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Analysis

Summary

ANALYSIS

The FMS performance reported by the crew was consistent with FMC 2 performing a progressive series of resets before it latched. A similar progression of resets then occurred for FMC 1 but, in accordance with its design, it did not latch and was available for use but with the flight plan information cleared. During the resetting process, the FMS response would have been confusing to the crew and consistent with the crew’s observation of the ‘FMS locking us out’.

With FMC 2 latched, the MENU page would have been displayed on the copilot’s multi-function control and display unit (MCDU) but with the FMC 2 prompt missing from the top left data field. The standby navigation/radio (STANDBY NAV/RAD*) prompt would also have been displayed on the MCDU and ‘MAP FAIL’ displayed on the copilot’s navigation display. Once the resetting was completed, then an ILS frequency could be tuned using the STANDBY NAV/RAD page on either the pilot in command’s (PIC) or copilot’s MCDU.

The PIC was the handling pilot during the flight. Normally the PIC’s MCDU will interact with FMC 1 and the copilot’s MCDU will interact with FMC 2. With FMC 2 latched, it was necessary for the copilot to change his source select switch to ‘FO ON 1’ to access FMC 1.

During the investigation the VIA manufacturer advised that no other FMS problems of this nature had been reported.

 
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