A DSNDING B737-300 FAILS TO CROSS THE DALAS INTXN AT THE REQUIRED ALT OF 12000 FT. CAPT THOUGHT IT WAS A 13000 FT ALT ASSIGNMENT; THE FO PF THOUGHT 12000 FT.

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WE WERE FLYING THE ROME 2 ARR PROC TO ATL. RECEIVED A CLRNC TO CROSS ERLIN AT 13000 FT. LATER WERE GIVEN A CLRNC DIRECT TO DALAS (11 MI BEYOND ERLIN). THIS CLRNC DID NOT ALLOW US TO CROSS ERLIN SO WE WERE GIVEN 12 MI W OF DALAS AT 13000 FT. SO WE COULD MAKE THIS RESTR; I WAS TRYING TO PROGRAM A FIX FOR THE RESTR WHEN I NOTICED THE FO HAD DIALED 12000 FT INSTEAD OF 13000 FT. WE WENT TO 12500 FT THEN BACK TO 13000 FT. MY FO REMEMBERS THE RESTR TO BE 12000 FT. I REMEMBER 13000 FT. THE CTLRS WERE VERY BUSY; WE WERE VERY BUSY AS WELL. COULDN'T GET A WORD IN TO CONFIRM THE ALT. WE WERE THEN XFERRED TO APCH AND RECEIVED A NEW CLRNC TO 10000 FT. WE DIDN'T SAY ANYTHING; THE CTLRS DIDN'T SAY ANYTHING. THEY WERE PROBABLY TOO BUSY TO NOTICE. THIS IS A CLASSIC EXAMPLE OF AUTOMATION- DISTR DEVELOPING INTO A POSSIBLE BUSTED ALT. I DO INDEED BELIEVE AUTOMATED COCKPIT; AT LEAST NAV; IS GREAT. BECAUSE WE HAD AN AUTO-COCKPIT; THE CTLR KNEW THIS AND GAVE US DIRECT; WHICH SHORTENED OUR RTE A LITTLE BUT PUT OUR ABILITY TO MAKE THE RESTR IN DOUBT. WE WERE BUILDING THE NEW FIX TO CHK IF WE WERE GOING TO MAKE THE RESTR WHEN CONFUSION DEVELOPED AND THINGS WENT 'SOUTH' FROM THERE. SUPPLEMENTAL INFO FROM ACN 411964: OUR FMC WAS PROGRAMMED TO LATERALLY NAV THE ROME 2 ARR PROFILE AS WELL AS THE VERT PROFILE TO COMPLY WITH THE ERLIN RESTR OF 13000 FT AT 250 KTS AS DICTATED ON THE PLATE FOR A PLANNED LNDG TO THE E. AT 40 NM FROM THE RMG VOR; WE WERE CLRED DIRECT TO DALAS INTXN AND INSTRUCTED TO CROSS 10 NM W OF DALAS AT 12000 FT. AT THE TIME OF THIS CLRNC; WE WERE LEVEL AT FL240. AS MY CAPT BEGAN TO REPROGRAM OUR FMC; I ENTERED 12000 FT IN THE ALT WINDOW OF OUR MCP AND BEFORE I STARTED MY DSCNT; I REQUESTED THAT THE PNF/CAPT VISUALLY CONFIRM MY ALT ENTRY. TO MY RECOLLECTION; HE CONFIRMED THE 12000 FT ENTRY. WHILE MY CAPT WAS BUSY WITH THE FMC; I BEGAN A RAPID VERT SPD CTLED DSCNT TO CROSS 10 MI W OF DALAS AT 12000 FT. ATC FURTHER COMPLICATED OUR SIT BY ISSUING A SPD REDUCTION TO 250 KTS. AFTER ASSESSING OUR SIT; I DETERMINED WE COULD MEET THIS RESTR; PROVIDED THE SPD BRAKE WAS DEPLOYED AND AN IMMEDIATE 3000-4000 FPM DSCNT RATE WAS INITIATED. PASSING THROUGH 13000 FT AND 14 NM W OF DALAS INTXN; MY CAPT SUDDENLY TOOK CTL OF THE ACFT AND LEVELED OFF AT 13000 FT; AS OPPOSED TO OUR ASSIGNED ALT OF 12000 FT. DUE TO THE FACT THAT IT WAS VMC; AND MY SITUATIONAL AWARENESS CONFIRMED THAT NO OTHER ACFT WERE IN OUR VICINITY; I ELECTED NOT TO OVERRIDE MY CAPT'S ACTIONS. CONTRIBUTING FACTORS: BOTH PLTS RELATIVELY INEXPERIENCED IN ATL AIRSPACE; HVY NAV AND COM WORKLOAD; CAPT'S CONFUSION BTWN PAST AND CURRENT CLRNC; PNF DISTR BY LAST MIN AUTOMATION WORKLOAD; END RESULT BEING AN ASSIGNED ALT DISAGREEMENT BTWN THE PF AND PNF.