B757 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Flight Instructor
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Aircraft
I was the pilot monitoring. We flew the RIIVR 2 STAR and were cleared for the ILS to 24R into LAX. We were on a very long final having previously completed all our checklists and briefings and after we reviewed the waypoints in the FMS crosschecked with the charts. All of a sudden the aircraft rolled to the left with the autopilot on in a way that was deliberate. We were in LNAV and VNAV performing 'VNAV to the feather.' Prior to the roll we were centered on the lateral course and centered on the vertical path. I noticed that the aircraft banked to the left and I saw the 'noodle' or white lateral course indicator curve in a circle to the left on the map display. I said something to the Captain and then almost immediately ATC asked us if we were on course. I replied that our RNAV system banked to the left on its own. Then the Captain went into heading mode and armed the localized to capture. That put us back on course and we continued the approach and landed normally. It all happened very quickly but I would guess that we were still 'established' on the LOC course the entire time especially considering how far away from the runway we were.I don't know why this occurred. The roll was not associated with an automation mode change and the FMS points had been verified using the appropriate charts. I was impressed by how fast ATC noticed a slight course change and asked us about it. It may have been wake but it was so smooth and deliberate of a roll that my impression is that this was not wake turbulence.
B757 First Officer reported a track excursion while on autoflight for unknown reasons.