When to switch to VLOC on an ILS or VOR approach?

May 5th, 2014 by Max Trescott

VLOC SAC ILS VORHard to believe, but the ubiquitous workhorse IFR GPS receiver, the Garmin 430, was introduced 17 years ago in 1997. With more than 100,000 Garmin 430s and 530s shipped, it still has the largest installed base of any IFR-capable GPS. Yet despite its longevity, pilots are still asking basic questions about it, such as “When should I Load versus Activate?” or “When do I switch to VLOC on an ILS or VOR approach?”

Lest you think any of these questions are trivial, the former question became a full page in my Max Trescott’s GPS and WAAS Instrument Flying Handbook. As for the latter question, there’s finally an official FAA answer and surprisingly, it’s different depending upon whether you’re flying an ILS or a VOR approach.

For a lot of people flying mostly ILSs into the same few airports, the answer may seem simple. They might respond “Well the CDI just switches automatically to VLOC as I’m about to intercept the final approach course.” That is true some of the time, though only for ILS approaches and only if you’ve turned on the ILS CDI Autocapture in the Garmin 430 or 530’s AUX group.

But the automatic switching on an ILS only occurs if you intercept the final approach course between 2 to 15 miles outside the Final Approach Fix (FAF). That’s not a problem for most ILSs, but for a really long one with a large descent of perhaps 5,000 feet or more (e.g. the ILS 31 at Salinas, Calif. or the ILS 32R at Moffett Field, Calif.) the CDI won’t switch automatically as you join the final approach course. In these cases, you’ll need to manually switch it. Of course, you’ll always need to manually switch it for any non-ILS approach that uses a Nav radio, such as Localizer, VOR, VOR/DME, LDA, SDF, and Localizer back course approaches.

How Late Can You Switch?
But when are you required to switch to the Nav radio for primary guidance? Imagine you’re on a checkride and you forget to switch the CDI from GPS to the Nav radio. How far can you proceed along the approach before you fail the checkride because you didn’t switch the CDI to the Nav radio?

The story I heard years ago—but never confirmed so I don’t know if it’s true—was that Garmin and Cessna gave differing guidance on this point, because they were located in different FSDOs and got different guidance from their local FAA regional offices. One said you had to switch the CDI or HSI to the NAV radio as soon as you turned onto the final approach course. The other said that you didn’t have to make the switch until you reached the FAF. Which is correct? Like most things in life, it depends!

The FAA reference for this is AC 90-108, dated March 3, 2011. For an ILS, localizer, LDA, or localizer back course, Section 8. c. says that an RNAV System (e.g. a GPS) cannot be used for “Lateral navigation on LOC-based courses (including LOC Back-course guidance) without reference to raw LOC data.” This means that as soon as you turn onto a localizer or ILS, you need to display course guidance from the Nav radio. On the Garmin 430/530, that means as soon as you turn onto the localizer, you must push the CDI button so VLOC is displayed.

But oddly for a VOR approach, the answer is different. Section 8. b. says that an RNAV System (e.g. a GPS) cannot be used as a “Substitution for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment.” The final approach segment always starts at the FAF, which is marked with a Maltese cross. So on a VOR approach, you can fly all the way to the FAF before you need to switch the CDI or HSI to the Nav radio. Fly past the FAF using just the GPS (as I saw a client do a few days ago) and you’ve busted your checkride, and the regulations if you were to do it for real on an IFR flight plan.

How Early Should You Switch?
Waiting until the last possible time to switch the CDI or HSI to the Nav radio rarely makes sense. My guidance to clients is when the controller first begins issuing vectors—meaning you’re no longer using the GPS for primary guidance—switch the CDI or HSI to the Nav radio (unless of course you’re flying a GPS approach). That gives you time to verify that the course is set correctly before you join the approach course.

