ACFT EQUIP PROB MALFUNCTION GOES UNNOTICED UNTIL THE LOM IN APCH PROC.

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ON DSCNT INTO ORD WHILE STILL S OF OXI ON THE KOKOMO ARR ATC SLOWED US TO 190 KTS. THE FO WAS FLYING AND CALLED FOR 2 DEGS FLAPS. THE INBOARD FLAPS EXTENDED PROPERLY; HOWEVER; THE OUTBOARD FLAPS REMAINED UP THUS LOCKING ALL LEADING EDGE DEVICES IN THE UP POS. THE ACFT CONTINUED TO FLY NORMALLY AND THE PROB WENT UNNOTICED. LATER WE WERE SLOWED TO 170 KTS. 5 DEGS FLAPS WERE CALLED FOR AND THE INBOARD FLAPS CONTINUED TO EXTEND NORMALLY WHILE THE OUTBOARDS REMAINED UP ALONG WITH ALL LED'S. THE PROB CONTINUED TO GO UNNOTICED AND THE ACFT CONTINUED TO HANDLE NORMALLY. THIS SCENARIO CONTINUED THROUGHOUT THE APCH PHASE OF THE FLT UNTIL THE 'FINAL APCH CHKLIST' WAS CALLED FOR AT THE OM. SINCE THE ACFT WAS STILL HANDLING 'NORMALLY' WE ADDED THE PRESCRIBED EXTRA AIRSPD PER THE ABNORMAL CHKLIST AND LANDED UNEVENTFULLY. PREVENTION: WE AS A CREW WERE SO USED TO THE ACFT SYS WORKING NORMALLY THAT WITHOUT A SUDDEN CHANGE IN ACFT HANDLING THE FLAP/LED PROB WENT UNNOTICED. A GOOD AND VIGILANT SCAN IS STILL VITAL EVEN WHEN ALL APPEARS NORMAL. ALSO; THE APCH SHOULD HAVE BEEN DISCONTINUED AND THE FULL ABNORMAL PROC COMPLETED EVEN THOUGH THERE WERE NO 'HANDLING' PROBS.