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.