AN A319 CREW; DSNDING INTO LAS; MISINTERPED ATC'S INSTRUCTIONS; OVERSHOOTING A PUBLISHED XING RESTR.

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CLRED BY ZLA TO CROSS LYNSY INTXN AT 12000 FT; 250 KTS. APCHING LYNSY; CHANGED TO LAS APCH CTL. CLRED TO DSND VIA LYNSY 1; EXCEPT CROSS UTARE AT 8000 FT. I READ BACK CLRNC AND CHKED PUBLISHED ALT; ON STAR; AT LYNSY. STAR INDICATED UTARE AT 8000 FT; 210 KTS. FO AND I WERE BOTH UNSURE OF THIS CLRNC. THE STAR SHOWS FIX AFTER UTARE; DUSJO; AT 6500 FT. I ASKED IF CTLR MEANT; CROSS UTARE AT AND MAINTAIN 8000 FT. NEW CLRNC GIVEN; CROSS UTARE AT AND MAINTAIN 8000 FT. DURING THIS EXCHANGE; THE ACFT BEGAN TO CAPTURE 12000 FT AND SLOW TO 250 KTS. AFTER THE CLRNC WAS VERIFIED; FO SELECTED 8000 FT ON FCU AND WE VERIFIED CHANGE USING ALT AWARENESS PROCS. I CHKED STAR AGAIN TO BE SURE WE WERE SET UP CORRECTLY FOR THE ARR. DURING THIS TIME; I REALIZED THE ACFT WAS NOT IN MANAGED DSCNT MODE. WE WERE NOW IN VERT SPD APCHING 11000 FT. WE HAD NOT REACHED MIROK INTXN; WITH ITS RESTR OF AT OR ABOVE 11000 FT. I INSTRUCTED FO TO LEVEL UNTIL MIROK AND THEN RESUME A MANAGED DSCNT TO 8000 FT. IF THE ALT ALERTER IS CHANGED ON THIS ACFT WHILE IN THE CAPTURE MODE; IT REVERTS TO VERT SPD. THIS APPEARS TO BE WHAT OCCURRED. APCH INDICATED OUR ALT AT LYNSY WAS 11200 FT INSTEAD OF THE REQUIRED 12000 FT. SEVERAL FACTORS CONTRIBUTED TO THIS ERROR. THE WORDING OF THE ORIGINAL CLRNC CREATED CONFUSION IN THE COCKPIT. APCH INDICATED THE ACFT AHEAD OF US HAD A SIMILAR PROB UNDERSTANDING WHAT THE CTLR WANTED. THIS OCCURRED AT XA00; BUT THE CREW WAS ON LAST LEG OF A LONG DAY WITH BODY CLOCKS ON XD00. I THINK FATIGUE SLOWED OUR RESPONSE TIME. THE REVERSIONS ON THIS ACFT ARE A KNOWN PROB AND WE HAVE BEEN WELL TRAINED TO BE VIGILANT FOR THEM. STILL WE FAILED TO CATCH IT PROMPTLY. I BELIEVE IT WOULD BE SAFER NOT TO BE CLRED TO DSND VIA A STAR IF INTERMEDIATE LEVELOFFS ARE DESIRED BY ATC. THIS WAS THE FIRST TIME EITHER OF US HAD FLOWN THIS STAR. WE HAD THOROUGHLY BRIEFED IT AND HAD WE BEEN TOLD TO DSND WITHOUT THE UNPUBLISHED RESTR; I DON'T THINK WE WOULD HAVE HAD THIS DEV. THIS IS THE FIRST TIME EITHER OF US COULD REMEMBER RECEIVING A CLRNC LIKE THIS. SUPPLEMENTAL INFO FROM ACN 558465: ABOUT THIS TIME; LAS APCH CLRED US TO LAS VIA THE LYNSY RNAV 1 ARR; EXCEPT TO CROSS UTARE WAYPOINT AT 8000 FT. I IMMEDIATELY PUT 8000 FT IN THE ALT WINDOW IN THE FCU PANEL. I DID NOT NOTICE ON THE FMA THAT THE ALT CAPTURE HAD BEEN ARMED TO LEVELOFF AT 12000 FT. WHEN I SELECTED 8000 FT IN THE ALT WINDOW; THE ALT CAPTURE WAS NO LONGER ARMED AND THE AIRPLANE CONTINUED TO DSND IN A VERT SPD MODE TO 8000 FT. WE DID NOT REALIZE WE HAD DSNDED BELOW 12000 FT BEFORE WE CROSSED THE LYNSY INTXN. WHAT WE BOTH MISSED FROM THE ORIGINAL CLRNC WAS TO MAINTAIN 8000 FT TO UTARE. I THINK FACTORS CONTRIBUTING TO THIS INCIDENT WERE OUR MISUNDERSTANDING OF THE INITIAL CLRNC AND MY FAILURE TO MONITOR THE FMA ON THE PFD.