Narrative:

Flight from mco to jfk. Prior to departure the captain's altimeter indicated 100 ft high in the standby mode. The standby mode is put on maintenance carry over by maintenance. Now we have the first officer's altimeter available and the captain's altimeter in central air data computer mode. Earlier in the day the captain's altimeter was worked on, and I believe replaced. The flight departs mco normally and climbs to FL250, both altimeters are set to 29.92 inches at 18000 ft MSL, however the first officer's altimeter is lagging slightly behind the captain's altimeter. Indicating level at FL250 on the captain's altimeter and slightly lower (maybe 100-150 ft) on the first officer's altimeter, the first officer asks ATC what altitude ATC shows the flight on their radar. ATC replies level at FL250. We complete the appropriate checklist for unreliable altitude indications, and suspect the first officer's altimeter of the malfunction. The checklist calls for using the lowest indicated altitude, however as a crew, we decided to use the captain's altimeter, although higher, it was in agreement with ATC's readout from our transponder. Further along in the flight the first officer's altimeter starts descending at a rate of approximately 10 FPM. At approximately 150 NM south of norfolk, va, the first officer's altimeter is about 600 ft low and still descending. Approximately 20 mins later, in level flight, the captain's altimeter rolls rapidly (less than 2 seconds) to show an indicated altitude of approximately 24000 ft MSL. We notified ATC of the situation and that we were unable to verify our altitude. ATC replied they still showed us level at FL250. We told ATC that they should be suspicious of any altitude readouts received from our aircraft. ATC replied they understood and would not clear any aircraft within 2000 ft of our flight path. The WX at jfk was reported as good VFR so we as a crew decided to proceed to our destination. On leveloff during our descent ATC generally showed us approximately 1000 ft off what we were indicating and asked us to turn off our altitude reporting, but we elected to leave it on to get some information from the TCASII. The flight completed the VOR runway 13L approach and landed runway 13R at jfk. During the approach the first officer's altimeter was indicating closer than the captain's to the radar altimeter (terrain approximately sea level). I called maintenance the next day and was told the captain's CADC failed and the first officer's altimeter failed. My hypothesis to this situation is that the problem was created earlier in the day, prior to the flight, when the system was worked on by maintenance, either by faulty parts, incorrect installation, or both. One possible piece of equipment that could have helped in this situation would have been GPS.

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Original NASA ASRS Text

Title: A B727-200 FLC EXPERIENCES A LOSS OF ALT INFO WHEN THEIR 2 ALTIMETERS DISAGREE WHILE INFLT AT FL250. APCH TO JFK IS DONE IN VMC CONDITIONS.

Narrative: FLT FROM MCO TO JFK. PRIOR TO DEP THE CAPT'S ALTIMETER INDICATED 100 FT HIGH IN THE STANDBY MODE. THE STANDBY MODE IS PUT ON MAINT CARRY OVER BY MAINT. NOW WE HAVE THE FO'S ALTIMETER AVAILABLE AND THE CAPT'S ALTIMETER IN CENTRAL AIR DATA COMPUTER MODE. EARLIER IN THE DAY THE CAPT'S ALTIMETER WAS WORKED ON, AND I BELIEVE REPLACED. THE FLT DEPARTS MCO NORMALLY AND CLBS TO FL250, BOTH ALTIMETERS ARE SET TO 29.92 INCHES AT 18000 FT MSL, HOWEVER THE FO'S ALTIMETER IS LAGGING SLIGHTLY BEHIND THE CAPT'S ALTIMETER. INDICATING LEVEL AT FL250 ON THE CAPT'S ALTIMETER AND SLIGHTLY LOWER (MAYBE 100-150 FT) ON THE FO'S ALTIMETER, THE FO ASKS ATC WHAT ALT ATC SHOWS THE FLT ON THEIR RADAR. ATC REPLIES LEVEL AT FL250. WE COMPLETE THE APPROPRIATE CHKLIST FOR UNRELIABLE ALT INDICATIONS, AND SUSPECT THE FO'S ALTIMETER OF THE MALFUNCTION. THE CHKLIST CALLS FOR USING THE LOWEST INDICATED ALT, HOWEVER AS A CREW, WE DECIDED TO USE THE CAPT'S ALTIMETER, ALTHOUGH HIGHER, IT WAS IN AGREEMENT WITH ATC'S READOUT FROM OUR XPONDER. FURTHER ALONG IN THE FLT THE FO'S ALTIMETER STARTS DSNDING AT A RATE OF APPROX 10 FPM. AT APPROX 150 NM S OF NORFOLK, VA, THE FO'S ALTIMETER IS ABOUT 600 FT LOW AND STILL DSNDING. APPROX 20 MINS LATER, IN LEVEL FLT, THE CAPT'S ALTIMETER ROLLS RAPIDLY (LESS THAN 2 SECONDS) TO SHOW AN INDICATED ALT OF APPROX 24000 FT MSL. WE NOTIFIED ATC OF THE SIT AND THAT WE WERE UNABLE TO VERIFY OUR ALT. ATC REPLIED THEY STILL SHOWED US LEVEL AT FL250. WE TOLD ATC THAT THEY SHOULD BE SUSPICIOUS OF ANY ALT READOUTS RECEIVED FROM OUR ACFT. ATC REPLIED THEY UNDERSTOOD AND WOULD NOT CLR ANY ACFT WITHIN 2000 FT OF OUR FLT PATH. THE WX AT JFK WAS RPTED AS GOOD VFR SO WE AS A CREW DECIDED TO PROCEED TO OUR DEST. ON LEVELOFF DURING OUR DSCNT ATC GENERALLY SHOWED US APPROX 1000 FT OFF WHAT WE WERE INDICATING AND ASKED US TO TURN OFF OUR ALT RPTING, BUT WE ELECTED TO LEAVE IT ON TO GET SOME INFO FROM THE TCASII. THE FLT COMPLETED THE VOR RWY 13L APCH AND LANDED RWY 13R AT JFK. DURING THE APCH THE FO'S ALTIMETER WAS INDICATING CLOSER THAN THE CAPT'S TO THE RADAR ALTIMETER (TERRAIN APPROX SEA LEVEL). I CALLED MAINT THE NEXT DAY AND WAS TOLD THE CAPT'S CADC FAILED AND THE FO'S ALTIMETER FAILED. MY HYPOTHESIS TO THIS SIT IS THAT THE PROB WAS CREATED EARLIER IN THE DAY, PRIOR TO THE FLT, WHEN THE SYS WAS WORKED ON BY MAINT, EITHER BY FAULTY PARTS, INCORRECT INSTALLATION, OR BOTH. ONE POSSIBLE PIECE OF EQUIP THAT COULD HAVE HELPED IN THIS SIT WOULD HAVE BEEN GPS.

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.