AN MD80 DSNDING INTO FLL; DSNDS BELOW CLRED ALT.

Narrative

WE WERE BEING VECTORED FOR ARR TO FLL BECAUSE TSTMS CLOSED THE MRLIN 4 STAR. WE WERE DEVIATING E OF ZBV FOR TSTMS WITH CLRNC DIRECT ZBV WHEN ABLE. WE HAD BEEN CLRED TO 10000 FT IN A DSCNT FROM CRUISE. 10000 FT WAS SET IN THE ALT ALERT WINDOW AND CONFIRMED BY BOTH PLTS. FO WAS FLYING THE ACFT ON AUTOPLT WITH THE CAPT HANDLING RADIOS AND TUNING THE WX RADAR. BOTH CREW MEMBERS WERE DISCUSSING THE BEST RTE AROUND THE TSTMS WHEN ZMA XMITTED TO MAINTAIN 10000 FT. AT THAT TIME THE CAPT AND FO BOTH CHKED THE ALTIMETERS WHICH READ 9200 FT. WE IMMEDIATELY RETURNED THE ACFT TO 10000 FT. WE BOTH CONFIRMED THE ALT ALERT WINDOW NOW READ 9000 FT. WE DID NOT KNOW HOW THE CHANGE OCCURRED BUT SUSPECT THAT THE FO MAY HAVE BUMPED THE ALERTER; WHICH DOES CHANGE IN 1000 FT INCREMENTS WITH 1 NOTCH TURN; IN THE DSCNT AS HE WAS MAKING HDG OR VERT SPD CHANGES WHILE MANEUVERING AROUND WX. THE ERROR WAS NOT NOTICED BECAUSE BOTH PLTS RELIED ON THE AUTOPLT TO FLY THE DSCNT AND LEVEL OFF TO THE ASSIGNED ALT THAT WAS SET AND CONFIRMED BY BOTH PLTS. TO PREVENT FURTHER DEVS; AT LEAST 1 PLT MUST ALWAYS FLY THE AIRPLANE EVEN THOUGH IT IS HOOKED UP TO THE AUTOPLT. SOMEONE MUST MONITOR PERFORMANCE RELATIVE TO THE ASSIGNED CLRNC.