Narrative:

Aircraft was cruising at 35,000' MSL on 'a' autoplt using #1 INS for navigation. The altitude alert light along with the altitude comparator light came on. 31 (captain's) altimeter was steady on 35,700'. Standby altimeter and #2 (first officer's) altimeter were both indicating a descent (approximately 34,000'). Both flight directors were calling for a slight pitch down. The autoplt was immediately disconnected and the aircraft pitch attitude increased to normal cruise. Center was asked what altitude he showed for us and he replied that he was just about to query us because he showed 35,700. The transponder was switched to #2 altitude reporting and center showed 33,000, the same stabilized altitude shown on the standby altimeter and #2 altimeter. Center was requested to block 33,000 to 35,000 for us. #1 altimeter then left 35,700 and headed down to join the other 2 altimeters. (Note: no fail flags showing yet.) the aircraft was then put in a climb back to 35,000 using the #2 altimeter with the standby altimeter backing it up. #1 altimeter then went back up to 35,700 and a red fail flag appeared in the window along with a mach fail flag and the others associated with a CADC failure. As our central air data computer failure procedure was accomplished, the #1 altimeter returned to 35,000 on its own and all associated fail flags disappeared. The flight proceeded to destination using the 'B' autoplt and #2 INS as the primary INS. #1 altimeter and #1 CADC system operated normally for the duration of the flight. I contribute this problem to a temporary failure of the #1 CADC. The movement of the #1 altimeter was rapid and not normal for the situation. The fact that it showed an increase in altitude caused the autoplt and flight director to descend (the altitude hold function was being utilized), to the altitude at which the altitude hold switch was engaged. Please note that the entire time lapse from recognition of the problem to the aircraft being stabilized back at 35,000' was not long, probably less than a minute and a half.

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Original NASA ASRS Text

Title: ALT DEVIATION EXCURSION FROM ASSIGNED. WDB HAD CENTRAL AIR DATA COMPUTER FAILURE.

Narrative: ACFT WAS CRUISING AT 35,000' MSL ON 'A' AUTOPLT USING #1 INS FOR NAVIGATION. THE ALT ALERT LIGHT ALONG WITH THE ALT COMPARATOR LIGHT CAME ON. 31 (CAPT'S) ALTIMETER WAS STEADY ON 35,700'. STANDBY ALTIMETER AND #2 (F/O'S) ALTIMETER WERE BOTH INDICATING A DSCNT (APPROX 34,000'). BOTH FLT DIRECTORS WERE CALLING FOR A SLIGHT PITCH DOWN. THE AUTOPLT WAS IMMEDIATELY DISCONNECTED AND THE ACFT PITCH ATTITUDE INCREASED TO NORMAL CRUISE. CENTER WAS ASKED WHAT ALT HE SHOWED FOR US AND HE REPLIED THAT HE WAS JUST ABOUT TO QUERY US BECAUSE HE SHOWED 35,700. THE XPONDER WAS SWITCHED TO #2 ALT REPORTING AND CENTER SHOWED 33,000, THE SAME STABILIZED ALT SHOWN ON THE STANDBY ALTIMETER AND #2 ALTIMETER. CENTER WAS REQUESTED TO BLOCK 33,000 TO 35,000 FOR US. #1 ALTIMETER THEN LEFT 35,700 AND HEADED DOWN TO JOIN THE OTHER 2 ALTIMETERS. (NOTE: NO FAIL FLAGS SHOWING YET.) THE ACFT WAS THEN PUT IN A CLIMB BACK TO 35,000 USING THE #2 ALTIMETER WITH THE STANDBY ALTIMETER BACKING IT UP. #1 ALTIMETER THEN WENT BACK UP TO 35,700 AND A RED FAIL FLAG APPEARED IN THE WINDOW ALONG WITH A MACH FAIL FLAG AND THE OTHERS ASSOCIATED WITH A CADC FAILURE. AS OUR CENTRAL AIR DATA COMPUTER FAILURE PROC WAS ACCOMPLISHED, THE #1 ALTIMETER RETURNED TO 35,000 ON ITS OWN AND ALL ASSOCIATED FAIL FLAGS DISAPPEARED. THE FLT PROCEEDED TO DEST USING THE 'B' AUTOPLT AND #2 INS AS THE PRIMARY INS. #1 ALTIMETER AND #1 CADC SYSTEM OPERATED NORMALLY FOR THE DURATION OF THE FLT. I CONTRIBUTE THIS PROBLEM TO A TEMPORARY FAILURE OF THE #1 CADC. THE MOVEMENT OF THE #1 ALTIMETER WAS RAPID AND NOT NORMAL FOR THE SITUATION. THE FACT THAT IT SHOWED AN INCREASE IN ALT CAUSED THE AUTOPLT AND FLT DIRECTOR TO DSND (THE ALT HOLD FUNCTION WAS BEING UTILIZED), TO THE ALT AT WHICH THE ALT HOLD SWITCH WAS ENGAGED. PLEASE NOTE THAT THE ENTIRE TIME LAPSE FROM RECOGNITION OF THE PROBLEM TO THE ACFT BEING STABILIZED BACK AT 35,000' WAS NOT LONG, PROBABLY LESS THAN A MINUTE AND A HALF.

Data retrieved from NASA's ASRS site as of August 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.