B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Wake Vortex Encounter
Flight Crew Returned To Clearance
Aircraft; Environment - Non Weather Related
Get vectors in terminal area for [SFO] ILS 28L; told we are following B747. Captain said to stay a dot high on glideslope (GS) to avoid wake. I; pilot flying; agreed. As we started intermediate descent from about 6;000 FT to 5;000 FT we encountered moderate turbulence with rolling tendencies. Without hesitation I disengaged autopilot (AP); as I correctly interpreted encountering the 747's wake. Told the Captain (pilot not flying) that I was going to hand fly. He said no problem. A final vector/heading we were told to intercept localizer for Runway 28L. I engaged the VOR/LOC to arm and capture localizer. After getting LOC CAP displayed on FMA and following FD commands we were then told cleared for ILS Approach. I then armed APCH to capture GS. Captain said; 'GS alive' followed by GS CAP. I followed FD commands with AP off; continuing to configure with gear and flaps; fully configured in the slot by 1;500 FT AFL. AT approximately 700 FT AFL on GS Captain said LOCS not armed and that I was in HDG SEL with a 3/4 - full scale deflection to the right; meaning I was left of course. I began to maneuver the aircraft back to the right to get back on the LOC immediately and at almost the same time we broke out of the clouds and I maneuvered the aircraft back onto the LOC for a safe and smooth landing. We do not know why the aircraft fell out of LOC CAP mode and defaulted to HDG SEL. We never lost GS CAP mode. I was fully configured and on speed with no excessive maneuvering. We talked about possible go around but broke out of the clouds almost immediately when all this took place. While a go around may have been the better option; I felt due to rising terrain to the left and right of the aircraft may have put us in an unsafe situation since we were off the LOC and we did break out by 500 FT AFL. We advised ATC after landing they said no one reported problems with localizer. We took same aircraft [out] and had no similar problems.
B737-800 First Officer reported losing LOC capture on approach to SFO for unknown reasons.