Using RNAV/GPS as substitute for VOR cross-radial on an ILS/LOC approach

6 Answers

1. Best Answer
   
   

   
   

   Russ Roslewski on Oct 12, 2021

   1) Why do you want to do this? (Use the GPS in OBS mode to make a cross-radial). Is this a very old GPS? Otherwise, if it’s any GPS like a Garmin 430/530 or more recent, just load the ILS procedure and identify the fixes by their actual location. No cross-radials needed.

   But if it’s something like a Garmin 300XL or other vintage GPS that doesn’t have non-GPS procedures in the database, then I still don’t think it’s legal (or wise) to create a VOR radial in the way you suggest. The primary reason is that the Mag Var of the VOR and the Mag Var the GPS is using for calculations could be quite a bit different (I’ve seen 7 degrees), which could greatly affect where you “think” the fix is.

   What I would do in this second situation (and what I used to do when I flew with a GNC-300XL), would be to dial in the NAVAID providing DME and use that distance readout – since GPS is a suitable substitute for DME. So, have it show distance to the CYN VOR and you can then identify SOITE and XOGUR – but interestingly, not ZOMEV.

   2) Using the OM – The OM on this procedure is shown in half-tone. This means that while you will fly over it, it is not specifically part of the procedure. It’s intended kind of like a heads-up that you might hear beeping when you fly over it. But it could be halfway down final or something else. In this case, it does appear that the OM is at the same location at ZOMEV, or at least really close. So realistically you’d probably be just fine, but since it’s not actually part of the procedure, I can’t see how it would be legal to use it.

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2. 
   

   
   

   mcapellupo on Oct 12, 2021

   @Russ, thank you for your insight! This is very helpful!

   To clarify, yes, I am working with a very old GPS unit, a KLN-89B (interestingly enough, BendixKing, through Honeywell, still provides current updates for this!). It does not have ILS overlays or any other non-GPS procedures.

   I see what you mean about the magnetic variation. I don’t think my GPS unit (or others, for that matter) would account for the difference in the magnetic variation setting on the VOR and the actual, present-day magnetic variation. Nevertheless, the FAA has said you can legally use GPS as a substitute for VOR to “[d]etermine aircraft position relative to … a VOR,” to “[d]etermine … a named fix defined by a VOR radial … intersecting a VOR or Localizer (LOC) course,” and to “[n]avigate to or from a VOR….” (AC 90-108 at §7[a][1&2]). The only exception that seems relevant to my situation is that I can’t use GPS as a substitute for VOR “providing lateral guidance for the final approach segment” (§8[b]).

   This should mean that I can legally use my GPS to fly at least the procedure turn/course reversal at Coyle VOR. I’m not so sure about legally using GPS to identify ZOMEV intersection because that is the FAF, which would be the start of the “final approach segment”.

   Also, forget about GPS for a moment, is there any way to legally fly this approach with only one VOR/LOC/GS indicator? Presumably you should be able to, because dual VOR receivers or DME are only required if you are flying the LOC-only and want to be able to identify XOQUR for the lower minimums, and GPS obviously is not required to shoot this approach. Then how on earth do you identify ZOMEV if you only have one VOR receiver? I guess the FAA expects you to flip between McGuire VOR and the LOC frequency all the way to ZOMEV.

   I’ve never heard of a “half-tone” marker beacon. I see it doesn’t have a standalone frequency like some other marker beacons, but why would that not be a legal way to identify ZOMEV? Is there anything published by the FAA about “half-tone” marker beacons?

   By the way, I appreciate the difference between what is legal and what is “wise”. These questions are to figure out what the FAA would deem bare minimum-legal to fly this particular approach.

   Thanks again,
   -Mike

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3. 
   

   
   

   John D Collins on Oct 13, 2021

   Most GPS will use the VOR station magnetic declination which is the variation at the time the VOR was installed or last aligned. This is to align with VOR radials which don’t change with magnetic variation drift over time. The KLN-89B is no exception and includes this statement in the OBS section description in the Pilot Guide:

   “When the active waypoint is a VOR, and the AUTO magnetic variation mode is active (see section 4.10), the published magnetic variation for the VOR is utilized rather than the calculated magnetic variation, so that the KLN 89(B) indication will be identical to a NAV receiver indication.”

   When in OBS mode, the GPS will set the course based on the OBS setting on the HSI/CDI. There can be considerable error in what course is selected as seen on the HSI/CDI verses the course set into OBS mode. Rather than use the visual setting of the OBS setting as shown on the CDI/HSI, I recommend using the OBS value displayed on the GPS to set the cross radial.

   Using GPS Substitution is permitted to determine the Along Track Distance (ATD) for a fix location that is part of the Final Approach Segment (FAS). What is not permitted on the FAS is the use of substitution of GPS for the lateral guidance, which on this procedure is provided by the localizer.

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4. 
   

   
   

   Russ Roslewski on Oct 13, 2021

   John – good information on the magnetic variation issue, I was unaware of that. Do you know if the same is true of the Garmin GPS units? I looked through the 430W and 650 manuals but did not immediately find anything similar. I did find in both that the magnetic variation (station declination) of the VOR is shown on the info page, but both had the following statement later on in the manual: “If “Magnetic” is selected, all track, course and heading information is corrected to the magnetic variation computed by the GPS receiver.” Although this mentions the “calculated” value, a VOR radial is not necessarily a “track, course or heading” so I’m not sure if it’s relevant or not.

