Is this this something the real plane does? I have noticed over long distances between way points the plane will slowly flyaway from the magenta line. I am about to try to do zero wind and resetting the plane on path and I will update if it happens again. P.S This picture is using default fsx fair weather.

Looking at your ANP of 0.06, your plane thinks that its flying on the correct path, the magenta line is probably not being rendered correctly due to map projection and the great circle

So you are saying that I am actually flying "on path" but the lnav path is not updating cause it to look like I am not on path?

No, he is saying that the line is not being rendered correctly due to the earths spherical shape, but your plane is on course. You are nearly 1700nm from your next waypoint, and this kind of thing is to be expected. A distance like that is extremely unlikely to ever happen in real-world. This can be avoided be using flightplans, nat-tracks, and not going direct on that long of a route.

Edited September 7, 201114 yr by NickC

No, he is saying that the line is not being rendered correctly due to the earths spherical shape, but your plane is on course. You are nearly 1700nm from your next waypoint, and this kind of thing is to be expected. A distance like that is extremely unlikely to ever happen in real-world. This can be avoided be using flightplans, and not going direct on that long of a route.

That's what I was thinking.

No. No. And No. Heading up and track up have nothing going on here. Neither does ANP. You're 18.1 miles left of course. That's your cross-track or technical error. The plane won't go off track because of great circle or rhumb line or non-cartesian planes or whatever. My bet is you're not in LNAV.

No Lnav and vnav armed both flight directors on and A/T on.

I will also confirm that this has happened to me twice, once eroute KSEA-PHNL, and KSEA-PANC, both flights were done in clear weather (so no wind), and the plane flew off course slowly (I was in LNAV) to the point where LNAV dissengaged.

I will also confirm that this has happened to me twice, once eroute KSEA-PHNL, and KSEA-PANC, both flights were done in clear weather (so no wind), and the plane flew off course slowly (I was in LNAV) to the point where LNAV dissengaged.

What was the distance of the leg you were flying?

In both cases the distance between waypoints was greater than 400nm (I dont remember the exact distance). This seems to only happen when there are large distances between waypoints. I think I had just past ARRIE on the BANGR departue out of KSEA an went direct LAIREon my way to PANC which if I remember correctly was 700 and something nm. Being cleared direct LAIRE often happens to flights out of KSEA going to PANC.

Well, I'm gonna call that one a bug. I've done KSEA-PHOG, PHLI, PHNL, RW, and we would not be having a good day if we were 18.1L of track. There's no reason why the plane would go off-track. Even if the legs are farther than the typical Class-2 Nav stuff, it shouldn't make a difference.

I'm thinking it is as well.

The real aircraft, regardless of RNP/ANP values, would never deviate from the purple line when flying with A/P in LNAV mode. The IRS only -300/400's would sometimes have a rethink of the FMC position after a long period without radio updates, discover it was off track, but swiftly move to get back on track. Edit to add: And then you would see this: The aircraft would start a turn towards the purple line, which would have 'jumped' away from the aircraft. In theory this could occur on an -800 as well, if both GPS's had failed, and no radio updates had been received for a little while. But again, the aircraft will always try to be on the line, it will never let itself drift away from the line in LNAV mode.