Narrative:

On routine descent, we were given a crossing restr 65 mi south of the richmond VOR at FL230. After leveling off at FL230 I was distraction by a minor cabin temperature problem. The captain was writing down the ATIS. It was at this time that I heard the aural altitude warning go off. I looked at the altimeter and it showed us at FL227 and descending. I immediately initiated a climb back to FL230. ZDC did ask what my altitude was at that time. We were level at FL230 and I responded FL230. Nothing more was said by the controller. I still do not know why the autoplt did not maintain the assigned altitude. I do know, however, that the captain and myself were busy doing other tasks and nobody was watching the aircraft at that moment. Interestingly enough we had a jump seat rider on board and he didn't notice anything until the altitude warning went off. It was obviously a very gradual descent that was not noticed until it was too late. There was no conflict with other aircraft. Supplemental information from acn 356025: after a brief discussion, the crew could not ascertain the reason that the autoplt descended. My guess is that the PF (first officer) bumped the thumbwheel (vertical speed) as he made a heading change, starting a very gradual descent which went unnoticed until the aural altitude warning.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION FROM ASSIGNED. UNMONITORED AUTOPLT DSCNT WHEEL SETS UP A SLOW, UNNOTICED DSCNT.

Narrative: ON ROUTINE DSCNT, WE WERE GIVEN A XING RESTR 65 MI S OF THE RICHMOND VOR AT FL230. AFTER LEVELING OFF AT FL230 I WAS DISTR BY A MINOR CABIN TEMP PROB. THE CAPT WAS WRITING DOWN THE ATIS. IT WAS AT THIS TIME THAT I HEARD THE AURAL ALT WARNING GO OFF. I LOOKED AT THE ALTIMETER AND IT SHOWED US AT FL227 AND DSNDING. I IMMEDIATELY INITIATED A CLB BACK TO FL230. ZDC DID ASK WHAT MY ALT WAS AT THAT TIME. WE WERE LEVEL AT FL230 AND I RESPONDED FL230. NOTHING MORE WAS SAID BY THE CTLR. I STILL DO NOT KNOW WHY THE AUTOPLT DID NOT MAINTAIN THE ASSIGNED ALT. I DO KNOW, HOWEVER, THAT THE CAPT AND MYSELF WERE BUSY DOING OTHER TASKS AND NOBODY WAS WATCHING THE ACFT AT THAT MOMENT. INTERESTINGLY ENOUGH WE HAD A JUMP SEAT RIDER ON BOARD AND HE DIDN'T NOTICE ANYTHING UNTIL THE ALT WARNING WENT OFF. IT WAS OBVIOUSLY A VERY GRADUAL DSCNT THAT WAS NOT NOTICED UNTIL IT WAS TOO LATE. THERE WAS NO CONFLICT WITH OTHER ACFT. SUPPLEMENTAL INFO FROM ACN 356025: AFTER A BRIEF DISCUSSION, THE CREW COULD NOT ASCERTAIN THE REASON THAT THE AUTOPLT DSNDED. MY GUESS IS THAT THE PF (FO) BUMPED THE THUMBWHEEL (VERT SPD) AS HE MADE A HDG CHANGE, STARTING A VERY GRADUAL DSCNT WHICH WENT UNNOTICED UNTIL THE AURAL ALT WARNING.

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.