A320 FLT CREW ISSUED CROSSING RESTRICTION AT SLIDR ON BUNTR ARRIVAL INTO PHX FAILS TO INITIATE DESCENT IN A TIMELY MANOR.

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DAY 4 OF A 4 DAY TRIP. HAD 30 HR LAYOVER. WX WAS GOOD ALL THE WAY TO PHX. WE WERE IN THE DSCNT PHASE OF FLT. ZAB GAVE US DSCNT INSTRUCTIONS TO DSND FROM FL360 TO FL340 AND TO CROSS SLIDR INTXN AT FL300. I WAS THE PM AND I BEGAN THE PRE-ARR ROUTINE OF GETTING PHX ATIS AND MAKING THE STANDARD PRE-ARR PAX ANNOUNCEMENT. MY FO BRIEFED THE ARR AND RWY AND TAXI RTE. WE DIDN'T HAVE A GATE ASSIGNMENT AS OF YET. WE HAD DSNDED TO FL340 AND WERE WAITING UNTIL THE DSCNT POINT CALCULATED BY THE FMGC CAME UP TO BEGIN OUR DSCNT TO MAKE THE SLIDR RESTR. AS A SIDE NOTE; THIS ALT RESTR IS NOT ON THE ARR. WE MADE SURE ALL OF THE APPROPRIATE ALT DSCNT RESTRS WERE IN THE FLT PLAN ON THE FMGC. WE HAD APPROX 10-15 MINS BEFORE WE WOULD REACH THE TOD POINT ON THE COMPUTER. KNOWING WE HAD THIS XING RESTR; I ALWAYS STAY VERY AWARE OF THIS POINT IN OUR FLT. HOWEVER; THIS TIME WAS DIFFERENT. WE BOTH JUST FORGOT ABOUT MAKING SURE WE BEGAN THE DSCNT AT THE PROPER TIME AND WE FLEW PAST THE TOD POINT. THE FO SUDDENLY REALIZED WE HAD PASSED THAT POINT AND I TOLD HIM TO BEGIN HIS DSCNT NOW. I CALLED ZAB AND TOLD THEM WE WOULD BE HIGH AND NEEDED SOME RELIEF ON THIS XING RESTR. THEY SAID OK AND WE CONTINUED OUR DSCNT. WE WERE HIGH XING SLIDR BY APPROX 1900 FT. I THINK THE ONLY THING THAT COULD PUT THIS SITUATION IN ITS PROPER PERSPECTIVE IS COMPLACENCY. WE WERE NOT TIRED AND HAD A VERY GOOD FLT FROM ZZZ. THE TRIP AS A WHOLE WAS UNEVENTFUL. WE BOTH JUST FORGOT ABOUT THE XING RESTR. THE AIRBUS DOES NOT PERMIT AUTO START OF A DSCNT EVEN AS AUTOMATED AS IT IS. IT MUST BE MANUALLY STARTED BY MANAGED; OPEN OR VERT SPD DSCNT. MY FO AND I HAD PREVIOUS EXPERIENCE ON THE B737 WHERE THE VNAV DSCNT PROFILES WERE AUTOMATICALLY STARTED IF YOU HAVE VNAV SELECTED ON THE MCP. DID WE REVERT BACK TO OUR BOEING DAYS? I DON'T THINK SO. WE JUST GOT COMPLACENT AND FAILED TO MONITOR THE FLT PROGRESS JUST FOR THAT FEW MINS. I THINK THIS CAN HAPPEN TO ANYONE. A PARTICULAR FACTOR THAT I SEE EVERY TIME I FLY IS THE AMOUNT OF SMALL MISTAKES THAT OCCUR ON THE 4TH DAY OF FLYING. I INVARIABLY SEE THIS ALL THE TIME. WHETHER IT IS THE PF OR THE PM SMALL MISTAKES ARE MADE ON DAY 4. I AM CURIOUS IF OTHER CREWS EXPERIENCE THIS PHENOMENON ON DAY 4 FLYING. I AM CURIOUS IF OUR FLT SAFETY DEPT IN OUR COMPANY COMPILES ANY STATISTICAL INFO CONCERNING A HIGH RATE OF MINOR ERRORS ON DAY 4? IF YOU DON'T; I THINK YOU SHOULD. THIS COULD BE A VERY TELLING INDICATION OF WHAT OUR CREWS ARE EXPERIENCING ON THESE 4 DAY FLTS AND IF IT IS A SAFETY OF FLT CONCERN. IS IT UNRECOGNIZED FATIGUE? IS IT 'I WANT TO GET THERE AND GET DONE' PHENOMENON? I DON'T KNOW; BUT I DO KNOW THAT IT OCCURS AND IT OCCURS MORE OFTEN THAN I LIKE. WE ALL KNOW SAFETY IS PARAMOUNT BUT WHY ARE THESE ERRORS ENCOUNTERED TIME AND TIME AGAIN ON DAY 4. I WOULD LIKE TO KNOW WHAT IS THE PROB AND CERTAINLY WHAT IS THE SOLUTION.