ACFT EQUIP PROB. DURING THE APCH THE ACR CREW REALIZED THAT THEY HAD TO USE A GREAT DEAL OF RUDDER TRIM. ALSO; THE ACFT WOULD YAW ABRUPTLY.

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ON FINAL TO RWY 26R IN ATL; IT WAS NOTED THAT AN UNUSUAL AMOUNT OF L RUDDER TRIM WAS NEEDED (3 1/2 UNITS). THE WIND WAS 310 DEGS AT 7 KTS AND FUEL AND ENGS WERE EVEN. AS I TOOK OUT 1 UNIT TO HOLD MANUALLY; THE ACFT YAWED TO THE R TWICE. THE LNDG WAS UNEVENTFUL. THINKING THAT WE HAD A RUDDER PROB MAINT WAS CALLED. MAINT ADVISED US THAT THEY HAD FOUND THE #5 LEADING EDGE SLAT WAS NOT FOLLOWING #4 AND #6. AFTER THEY WERE FULLY DOWN; #5 WOULD GO TO FULL DOWN; BUT THEN IT WAS NOT LOCKING IN PLACE. WE HAD NO INDICATION IN THE COCKPIT. THE YAW ON SHORT FINAL WAS DISTURBING; ESPECIALLY IN A 737. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE FIRST INDICATION THE RPTR HAD OF THE ACFT PROB WAS OVER THE OM WHEN THEY WENT TO FLAPS 30 DEGS. HE SAID THAT THE ACFT 'JUST DID NOT FEEL RIGHT SO I LOOKED AROUND AND NOTICED THAT THE TRIM WAS 3.5 UNITS TO THE L.' THE CAPT FURTHER STATED THAT WHEN HE TOOK OUT 1 UNIT OF TRIM THE ACFT YAWED TO THE R ABRUPTLY TWICE SO HE PUT THE TRIM BACK IN AND LEFT IT ALONE. THE CAPT DID NOT OBSERVE THE YAW DAMPER INDICATOR ON THE OVERHEAD PANEL DURING THE YAW TO SEE IF THE YAW DAMPER INDICATOR ON THE OVERHEAD PANEL DURING THE YAW TO SEE IF THE YAW DAMPERS MOVED. THE LATER REVELATION OF THE #5 SLAT PROB AND THE NATURE OF ITS MOVEMENT LEAD THE RPTR TO SUSPECT IT TO BE THE CAUSE OF THE YAW. THE SLAT; DURING MAINT TESTS; DID NOT FULLY EXTEND INITIALLY; BUT WOULD DELAY UNTIL ALL OF THE OTHER LED DEVICES HAD FULLY EXTENDED THEN THE #5 SLAT WOULD FULLY EXTEND; BUT NOT LOCK. INSTEAD THE SLAT; IN THE MAINT TESTS; WOULD OCCASIONALLY CYCLE FROM FULL EXTENSION TO PARTIAL EXTENSION AND BACK TO FULL. THIS COULD GIVE A YAW MOMENT TO THE ACFT AT NORMAL APCH SPDS. THE RPTR DOES NOT THINK THAT THE ACFT WAS FLT TESTED WITH THIS CONDITION. THE MANUFACTURER DID CONTACT HIM AND THE MAINT PERSONNEL FOR THEIR OWN INFO. THE ACFT WAS A B-737-200. THIS SAME ACFT HAD WHAT TO THE RPTR SOUNDED LIKE A SIMILAR SIT WITH THE #2 SLAT AND THE MAINT DEPT HAD CHANGED THAT SLAT'S ACTUATORS BEFORE HIS FLTS.