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When performing one of the procedures shown in the pictures below, when exactly do we consider ourselves to be on the next leg?

For example: for the fly-by waypoint, when do we consider that we are no longer flying from the runway to Alpha, but we're now on the Alpha to Bravo leg instead? Is it in the middle of the arc, once we finish the arc...? For fly-over waypoints, I guess it's when we pass Alpha, but I'm not sure about that either.

enter image description here

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The transition point as defined in RTCA DO-283 RNP MOPS is the point where the aircraft's path passes through a vertical plane that bisects the inbound and outbound legs going through the waypoint.

That generic description covers all the possibilities of the aircraft passing the waypoint regardless of how good or bad it is following the defined path.

Realistically, in the case of the fly-by waypoint, it's the point half way through the transition turn where the waypoint is directly abeam the flight path.

The same rule (crossing the bisecting plane) applied to the fly-over waypoint results in the transition occurring at the waypoint.

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    $\begingroup$ I think a diagram would support the explanation, as some people aren't great at turning the verbal description into a geometric description. $\endgroup$ – Nij Nov 22 '17 at 9:36

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