Narrative:

While in descent for landing, we were cleared to 11000'. This was set in the altitude select window and confirmed as being correctly set. Later, I noticed our altitude was about 10500' and still descending, I confirmed with my first officer that we were only cleared to 11000', and immediately initiated a climb back up to 11000'. It is possible that we missed out 'two to go' and 'one to go' calls, however, I can't say for sure. We were quite puzzled, though, how the altitude window became set at 10000'. Since, the knob was set to adjust the altitude at 1000' at a time (as opposed to 100), and considering the 'softness' of the detents on this aircraft, I theorize that the altitude knob may have been accidentally bumped, moving the knob to the new position. The FAA should take a better look at the tactile properties of cockpit knobs. The widebody transport has several control knobs that are not easy to distinguish from each other by touch, and are not necessarily intuitive in operation. They are certainly not standardized with other (widebody transport/heavy type) aircraft that have been operating for a good number of yrs. Standardization should have been a bigger factor in design and certification considerations. In this particular incident, the detents, or clicks, in the altitude knob should be 'harder' and not so susceptible to accidental knocks. Callback conversation with reporter revealed the following information. Primary problem has to so the the lack of a definite detent as the altitude alert is set. Just moving your hand away from the control it can inadvertently be changed. He had changed his procedure in setting the 100' activated versus 1000. Feels it is a very poor design. Heading and course select knobs practically the same feel and be a hidden trap. No problem with the side stick controller. Would like to see things moving when the aircraft is in autoflt, throttles, etc. Supplemental information from acn 157388. I overheard our clearance to descend to 11000'. The captain acknowledged and I did notice he set 11000' in the altitude select and engaged the airplane to automatically to descend to and level off at 11000'. I believe this occurrence was a malfunction of our automatic system (FMGC) to perform properly.

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Original NASA ASRS Text

Title: ALT DEVIATION ACCOUNT ALT ALERTER CHANGED 1000' WITH NO INPUT.

Narrative: WHILE IN DSNT FOR LNDG, WE WERE CLRED TO 11000'. THIS WAS SET IN THE ALT SELECT WINDOW AND CONFIRMED AS BEING CORRECTLY SET. LATER, I NOTICED OUR ALT WAS ABOUT 10500' AND STILL DSNDING, I CONFIRMED WITH MY F/O THAT WE WERE ONLY CLRED TO 11000', AND IMMEDIATELY INITIATED A CLB BACK UP TO 11000'. IT IS POSSIBLE THAT WE MISSED OUT 'TWO TO GO' AND 'ONE TO GO' CALLS, HOWEVER, I CAN'T SAY FOR SURE. WE WERE QUITE PUZZLED, THOUGH, HOW THE ALT WINDOW BECAME SET AT 10000'. SINCE, THE KNOB WAS SET TO ADJUST THE ALT AT 1000' AT A TIME (AS OPPOSED TO 100), AND CONSIDERING THE 'SOFTNESS' OF THE DETENTS ON THIS ACFT, I THEORIZE THAT THE ALT KNOB MAY HAVE BEEN ACCIDENTALLY BUMPED, MOVING THE KNOB TO THE NEW POS. THE FAA SHOULD TAKE A BETTER LOOK AT THE TACTILE PROPERTIES OF COCKPIT KNOBS. THE WDB HAS SEVERAL CTL KNOBS THAT ARE NOT EASY TO DISTINGUISH FROM EACH OTHER BY TOUCH, AND ARE NOT NECESSARILY INTUITIVE IN OPERATION. THEY ARE CERTAINLY NOT STANDARDIZED WITH OTHER (WDB/HVY TYPE) ACFT THAT HAVE BEEN OPERATING FOR A GOOD NUMBER OF YRS. STANDARDIZATION SHOULD HAVE BEEN A BIGGER FACTOR IN DESIGN AND CERTIFICATION CONSIDERATIONS. IN THIS PARTICULAR INCIDENT, THE DETENTS, OR CLICKS, IN THE ALT KNOB SHOULD BE 'HARDER' AND NOT SO SUSCEPTIBLE TO ACCIDENTAL KNOCKS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. PRIMARY PROB HAS TO SO THE THE LACK OF A DEFINITE DETENT AS THE ALT ALERT IS SET. JUST MOVING YOUR HAND AWAY FROM THE CTL IT CAN INADVERTENTLY BE CHANGED. HE HAD CHANGED HIS PROC IN SETTING THE 100' ACTIVATED VERSUS 1000. FEELS IT IS A VERY POOR DESIGN. HDG AND COURSE SELECT KNOBS PRACTICALLY THE SAME FEEL AND BE A HIDDEN TRAP. NO PROB WITH THE SIDE STICK CTLR. WOULD LIKE TO SEE THINGS MOVING WHEN THE ACFT IS IN AUTOFLT, THROTTLES, ETC. SUPPLEMENTAL INFO FROM ACN 157388. I OVERHEARD OUR CLRNC TO DSND TO 11000'. THE CAPT ACKNOWLEDGED AND I DID NOTICE HE SET 11000' IN THE ALT SELECT AND ENGAGED THE AIRPLANE TO AUTOMATICALLY TO DSND TO AND LEVEL OFF AT 11000'. I BELIEVE THIS OCCURRENCE WAS A MALFUNCTION OF OUR AUTOMATIC SYS (FMGC) TO PERFORM PROPERLY.

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.