CAPT OF AN AIRBUS A319 ALLOWED ACFT TO DRIFT THROUGH THE FINAL APCH COURSE AND TOWARD THE R ACTIVE PARALLEL RWY CAUSING APCH CTLR TO ALERT THE CREW TO TURN BACK TO THE L RWY.

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I WAS THE CAPT AND PF. THE FO WAS A CHK AIRMAN GIVING ME A LINE CHK ON THE FINAL LEG OF A 5 DAY TRIP. THE ENTIRE TRIP WAS IOE (INITIAL OPERATING EXPERIENCE) FOR ME WITH THE SAME CHK AIRMAN. WE APCHED DTW ON THE SPICA TWO (10-2C COMMERCIAL CHART) STAR; BUFFALO TRANSITION. WE WERE RADAR VECTORED FROM OVER VERNR INTXN (APPROX 36 MI FROM DTW). WX WAS RPTED AS 15000 FT OVCST; 5 MI VISIBILITY; HAZE; ENOUGH IT HAD BEEN LOW OVCST ALL DAY. I (PF) WAS FOCUSED IN ON THE APCH; AS THIS WAS A LINE CHK. WE WERE ASSIGNED 180 KTS UNTIL FAF. WE WERE FLAPS 2 DEGS; 180 KTS SELECTED SPD; ON GS; ON LOC; APPROX 4-5 MI FROM FAF; WHEN CHK PLT TOLD ME TO GO TO GEAR DOWN; FLAP 3 DEGS. I DID THIS; BUT WAS ANNOYED AND BEWILDERED AS TO THIS COMMAND; SINCE I HAD NOT PLANNED ON DOING THIS FOR ANOTHER 2 OR 3 MI; AND 500 OR 600 FT AS PER COMPANY PROCS (FAF ALT IS 2509 FT; AND WE WERE AT 3400 FT). PREVIOUS TO THIS; WE HAD OVERHEARD RADIO TALK OF A PLAN TO CLOSE RWY 21L; AND I HAD OBSERVED FO (CHK PLT) DOING SOMETHING TO THE MCDU (THE BOX WE USE TO PROGRAM THE NAV COMPUTERS). IT TURNS OUT HE WAS TRYING TO INSERT RWY 21R ILS INTO THE SECONDARY FLT PLAN. I BELIEVE NOW THAT HE GOT CONFUSED SOMEWHERE AND THOUGHT WE WERE CLOSER THAN WE WERE; THUS HIS COMMAND TO GO GEAR DOWN/FLAPS 3 DEGS. AS FATE WOULD HAVE IT; AT THIS EXACT TIME; AS I WAS WONDERING WHY HE TOLD ME TO GO GEAR DOWN/FLAPS 3 DEGS (I HAD ALL OF A SUDDEN BEEN DISTRACTED AT A CRITICAL TIME); THE PLANE WENT INTO A TURN TO THE R (TOWARD ACTIVE RWY 21R). NEITHER OF US NOTICED THIS IMMEDIATELY. TWR TOLD US TO TURN L AND GET ON RWY 21L ILS. (WE DID NOT HAVE ARPT IN SIGHT.) THIS WE DID AND LANDED SAFELY. SOME MANEUVERING WAS NECESSARY. PNF SOMEHOW ACTIVATED RWY 21R ILS. HUMAN PERFORMANCE: PF WAS DISTRACTED BY A COMMAND FROM PNF (CHK PLT) AND PNF WAS DISTRACTED BY MCDU. NEITHER OF US GRASPED THE SERIOUSNESS OF AN UNWANTED TURN TOWARD AN ACTIVE PARALLEL RWY IMMEDIATELY. LESSON: NEVER WILL I ALLOW MY PARTNER TO DO ANYTHING IN THE MCDU UNLESS I KNOW WHAT AND WHY.