Narrative:

In cruise at FL350 first officer's leg using autoplt/flight guidance #2 system. We had been cleared pilot discretion descent to FL240 and 240 had been set and armed in altitude window. Aircraft remained in altitude hold mode as it should have at 35000'. I was getting the airport ATIS and the first officer was looking over the STAR when I noticed the aircraft had climbed to 35480' while the flight director still indicated altitude hold. I immediately started a descent when the center controller called us to ask about our altitude. He told us there was no conflict, no problem, and no alarms had been set off. On arriving at dfw I wrote the altitude hold problem up in the aircraft log book for maintenance action. Also we had not looked away from the panel but for only a very short time. We never felt or noticed the aircraft climb. We were flying into an area of rapidly increasing barometric pressure. Dfw was 30.66 and increasing rapidly. I believe now that it could have been a glitch in the CADC (central air data computer) correcting for the rapid rise in barometric pressure that created the indication problem. The problem could not be duplicated later and the fact remains that the autoplt remained in altitude hold and had leveled at 35480' which just about corresponds to the change in altimeter settings in the area we had just flown through.

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Original NASA ASRS Text

Title: ACR MLG ALT EXCURSION.

Narrative: IN CRUISE AT FL350 F/O'S LEG USING AUTOPLT/FLT GUIDANCE #2 SYS. WE HAD BEEN CLRED PLT DISCRETION DSCNT TO FL240 AND 240 HAD BEEN SET AND ARMED IN ALT WINDOW. ACFT REMAINED IN ALT HOLD MODE AS IT SHOULD HAVE AT 35000'. I WAS GETTING THE ARPT ATIS AND THE F/O WAS LOOKING OVER THE STAR WHEN I NOTICED THE ACFT HAD CLBED TO 35480' WHILE THE FLT DIRECTOR STILL INDICATED ALT HOLD. I IMMEDIATELY STARTED A DSCNT WHEN THE CENTER CTLR CALLED US TO ASK ABOUT OUR ALT. HE TOLD US THERE WAS NO CONFLICT, NO PROB, AND NO ALARMS HAD BEEN SET OFF. ON ARRIVING AT DFW I WROTE THE ALT HOLD PROB UP IN THE ACFT LOG BOOK FOR MAINT ACTION. ALSO WE HAD NOT LOOKED AWAY FROM THE PANEL BUT FOR ONLY A VERY SHORT TIME. WE NEVER FELT OR NOTICED THE ACFT CLB. WE WERE FLYING INTO AN AREA OF RAPIDLY INCREASING BAROMETRIC PRESSURE. DFW WAS 30.66 AND INCREASING RAPIDLY. I BELIEVE NOW THAT IT COULD HAVE BEEN A GLITCH IN THE CADC (CENTRAL AIR DATA COMPUTER) CORRECTING FOR THE RAPID RISE IN BAROMETRIC PRESSURE THAT CREATED THE INDICATION PROB. THE PROB COULD NOT BE DUPLICATED LATER AND THE FACT REMAINS THAT THE AUTOPLT REMAINED IN ALT HOLD AND HAD LEVELED AT 35480' WHICH JUST ABOUT CORRESPONDS TO THE CHANGE IN ALTIMETER SETTINGS IN THE AREA WE HAD JUST FLOWN THROUGH.

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.