GPS secondary areas 1

25.10.2004

Submission:

Determining the applicable OC for the obstacles in secondary area in a situation like this, when the nominal flight path is a combination of the turn and a straight segment, will always be a challenge, to say the least.

Rather than trying to give a definite answer to Mr. Whitworth 's question, I will explain how we do it in New Zealand. Our construction is somewhat different to the Figure III-33-4 in Doc 8168 and is shown in Figure 1.

No attempt is being made to meld initial and intermediate segments on the inside of the turn. Instead the whole areas applicable to the segments (red for the initial, blue for intermediate segment) are evaluated for obstacles. Standard expansion of the primary area associated with the turn anticipation is applied on the inside of the turn.

The obstacles in the overlapping area are evaluated twice.

Full OC applicable to intermediate segment is applied to the obstacles within that portion of the intermediate segment secondary area overlapping the primary area of the initial segment.

Applicable OC in the secondary areas is calculated based on the distance perpendicular to the nominal, straight-line flight path (Figure 2, d'' for initial segment and d" for intermediate segment). This methodology is similar to the existing method of joining DME arcs with the intermediate segment of the approach (PANS-OPS Vol. II, Figure III-4-2). I believe it is conservative enough and much simpler than the one illustrated in Chapter 33, Figure III-33-4.

Talking about complexity; has anyone tried to construct the protection area using Figure III-33-4 method, with a 4NM intermediate segment?

Obrad Puskarica
Senior Navigation Procedures Specialist, Navigation Development Unit

Airways Corporation of New Zealand