Australian investigators have catalogued a series of missed opportunities to catch the weight data error which led to the serious Emirates Airbus A340-500 tail-strike at Melbourne.
During the flight preparations, the base weight from the flight-management system, 361.9t, was augmented with a 1t allowance for last-minute changes, to produce a figure of 362.9t.
Probably through a simple miskeying, the first officer inadvertently entered the incorrect take-off weight for the aircraft - using the figure 262.9t rather than 362.9t - when calculating the take-off performance data through the A340's electronic flightbag.
This incorrect weight, 100t below the actual figure, was transcribed on to the flightplan, along with the associated performance parameters.
While the single electronic flightbag was handed to the captain so he could check the figures, the Australian Transport Safety Bureau said: "There was a lot of activity in the cockpit at that time and it is likely that the associated distractions degraded the captain's checks, and the weight error remained undetected."
The pilots' procedures were also supposed to include a verbal check between them to compare the take-off weight in the flight-management system with that entered into the electronic flightbag.
However the "various distractions", including the first officer discussing departure clearance with air traffic control, meant this check was "omitted", said the ATSB.
The loadsheet confirmation procedure provided two more chances to pick up the error - the first when the first officer read the take-off weight from the flight-management system and then from calculations on the flightplan.
But having correctly read the former as 361.9t, the first officer initially misread the flightplan as 326.9t, then re-read it as 362.9t - the correct figure, even though this was not the one written on the plan. The first officer thought he had simply written the wrong figure, and corrected it, but this left the miscalculated performance data unchanged.
The second chance to capture the error came with a check of the "green dot" speed from the flight-management system and electronic flightbag.
While the check is intended to ensure these speeds are within 2kt (4km/h), the pilots failed to notice the two systems were displaying "green dot" speeds differing by 40kt.
The flight-management system read 265kt and the flightbag 225kt. "Because they both ended in a '5', the captain may not have noticed the difference in the values," the ATSB said.
During the take-off roll on 20 March 2009, the aircraft failed to accelerate sufficiently, using almost the entire runway before over-rotating and suffering a tail-strike 265m from the runway end, followed by two more strikes at 173m and 110m. The A340 overran, hitting infrastructure, before becoming airborne and eventually returning to land safely.
Investigators pointed out that the variations in parameters experienced by the crew during normal mixed-fleet operations "increased the difficulty" of the pilots to recognise suspect outputs from the electronic flightbag.
In the two months prior to the accident, the crew had been exposed to take-off weights varying from 150-370t, and the erroneous take-off weight of 262.9t "would not have been sufficiently conspicuous" to alert them, said the ATSB.
"This problem is not unique to this accident," it stated. "Previous investigations into similar data entry error and tail-strike occurrences have highlighted the inability of flightcrew to conduct a 'rule of thumb' or reasonableness check of speeds when moving between aircraft types.
"An unintended consequence of mixed-fleet flying appears to be a reduction in a flightcrew's ability to build a model in long-term memory to facilitate recognition of 'orders of magnitude', or a 'rule of thumb', in respect of take-off performance data."