B737 Flight Crew reported a failure of an FMC with the other FMC already on MEL. The Captain exercised his authority to reactivate the MEL'd FMC and continued the flight to destination airport.
Synopsis
B737 Flight Crew reported a failure of an FMC with the other FMC already on MEL. The Captain exercised his authority to reactivate the MEL'd FMC and continued the flight to destination airport.
Narrative
[Our flight] from ZZZ-ZZZ1 had a MEL for the right FMC. It was inoperative but not collared or required to be per the MEL. The flight was uneventful and normal until about 80 miles west of ZZZ2. The left FMC then failed; the auto-throttles failed; VNAV went to Altitude Hold; and the Autopilot went into CWS Roll mode. ATC was notified we had a dual FMC failure losing all long range NAV and we requested radar vectors until further notified. The ATC request was granted. First Officer was great at making suggestions for our situation and recommended we requested priority handling to see if the other FMC still worked. First Officer and I considered our options and we made a plan. Our workload was increased and we had lost our normal mode of navigation so we [requested priority handling]. We consulted the Quick Reference Handbook; but considered the fact that the MEL effectively prevented us from potentially using the other FMC if it did work due to the restriction to not move the switch from the operative side for all 'phases of flight'. The Quick Reference Handbook also did not consider the fact we had a MEL. In light of our situation; and priority handling; I decided to use my Captains authority to see if the right FMC was working normally. The FMC source select switch was placed to normal and it was confirmed the right FMC worked normally and then the switch was placed to both on the right FMC. The rest of the flight was uneventful. The [priority landing] was continued all the way to the gate since we had to deviate from the MEL using [the] Captains authority. The passengers were never notified or made aware of the above issue. The Fire Rescue vehicles were requested to stay out of sight. The Flight Attendants were made aware of our situation in case the fire trucks did decide to converge on us; but were briefed that the flight was operating mostly normal.
Second reporter narrative
The airplane came into ZZZ with a write up for dual FMC fail on ground during taxi in. The left FMC was changed out earlier in the day for a different issue. Maintenance reset the FMCS in ZZZ and did some tests and thought the right FMC had failed so they deferred the right FMC. MEL was complied with and the FMC switch was placed in both on left position. Once in cruise; and near ZZZ2; we were presented with a FMC caution light and flags on the Navigation Display. The auto throttle disconnected. Captain was Pilot Flying. First Officer was monitoring. We both noticed the failure at the same time and had somewhat expected this to happen since there was history of this. Pilot Flying assumed control of the airplane in anticipation that the Autopilot would kick off. A heading was requested from ATC and the Autopilot remained on and a known power setting was set. We referenced the Quick Reference Handbook but the Quick Reference Handbook did not fully apply to the situation since we were already on both of left for the FMC. The Captain requested priority handling out of caution as our workload had increased and we and lost all long range NAV. Captain and First Officer both questioned the validity of the maintenance deferral and felt that it would be best to try the deferred right FMC. The Captain exercised authority to deviate from the MEL and while still in heading select; we positioned the FMC switch to both on right. Within 10 seconds the right FMC came up and appeared valid. We agreed that the best course forward was to continue to operate on FMC right. The crew exercised excellent CRM. The Captain showed excellent leadership effectiveness in handling the situation calmly and methodically. Both crew exercised excellent situational awareness as we both noticed the fault and were able to quickly react. We had great communication and the Captain welcomed all input from the First Officer. Monitor Crosscheck was good as we verified that the right FMC did indeed present valid data. Workload management was good as we split tasks and kept each other in the loop. Captain temporarily gave control of the airplane to the First Officer while he managed the situation with ATC; Dispatch; and Maintenance Crew. We used the highest level of automation available during each phase of the event. I honestly do not think I would handle the situation any differently next time except maybe query maintenance in ZZZ more thoroughly as I did not have a great gut feeling about the deferral. However; we knew that conventional NAV was available as well as help from ATC with many divert options available encountered as we were not over water.
More incidents for this aircraft family →
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.