AN ACR ACFT APCHING DFW WAS GIVEN A RWY CHANGE BUT HAD A TRACK DEV WHEN THE FMS DID NOT QUICKLY ENOUGH SWITCH TO THE NEWLY ASSIGNED RWY ARR.

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WE WERE FLYING THE ABC ARR TO DFW. WE HAD ILS RWY XXC SELECTED AS THE FINAL APCH PROC AND BRIEFED THAT APCH. JUST PRIOR TO CDE; DFW APCH CTL GAVE US OUR LNDG RWY XXR. MY FO ASKED IF I WANTED HIM TO CHANGE THE RWY IN THE FMS AND I ANSWERED YES. I WATCHED AS HE SELECTED ILS XXR; THEN I TURNED TO THE ILS XXR PAGE IN THE FLT MANUAL. RATHER THAN THE FAMILIAR 'HOW DOES THAT LOOK' THE FO SAID SOMETHING (I DON'T REMEMBER WHAT) THAT BROUGHT MY ATTN BACK TO THE PFD AND NAV DISPLAY WHERE I SAW THE ACFT IN A L HAND BANK; DUTIFULLY FOLLOWING THE MAGENTA LINE TO A MORE SOUTHERLY DIRECTION. EFG (THE NEXT POINT ON THE ARR) WAS MISSING. BY THE TIME I SELECTED HDG; AND ROLLED THE ACFT BACK TO A R TURN WE WERE 15 T0 20 DEGS OFF THE APCH TRACK. THE FO MANUALLY TYPED EFG IN THE CDU AS A POINT OF REF; THEN CDE TO REESTABLISH THE APCH TRACK. WE REJOINED THE TRACK JUST PRIOR TO EFG. THE REMAINDER OF THE APCH AND LNDG WERE UNEVENTFUL. AFTER LNDG WE CALLED TRACON AND TALKED TO THE SUPVR WHO ADVISED US THAT ALTHOUGH THERE WAS NOT A PROB WITH SEPARATION OF ACFT; WE DID DEVIATE SIGNIFICANTLY FROM THE APCH TRACK. I DIDN'T OBSERVE THE SEQUENCE OF KEY STROKES THAT LED TO DELETING FIXES BTWN PRESENT POS AND FINAL APCH POINTS; AND THIS EVENT (AGAIN) EMPHASIZES THE NEED TO XCHK WITH THE OTHER CREW MEMBER PRIOR TO PUSHING THE EXECUTE BUTTON. NO MATTER HOW EXPERIENCED WE ARE; AND HOW FAMILIAR WITH THE ARPT AND ARRS; THERE IS ALWAYS A WAY TO MAKE A MISTAKE. I WILL BE ESPECIALLY DILIGENT IN THE FUTURE TO ENSURE THIS ONE DOESN'T HAPPEN AGAIN. SUPPLEMENTAL INFO FROM ACN 763454: I AM NOT CERTAIN OF THE EXACT SEQUENCE OF EVENTS AT THIS POINT; BUT BELIEVE THAT WHAT HAPPENED WAS THE ACFT SEQUENCED CDE JUST AS I WAS EXECUTING THE CHANGE. THIS CAUSED THE AIRPLANE TO BEGIN TRACKING TOWARD THE POINT AFTER EFG HIJ RATHER THAN CONTINUING TO TRACK TOWARD EFG. THE ACFT BEGAN TO TURN L TO GO DIRECT TOWARD HIJ. RATHER THAN INTERVENE DIRECTLY ON THE MCP (SINCE IT WAS NOT MY LEG) I IMMEDIATELY BEGAN TELLING THE CAPT THAT WE NEEDED TO CORRECT BACK TO THE RIGHT TO GET BACK ON THE COURSE. AS WE REVERSED THE TURN BACK TO THE R; ATC CALLED AND NOTIFIED US THAT WE APPEARED TO BE LEAVING THE COURSE TO HEAD S TOO EARLY. WE ACKNOWLEDGED; COMPLETED OUR CORRECTION BACK TO COURSE; AND COMPLETED THE REMAINDER OF THE ARR AND LNDG UNEVENTFULLY. LESSONS LEARNED INCLUDE THESE: 1) WHEN MAKING RWY CHANGES; THE NAV DISPLAY CAN BE VERY DIFFICULT TO INTERPRET EXACTLY WHAT WILL HAPPEN DUE TO THE MULTIPLE ARR TRACKS THAT ARE SOMETIMES DISPLAYED; AS THEY WERE IN OUR CASE. I AM IN THE HABIT OF NOT NECESSARILY VERIFYING RWY CHANGES PRIOR TO EXECUTING THEM; AS IT IS NORMALLY EASIER TO REVIEW THOSE CHANGES AFTER THEY HAVE BEEN ACTIVATED. THEY SHOULD STILL BE VERIFIED; HOWEVER; BY BOTH PLTS AS THIS CASE REINFORCES. 2) IN ADDITION; IT IS IMPORTANT TO BE AWARE OF PROX TO THE NEXT WAYPOINT TO ENSURE UNINTENDED ADVANCEMENT OF FMC POINTS DOES NOT OCCUR AT THE SAME TIME CHANGES ARE BEING MADE. 3) WE COULD EASILY HAVE DELAYED MAKING THE RWY CHANGE IN THE FMC UNTIL AFTER THE TURN POINT; BUT IN MOST CASES THAT IS UNNECESSARY. IT IS IMPERATIVE NOT TO RUSH SUCH CHANGES IN ANY CASE. I BELIEVE THIS PARTICULAR CASE WAS MY FAULT FOR TRYING TO GET THINGS DONE MORE QUICKLY THAN WAS REQUIRED. 4) FINALLY; I SHOULD HAVE BEEN MORE DIRECTIVE AND LESS DIPLOMATIC AS I SAW WHAT WAS HAPPENING AS IT OCCURRED. THIS WOULD HAVE EXPEDITED THE CORRECTION. AFTER THE INCIDENT; WE WERE ASKED TO CONTACT THE ATC TO DISCUSS THE POSSIBLE FILING OF A RPT OF GND TRACK DEV. WHEN WE CALLED; THE INDIVIDUAL ASSURED US THAT NO LOSS OF TFC SEPARATION HAD OCCURRED.