Flyover vs Fly-by

[fart]

By how much can you undershoot or overshoot a Fly-By waypoint? What is the criteria for a Fly -By waypoint as opposed to a Flyover waypoint which you have to pass directly overhead???
Thanks

[PPRuNe Radar]

fart

The whole idea of a Flyby waypoint is that you don't overshoot the desired track after the waypoint. Instead the aircraft will 'anticipate' when the turn needs to be made in order to smoothly roll out on the new track. This actual point will depend on many things, such as aircraft speed, wind speed/direction, size of turn, FMC algorithm and database coding (leg type and track terminator) and a few other things probably as well.



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PPRuNe Radar
ATC Forum Moderator
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[fart]

Thanks for the reply, it is quite helpful. From your own experience, if the aircraft is not going to overshoot the track, can you say how close most aircraft normally pass by a FLy-By? Most aircraft in our airspace will be 250 knots or less and on a SID or STAR and I guess what I am asking is what can the controller expect the aircraft to do when it is approaching a Fly-By? Can you clear an aircraft to the FLy-By point for instance although it is not actually required to Fly over it? In some of our procedures, a FLy By is the start of an approach and it is only the final approach fix and a waypoint beyond the runway after a go-around that are designated as Flyover points. We are in the process of redesigning our airspace and introduce SID/STARS and RNP5. Should one clear an aircraft to the Flyover via the Fly -By or what do you do in such a scenario?

Thanks, your reply is much appreciated.

Cheers

[PPRuNe Radar]

fart

First thing to note is that a Flyby waypoint has to be coded into the aircraft FMC as part of a procedure. The waypoint can still exist in the FMC database for use by the pilot at any time but (at least to my knowledge) if he inserts it as a waypoint himself then he has no way of making it anything other than Flyover waypoint. All to do with the ARINC 424 coding protocols, which gets very heavy ;)

If you are clearing a pilot to an intermediate waypoint on a coded route and it is a Flyby waypoint, then it makes no difference, the FMC will continue to treat it as such.

In terms of what you can expect the aircraft to do at a Flyby, it will probably commence the turn a couple of miles before the waypoint, passing maybe a mile or so abeam it in the turn.

At the moment there is no clear guidance on the separation (spacing) to be applied between SID/STAR routes, however PANS-OPS does provide containment formulae albeit primarily to do with obstacle clearance calculation. These calculations can be made for Flyby waypoints to show the likely swathe where aircraft following the procedure can be found, or in other words how far they are likely to deviate from the nominal track. At the moment route spacing 'rules' only exist for airspace outside TMAs although it is now recognised that similar guidance needs to be supplied for routes such as those you are asking about.

If you're into maths, geometry, and trig, then procedure design is for you !! ICAO DOC 8168 has all the gory details.



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PPRuNe Radar
ATC Forum Moderator
[email protected]

[fart]

A big thank you for your reply, it is all beginning to make sense now and it is nice to get it staright from the horse's mouth - somebody whom has actually worked these procedures. Many thanks again and it sounds much better they way you describe it than it looks on paper!

Cheers
Fart

[PPRuNe Radar]

Whilst European specific, there is a lot of useful explanation about waypoint types (including RNP diagrams) in Eurocontrol's RNAV Standard. Also there is a Eurocontrol Terminal Airspace Design document.

Both found here :


http://www.ecacnav.com/library.htm

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PPRuNe Radar
ATC Forum Moderator
[email protected]