AN MD88 IN CRUISE AT FL290 EXPERIENCED AN EXCURSION FROM THE ASSIGNED ALT DUE TO THE GRADUAL FAILURE OF THE #1 AIR DATA COMPUTER.

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I WAS THE PF ENRTE TO EWR ON J208; 20 NM SW OF THE HOPEWELL VORTAC AT FL290. WE WERE ON AUTOPLT AND USING LNAV AND VNAV. A MESSAGE CAME UP ON THE FMS MCDU SCRATCH PAD INDICATING A CADC 1 DATA FAILURE. MY ALTIMETER AND VSI BEGAN FLUCTUATING WITH AN INTERMITTENT ALTIMETER OFF FLAG. THE AUTOPLT AND AUTOTHROTTLES DISENGAGED AND MY VSI WAS INDICATING A CLB. AS I WAS CORRECTING FOR THE PERCEIVED CLB; THE FO'S INSTS INDICATES WE WERE DSNDING. I GAVE HIM CTL AND HE LEVELED OUT AT FL285 AND WAS CORRECTING TO FL290 WHEN ZDC INQUIRED ABOUT OUR ALT. WE STATED THAT WE WERE AT FL285 AND CORRECTING TO FL290 AND ADVISED HIM OF OUR PROB. AT FL290 ZDC SAID HE SHOWED US AT FL300! WE VERIFIED OUR ALT USING THE FO'S INSTS AND OUR STANDBY ALTIMETER AND SWITCHED OUR XPONDER TO THE #2 POS. ZDC THEN CONFIRMED FL290. CTR DID NOT MENTION ANY TFC CONFLICT AND OUR TCASII DID NOT INDICATE A CONFLICT. THE FO CONTINUED MANUALLY FLYING THE ACFT AS I CONTINUED TROUBLESHOOTING AND COMPLETING THE ABNORMAL PROCS. THE #2 CADC ALSO APPEARED TO BE DEGRADED. IF WE ENGAGED THE AUTOPLT AND AUTOTHROTTLES AND USED LNAV AND VNAV; WE WOULD GET A CADC #2 DATA FAIL MESSAGE; MINOR ALTIMETER FLUCTUATIONS AND THE AUTOPLT AND AUTOTHROTTLES WOULD DISENGAGE. WE WERE ABLE TO OPERATE WITH THE #2 CADC; #2 DFGC ON AUTOPLT WITH LNAV AND ALT HOLD. WE BACKED UP LNAV WITH RAW DATA. WE USED THE AUTOTHROTTLES BUT NOT VNAV. THE WX WAS CLR ENRTE AND FOR OUR APCH SO WE CONTINUED TO EWR WITH NO FURTHER PROBS. BOTH CADC'S WERE SCHEDULED TO BE REPLACED AT EWR.