MALFUNCTIONING ALT ALERTER SETS UP CREW FOR AN ALT OVERSHOOT DURING DSCNT.

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ON DSCNT INTO MCO; ZJX GAVE CLRNC TO CROSS LAMMA INTXN (BITHO ARR) AT 12000 FT. WITH THE AUTOPLT ENGAGED AND THE COPLT FLYING; HE SET 12000 FT IN THE ALT ALERTER AND WE BOTH POINTED TO IT PER COMPANY GUIDELINES. DURING THE DSCNT; WE WERE HANDED OFF TO ORLANDO APCH. AS I CHKED IN; WE WERE DSNDING THROUGH 12000 FT AND AS I LOOKED AT THE ALT ALERT WINDOW; IT INDICATED 11000 FT. ON CHK-IN WITH ORLANDO; I RPTED OUT OF 12000 FT FOR 11000 FT WITH THE INFO. ORLANDO REPLIED ROGER; EXPECT RWY 17. A FEW MINS LATER; THE APCH CTLR QUERIED US ABOUT OUR CLRED ALT AND WE REPLIED 11000 FT. AT THAT POINT WE QUESTIONED OURSELVES; TRYING TO FIGURE OUT IF WE WERE CLRED TO 12000 FT (NORMAL) OR 11000 FT AND DID WE SET THE ALERTER WRONG. IT WAS ONLY WHEN CLRED DOWN TO 2000 FT THAT WE DISCOVERED THE PROB. WHEN TRYING TO SET 2000 FT IN THE ALT ALERTER; THE THOUSANDS DIGIT SKIPPED FROM 3 TO 1 AND WOULD NOT GO TO 2. IN OTHER WORDS; IT WENT FROM 3000 FT STRAIGHT TO 1900 FT AND THEN BACK. WE FIGURED THEN THAT WE SET 12000 FT. AT SOME POINT DURING DSCNT IT JUMPED TO 11000 FT AND WE NEVER SAW THAT; I'VE NEVER SEEN A PROB WITH AN ALERTER AND THUS WE WERE BOTH IN A STATE OF DISBELIEF. WHILE IT'S EASY TO SAY WE SHOULD HAVE JUST 'FLOWN THE PLANE;' IT'S OBVIOUS THERE'S GREAT TRUST AND ACCEPTANCE AND RELIANCE UPON ALT ALERTERS. THIS ONE; FOR SOME REASON; HAD A MECHANICAL QUIRK THAT BIT US. (YES WE WROTE IT UP.) A FOLLOW UP CONVERSATION WITH THE ORLANDO SUPVR INDICATED THAT NO CONFLICT OCCURRED AS A RESULT OF OUR PROB. AN ADDITIONAL FACTOR AS TO WHY I MAY NOT HAVE THOUGHT TWICE ABOUT DSNDING TO 11000 FT IS DUE TO THAT BEING THE CORRECT ALT ON A DIFFERENT ARR INTO MCO. I WILL NOT BE SO TRUSTING OF ALERTERS IN THE FUTURE. SUPPLEMENTAL INFO FROM ACN 447718: THE HUMAN FACTORS INVOLVED WERE FATIGUE ON OUR PART AND THE CTLR'S. WE DID NOT NOTICE THE CHANGE AND WERE NOT SURE DURING THE SWITCHOVER THAT WE HAD NOT BEEN CLRED TO 11000 FT. THE CTLR DID NOT CATCH OUR UNUSUAL CHK IN OF '12000 FT FOR 11000 FT' FOR SEVERAL MINS. IT WAS LATE AT NIGHT AND I BELIEVE THAT WE WERE BOTH TIRED.