   Mike – to your question about flying the LOC procedure with just one VOR/ILS receiver, yes, you can. Yes, you would be expected to flip back and forth prior to ZOMEV to identify it. Yes, this is challenging. But it’s all you can do with one VOR/ILS receiver. That’s why all “serious” IFR aircraft in the past had two VOR/ILS receivers. Note that you don’t need to identify ZOMEV to fly the ILS procedure, however, just the LOC.

   As to the “half-tone” charting of the OM. It’s not a “half-tone OM”, it’s an OM charted in “half-tone” – meaning “partially grayed out”. It is briefly mentioned in the chart legend, and on page 119 of the Aeronautical Chart User’s Guide. It means that the OM isn’t used on the procedure, but still physically exists. Typically this means that the procedure was redesigned from when the OM was originally installed, and the FAF is now at a different spot – but the OM for various reasons hasn’t been decommissioned.

   A better example of this is at ATL, ILS OR LOC RWY 9L: https://skyvector.com/files/tpp/2110/pdf/00026IL9L.PDF

   The FAF is VINII, but the OM looks like it’s a good mile further down final. So it’s not usable on the procedure, but your MB receiver will beep as you’re descending.

   In your example, of course, the OM appears to be right there at the FAF, so realistically it shouldn’t be a problem using it, but may not be strictly legal.

   And OMs never had a frequency assigned. They’re all on the same frequency. When you do see a frequency it’s because it’s actually a Compass Locator Outer Marker, which is an NDB collocated with the OM. The NDB of course, has a frequency.

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   John D Collins on Oct 13, 2021

   Russ,

   With the early GPS systems particularly the TSO C129 variants, the TSO does not specify the magnetic variation to be used, even though most will use the VOR declination, but in TSO C145/146, the TSO is explicit and requires that a VOR use the station declination. This is from DO 229E, the latest version of the TSO C145/146 specification source document:

   2.2.1.3.12 Magnetic Course
   The source of the magnetic variation used for paths that are defined using magnetic course
   shall be in accordance with the following:

   a) If the leg is part of a database terminal area procedure and the magnetic variation is specified by the State for that procedure, the magnetic variation to be used is the value specified.

   b) If the leg is not part of a procedure and the active fix is a VOR or TACAN, the magnetic variation to be used is the published station declination for the VOR or TACAN.

   c) If the leg is not part of a procedure and the terminating fix is not a VOR or TACAN, the magnetic variation to be used shall be defined by the equipment using an internal model.

   So, for example if the waypoint is a VOR, then direct to the VOR using OBS mode or Course to a VOR, it will be a radial. On the other hand, if the active fix is a named fix, the current variation is used IAW the built in model. You can play with this using the Garmin trainers.

   Also if you enter a FRD such as CTF132040 you will see a course along the route CTF to CTF132040 aligns with the airway V259.

   All this is intended to match up OBS mode or CRS TO to a VOR radial.

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   mcapellupo on Nov 10, 2021

   John,

   Thank you for the information. I have a follow-up question on your statement below:

   “Using GPS Substitution is permitted to determine the Along Track Distance (ATD) for a fix location that is part of the Final Approach Segment (FAS). What is not permitted on the FAS is the use of substitution of GPS for the lateral guidance, which on this procedure is provided by the localizer.”

   Just to be clear, on this particular procedure, I want to use GPS as a substitute for GXU VOR to identify ZOMEV intersection, which is based on the 141 radial from GXU VOR (see updated link to the procedure below). There is no designated distance from the VOR in this procedure–only a cross-radial–so I would not be using GPS for along track distance (ATD). Can I still use GPS for this purpose, to identify my position relative to GXU VOR? That would seem to be acceptable under AC 90-108 (you can use GPS as a substitute for VOR to “[d]etermine aircraft position relative to … a VOR”, AC 90-108 at §7[a][1]), but is this using GPS for lateral guidance in the final approach segment (FAS)? I agree that lateral guidance in this procedure is provided by the localizer, but is using the GPS as I described above considered “lateral guidance”? In a sense, yes, I am using GPS to determine where I am laterally on the procedure–to identify ZOMEV–but I am primarily receiving lateral guidance from the localizer, not the GPS.

   https://aeronav.faa.gov/d-tpp/2111/05623il6.pdf#nameddest=(MJX)

   Related question: assuming your answer is that yes I can use GPS to substitute for GXU VOR on this procedure, do I need to change the sensitivity on my GPS unit for that purpose? The default sensitivity for my KLN 89-B is +/-5 nm, basically an en route sensitivity. I don’t think the KLN 89-B changes sensitivity when using it as a substitute for a VOR, but there is an option to change the sensitivity manually to either +/-1 or +/- 0.3 nm (when not flying an RNAV/GPS approach, of course). It would make sense to change the sensitivity to at least +/- 1 nm for better accuracy in identifying ZOMEV intersection from the GXU radial, but I don’t see any requirement to do this in the regulations. I guess the default sensitivity is sufficient?

   Thanks in advance,

   -Mike

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