Localizer to Threshold

16.01.2008

Submission:

The position of the LOC only area when the distance from the LOC-threshold is shorter than 2000m. The shortest distance available in the PANS-OPS criteria between the LOC and the threshold is 2000m. If I have a shorter runway and/or the LOC is offset, how do I position the LOC-area? I guess I have to make a fictitious threshold somewhere. Can anyone tell me how to think in these cases? Does the width of the LOC-signal at a certain point (for ex threshold ) have something to do with the PANS-OPS areas?
Susanne Westman - Procedure designer, Sweden

Answer or Commentary:

(I.W.

Pans-Ops states:

The PANS-OPS OAS CD-ROM gives coefficients for ..... any localizer-threshold distance between 2 000 m and 4 500 m. Extrapolation outside these limits is not permitted. If a localizer threshold distance outside this range is entered, the PANS-OPS OAS CD-ROM gives the coefficients for 2 000 m or 4 500 m as appropriate, which Must be used.

So one should use the 2000 m constants.

As to the LLZ course width at the threshold, this is covered by the 210 m LLZ course width section (1.4.8.7.5). This section basically says, use the CRM and what it does internally I do not know. Certainly the physical width of the LLZ signal at the threshold will affect the Pans-Ops areas but if you are using GBAS criteria for example one can set up the threshold and LLZ at any place as these positions are just Latitude / Longitude positions and are not dependent on an actual physical ground based navigational aid.

That is why I take great care with these Latitude / Longitude positions as a single digit mistake could result in a highly controlled precision approach to the taxiway. This is also why a accurate survey of the aerodrome is needed because if you do not know the position of the runway how can you design an approach to it especially as there is no physical beam to follow.)