ILS CAT II

28.10.2004

Submission:

I have a question for you or any of your readers who have experience with the design of Cat II ILS procedures and the associated application of PANS ops. PANS ops Vol II Part 3 Chapter 21 Para 21.4.8.8.1 describes how Cat II ILS OAS are to be protected:

by the "Cat II OAS and those portions of the ILS Cat I (surfaces ?) which lie above the limits of the ILS Cat II (surfaces)".

The only common surface equation between Cat I and II OAS for a given THR-LLZ distance/GP angle/MAP gradient configuration is the Z (missed approach) surface. I am having difficulty understanding how to apply "and those portions ...which lie above the limits ... . ". Given that the Cat I and II surfaces generally do not have a common intersection at the 150 metre contour. When viewed in cross section profile from the front, back or side, the Cat II surfaces seem narrower than the Cat I surface, so I can only assume that the intention of the reference is to join the W, X and Y surfaces where necessary by a horizontal line at z = 150m. Where the FAP is beyond the range of the 300 metre contour for the W and X surfaces, I assume that the Cat I surfaces extend back to the FAP altitude.

Molly Bushell _ Australia

Answer or Commentary:

(I.W.

Yes as far as I can see you have made the correct interpretation, in fact this is a little known fact. Most people just assume that if the Category I surfaces are clear then a Category II approach can be designed and conversely if the the Category II surfaces are penetrated Cat II is nor possible. However you have shown how even though a Cat I approach may be impracticable (i.e. 149m + height loss -> 500+ ft) a Cat II approach could be a very viable option. Following is a plan view of the problem showing the flat150m transition area, it is not very wide about 200m but still..)