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While studying FAA's PBN Manual (Order 8260.58A), I came across the concept of early/inside and late/outside turn construction (Section 5-3. Turning Departures).

For each of the possible cases, it provides instructions for constructing different Obstacle Clearance Areas.

Here are some examples:

a.) Early and inside turn construction

1. Turn following VA leg.

2. Turn follow VI leg.

3. Turn-at-fix (FO or FB).

b.) Late and outside turn construction

1. Wind spiral application

My question is, what are these kind of turns exactly? Are they special cases of a "normal" (expected) turn? In addition, how do pilots know when they have to perform such turns?

What also confuses me is the fact that the PBN manual, which describes the design of IFPs, covers such cases that have to do with a pilot's/ATC's "on the spot" decision. When designing, for example, a SID, shouldn't you end up with one (multi-branched or not) Departure Procedure, with specific waypoints selected after the evaluation of specific Obstacle Clearance Areas covering "normal" turns and not special cases?

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I think what you're talking about are what pilots refer to as "fly-past" and "fly-over" waypoints.

Up until recently, all waypoints in the world were "fly-past", ie you could cut the corner from one leg to the next and not actually fly over the waypoint itself. This is still the standard, unless otherwise stated.

But then some SIDs and STARs needed people to actually fly OVER the waypoint before turning onto the next leg, and so coding was included in FMCs etc to get the autopilot to not start the turn until AFTER the waypoint. The difficulty is that there's usually not any differentiation in the presentation on the CDU between "fly-over" and "fly-past" - you have to look at the produced track on the ND to then see whether you're cutting the corner or turning after the waypoint.

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  • $\begingroup$ thank you for your feeback but I am not talking about FB or FO waypoints. I am talking about different Obstacle Clearance Areas construction shortly after the Initial Climb Area, depending on an aircraft's "early" or "late" turn. What I am having trouble understanding is what early and late turns actually mean. $\endgroup$ – Vector Zita Dec 25 '18 at 6:48

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