I saw a great example of why that’s important while teaching last weekend at a Cirrus Pilot Proficiency Program (CPPP) in Concord, Calif. One of the attendees I flew with didn’t switch the HSI to the Nav radio until the moment he turned onto the final approach course for the LDA RWY 19R at KCCR. At that time, I noticed that the HSI’s course pointer was incorrectly set for 191 degrees rather than the 181 degrees required for the approach, but didn’t say anything because I wanted to see if and when he’d catch the error. Had he made the switch earlier, he would have had more time to review his setup and possibly catch this error.

The needle remained centered, though it was pointed 10 degrees away from our heading. As we crossed the FAF, he asked “Now do I turn ten degrees to follow the pink line to the airport?” I was stunned that he came up with that as a possibility, since localizer signals are always beamed out in a straight line with no turns. Clearly he knew there was a problem in the conflicting information he was seeing, but he never considered the possibility that the course was set incorrectly.

The mantra I teach clients is to review “MORSE, Source, Course” as part of their setup for an instrument approach. There’s no need to check the MORSE code ID or to set the CDI Course when flying a GPS approach, but they’re absolutely essential to check and set anytime you’re using the Nav radio.

Why Does the FAA Allow the Switch to Occur Later for a VOR
So why must you switch to the Nav radio as soon as you turn onto an ILS or localizer, but can wait until the FAF to make the switch when flying a VOR approach? Consider an instrument approach with a VOR at the FAF. You might guess that when on the approach outside the VOR, a GPS signal keeps you closer to the centerline than a VOR signal, but that’s only true when you’re more than 6 NM from the VOR. At that point, the GPS is in Terminal mode and full scale CDI deflection is ±1 NM, which matches the ±10° full-scale deflection for a VOR signal at that distance.

Six miles is probably close to the average length of an intermediate segment, so while I have trouble saying these words [choke], the VOR would actually be more precise for navigating the last six miles to the FAF. Yes, a VOR signal scallops around a lot, but usually not much when you’re that close to a VOR.

The real benefit of GPS accuracy when flying a VOR approach occurs when you’re flying the initial segment, almost all of which would be more than 6 NM from a VOR at the FAF. Not only would GPS keep you closer to the centerline, but more scalloping occurs on a VOR signal at that distance.

It’s a little tougher to do the same analysis on an ILS or localizer approach, since the beamwidth of the localizer varies between about 3 to 6°, depending upon the particular installation. Suffice it to say that any approach with a localizer will have a narrower beamwidth, keeping you closer to the centerline, than a VOR approach when at the same distance from the antenna site. Just remember that localizers are more precise, so the FAA wants you to start using the Nav radio as soon as you turn onto one. But VORs are less precise, so you don’t have to switch to the Nav radio until you reach the FAF.

After reading this post, a friend emailed suggesting I’d misinterpreted AC 90-108 and came to the wrong conclusion about needing to switch to localizer data as soon as you turn onto the final approach course. I sought clarification from AFS-470 at FAA HQ and they quickly responded confirming that pilots MUST use raw localizer data for primary guidance along the entire localizer. They raised an additional point that a reader also mentioned  in the Comments section. Both pointed out that a pilot can always monitor RNAV (GPS) data as they fly along a localizer. However they cannot use it for primary navigation. The pilot must have raw LOC data displayed on their primary instrumentation and  must use that LOC/VOR data for primary navigation. My thanks to everyone who contributed to this discussion!

Max Trescott

Max Trescott specializes in teaching in glass cockpit aircraft. He is best known for his Max Trescott's G1000 Glass Cockpit Handbook and Max Trescott's GPS and WAAS Instrument Flying Handbook. He formerly worked for Hewlett-Packard and now is a full-time flight instructor. He is the 2008 National CFI of the Year. Visit Max’s website.

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The opinions expressed by the bloggers do not reflect AOPA’s position on any topic.

