B737-300 FLT CREW EXPERIENCES UNUSUAL AILERON DISPLACEMENT ON APCH.

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AT APPROX 1500 FT MSL ON AN AUTOPLT COUPLED APCH TO RWY 33L AT BWI; I CALLED FOR FLAPS 30 DEGS AND THE BEFORE LNDG CHKLIST. THE FO LOWERED THE FLAPS TO 30 DEGS; AND THEN READ THE CHKLIST AND I REPLIED; 'I NOTICED THE AUTOPLT WAS MAKING L AND R CORRECTIONS.' AT FIRST I THOUGHT THE AUTOPLT WAS MAKING COURSE CORRECTIONS. THEN I TOOK NOTE OF THE MORE L CORRECTION IT WAS MAKING. I ATTRIBUTED THIS TO POSSIBLY THE L WING WAKE VORTEX OF THE ACFT THAT WAS APPROX 4 MI AHEAD. I MADE A COMMENT REGARDING THIS TO THE FO; AND THEN DISENGAGED THE AUTOPLT. WE MADE THE 1000 FT CALL. AT ABOUT 700 FT; I RELAXED YOKE PRESSURE TO SEE HOW THE ACFT WOULD RESPOND. THE ACFT ROLLED GENTLY TO THE R AS IF IT WERE A LITTLE BIT OUT OF TRIM. I MADE PWR ADJUSTMENTS TO ACHIEVE SYMMETRICAL THRUST. I THEN PUT IN NOT MORE THAN 1 UNIT OF L RUDDER TRIM. INSIDE 300 FT; I TOOK NOTE TO HOW MUCH L AILERON IT WAS TAKING TO HOLD WINGS LEVEL. IT WAS APPROX 3-4 UNITS L AILERON. THERE WAS NO CONCERN AT THIS POINT BY EITHER ONE OF US. IT WAS MORE OR LESS A MATTER OF FACT. THE FO CONTINUED WITH HIS CALLOUTS. HE MADE A 200 FT CALL; 100 FT; 50 FT; 30 FT; AND 10 FT AND WE WERE ON THE DECK. IT WAS APPROX 18 SECONDS FROM THE TIME; I REALLY STARTED TO PROCESS WHY I HAD SO MUCH L AILERON; UNTIL WE WERE ON THE RWY. AS WE TAXIED OFF THE RWY; I INSTINCTIVELY CALLED FLAPS UP; THEN CHANGED MY COMMAND TO 'LEAVE THE FLAPS DOWN.' THE FO SAID 'YOU MEAN FLAPS UP' AND I SAID 'NO; FLAPS DOWN;' THEN PROCEEDED TO EXPLAIN MY THOUGHTS. I MENTIONED TO THE FO THAT WE MIGHT HAVE HAD A FLAP PROB THAT MAY HAVE BEEN MASKED BY THE AUTOPLT AND/OR THE WAKE VORTICES OF THE PRECEDING ACFT. I INSTRUCTED THE FO TO ADVISE GND CTL THAT WE WOULD BE TAXIING TO THE GATE WITH THE FLAPS DOWN AND TO ADVISE BWI MAINT. IT WASN'T UNTIL WE WERE TAXIING TO THE GATE THAT I COULD REALLY DIGEST THE APCH. I COULD NOT HELP BUT THINK THAT THE INITIAL ROLLING THAT OCCURRED INSIDE 1500 FT MAY HAVE BEEN THE FLAPS EXTENDING FROM 15 DEGS TO 30 DEGS AND NOT THE AUTOPLT OR WAKE VORTEX THAT I HAD INITIALLY THOUGHT; OR SIMPLY JUST AN OUT-OF-TRIM ACFT. IF IT WEREN'T FOR THE RECENT SPINDLE FAILURES IN OUR FLEET; I PROBABLY WOULD NOT HAVE HAD A SECOND THOUGHT. IN ANY CASE; I FIGURED I WOULD GET THE FLAPS CHKED. I HAVE HAD THE SPINDLES CHKED IN THE PAST AND IT TAKES NOT MORE THAN A FEW MINS TO DO. MAINT MET US AT THE GATE. I BRIEFED HIM ON WHAT WE THOUGHT AND HE WENT ON TO DO THE CHKS. I CALLED DISPATCH AND BRIEFED THEM AS WELL. THE FLAPS AND SPINDLES APPEARED TO BE OK. I JUST CHALKED IT UP TO WHAT I HAD ORIGINALLY THOUGHT AND FIGURED WE WOULD BE ON OUR WAY. AS WE WERE PREPARING THE ACFT FOR OUR NEXT LEG; WE RECEIVED ANOTHER CALL FROM COMPANY. THEY WANTED ME TO BRIEF THEM ON WHAT HAPPENED AND WHAT I THOUGHT. THEY CHOSE TO GND THE ACFT AND INVESTIGATE IT FURTHER. WE CHANGED ACFT AND CONTINUED ON WITH OUR FLT ASSIGNMENTS. IT WASN'T UNTIL THE FOLLOWING DAY THAT WE RECEIVED A MESSAGE TO CALL OUR CHIEF PLT. HE HAD ADVISED US TO FILL OUT AN ASAP RPT AND; IN SO MANY WORDS; EXPLAIN WHY WE DID NOT COMPLY WITH PROPER FLAP SPINDLE PROCS. IF WE HAD DETECTED AN OBVIOUS ROLL-OFF; WE WOULD HAVE APPLIED THE NEW FLAP CARRIAGE SPINDLE FAILURE PROCS. IT WAS ONLY AFTER WE WERE ON THE GND THAT WE THOUGHT WE MIGHT HAVE HAD A POSSIBLE SPINDLE FAILURE. SUPPLEMENTAL INFO FROM ACN 615464: THE ACFT WAS IN THE SLOT AND ON PROFILE; ALWAYS IN CTL; AND I NEVER RECOGNIZED OR PERCEIVED AN OBVIOUS OR UNUSUAL ROLL OF THE ACFT.