A B737-800 flight crew reported confusion on how to properly set up the autoflight system for a visual approach (ILS out of service) to SFO at night. They improperly armed VOR/LOC mode and a track deviation resulted.
Synopsis
A B737-800 flight crew reported confusion on how to properly set up the autoflight system for a visual approach (ILS out of service) to SFO at night. They improperly armed VOR/LOC mode and a track deviation resulted.
Narrative
Was cleared 'present heading till the shoreline; 4000 feet at the bridge; cleared visual 28L'. Heading was whatever heading was to track course from BSR to MENLO. Was also pointed out traffic to maintain visual with. Traffic was slightly ahead of us; converging from the east for Runway 28R. Runway 28L ILS OTS; extended centerline from final approach fix; L NAV and V NAV were both engaged. Also LOC/VOR was engaged. I engaged LOC from past experience in 767 that if both were engaged it would capture whatever came within parameters first. Also since Runway 28L was OTS and no signal; I wasn't concerned. Though I think now this is a bad habit for 737. No one during initial training; nor am I aware of anywhere in manuals; is [the] interaction of VOR/LOC and L NAV when both engaged discussed . Anyway; as we approached the magenta line of extended centerline; the plane began to turn towards anticipated final course; so much so that I slewed heading bug to final course. Then I focused on the traffic as we were on converging course and he appeared to be getting closer. Then the Captain said 'hey it's not working; we aren't tracking'. I looked towards the airport and saw we were clearly through the 28L final. Now I knew why traffic appeared to be getting closer! We were still on converging course. No TA or RA was ever issued; but as we were converging and I now realized we were past final course where we didn't belong; I had great concern for the traffic and clicked off autopilot and visually rapidly rolled the plane back towards Runway 28L final without regard to inside instruments. 'bank angle bank angle' was annunciated. Again my primary concern at that point was to get traffic separation. I estimate bank was no more then 40. At 'bank angle' I shallowed bank and no more annunciations and landed uneventfully with no one on radio saying anything nor any remarks from passengers or Flight Attendant's.
Second reporter narrative
On final I noted that LNAV and VNAV were still active and VOR/LOC was still armed. At the gate we had a brief discussion as to why the autopilot did not intercept the LNAV final. We both agreed that it must have had something to do with VOR/LOC being armed with the ILS being OTS although neither of us was completely sure being relatively new to the airplane. We had both done that approach to SFO many times (with the ILS working); so out of habit we had both set our Nav radios to the ILS Runway 28L even after briefing the ILS OTS. In retrospect (and after a good night's sleep); it seems obvious that we should not have set those frequencies and it should have raised a red flag to me when the PF armed VOR/LOC after briefing LNAV/VNAV.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.