  • http://www.rapp.org/ Ron Rapp

    Great post, Max. I’ve flown with pilots who rely on the Garmin to switch to VLOC on its own, and that always makes me uneasy. I like to see them proactively check the nav source to ensure it’s correct. It’s pretty important. While it’d be bad enough to shoot a VOR-only approach using the GPS, it could be much worse to inadvertently fly a GPS procedure in VLOC mode!

    • Max Trescott

      Ron, the problem with relying upon the Garmin to make the switch is that it gives pilots a false since of security, since it won’t switch on many long ILSs and it never does it on VOR approaches. Of course failing to make the switch and using GPS instead of the Nav radio would probably rarely kill you….though all it would take is once! True on your last point….it would be much worse to set to VLOC for a GPS approach!

  • WestlandAviation

    That button should be painted red……maybe like the Boeing FL Change/level change/open descent mode that bit Asiana (besides other basic airmanship issues……

    • Max Trescott

      That’s an excellent point–why are these important switches so ordinary looking? In older planes with an external CDI and a switch that allowed you to select whether VOR or GPS data was displayed on the CDI, I often it heard that switch referred to as the “Kill Switch,” since it could kill you if it were in the wrong position. The CDI key on a Garmin 430/530 serves the same function and having it in the wrong position could in some cases be deadly.

  • John Collins

    Good post. I have one observation, AC 90-108 lists the following restriction in the section detailing the “USES OF SUITABLE RNAV SYSTEMS NOT ALLOWED BY THIS AC”

    “c. Lateral Navigation on LOC-Based Courses. Lateral navigation on LOC-based courses
    (including LOC back-course guidance) without reference to raw LOC data.”

    One can use the GNS430W in GPS mode if there is a number two CDI/Nav receiver that displays the localizer course in question on it. This can be particularly helpful if you are flying the full procedure and tracking the localizer course outbound and using a roll steering guided autopilot to fly the outbound, PT, and inbound intercept. If there is ever a course discrepancy on the CDI displaying the localizer data, it wins and must be followed anytime one is following a localizer course.

    I personally prefer to do my own CDI source switching and don’t use the auto switch feature. I like your mantra Morse,Source, Course. As an aside, on some autopilots, in particular the KFC225 and KAP140, the auto-switch function is not supported if the installation follows the IM.

    • Max Trescott

      John, you raise an excellent point, which I’ll probably go back and add to the blog post.
      You can always refer to RNAV data when flying a localizer (or VOR for that matter) as long as:
      1) RNAV data is not the primary means of navigation
      2) Localizer (or VOR) raw data is displayed on primary instrumentation
      3) The localizer (or VOR) raw data is use for primary navigation.

  • Vernon Johnson

    Great article. I agree with John Collins too (if you have 2 Nav radios). I trained for the Instrument rating in a bird with GS on the #2 CDI. My CFII took delight in failing the #1 Nav (G530W) at various points on the ILS. Whether failing to switch to LOC or losing the #1 CDI, If the second Nav radio is set to the ILS, it’s a non event. If your #2 CDI has no GS and you are still far enough out to switch to the Localizer profile, not quite a non event but perhaps an option.

  • Jaykey Boii

    With that being said, are you allowed to fly the outbound course and pt via gps on ils/loc?

  • Paul Stein

    John Collins is correct regarding the use of two receivers.

    90-108 deserves careful reading in its entirety. For example, Section 1. a. (1) and (2) articulate in context what “Substitute and “Alternate means of navigation” are. Section 3. a. and g. define those terms again. A substitution occurs with inoperative equipment. Armed with that knowledge, the Section 8. b. restriction means something very different (and specific). Thus, 8. b.. all by itself, provides guidance against using the RNAV system on the final approach course only in the instance of inoperative equipment. Nothing here says you can not have your RNAV in #1 display and the ground-based NAVAID in the #2.

    As far as 8. c., goes, it is plainly written: The only restriction there applies if you do it “without reference to raw LOC data.” It does not specify which display it needs to be presented in. Thus, if you reference raw LOC data in your #2 display, the restriction in 8. c. clearly does not apply.

