Narrative:

I was the PF en route to ewr on J208, 20 NM southwest 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 climb. As I was correcting for the perceived climb, the first officer's instruments indicates we were descending. I gave him control and he leveled out at FL285 and was correcting to FL290 when ZDC inquired about our altitude. We stated that we were at FL285 and correcting to FL290 and advised him of our problem. At FL290 ZDC said he showed us at FL300! We verified our altitude using the first officer's instruments and our standby altimeter and switched our transponder to the #2 position. ZDC then confirmed FL290. Center did not mention any traffic conflict and our TCASII did not indicate a conflict. The first officer continued manually flying the aircraft as I continued troubleshooting and completing the abnormal procedures. 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 altitude hold. We backed up LNAV with raw data. We used the autothrottles but not VNAV. The WX was clear en route and for our approach so we continued to ewr with no further problems. Both CADC's were scheduled to be replaced at ewr.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.