B757 FLT CREW EXPERIENCES LE SLAT DISAGREE DURING APPROACH AND AFTER COMPLYING WITH QRH; LANDS SAFELY WITH FLAPS 20.

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FO WAS PF AND CALLED FOR FLAPS 1 DEG. I SELECTED AND IMMEDIATELY GOT THE MASTER CAUTION FOR LEADING EDGE SLAT DISAGREEMENT EICAS AND LEADING EDGE FLAP LIGHT. THIS SAME THING WAS IN THE LOG HISTORY RECENTLY AND WAS CLRED BY MAINT. THE ACFT SEEMED TO BE FLYING NORMALLY; SO WE TOLD APCH TO TAKE US OFF THE ARR SO WE COULD WORK ON A FLAP ISSUE. WE SURPRISED THE CTLR WITH OUR REQUEST AND HAD DSNDED BELOW 7000 FT TO OUR CLRED ALT OF 6000 FT WHEN HE CLRED US TO CLB TO 8000 FT. I KNOW HE WAS PRETTY BUSY AT THIS POINT BUT I DID NOT WANT TO CLB AND KNEW THAT 6000 FT WOULD BE A GOOD ALT FOR TERRAIN WHILE WE RAN CHKLISTS. WE WERE VECTORED THROUGH FINAL AND WERE GIVEN TURNS W OF THE ARPT WHILE I RAN THE 2 CHKLISTS FOR THIS PROB. AFTER COMPLETING THEM; THE FO ASKED IF HE COULD REVIEW. EVERYTHING WAS DOUBLECHKED AND WE PROCEEDED TO COMPLETE THE APCH AND LNDG. WE WERE TOLD THAT WE WOULD GET RWY 28L BUT I REQUESTED RWY 28R. NO FURTHER ASSISTANCE FROM ATC WAS REQUIRED AND CONSIDERING THE PERFECT WX; EXCESS RWY OVER WHAT OUR CONDITION NEEDED; WE DECIDED THAT THERE WAS NO NEED TO ADVISE THE CABIN. LNDG WAS UNEVENTFUL. SUPPLEMENTAL INFO FROM ACN 784155: ON APCH INTO SFO ON MOD3 STAR; SLOWING THROUGH 220 KIAS; I CALLED FOR FLAPS 1 DEG. LEADING EDGE FLAP DISAGREEMENT EICAS MESSAGE APPEARED (REPEAT WRITE-UP; PREVIOUSLY BRIEFED). CAPT AND I AGREED TO BREAK OFF APCH; LEVEL OFF AT 7000 FT MSL AS ASSIGNED; MAINTAIN 210 KIAS; AND ASK ATC TO VECTOR US AROUND FOR A LONG DOWNWIND TO RUN APPROPRIATE CHKLIST. WE RAN THE CHKLIST; WHICH LED US TO THE LEADING EDGE FLAP ASYMMETRICAL CHKLIST. SUBSEQUENT APCH AND LNDG AT FLAPS 20 WAS UNEVENTFUL; NO EMER DECLARED; NO CABIN ADVISORY REQUIRED OR NECESSARY. EXCELLENT CRM ON CAPT'S PART. WE EACH RAN THE CHKLIST TO INDEPENDENTLY CONFIRM THE PROB; AND WORKED TOGETHER TO TURN AN ABNORMAL INTO A SAFE OP AND NORMAL LNDG. CONCLUSION: YRS OF CRM TRAINING; GOOD WX; AND PLENTY OF EXTRA FUEL (FUEL = TIME TO THINK; AND WE FELT NO EXTRAORDINARY TIME PRESSURE TO RUSH THROUGH CHKLISTS) MADE THIS A NON-EVENT.