A319 CAPTAIN RELATES PROBLEMS ENCOUNTERED BY BRAND NEW CABIN ATTENDANT FLYING AS PURSER; INCLUDING FAILING TO DISARM THE ESCAPE SLIDE RESULTING IN ITS DEPLOYMENT WHEN THE DOOR WAS OPENED BY GND SERVICE PERSONNEL.

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I WAS THE CAPT OF THE FLT TO PDX. THE FLT ATTENDANT CREW CONSISTED OF 2 BRAND NEW-HIRE FLT ATTENDANTS 'A;' 'B;' AND 1 VETERAN FLT ATTENDANT 'C.' I MET FLT ATTENDANTS 'A' AND 'B' DURING THE CREW BRIEF. THEY WERE BOTH IN THE FORWARD GALLEY. I NEVER MET FLT ATTENDANT 'C' OR HAD ANY INTERACTION WITH HER BEFORE; DURING OR AFTER THE FLT. THE 2 NEW-HIRES SEEMED UNSURE OF THEMSELVES AND WERE CONTINUOUSLY REFERRING TO THEIR MANUALS AS THEY SET UP THE FORWARD GALLEY. I ASKED FLT ATTENDANT 'A' IF HE HAD EVER FLOWN PURSER/'A' AND IF IT WAS LEGAL FOR HIM TO FLY THAT POS SINCE HE WAS SO NEW. HE SAID YES UNDER CERTAIN CIRCUMSTANCES THAT HE COULD FLY PURSER/'A' AND THAT HE HAD BEEN JUNIOR MANNED INTO IT. AT THAT POINT I DROPPED THE ISSUE AS I WAS NOT COMPLETELY FAMILIAR WITH THE COMPANY POLICY OR THE FLT ATTENDANT CONTRACT REGARDING NEW-HIRES AND FLYING THE PURSER/'A' POS. THE FLT WAS UNEVENTFUL WITH THE EXCEPTION OF A MINOR DEV WHEN OUR MEALS WERE DELIVERED. AT CRUISE; FLT ATTENDANT 'A' CALLED AND SAID OUR MEALS WERE READY. WE WAITED FOR THE COCKPIT ACCESS SIGNAL FOR ABOUT 3 MINS. I ASKED THE FO TO CHK THE VIEW PORT TO SEE WHAT WAS GOING ON. THE FO SAID THE CART WAS IN PLACE BUT THERE WAS NO SECOND FLT ATTENDANT TO GUARD THE CART AND FLT ATTENDANT 'A' LOOKED AS THOUGH HE WAS NOT SURE HOW TO ACCESS THE COCKPIT (IE; SIGNAL US). I CALLED FLT ATTENDANT 'A' TO LET HIM KNOW THAT A SECOND FLT ATTENDANT WAS NEEDED AND HOW TO ACCESS THE COCKPIT. ONCE OUR MEALS HAD BEEN DELIVERED; I DEBRIEFED BOTH FLT ATTENDANT 'A' AND FLT ATTENDANT 'B' ON COCKPIT ACCESS SOP. THE REST OF THE FLT WAS UNEVENTFUL UNTIL PARKING AT PDX. ONCE PARKED AT PDX; THE FO AND I STARTED THE PARKING CHKLIST. WE NOTICED THAT THE FORWARD DOORS HAD NOT BEEN DISARMED. THE FO CALLED FLT ATTENDANT 'A' TO REMIND HIM THAT HE HAD NOT DISARMED THE FORWARD DOORS. APPROX 5 SECONDS LATER DOOR 1L WAS OPENED BY THE PDX CUSTOMER SVC REPRESENTATIVE AND ABOUT 10 SECONDS LATER WE HEARD THE DISTINCT SOUND OF A SLIDE BEING DEPLOYED; FOLLOWED BY A COCKPIT INDICATION THAT THE DOOR 1R SLIDE HAD DEPLOYED. AFTER DISCUSSING THIS ISSUE WITH SEVERAL FLT ATTENDANTS AND FLT ATTENDANT SUPVRS; WE LEARNED THAT NEITHER FLT ATTENDANT 'A' NOR FLT ATTENDANT 'B' SHOULD HAVE BEEN FLYING THE PURSER/'A' POS. BOTH OF THESE GENTLEMEN WERE DOING THEIR VERY BEST TO BE GOOD FLT ATTENDANTS. I AM NOT SURE WHO LED FLT ATTENDANT 'A' TO BELIEVE THAT HE COULD BE JUNIOR MANNED INTO THE 'A' POS; BUT HE SHOULD NOT HAVE BEEN FORCED INTO IT ON HIS SECOND DAY ON THE LINE. AS PLTS; DESPITE OUR EXPERIENCE; WE ARE RESTR TO HIGHER LNDG MINIMUMS WHENEVER WE TRANSITION TO A NEW FLEET AND SEAT; YET SOMEHOW A BRAND NEW HIRE GOT JUNIOR MANNED INTO A POS THAT HE SHOULDN'T HAVE BEEN FLYING. I BELIEVE THAT HAD PROPER FLT ATTENDANT STAFFING PROCS BEEN FOLLOWED THIS UNFORTUNATE EVENT WOULD NOT HAVE TAKEN PLACE. AS A SIDE NOTE; THIS IS THE THIRD TIME IN THE LAST 2 MONTHS I HAVE HAD FIRST/SECOND MONTH FLT ATTENDANTS FLYING PURSER/'A' WITH A SENIOR (IE; 10+ YRS SENIORITY) FLYING IN THE BACK OF THE ACFT.