A B757 FLC ALLOWS THEIR ACFT TO MAKE A PREMATURE TURN OFF THE ARR RTE INTO MSP. FMC PROGRAMMING PROB.

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WE WERE ON THE SKETR 1 ARR TO MSP. FOR LNDG ON RWYS 30L/R THERE IS A 105 DEG HDG AFTER THE FCM VOR WHICH IS NOT PRELOADED IN THE FMC. WHEN I ADDED A WAYPOINT 10 MI FROM FCM ON A 105 DEG HDG; I APPARENTLY DROPPED THE FCM WAYPOINT. CONSEQUENTLY; AFTER PASSING FUTON; THE ACFT TOOK UP A HDG FOR THE FCM 105 DEG BEARING; 10 DME OUT. THIS HAPPENED AS THE CAPT WAS BRIEFING THE APCH SO IT WENT UNNOTICED FOR SEVERAL MINS UNTIL APCH CTL CALLED US WITH A VECTOR TO GET 'BACK ON THE ARR.' MY INITIAL MISTAKE WAS MADE MOSTLY BECAUSE FCM AND THE NEW WAYPOINT 'FCM 01' WERE ON 2 DIFFERENT DISPLAY PAGES SO I COULDN'T VIEW THEM SIMULTANEOUSLY BEFORE EXECUTING. MY OTHER MISTAKE WAS NOT HAVING THE CAPT VERIFY THE ADDITIONAL WAYPOINT WHICH IS NORMAL OPERATING PROC. I THINK I FELT IT WAS AN ADDITIONAL 'EXTRA' WAYPOINT SO I COULD JUST ENTER IT AND NOT 'BOTHER' HIM. 2 LESSONS LEARNED BY ME. SUPPLEMENTAL INFO FROM ACN 426827: WHAT PROBABLY HAPPENED IS THAT AS THE PNF ENTERED THE FIX (FCM1) A RTE DISCONTINUITY WOULD HAVE BEEN GENERATED; BECAUSE SOME OF THE FIXES WERE ON ANOTHER PAGE. SOME CONFUSION WAS EXPERIENCED WHEN CLOSING RTE DISCONTINUITY OVER 2 PAGES. I; THE PF; NEVER CHKED THE WORK UNTIL WE WERE ALREADY HEADED OFF COURSE AS I WAS BUSY SETTING UP THE APCH AND FLYING THE ACFT. WE GOT BUSY AND DIDN'T FOLLOW OUR NORMAL PROCS WHICH WOULD HAVE ALLEVIATED THIS PROB.