These last few weeks I have been flying a couple of different aircraft that had inoperative glideslope capabilities, but I was able to fly LOC approaches without trouble. In my career as a pilot and mechanic, I have always had the—possibly inaccurate—understanding that a VHF NAV receiver had the capability of receiving and processing both VOR and LOC signals.
My question is prompted by this answer to a question regarding flight plan equipment codes—and particularly the comment section of that answer. In the comments of that answer it was stated that an ILS capable VHF NAV receiver is necessary to fly a LOC approach. For the purpose of this question let's overlook the inaccuracy of the ILS part of that statement and instead consider the LOC capabilities of a VHF NAV receiver.
I understand that the additional requirements needed for ILS capability include a glideslope antenna (possibly included in the existing NAV antenna), equipment to process the frequency pairing, and a glideslope display. I know that all LOC frequencies fall within the range of VOR frequencies (108.10–111.95 MHz and 108.0–117.95 MHz respectively) and the frequency spacing is compatible. I also understand that the circuitry required for VOR signal processing includes circuitry not required for LOC signal processing (that circuitry which can deteriorate over time requiring the periodic VOR accuracy checks).
Given all this, my questions are:
- Is LOC signal processing an inherent capability of a NAV receiver: Will any given VHF NAV radio receiver be inherently capable of processing a LOC signal, or is there additional circuitry required for that capability beyond that required for VOR signal processing?
And if not,
- Are there NAV receivers incapable of LOC signal processing: If LOC signal processing is not an inherent capability of a VHF NAV receiver, are there NAV receivers that were built without the LOC signal processing capability? In other words, is it possible that a pilot or mechanic would encounter a NAV receiver that did not have LOC signal processing capability? More precisely—from a pilot's standpoint—might I encounter a NAV radio that I could use to navigate tuned to a VOR station but not a LOC station?