A320 FLC MISSES XING RESTR DURING STANDARD TERMINAL ARR RTE (STAR) INTO LAS.

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CLRED FOR LYNSY PROFILE DSCNT INTO LAS. FO WAS PF AND CAPT PNF. ACFT ALLOWED TO DSND PREMATURELY AFTER XING LYNSY BECAUSE: 1) ALT ALERTER NOW SET FOR BOTTOM OF PROC (8000 FT MSL); 2) FO WAS FLYING THE VERT PROFILE IN 'OPEN DSCNT' MODE AS HE HAD BEEN INSTRUCTED TO EXPEDITE HIS DSCNT AND HAD NOT RETURNED TO 'MANAGED' DSCNT AT LYNSY; AND 3) CAPT DID NOT PROPERLY MONITOR AND PROVIDE A BACK-UP FOR PROPER MODE. MITIGATING FACTORS: 1) CAPT HAD JUST RECENTLY BEEN OFF FREQ WHILE GETTING THE IN-RANGE CALL; LEAVING A LOW EXPERIENCE LEVEL PLT IN A HIGH WORKLOAD SIT. 2) BOTH PLTS RELATIVELY LOW EXPERIENCE LEVEL IN TYPE. 3) FIRST FLT TOGETHER AND AT LATE NIGHT HRS LCL TIME. 4) BOTH PLTS ENGROSSED IN THE UPCOMING APCH AND RWY/TXWY TO USE WHEN MORE STRINGENT MONITORING OF PRESENT CONDITIONS SHOULD HAVE BEEN ACCOMPLISHED. AFTER QUESTIONING; CTR CTLR IMMEDIATELY CLRED FLT TO CONTINUE TO 8000 FT MSL. NO TFC NEARBY DUE TO LATE HR. XCHK; XCHK; XCHK OTHER PLTS ALWAYS! NEVER FORGET THE IMPORTANCE OF THE A320 FMA ON SAFETY OF FLT. SUPPLEMENTAL INFO FROM ACN 564403: ALWAYS BACK EACH OTHER UP IN HIGH WORKLOAD SITS. ALWAYS CONFIRM AUTOPLT/FMS SELECTION ON AIRBUS A320/19 FMA. ALWAYS DETERMINE PRESENT POS WHEN 1 PLT HAS BEEN ABSENT OR OUT OF THE LOOP.