As the captain was readying to change the data in the navigation computer, cabin crew called the cockpit to advise they had a problem arming one of the doors for take-off, which might require an aircraft engineer's attention to fix.
As a result of the interruption to sort out the door issue, the captain had not followed all the procedures to enter data for the change of runway before taxiing for the runway.
The first officer twice dismissed computer alerts about the plane's take-off data.
The first time he believed the take-off data would be checked, and the second time believed it had been checked.
"There were no other warnings in place to alert the crew that they were commencing the take-off without the take-off speeds in the aircraft's navigation systems," investigators said.
As the Airbus reached 100 knots down the runway, the pilots noticed there were no readings of take-off thrust, speed, or the status of the auto-thrust.
Pilots initially thought the lack of instrument readouts was due to an auto-thrust failure.
The first officer asked the captain whether take-off should continue, but the captain said the engines were on full thrust anyway, and there were no other warnings or alerts of serious problems.
The heavily-laden aircraft lifted off at 159 knots (294km/h), in line with the pilots' manual calculations.
The flight eventually landed in Melbourne without further incident.
"The captain reported that there were a number of distractions and interruptions from the time they were pushed back from the gate, until the take-off commenced," investigators said.
A Qantas spokesman said the incident "was a process error with no operational safety risk".
"The aircraft took off normally and safely. Both Airbus and Qantas have made changes to ensure that the process error cannot occur again."
Qantas contacted Airbus, which said it had just introduced revised software to warn pilots if they commence a take-off without take-off speeds entered in the flight computers.
That software patch has been retrofitted to all Qantas A380s.
Pilots' pre-flight procedures for runway changes have also been amended.