AN ACR A320 CREW MISSET THEIR ALT SELECTOR AND DEVIATED FROM THEIR ASSIGNED ALT. ERROR ADMITTED. ALTDEV. ALLEGED ACFT DESIGN PROB THE INCREMENTAL RANGE SELECTOR CAN BE SELECTED FROM 1000 FT TO 100 FT TOO EASILY FOR THESE RPTRS.

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I HAD BEEN CLRED TO DSND AND MAINTAIN 10000 FT; BUT DSNDED TO 9000 FT MSL. WE WERE ON A FLT FROM DTW TO LGA. ON THE PREVIOUS LEG; I HAD DEVELOPED A STRONG/PAINFUL SINUS HEADACHE AND CONSIDERED ASKING THE CAPT TO FLY THE DTW/LGA LEG SINCE I HAD BEEN UNABLE TO GET ANY ASPIRIN TO RELIEVE THE PAIN. I USED POOR JUDGEMENT AND DECIDED NOT TO 'WIMP OUT' AND FLY THE LEG MYSELF AS PLANNED. DURING THE ARR; ZNY CLRED US TO CROSS FJC VOR AT 10000 FT AND THE CAPT PUT THAT IN THE FMGC. I SET THE FLT CTL UNIT (FCU) ALT AT 10000 FT AND CHKED IT ON BOTH THE INSERTION WINDOW AND ON THE FMA (FLT MODE ANNUNCIATOR). THEN I MOVED THE SELECTION KNOB COUNTER CLOCKWISE FROM THE 1000 FT PER CLICK TO THE 100 FT PER CLICK SETTING. THAT'S WHEN I PROBABLY; INADVERTENTLY MOVED THE COAXIALLY MOUNTED ALT SET KNOB FROM 10000 FT TO 9000 FT. I MONITORED THE LEVEL OFF AT 10000 FT (CAUSED BY THE MANAGED NAV MODE) AND FAILED TO NOTICE ON THE ALTIMETER THAT 9000 FT WAS THE BLUE HIGHLIGHTED ALT. I THEN BURIED MY HEAD TO REVIEW THE APCH PROC AS THE CAPT WAS PROGRAMMING THE FMS. A SHORT WHILE LATER; I LOOKED UP AND SAW THE ALT PASSING 9800 FT. I LOOKED UP AND SAW THE FLT CTL UNIT WINDOW WAS SET TO 9000 FT. I STUPIDLY ASSUMED THE CAPT HAD CHANGED IT (REALLY DUMB SINCE; WHEN THE AUTOPLT WAS ENGAGED; IT WAS MY RESPONSIBILITY TO CHANGE FLT CTL UNIT SETTINGS). I NOTICED THE CAPT WAS BUSY; SO I DIDN'T INTERRUPT HIM; THOUGH I HAD A REAL DOUBT ABOUT OUR CLRED ALT. SHORTLY AFTER WE LEVELED AT 9000 FT; CTR QUESTIONED OUR ALT AND THEN DIRECTED US BACK TO 10000 FT. THERE IS REALLY NO VALID EXCUSE FOR THIS. THERE IS A HARDWARE DESIGN PROB; BUT HAD I CHALLENGED THE CAPT OR QUESTIONED CTR WHEN I WAS IN DOUBT; IT WOULD NOT HAVE OCCURRED. SUPPLEMENTAL INFO FROM ACN 342936: I LEFT THE FREQ TO DO A FINAL PA FOR OUR PAX. AS I WAS OFF FREQ; I NOTICED THE ACFT DSNDING OUT OF 10000 FT. THE ALT WINDOW SHOWED 9000 FT MSL; SO I ASSUMED WE HAD BEEN GIVEN A CLRNC TO 9000 FT DURING MY PA. THE INCREMENTAL RANGE SELECTOR IS CONCENTRICALLY MOUNTED ON THE SAME SHAFT AS THE ALT SELECTOR KNOB. I FEEL THAT THE ALT SELECTOR ON THE A320 COULD BE BETTER DESIGNED TO MAKE IT MORE DIFFICULT TO INADVERTENTLY SELECT A DIFFERENT ALT FROM THAT ASSIGNED.