A B737-800 IN CRUISE AT FL350 EXPERIENCED DUAL FMS FAILURE. CTL DISPLAY UNITS BLANK WITH NO MESSAGES. STARTED CHKLIST WHEN SYS RETURNED.

Narrative

NOT LONG AFTER TOP OF CLB EARLY IN CRUISE ON OUR TRANSCONTINENTAL FLT FROM SEATTLE TO MIAMI WE HAD A DUAL FMC FAILURE. WE HAD BEEN CLRED TO SNY AND ADDED THE ABEAM POINTS AND THE FORECAST WINDS OVER NAV POINTS; AS WELL PLOTTED AN AREA OF TURB ALONG OUR RTE OF FLT. ATC THEN CLRED US DIRECT TO SZW WHICH WE LOADED WHEN BOTH CDU'S WENT BLANK EXCEPT FOR THE ACARS DISPLAY. NO OTHER MESSAGES (SINGLE FMC; DUEL FMC FAILURE NOTICES) WERE DISPLAYED. WE NOTICED THAT THE CAPT'S VTK MESSAGE WAS DISPLAYED; FOLLOWED BY THE SAME MESSAGE ON THE FO'S SIDE. NO 'MAP' DISPLAY MESSAGE; LIKE WE WOULD SEE ON A CLASSIC DUAL FMC FAILURE. CRUISE DISPLAY TRIPPED TO AUTOTHROTTLE LIMIT. WE LOST LNAV AND VNAV. ALT HOLD ENGAGED AND CTL WHEEL STEERING FOR LATERAL NAV. AT FIRST WE THOUGHT THAT IT WAS A CAPT'S FMC FAILURE BECAUSE INITIALLY ONLY THE CAPT'S SIDE DISPLAYED A VTK FLAG. SELECTED BOTH FMC SWITCHES ON R ON THE XFER SWITCHES. SHORTLY AFTER BOTH SIDES SHOWED A VTK FLAG. CALLED FOR THE DUAL FMC FAILURE CHKLIST IN THE QRH; WHEN THE FMC'S SEEMED TO 'FIX' THEMSELVES AND WE WERE ABLE TO ONCE AGAIN GAIN ACCESS TO THE FMC. EVERYTHING HAD BEEN DUMPED AND WE HAD TO REBUILT THE ENTIRE RTE AND PERFORMANCE PAGE. WE THEN PROCEEDED TO MIA WITHOUT ANY FURTHER PROBS WITH EITHER FMC. I THOUGHT IT WAS WORTHY OF A REPORT BECAUSE THE 'FAILURE' WAS UNLIKE ANY I HAD EVER EXPERIENCED IN TRAINING. I DON'T THINK THAT IT WAS A FULL FAILURE; BUT A KIND OF SOFT FAILURE; WHICH WAS DIFFICULT TO DIAGNOSE. WE NEVER RECEIVED ANY OF THE USUAL FMC MESSAGES; AKA; 'SINGLE FMC' FAILURE; 'DUAL OPS RESTORED;' NOR DID WE EITHER OF US HAD THE 'MAP' DISPLAY.