B777 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Total 15854; Flight Crew Type 2784
Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft
Approaching ZZZZZ1 waypoint the first officer established VHF communication with ATC. ATC announced radar contact proceed ZZZZZ1 ZZZZZ2. I selected direct ZZZZZ2 with abeam points just after we passed ZZZZZ1. Next active waypoint ZZZZZ3 was 38 miles ahead. As we approached ZZZZZ3 I selected Economy cruise and executed the command while voicing my actions to the first officer. I was monitoring the progress page to see what our new ETA and fuel remaining projections would be with the new speed. The new time was +12 minutes and new fuel was -4;000 lbs. I was pointing this out to the first officer when the FMC blanked and refigured the ETA and fuel back to the original estimates. Our original speed was Mach .84 and new Economy speed was Mach .834; therefore I didn't expect much change in ETA or fuel remaining. After the FMC momentary blanking and re computing ETA and fuel; the autopilot began a sharp turn to the right. The first officer and I both said; why are we turning? The first officer was quick to push heading select and dial a heading towards the magenta line. Our deviation off the line was less than 2 nm. Once we were established on the line I reselected LNAV and the autopilot functioned properly for remainder of the flight. We did not notify ATC of the less than 2 mile deviation. We did contact our maintenance personnel with a full report via ACARS. We also made a maintenance entry for the next crew. I referenced our Flight Manual bulletin; of which I am familiar; which cautions certain actions to preclude any uncommanded turns in LNAV. This bulletin is 4 years old and I think it's time to fix this anomaly. The bulletin warns against making vertical or lateral changes in the FMC when within 4 nm of an active waypoint. I AM familiar with this bulletin and believed making a speed change wouldn't be an issue. This should be addressed.
[Report narrative contained no additional information.]
Boeing 777 flight crew reported after selecting 'Econ Cruise;' the Flight Management Computer blanked and recomputed the ETA and fuel; followed by the autopilot commencing an uncommanded right turn.