A320 CREW INBOUND TO LAS ON NEWLY DESIGNED RNAV PROCS; WITH HVY COCKPIT WORKLOAD; OVERSHOT ALT XING.

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ON THE LYNSY ARR INTO LAS; WE DSNDED BELOW THE 12000 FT MSL LYNSY ALT RESTR. THIS WAS BECAUSE OF AIR CREW DISTR INVOLVING TURB; PLANNING FOR STRONG GUSTY SURFACE WINDS; CHANGING RWYS; REPROGRAMMING FMGC; BRIEFING A POSSIBLE DIVERT; AND BRIEFING THE CABIN CREW. AT THE START OF THE DSCNT; CAPT COPIED ATIS AND CONTACTED OPS; WITH THE FO FLYING THE ACFT AND TALKING TO ZLA. ATIS INFO WAS COPIED; INDICATING LNDG RWYS WERE RWYS 19 AND RWYS 25. A CHK OF THE WIND CHARTS INDICATED WINDS WERE OUT OF LIMITS FOR THE A320. CAPT RECHKED ATIS; WHICH CHANGED; INDICATING LNDG RWYS WERE RWYS 1 AND RWYS 25. WINDS WERE IN LIMITS FOR RWYS 1. CAPT ASKED FOR THE LYNSY RWYS 1 TRANSITION. ZLA THEN COORDINATED WITH LAS APCH CTL AND CAME BACK WITH CLRNC FOR THE RWYS 1 TRANSITION AND A RPTED MICROBURST AND MODERATE TURB AT LAS. ANY FURTHER INFO FROM LAS APCH CAPT COORDINATED WITH THE CABIN CREW WHILE FO FLEW AND REPROGRAMMED THE FMGC. HE HAD GONE TO OPEN DSCNT MODE WITH AN FCU ALT OF 8000 FT SET; TO SLOW EARLY AT ATC'S REQUEST AND TO MEET ALT RESTR. SUBSEQUENT XCHK REVEALED DSCNT BELOW 12000 FT. A CLB HAD BEEN INITIATED. ATC QUERIED THE ALT. THEN GAVE DSCNT CLRNC TO 10000 FT MSL. COCKPIT CREW WAS VERY BUSY DEALING WITH CHANGING WX; RWYS; TURB; POSSIBLE DIVERSION; REPROGRAMMING THE FMGC; BRIEFING APCHS; AND COORDINATING WITH CABIN CREW. THIS WAS COUPLED TO A COMPLICATED ARR PROC WITH MULTIPLE OPTIONS REQUIRING REPROGRAMMING THE FMGC AT A CRITICAL TIME NEARING AN ALT RESTR. A SIMPLIFIED ARR PROC WITH AN EARLIER TRANSITION TO APCH CTL WOULD LESSEN THE CHANCE OF COCKPIT CREWS MAKING ALT ERRORS. SUPPLEMENTAL INFO FROM ACN 540944: I WAS PF ON LYNSY RNAV 1 ARR TO LAS. APPROX 20 MI NW OF KINGMAN WE WERE CLRED TO DSND VIA THE LYNSY 1. I DIALED 8000 FT ON THE FCU AND SELECTED MANAGED ALT FOR THE CONSTRAINTS. ZLA THEN SLOWED US EARLY TO 250 KTS; SO I SELECTED OPEN DSCNT. THE CAPT WAS TALKING TO ATC ABOUT A MICROBURST AT LAS; AND SETTING THE ATIS. WE WERE EXPERIENCING MODERATE TURB. I WAS TEMPORARILY DISTR WITH OUR CONCERN AND CONVERSATION REGARDING SEVERE WX AT LAS AND THEN REMEMBERED THAT THE ACFT WAS IN OPEN DSCNT AND NOT MANAGED DSCNT. WE HAD BEGUN TO DSND BELOW THE 12000 FT RESTR AT LYNSY APPROX 20 MI PRIOR TO LYNSY. I IMMEDIATELY SELECTED OPEN CLB BACK TO 12000 FT; AND WAS ABOUT TO RPT IT TO ATC WHEN ATC QUERIED US. THE CAPT RPTED WE WRE CORRECTING AND ATC CLRED US DOWN TO 10000 FT. THE COMBINATION OF INSERTING THE RNAV ARR WITH ITS FIXES AND RESTRS INTO THE BOX AND COORDINATING A RWY CHANGE WITH ZLA AND LAS APCH WHILE DISCUSSING A RPTED MICROBURST AT LAS WAS VERY DISTRACTING. LESSON LEARNED; AND SOLUTION IS TO DO ONE THING AT A TIME; AND TO ALWAYS ENSURE THAT FLYING THE ACFT IS PRIORITY ONE. THE NEW RNAV PROCS AT LAS ARE TOO COMPLEX AND DISTRACTING WHEN INSERTING CHANGED RWYS.