Official Localizer coverage [25 vs 18 nm]
 

Official Localizer coverage..

Which one do you use?

Jeppesen booklet says 18 nm:
http://s8.postimage.org/sn21k1dc5/jepp.gif


Icao annex 10 says 25 nm:

http://s8.postimage.org/6mlp3euo5/icao.gif

Those illustrations are really meaningless. The localizer is useable for the distance set forth in any given ILS IAP. Flight inspection and facilities technicans make sure that is the case.

In some cases the localizer can be distorted sufficiently off centerline that radar vectors are mandatory.

Normally, as per ICAO standards, localizers are commissioned to 25 nm in the course sector and 17 nm out to 35 degrees off centerline.

In the U.S., and possibly other places, frequency congestion has brought about reduced coverage localizers which are 18 nm in the course sector and 10 nm out to 35 degrees off centerline.

There are also long-range facilities in the U.S., with coverage far exceeding the normal specifications.

Cheers,
/Fred

I have also seen the LAX localizer for 25L/R, which extends some 45 nm to the east?

172 driver:

More like 50 miles with the four ILSes from the east. That is called extended service volume (ESV) and requires more frequency protection and good localizer transmitters. You don't see ESV mentioned in any AIP, either.

In your practical flying if you use the localizer only where it is authorized on the approach chart, you then couldn't care less about that 18 miles.