A320 FLT CREW DEVIATES FROM PUBLISHED ALT RESTR ON RNAV ARR TO LAS.

Narrative

THE FO WAS FLYING AND BEGAN DSCNT ON THE KEPEC RNAV 1 ARR. OUR CRUISE ALT WAS FL230 WHICH CAN BE TOO LOW TO RECEIVE TERMINAL INFO (ATIS). JUST PRIOR TO OUR DSCNT; I (CAPT) INFORMED THE FO THAT I WAS GOING OFF THE ATC RADIO TO GET THE TERMINAL INFO (ATIS; GATE; ETC). WHILE I WAS RECEIVING INFO; THE FO RECEIVED DSCNT INSTRUCTIONS FROM LAS APCH TO 'DSND VIA THE KEPEC 1 ARR.' THE FO THEN PUT 8000 FT IN THE FCU WINDOW; WHICH WAS OUR CLRNC LIMIT; AND BEGAN AN 'OPEN DSCNT.' AN OPEN DSCNT ON THE AIRBUS WILL NOT MEET ALT RESTRS FOR THE RNAV ARR. WHEN I RETURNED TO THE ATC RADIO FREQ; THE FO STATED WE WERE CLRED TO 8000 FT. I ASKED IF WE RECEIVED A 'DSND VIA' CLRNC; AND HE SEEMED SOMEWHAT CONFUSED. I IMMEDIATELY STOPPED THE ACFT DSCNT WHICH HAD ALREADY GONE THROUGH THE FIRST ALT RESTR; AND RETURNED TO THE PROPER ALT FOR OUR LOCATION. WE HAD DEVIATED APPROX 2000 FT LOWER THAN ALLOWED. WE RECEIVED NO ATC INQUIRY OF OUR ALTDEV. NO OTHER CONFLICTING TFC WAS NEAR. FLT THEN PROCEEDED NORMALLY AT THE CORRECT ALTS. THIS IS AN AGE OLD PROB IN AVIATION. IT'S WHY WE NEED AT LEAST 2 CREW MEMBERS IN THE COCKPIT. IT WOULD HELP IF LAS ATIS COULD BE RECEIVED AT A GREATER DISTANCE FROM THE ARPT. THE DSCNT PHASE OF FLT IS NOT A GOOD TIME TO BE OFF THE ATC RADIO RECEIVING TERMINAL INFO.