Cat. 2 investigation report: simplified-format report, adapted to the circumstances of the occurrence and the investigation stakes.
For technical reasons, it is not possible to create two different pages for the same investigation concerning two flights on two different dates. Given that it was the repetition of the incident which led to a safety investigation being opened, we have chosen to show the date of the second event only on the site.
During the flight from Norwich airport, the failure of resolver 2 of the right AOA sensor led to an erroneous speed indication on the right PFD and the activation of the IAS DISAGREE alert during the take-off run, followed by the AOA DISAGREE and ALT DISAGREE alerts in the initial climb. The crew quickly identified the PFD giving the erroneous indications. They carried out the checklists corresponding to the last two alerts but not the IAS DISAGREE checklist as the erroneous indication had been identified and they considered that the safety of the flight was not threatened. They continued the flight to destination and asked for maintenance work.
The maintenance technician who worked on the aeroplane during the night did not strictly comply with the Fault Isolation Manual (FIM) troubleshooting procedures and did not identify the cause of the problem. The failure of resolver 2 of the right AOA sensor was therefore still present at the time of the flight the following day. During this flight out of Paris-Orly airport, the crew were confronted with the same symptoms. They also continued the take-off but decided to turn around after carrying out the checklists associated with the alerts.
The Boeing 737-800 flight manual specifies that if the aeroplane is “unsafe or unable to fly” when it has exceeded 80 kt during the take-off run, the crew must reject the take-off. The crews had little time to assess the capability of the aeroplane to continue the flight. They did not consider it opportune to reject the take-off. The favourable weather conditions during both take-offs probably furthered this decision. The identification of the erroneous indications and the use of the automatic systems linked to the functional sensor allowed the flight to be continued in the first case and a safe turn around in the second case.
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