Narrative:

I had been cleared to descend and maintain 10000 ft, but descended to 9000 ft MSL. We were on a flight from dtw to lga. On the previous leg, I had developed a strong/painful sinus headache and considered asking the captain 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 arrival, ZNY cleared us to cross fjc VOR at 10000 ft and the captain put that in the FMGC. I set the flight control unit (FCU) altitude at 10000 ft and checked it on both the insertion window and on the FMA (flight 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 altitude set knob from 10000 ft to 9000 ft. I monitored the level off at 10000 ft (caused by the managed navigation mode) and failed to notice on the altimeter that 9000 ft was the blue highlighted altitude. I then buried my head to review the approach procedure as the captain was programming the FMS. A short while later, I looked up and saw the altitude passing 9800 ft. I looked up and saw the flight control unit window was set to 9000 ft. I stupidly assumed the captain had changed it (really dumb since, when the autoplt was engaged, it was my responsibility to change flight control unit settings). I noticed the captain was busy, so I didn't interrupt him, though I had a real doubt about our cleared altitude. Shortly after we leveled at 9000 ft, center questioned our altitude and then directed us back to 10000 ft. There is really no valid excuse for this. There is a hardware design problem, but had I challenged the captain or questioned center when I was in doubt, it would not have occurred. Supplemental information from acn 342936: I left the frequency to do a final PA for our passenger. As I was off frequency, I noticed the aircraft descending out of 10000 ft. The altitude window showed 9000 ft MSL, so I assumed we had been given a clearance to 9000 ft during my PA. The incremental range selector is concentrically mounted on the same shaft as the altitude selector knob. I feel that the altitude selector on the A320 could be better designed to make it more difficult to inadvertently select a different altitude from that assigned.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

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.