FO OF A320 DOES NOT PROPERLY PROGRAM THE NAV COMPUTER RESULTING IN NOT MEETING A XING FIX ALT.

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OUR RTE WAS FROM ORD TO MSP VIA DIRECT DBQ; DIRECT ALO AND THE MEINZ 6 ARR TO MSP. THIS WAS A 1 HR FLT AT 39000 FT CRUISE. BECAUSE OF THE SHORT FLT AND HIGH ALT; THE CRUISE PART WOULD BE VERY SHORT. WHILE IN CRUISE I DID A WRITE-UP IN THE LOGBOOK AND MADE A PA TO THE PAX. DOING THIS PAPERWORK DURING A SHORT LEG KEPT ME OUT OF THE FLYING LOOP. I DID NOT HAVE A PROB WITH THIS BECAUSE THE FO HAD BEEN FLYING THIS HIGH TECH ACFT FOR MORE THAN 6 YRS. HE HAD TWICE THE TIME IN IT AS I DID. WE WERE HANDED OVER THE ZMP; CLRED DIRECT TO PEPPR INTXN ON THE MEINZ ARR TO CROSS PEPPR AT 10000 FT. I OBSERVED THE FO PROGRAMMING THE 10000 FT AT PEPPR IN THE VNAV. THE MAP SHOWED PATH CAPTURED WOULD BE PRIOR TO PEPPR. THERE SHOULD BE NO PROB IN MAKING THE RESTR. WHILE WE WERE IN THIS DSCNT; I WAS OFF ATC GETTING THE MSP ATIS. WHEN THAT WAS COMPLETE; I WROTE IT DOWN AND TOLD THE FO WHICH APCH WAS BEING USED. HE WENT INTO THE MAP TO PROGRAM THE APCH PAGE WHILE I PROGRAMMED THE PERFORMANCE PAGE; WITH THE ALTIMETER; TEMP AND WINDS. AS SOON AS THAT WAS DONE; I PULLED OUT THE APCH PLATE AND WENT INTO THE APCH BRIEF. WHILE IN THIS BRIEFING ATC CALLED AND ASKED IF WE COULD MAKE PEPPR AT 10000 FT. WE WERE MORE THAN A MI AWAY AND DSNDING THROUGH 22000 FT. NO WAY. CTR CALLED MSP APCH AND THEY ACCEPTED US AT A HIGHER ALT. WE WERE ON A VNAV PATH; BUT THE PATH WAS PROGRAMMED TO PUT US OVER FGT VOR AT 10000 FT WHICH IS 15 MI FURTHER AWAY. WHAT HAPPENED? WHEN THE FO PROGRAMMED THE APCH IN THE FLT PLAN; THE VNAV DEFAULTS TO A PATH FOR THE APCH. PREVIOUS VALUES THAT ARE PROGRAMMED ARE REMOVED. SINCE WE HAD NOT CAPTURED THE VERT PATH; THIS WAS NOT REALLY OBVIOUS TO THE PF. THE ACFT JUST CAPTURES ANOTHER VERT PATH. THE ONE THAT IT DEFAULTS TO WHEN THE APCH IS PROGRAMMED. THIS TRAP IN THE A320 VNAV IS KNOWN BY ALL THAT FLY IT. I WAS SURPRISED TO FIND THAT THE FO WHO HAD FLOWN THE ACFT FOR 6 YRS DID NOT KNOW ABOUT THIS TRAP AND DID NOT PROGRAM HIS VNAV AGAIN. AS CAPT; I GAVE MYSELF TOO MUCH WORK ON THIS SHORT LEG TO ADEQUATELY MONITOR THE FLYING.