Widebody; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Troubleshooting; Confusion; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Aircraft Equipment Problem Dissipated; Flight Crew Returned To Clearance; Flight Crew FLC Overrode Automation
Aircraft
We had passed 30W on track C. We were SLOP'ing 1 R. The next three points had been verified as per our procedures. I had just come up from break and double checked our clearance and verified all points ahead. They were correct and we were in LNAV. Shortly after passing 30W we received a ATC fault EICAS. The Captain Pilot Monitoring (PM) got QRH out and then we got a R GPS EICAS. I pulled up the Position page on the FMS to look at the GPS position. He completed the ATC fault checklist and he switched the transponder from #2 to #1. The R GPS EICAS had went out by this time with no action regarding it by us. Since we were using the right autopilot I stated I will select the center autopilot so it matched the transponder. Very shortly after selecting the center autopilot the aircraft began a left turn on its own. I stated this and both the captain and I reached up for heading select to stop the turn. He beat me to it. I reduced the scale on my ND to see if we were off course. WE were still good as we were between the dashed magenta SLOP line and the solid magenta line of our course. The captain had turned the heading perfectly and we lined up almost perfectly on the solid magenta line. I re-engaged LNAV; but it stayed ARMED (white) and would not engage. I stated this and he said I was not on an intercept course. However; we were on the solid line and it should have engaged. The captain then said our active waypoint is gone. I think our next point was N52 W40. He noticed the distance displayed on the ND was now indicating over 700 miles to our next point! Our active waypoint had simply disappeared! The distance indicated was to the subsequent waypoint AFTER 40W! He put it back in the FMS immediately and I was then able to engage LNAV. He then went to route 2 and put in the 30 W point we just passed and the 40 W point ahead to verify that we were on course and we still were. We then verified all points again and everything was as it should be. The captain asked if I had deleted the waypoint. I said NO! I was just looking at the GPS position after we got the R GPS EICAS. I did absolutely nothing on the legs or route page. I then noticed that we had a blue light on the HF #1 on the overhead call panel. I said I think we had a cell call. So the captain called [Oceanic] and they told us they had been trying to get a hold of us as ATC had some erroneous and strange readings from us regarding our position. The captain informed him what had happened. [ATC] said they were now getting good readings. Naturally the captain and I discussed what could have caused this. We were at a loss. Neither of us had ever seen anything like this on any aircraft we had flown. I have around 16 years of FMS experience; a fair amount of that is over the Atlantic. The captain then said he will call maintenance and dispatch. Maintenance was stumped and told us to hold on he wanted to check something. He came back shortly and said the aircraft had some sort of bus under voltage but the problem had cleared itself. He would notify maintenance and they would investigate. The whole thing is very odd. We still had a magenta line with no discontinuity or execute light. Obviously something happened; but exactly what caused this we could not figure out. There is nothing that we did that would have caused this. We switched transponders; switched autopilots; and I looked at the POS Ref page. Those were the only things we did. So how an active waypoint could have just dropped out and disappear from the FMS all together is beyond me.
The First Officer described a navigation anomaly that started with an ATC FAULT EICAS and then a R GPS EICAS. The warnings disappeared but the FMS and next waypoint were affected; and Gander Oceanic commented on some strange position reporting. The crew was able to reenter data and reengage everything; and then continued normally.