Distance From Waypoint for Boeing FMC Auto Sequencing

[extreme P]

Good day Gents,

Is there a published distance within which the aircraft must pass a waypoint in order for it to be auto sequenced? When approaching a waypoint I never like seeing the distance get larger.

[westhawk]

The distance at which an FMS/FMC/GPS navigator sequences to the next waypoint in the flight plan must certainly vary according to the navigational mode it's in at the time. (enroute, terminal. approach, RNP 10, 5, 1, .3 etc..) I think it also might matter whether the waypoint is coded as flyover or flypast.

I did a quick search of TSO 129 in hopes of finding a quick answer but alas no success. I feel certain the answer is hidden in a technical standard order somewhere...

westhawk

[Turbavykas]

I don't know about Boeing but lets just think in general.
I think just the distance doesn't make sense. You have to drive imaginary line that is perpendicular to the line to the next way-point. If you pass this line it should go to next way point.

[cav-not-ok]

on the bus FM, its within 7nm if i'm not mistaken.

[Pub User]

It depends on which Boeing, but the majority of modern ones sequence waypoints if they pass within 21 nm.

[vivnair]

Correct 21 nm on modern Boeing jets

[ImbracableCrunk]

How about on approach when you've got 4 fixes on final and you're vectored inside number 4? I've seen it where the FMC sequences and where it doesn't.

Surely it's not 21nm for this instance, Shirley.

[BOAC]

I think it also might matter whether the waypoint is coded as flyover or flypast. - I think that is your answer?

[JammedStab]

But there does not seen to be any indication on the Boeing FMC on whether the waypoint is a fly-by or fly-over. The Universal FMS did have this feature and as well, you could turn a fly-by into a fly-over waypoint quite easily.

[aterpster]

westhawk:

The distance at which an FMS/FMC/GPS navigator sequences to the next waypoint in the flight plan must certainly vary according to the navigational mode it's in at the time. (enroute, terminal. approach, RNP 10, 5, 1, .3 etc..) I think it also might matter whether the waypoint is coded as flyover or flypast.

I did a quick search of TSO 129 in hopes of finding a quick answer but alas no success. I feel certain the answer is hidden in a technical standard order somewhere...

TSO 129, like 145/145 is a shell behind proprietary RTCA technical documents.

Later generation equipment does much better job with fly by waypoints than the earlier stuff. The point at which the turn begins is a function of ground speed and angle of course change. The RNP value or the sensitivity mode has nothing to do with it.

[westhawk]

TSO 129, like 145/145 is a shell behind proprietary RTCA technical documents.

At least that explains my trouble with finding documentation.

Anyway, I learn something new most every day. This makes one more thing.

The point at which the turn begins is a function of ground speed and angle of course change.

Of course (pun!) it is, just like when we used to fly the jet airways using VOR/DME and choosing how much "turn anticipation" to use in order to remain within the lateral limits of the airway. 'cept it's all algorithms and binary logic gates now instead of Mark I cranium work.

The RNP value or the sensitivity mode has nothing to do with it.

So the maximum distance you can pass abeam of a fix and auto sequence to the next fix in the flight plan doesn't change according to the sensitivity mode then? Could you please verify that aterpster?

Thanks,

westhawk

[aterpster]

westhawk:

So the maximum distance you can pass abeam of a fix and auto sequence to the next fix in the flight plan doesn't change according to the sensitivity mode then? Could you please verify that aterpster?

It is all a matter of the course, not the width of the sensivity of the CDI (or such). In the case of RNP AR where TF to TF course changes are used, the designers must use expansion areas to account for TF to TF flyby, and often impose a speed limit. This is why RF legs are much preferred for RNP AR.