Investigators of last year's fatal Spanair Boeing MD-82 crash have disclosed that the pilots' failure to carry out proper flap and slat position checks contributed to the loss of the aircraft on departure from Madrid.
An update to the inquiry, published by Spanish investigation authority CIAIAC, shows the crucial checks were missed twice as the crew prepared for take-off.
The aircraft had been making a second attempt to depart on 20 August 2008 after returning to the apron to deal with a technical problem with the ram air temperature probe.
Before reaching the flaps/slats item on the 'after start' checklist, the captain instructed the co-pilot to request taxi clearance. The cockpit-voice recorder shows that the captain had not mentioned the word 'flaps' before reading the list.
CIAIAC says the flap/slat check was "omitted". It then states that, about 10min later, the co-pilot started the 'take-off imminent' checklist.
The cockpit-voice recording shows that the co-pilot ran through the final items on this list, calling out the figure 'eight' for the centre-of-gravity position and 'eleven' for the take-off flap setting of 11°.
While the pilot is supposed to read these figures directly from cockpit indicators and the flap/slat controls, investigators believe that these vital cross-checks were not performed. Flight-data recorder information shows the flaps were not deployed, and cockpit damage was consistent with the flap lever being in the 'retracted' position.
"The physical evidence and the data recorded for the flaps...contradict what the pilot is heard saying on the [voice recorder]," states the CIAIAC. "The check of the final items...is not considered to have represented an actual check of the cockpit indicators."
This supports the primary line of inquiry, that the aircraft crashed after failing to gain lift as a result of the pilots' failing to deploy the flaps and slats.
Investigators are still looking into the reasons why the take-off configuration alarm did not sound, but have yet to reach a detailed conclusion.
CIAIAC says, however, that the two checklist errors and the alarm failure meant that three safety barriers to prevent an improper take-off were "defeated".
"Improvement should be taken in the area of design and operations so that future accidents as this one can be prevented," it adds. Only 18 of the 172 occupants survived the crash.
Spanair has already twice amended its operations manual, the latest revision occurring in March this year. The changes include a specific clarification that a checklist has been completed, a requirement to check the take-off warning system on each flight, and a more rigorous check of final items on the 'take-off imminent' list.