DISTR DEALING WITH COCKPIT SECURITY ISSUES CONTRIBUTE TO A320 FAILING TO MAKE XING RESTR LEAVING CRUISE ALT.

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I (CAPT) LEFT THE FLT DECK TO USE THE TOILET. PER PROC; FLT ATTENDANT ENTERED FLT DECK AND REMAINED THERE UNTIL MY RETURN. WE HAD BEEN ISSUED THE SLIDE RESTR PREVIOUS TO MY LEAVING THE FLT DECK. ON MY RETURN AND JUST AFTER GETTING RE-ESTABLISHED IN MY SEAT; WE WERE ASKED BY ATC IF WE WERE GOING TO MAKE OUR RESTR AT SLIDR INTXN. WE WERE THEN AT FL360 JUST A FEW MI FROM SLIDR; WHICH WE WERE SUPPOSED TO CROSS AT FL310. MY FO SAID 'NO.' WHILE I WAS OUT OF THE FLT DECK; HE ALLOWED HIMSELF TO BE DISTR BY ENGAGING IN DUTIES HE WAS NOT AUTH TO PERFORM AT THAT TIME; AND THEREBY MISSED HIS DSCNT POINT. WHILE I WAS OFF THE FLT DECK; THE FO LOADED THE BOX WITH THE ARR INFO AND LISTENED TO ATIS ON THE #2 RADIO. HE ALSO TALKED TO COMPANY; I BELIEVE. HE WAS THE PF. THOSE WERE MY DUTIES AND EITHER WAY SHOULD HAVE BEEN PERFORMED WHILE THERE WERE 2 PLTS IN THE COCKPIT. SUPPLEMENTAL INFO FROM ACN 676319: AFTER CTR CLRED TO CROSS SLIDR INTXN AND JUST PRIOR TO TOP OF DSCNT FROM FL350 TO CROSS AT FL310; CAPT LEFT THE FLT DECK FOR PHYSIOLOGICAL NEEDS. FLT ATTENDANT CAME UP; DOOR WAS OPEN LONGER THAN NECESSARY AND ONCE CLOSED; FLT ATTENDANT BEGAN MONOLOGUE ABOUT AN UNRULY PAX. CAPT COMMUNICATED TO RETURN AND AGAIN DOOR WAS OPEN TOO LONG. AM AN FFDO AND MY ATTN WAS OCCUPIED LONG ENOUGH TO MISS THE TOP OF DSCNT TO MAKE THE XING RESTR. CONTRIBUTING FACTORS WERE TIMING OF PHYSIOLOGICAL BREAK (NEARING TOP OF DSCNT) AND AIRBUS FMGS LOGIC THAT DEMANDS THE PLT INITIATE THE DSCNT EVEN AFTER ENTERING INTO THE FMGS. IN A BOEING; ONCE ENTERED IN THE FMC; THE AUTOPLT WILL ENGAGE THE VERT PATH AUTOMATICALLY. FINALLY; AN FFDO'S PRIMARY JOB WHEN COCKPIT DOOR IS OPENING/CLOSING; IS NOT PLTING. I WOULD HOPE FUTURE ACFT DESIGNS WOULD NEGATE THIS FEEBLE AND FLAWED PROCESS OF FLT DECK DOOR OPENING AND CLOSING FOR NECESSITIES SUCH AS FOOD AND BATHROOM BREAKS.