AN ACR MD88 FLC HAD A 400 FT EXCURSION FROM THEIR ASSIGNED ALT WHEN THEY WERE DISTR BY AN FMC FAILURE.

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I WAS PF. I WAS IN THE PROCESS OF MANUALLY LEVELING THE ACFT AT FL350 (AUTOPLT OFF) WHEN WE HAD A DOUBLE MAP FAILURE WITH FMS TIMEOUT ANNUNCIATED. I LEVELED THE ACFT AND BELIEVE I ENGAGED THE AUTOPLT. I INSTRUCTED THE FO TO ATTEMPT TO REGAIN THE MAP. ATTITUDE INDICATIONS WERE NORMAL. THE FO WAS HAVING TROUBLE GETTING HIS FMS TO RESPOND (FMS TIMEOUT ANNUNCIATED); SO I REACHED DOWN TO MY FMS TO HELP. I NOTICED THE ACFT CLBING ABOVE OUR ASSIGNED ALT OF FL350 AND STOPPED IT AT FL354 AND PROMPTLY RETURNED THE ACFT TO FL350. AUTOPLT WAS RE-ENGAGED. I GAVE CTL OF THE ACFT TO THE FO AND RE-ESTABLISHED THE MAP AND RELOADED THE COMPUTER. SUPPLEMENTAL INFO FROM ACN 432328: APCHING LEVELOFF AT FL350; MAP FAILURE OCCURRED. PF ENGAGED ALT HOLD AND WAS ANNUNCIATED ON FMS. DURING ATTEMPT TO REINITIALIZE FMS; ACFT DEVIATED FROM ASSIGNED ALT BY APPROX 400 FT. NO TCASII CONFLICTS WERE ANNUNCIATED AND NO RPTS BY ATC WERE MADE. ACFT WAS MANUALLY FLOWN TO ASSIGNED ALT WHERE FMS WAS REINITIALIZED. DURING DEV; UNCERTAIN OF STATUS OF AUTOPLT. PRIOR TO LEVELOFF (APPROX 2000 FT) THE PF (CAPT) DISCONNECTED THE AUTOPLT TO TRIM THE ACFT. APCHING LEVELOFF; THE MAP FAILED. THE ACFT WAS FLOWN TO THE ASSIGNED ALT AND PF SELECTED ALT HOLD. ALT HOLD WAS ANNUNCIATED ON THE FMS.