Narrative:

We were descending from FL240 and were cleared to cross pvd at 11000 ft. 11000 ft was set in the altitude alert window by the captain and I verified. I continued descending and was using the vertical speed mode. I later went to the 'open descent' mode which requires pulling the knob which doubles as the altitude alert setting knob. The last time I checked the altimeter it said 10500 ft. I quickly checked the altitude alert window and it now read 10000 ft. I asked the captain to confirm 11000 ft with ATC as that was what I remembered being cleared to. As the captain queried he said to me he also remembered 11000 ft and that's what he set. I told him I agreed. I slowed the descent waiting for ATC to respond and changed the navigation display range to check TCASII for traffic. There were no targets and ATC confirmed 11000 ft. ATC gave us a frequency change and had no other comments. I stopped the descent at 10200 ft and climbed back to 11000 ft. I concluded the altitude alert window must have rotated when I pulled it to enter the 'open descent' mode. I've noticed similar occurrences with the heading and airspeed knobs when pulling them to change modes. However, they only change 1 degree or 1 KT per click. The altitude alert window can be set to 100 ft per click or 1000 ft per click. In cruise it is normally in the 1000 ft position. Depending how you place your hand on the glareshield you can hide the altitude window as you manipulate the vertical speed or pull the altitude alert knob to change modes. It is very easy to have it rotate 1 click and be off 1000 ft. If your hand is under the knob and resting on the mode control panel you can accidentally enter the 'expedite' mode. (Seen that happen several times.) I don't mind dual function knob/switches on the airspeed-heading or vertical speed but not on the altitude alert.

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Original NASA ASRS Text

Title: A DSNDING A320 OVERSHOT ITS ASSIGNED ALT WHEN THE ALT ALERTER IS ACCIDENTALLY MISSET TO A LOWER ALT.

Narrative: WE WERE DSNDING FROM FL240 AND WERE CLRED TO CROSS PVD AT 11000 FT. 11000 FT WAS SET IN THE ALT ALERT WINDOW BY THE CAPT AND I VERIFIED. I CONTINUED DSNDING AND WAS USING THE VERT SPD MODE. I LATER WENT TO THE 'OPEN DSCNT' MODE WHICH REQUIRES PULLING THE KNOB WHICH DOUBLES AS THE ALT ALERT SETTING KNOB. THE LAST TIME I CHKED THE ALTIMETER IT SAID 10500 FT. I QUICKLY CHKED THE ALT ALERT WINDOW AND IT NOW READ 10000 FT. I ASKED THE CAPT TO CONFIRM 11000 FT WITH ATC AS THAT WAS WHAT I REMEMBERED BEING CLRED TO. AS THE CAPT QUERIED HE SAID TO ME HE ALSO REMEMBERED 11000 FT AND THAT'S WHAT HE SET. I TOLD HIM I AGREED. I SLOWED THE DSCNT WAITING FOR ATC TO RESPOND AND CHANGED THE NAV DISPLAY RANGE TO CHK TCASII FOR TFC. THERE WERE NO TARGETS AND ATC CONFIRMED 11000 FT. ATC GAVE US A FREQ CHANGE AND HAD NO OTHER COMMENTS. I STOPPED THE DSCNT AT 10200 FT AND CLBED BACK TO 11000 FT. I CONCLUDED THE ALT ALERT WINDOW MUST HAVE ROTATED WHEN I PULLED IT TO ENTER THE 'OPEN DSCNT' MODE. I'VE NOTICED SIMILAR OCCURRENCES WITH THE HEADING AND AIRSPD KNOBS WHEN PULLING THEM TO CHANGE MODES. HOWEVER, THEY ONLY CHANGE 1 DEG OR 1 KT PER CLICK. THE ALT ALERT WINDOW CAN BE SET TO 100 FT PER CLICK OR 1000 FT PER CLICK. IN CRUISE IT IS NORMALLY IN THE 1000 FT POS. DEPENDING HOW YOU PLACE YOUR HAND ON THE GLARESHIELD YOU CAN HIDE THE ALT WINDOW AS YOU MANIPULATE THE VERT SPD OR PULL THE ALT ALERT KNOB TO CHANGE MODES. IT IS VERY EASY TO HAVE IT ROTATE 1 CLICK AND BE OFF 1000 FT. IF YOUR HAND IS UNDER THE KNOB AND RESTING ON THE MODE CTL PANEL YOU CAN ACCIDENTALLY ENTER THE 'EXPEDITE' MODE. (SEEN THAT HAPPEN SEVERAL TIMES.) I DON'T MIND DUAL FUNCTION KNOB/SWITCHES ON THE AIRSPD-HEADING OR VERT SPD BUT NOT ON THE ALT ALERT.

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.