GPS missed approach 2

25.10.2004

Submission:

Figure III-33-7 in Doc 8168, Vol II, Chapter 33, depicts the turning missed approach area for basic GNSS receivers. The area is derived by application of wind spirals to the edges of the primary area abeam the latest turning point. On the outside of the turn a constant width secondary area, equal to its width abeam the latest turning point, is applied to complete the protection area. Once the turn is completed, the area splays out as applicable. This is consistent with the general methodology used for the construction of the protection area for RNAV procedures (Chapter 31).

On the other hand, in Chapter 7 (Missed approach), turning protection area is derived by applying wind spirals to the outer edges of the total area (e.g. Figures III-7-23 to III-7-25).

Although this seems to be the similar methodology, the results are significantly different. In the accompanying figures, I constructed the protection area applying both methods and using parameters for Cat B airplanes, assuming NDB tracking and turn at an altitude.

Figure 1 shows the area resulting from application of the wind spirals to the outer limits of the total area (Chapter 7 methodology).

In Figure 2 the area is constructed by drawing the wind spirals from the edges of the primary area.

Figure 3 shows the difference (red area) between the two.

It is interesting to note that the lateral protection of the nominal flight path in case 1 actually reduces during the turn, i.e. W1<Wo. My opinion is that the correct methodology for construction of turn protection area should be the one in Figure 2.

I would like to hear opinions of other specialists.

Obrad Puskarica

Senior Navigation Procedures Specialist _ Airways Corporation of New Zealand

Answer or Commentary:

(I.W. A Very important point)