RNAV non-precision 4

25.10.2004

Submission:

I find your construction of the turn mostly correct interpretation of the Doc 8168, however there are two details that I would like to point out:

1. As Ingar Lie said, the width of the secondary area should remain constant throughout the turn.

2. According to Fig. III-31-3 (Doc 8168, Vol II), the tangent lines (red lines in fig. 4) should be parallel to the nominal tracks of the applicable segments before and after the turn. One of the lines in fig. 4 is the extension of the 30° intercept of the final track.

Now, I also have some questions.

First, what is the purpose of those tangent lines? It's hard to imagine an aircraft trajectory, even in the worst wind conditions emulating anything like a pointed corner. Personally, I think the wind spirals should do. I can't see reasoning behind the "extra protection" provided by the pointy bit formed by the tangent lines.

Second, why do we need to construct a turn expansion for such a small turn anyway? Turns in the initial segment, for example, do not require area expansion unless they are greater than 30° (Doc 8168, 31.4.1.3). Why are turns at FAF different? Besides, the area width is otherwise tapering towards MAPt.

It is interesting to note that in Appendix to Chapter 33 (GNSS), figure III-33-4 depicts a turn at IWP with no area expansion on the outside of the turn. One would expect that the same general RNAV criteria would apply, i.e. para 31.4.1.3 in this case. It is obvious, of course, that application of 31.4.1.3 would produce rather awkward geometry.

I see this as an inconsistency of the Doc 8168. My opinion is that the expansion of the protection area on the outside of the turn makes sense only when:

a) turn is for 30° or more, and

b) the segment following the turn is of the same or greater width than the segment preceding the turn.

Regarding the bank angle for turns in the approach segments, it is 25° for turns of 50° or more, and half the turn angle for turns less than 50°. (See attachment M, 2.4 and 4.1.2)

Obrad Puskarica

Senior Navigation Procedures Specialist - Airways Corporation of New Zealand