AN MD88 FLC WAS ISSUED A 13000 FT XING RESTR AT WOMAC INTXN WHILE INBOUND TO ATL. THE ACFT LEVELED OFF AT 12000 FT INSTEAD AND WAS DETECTED BY THE CTLR. BOTH CREW MEMBERS CLAIM THAT THE CORRECT ALT WAS SET IN THE FLT GUIDANCE WINDOWS; BUT ROLLED OVER TO 12000 FT DURING THE DSCNT. THEY MAINTAIN THAT THIS HAS BEEN A PROB IN THIS TYPE OF ACFT.

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DURING DSCNT; THE AUTOPLT WAS ENGAGED; THE ALT FOR XING AT WOMAC INTXN WAS SET AT 13000 FT. WE SET THIS ALT AT APPROX FL190 IN THE DSCNT. I SET THE 13000 FT ALT WHEN WE WERE CLRED FOR THE XING RESTR AT WOMAC; AND IT WAS VERIFIED VERBALLY BY THE CAPT. WE WERE GIVEN A FURTHER SPD REDUCTION DURING THE DSCNT TO 210 KTS. AT SOME TIME DURING THE DSCNT; THE ALT IN THE DIGITAL LED DISPLAY CHANGED TO 12000 FT; WHICH OCCURS OCCASIONALLY ON THE MD88 IN VARIOUS DIGITAL LED DISPLAYS ON THE AUTOFLT PANEL. THE AUTOPLT CONTINUED DSCNT AND LEVELED AT 12000 FT. APCH CTL QUERIED US AS TO OUR ALT; AND WE RPTED LEVEL AT 12000 FT; REALIZING THAT THE ALT WAS SUPPOSED TO HAVE BEEN 13000 FT. APCH THEN HAD US CONTINUE OUR DSCNT. NO CONFLICT WAS NOTED. HUMAN FACTORS: THE CREW HAD BEEN REROUTED FOLLOWING A 4-HR DELAY AT A PREVIOUS STATION AND WAS APCHING NEARLY 12 HRS ON DUTY. BOTH PLTS COMMUTE FROM WESTERN TIME ZONES AND HAD RPTED FOR DUTY VERY EARLY; HOWEVER; CREW COORD WAS VERY GOOD. POSSIBLE CAUSE WAS OVERRELIANCE ON THE AUTOFLT SYS WHICH SHOULD HAVE BEEN MONITORED MORE CAREFULLY DURING A BUSY ARR. I HAVE PERSONALLY EXPERIENCED SEVERAL OCCASIONS ON THIS ACFT WHERE VALUES SET IN THE LED WINDOWS HAVE CHANGED BY 1 OR MORE NUMBERS; WHICH REQUIRES INCREASED VIGILANCE BY THE CREW. I; AS THE PF; SHOULD HAVE MONITORED THE LEVELOFF MORE CAREFULLY RATHER THAN RELYING ON THE AUTOPLT.