B737 flight crew reported the failure of the Mode Control Panel resulted in the autopilot; autothrottle; and VNAV malfunctioning in cruise. The flight crew then decided to descend below RVSM airspace; and proceeded to continue the flight and safely landed at the airport.
Synopsis
B737 flight crew reported the failure of the Mode Control Panel resulted in the autopilot; autothrottle; and VNAV malfunctioning in cruise. The flight crew then decided to descend below RVSM airspace; and proceeded to continue the flight and safely landed at the airport.
Narrative
We were at FL370 approaching position ZZZZZ when our autothrottle started randomly disconnecting. We reconnected it and saw that it would revert between FMC SPD and ARM mode. This continued repeatedly until we decided to just stop using autothrottle. Next; the VNAV mode disconnected. We would reconnect it but after a few minutes or seconds it would come out of VNAV and revert to ALT HLD (Altitude Hold). Then the autopilot would disconnect again. We made several attempts to reconnect it only for it to disconnect again. We tried switching it to Autopilot B but the same thing happened. Around this time we were past position ZZZZZ1 and with ZZZZ Control. We decided that we needed to descend below RVSM airspace.We were given a clearance to cross ZZZZZ2 at 11000 ft. The First Officer set 11000 ft. in the MCP (Mode Control Panel). However; the altitude that I put in altitude preselect would jump to 50000 ft. and it would be stuck there for a minute before I was able to reset it to the proper altitude. This would happen repeatedly until the altitude was stuck at 50000 ft. Then; as we descended and were getting closer to landing; Heading Select would not work either. On the PFD (Primary Flight Display) we lost the FD (Flight Director). We had no guidance. In fact once we were cleared for the approach; APP Mode would not work initially. However; after intercepting the LOC; APP Mode did work. We continued and landed safely. We submitted write-ups for an inoperative Mode Control Panel; autothrottle; autopilot; and VNAV.Once we arrived at the gate we completed the parking checklist. We then contacted Company Dispatch to inform them of our mechanical problems. We were connected on the call with Maintenance Control where we explained our mechanical problem. They contacted Local Maintenance to troubleshoot the problem and came to the conclusion that the Mode Control Panel was broken and needed repairing. Unfortunately; the part needed to fix the problem was not available in ZZZ so the flight had to be canceled until the following day. We stayed in constant communication with all parties involved; including our passengers.
Second reporter narrative
[Report narrative contained no additional information.]
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.