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3 Answers

Localizer, Usage versus coverage

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General Aviation

Useable volume of the localizer: 18 NM at an angle of 10 degrees either side of runway centerline. 10 NM at angle of 35 degrees either side of runway centerline. The Question: If we receive the LOC way before the usable volume, say we receive it from 30 nm away, can we LEGALLY use it as our sole lateral NAV guidance? Reference Please.

3 Answers



  1. John D. Collins on Jul 20, 2013

    Many navigation signals can be received outside of the design service volume. The service volumes are determined by an analysis of many factors including the minimum transmission strength, the worst case receiver sensitivity, worst case signal to noise ratio, etc. Often the requirements can be met at greater distances than the service volume and when needed by the procedure or airway, they are flight tested at the required additional distances. When they are not required for use outside of the service volume, then flight testing is restricted to verifying that the criteria is met inside of the service volume. So, if you use the signal outside of the flight tested service volume, you are a flight test pilot.

    The signal doesn’t abruptly stop at the service volume and your receiver is likely to exceed the minimum requirements, which means that you will very likely find the localizer usable. However, since it isn’t flight tested by the FAA and it is not part of any procedure or route, you would technically be flying on an uncharted route. FAR 91.205(d)(2) requires that for IFR you must have “Two-way radio communication and navigation equipment suitable for the route to be flown.” To fly the portion of the route on the extended localizer path outside of the service volume, the localizer receiver would technically not be suitable for that portion of the route and other suitable navigation equipment would be required.

    The above is a technical analysis that probably has very little practical application as you would most likely be in radar coverage or have other equipment such as GPS to fill the gap, in which case, I would track the localizer. Also, there are many localizer routes that extend well beyond the service volume, but as I stated, they are flight tested for the purpose (example KLAX ILS or LOC 24L goes beyond 37 NM).

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  2. Lion on Jul 20, 2013

    Thank you John; you provided a very detailed answer which I can summarize it as follows:

    1. Out of the service volume, LOC indication is not reliable because it isn’t flight tested; therefore, it must not be used for navigation.

    2. I can intercept and track the LOC out of the service volume only if I am radar vectored because I am in radar coverage. In this case, do I have to remind the ATC that his/her vector will make me intercept the LOC out of its normal service volume or this is an unnecessary call?

    3. If the LOC service volume is extended, it will be depicted on the approach plate; it was flight tested and it is safe and legal to be used beyond the normal service volume.

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  3. Ale on Jul 21, 2013

    I personally won’t accept a vector to intercept and track any NAVAIDs including the ILS out of its Standard Service Volume (SSV) unless a NOTAM, the A/FD, or the approach plate indicates that the NAVAID is certified for an Extended Service Volume (ESV).

    That is because the area outside of the SSV is not tested by the FAA for accuracy and interference.

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