Narrative:

We were in the holding pattern waiting for the atl airport, which was closed due to trw to open. We were given a descent clearance from 22000 to 17000'. Due to the shifting WX and the change in altitude, we found our holding pattern was no longer habitable. There was a build up at our fix and one situation so as to conflict with our left turn in the holding pattern. The captain confirmed altitude capture on the FMA at 17000', then directed his attention to the arrival chart. We reached our altitude and the turning point for the outbnd heading approximately the same time. The first officer who was flying, confirmed the capture mode and then went outside to make his turn so as to avoid as much of the WX as possible. At this point ATC alerted us we were having an altitude excursion. The captain looked up and saw the altitude at 16500'. The captain also noted the altitude preset window showed 10000' instead of 17000'. It had displayed 17000' when the 18000' (1000' prior to level off) had been made. Within seconds the first officer looked up to see the display at 7000'. The altitude preset was reset to 17000'. The aircraft leveled off and a heading of 190 degrees per ATC instruction was established. It was assumed at that point we had brushed the altitude preset knob while the autoplt was in the capture mode, causing the aircraft to fail to level off. However, later in the same flight, while cruising at 12000', the altitude preset changed from 12000 to 11000'. Neither pilot was physically near the preset knob. I have been on the aircraft for 4 yrs and have seen the autoplt fail to capture altitude for a variety of reasons. I have never seen the altitude preset change. Two of the three unexplained changes of this flight could not be attributed to pilot input. If this is a failure that can occur, it is quite insidious as altitude alert is also changed. Callback conversation with reporter revealed the following: the aircraft was checked by company maintenance and the problem could not be duplicated. The reporter was really not sure what happened, but did admit it was possible for the problem to be flight crew related. Reporter said he never saw the altitude alert set at 7000' as claimed by the first officer. The instrument self test system was discussed, but not thought to have been a factor in the altitude deviation. The reporter was not very convincing about altitude present changes occurring west/O flight crew input.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT IN A HOLDING PATTERN.

Narrative: WE WERE IN THE HOLDING PATTERN WAITING FOR THE ATL ARPT, WHICH WAS CLOSED DUE TO TRW TO OPEN. WE WERE GIVEN A DSCNT CLRNC FROM 22000 TO 17000'. DUE TO THE SHIFTING WX AND THE CHANGE IN ALT, WE FOUND OUR HOLDING PATTERN WAS NO LONGER HABITABLE. THERE WAS A BUILD UP AT OUR FIX AND ONE SITUATION SO AS TO CONFLICT WITH OUR LEFT TURN IN THE HOLDING PATTERN. THE CAPT CONFIRMED ALT CAPTURE ON THE FMA AT 17000', THEN DIRECTED HIS ATTN TO THE ARR CHART. WE REACHED OUR ALT AND THE TURNING POINT FOR THE OUTBND HDG APPROX THE SAME TIME. THE F/O WHO WAS FLYING, CONFIRMED THE CAPTURE MODE AND THEN WENT OUTSIDE TO MAKE HIS TURN SO AS TO AVOID AS MUCH OF THE WX AS POSSIBLE. AT THIS POINT ATC ALERTED US WE WERE HAVING AN ALT EXCURSION. THE CAPT LOOKED UP AND SAW THE ALT AT 16500'. THE CAPT ALSO NOTED THE ALT PRESET WINDOW SHOWED 10000' INSTEAD OF 17000'. IT HAD DISPLAYED 17000' WHEN THE 18000' (1000' PRIOR TO LEVEL OFF) HAD BEEN MADE. WITHIN SECS THE F/O LOOKED UP TO SEE THE DISPLAY AT 7000'. THE ALT PRESET WAS RESET TO 17000'. THE ACFT LEVELED OFF AND A HDG OF 190 DEGS PER ATC INSTRUCTION WAS ESTABLISHED. IT WAS ASSUMED AT THAT POINT WE HAD BRUSHED THE ALT PRESET KNOB WHILE THE AUTOPLT WAS IN THE CAPTURE MODE, CAUSING THE ACFT TO FAIL TO LEVEL OFF. HOWEVER, LATER IN THE SAME FLT, WHILE CRUISING AT 12000', THE ALT PRESET CHANGED FROM 12000 TO 11000'. NEITHER PLT WAS PHYSICALLY NEAR THE PRESET KNOB. I HAVE BEEN ON THE ACFT FOR 4 YRS AND HAVE SEEN THE AUTOPLT FAIL TO CAPTURE ALT FOR A VARIETY OF REASONS. I HAVE NEVER SEEN THE ALT PRESET CHANGE. TWO OF THE THREE UNEXPLAINED CHANGES OF THIS FLT COULD NOT BE ATTRIBUTED TO PLT INPUT. IF THIS IS A FAILURE THAT CAN OCCUR, IT IS QUITE INSIDIOUS AS ALT ALERT IS ALSO CHANGED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE ACFT WAS CHKED BY COMPANY MAINT AND THE PROB COULD NOT BE DUPLICATED. THE RPTR WAS REALLY NOT SURE WHAT HAPPENED, BUT DID ADMIT IT WAS POSSIBLE FOR THE PROB TO BE FLT CREW RELATED. RPTR SAID HE NEVER SAW THE ALT ALERT SET AT 7000' AS CLAIMED BY THE F/O. THE INSTRUMENT SELF TEST SYS WAS DISCUSSED, BUT NOT THOUGHT TO HAVE BEEN A FACTOR IN THE ALT DEVIATION. THE RPTR WAS NOT VERY CONVINCING ABOUT ALT PRESENT CHANGES OCCURRING W/O FLT CREW INPUT.

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.