    The folks at AFS-470 may say something different when one speaks with them, but that does not change what 90-108 says. I believe it is important to know why you are doing things in an airplane. If you do it because you believe it is the best practice, or because you have received verbal information from an FAA employee, then you know how to weigh that source. If you are doing it because it is something a reg or an AC says, then it seems desirable to know what it actually says.

    For what it’s worth, I submit to three check rides a year – two with designees and one with an FAA inspector, and I get in the Sim twice a year. The procedure that is taught, and that I perform including during checkrides, is to leave the #1 display on the RNAV source throughout a VOR approach and use #2 for the raw VOR data. In the case of an ILS I allow the box to switch on it’s own and reconfirm that it has done so upon reaching the FAF. All examiners have thought this was great. (I say “For what it’s worth” to acknowledge that anecdotal information about a person’s checkride experience is extremely limited in its value for interpreting regs,)

  • frank clark

    I have a hard time believing that the GPS isn’t more accurate than a VOR all the way to touchdown or RNAV minimums wouldn’t be lower than VOR minimums. Case in point: KHTL VOR 27 vs RNAV 27. Because of Garmin coding problems the RNAV 27 is no longer in the 530 database requiring the VOR 27 approach. Although technically illegal, not switching to VLOC provides a much more stable tracking on the autopilot and appears to be more accurate when comparing the VOR CDI to the GPS CDI and looking at the runway.

    • Andrew Roberts

      If there is coding problems then something is probably wrong. I don’t see this as an issue though if you are handflying and also have the vor set on another nav.
      Certainly wouldn’t fly a poorly coded gps approach without the vor as a backup, especially not with an autopilot.

  • Warren Webb Jr

    Another good subject, Max. A thought on auto-switching from GPS to
    VLOC. First according to the manual, this can occur on more approaches
    than the ILS – according to Section 5 Points to Remember “Automatic
    switching of CDI output is available for ILS, localizer, SDF and LDA
    approaches”. I’m a believer in using these features to reduce workload
    and errors. Besides that, the automatic change occurs gradually to
    prevent abrupt CDI changes when coupled to an autopilot. This doesn’t
    mean than normal monitoring can be forgotten. Like other transitions
    that can be programmed or that are automatic (ex: transition to Approach
    Active mode within 2 miles of the FAF) the pilot must monitor and make
    sure the automation worked.

  • Andrew Roberts

    I agree with the article. However, I personally set my Nav 2 for the ILS/LOC and leave the GPS on Nav 1 throughout the approach. When the needles appear on NAV2 I follow that to the runway. Its the best option for situational awareness. You swap the gps completely away and you are losing a very helpful resource for no reason.
    Of course this only works if your NAV2 has a glideslope

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  • Richard DeMaria

    Question for you. Appreciate an answer if anyone can help me out. I fly a Skylane with a WAAS G1000. 1. Does then G1000 use “baro-aiding” for GPS approaches? Hence, I only need 4 satellites instead of 5 for RAIM, if WAAS is not available. There is no input for barometric pressure setting. I don’t know if the G1000 reads it off the bark knob. The GIV and Global types I fly, read it off the baro knob. I’m wondering if this system does the same. 2. Is it necessary to “activate’ a GPS approach to switch the scaling to .3NM? I have been “loading” the approach and flying the full procedure with all the correct annunciations to DA/MDA. I believe the system is scaling correctly. When I scour over the G1000 manual, it’s a bit vague. Thanks

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  • Chris O’Connor

    Would the requirement to switch to raw localizer data pertain to the outbound portion of the approach when executing the full approach with a procedure turn? In this case guidance provided by the gps provides more precise guidance than the vhf radio as it gives guidance throughout the procedure turn, while the vhf leaves one “blowing in the wind” during the course reversal. Once reintercepting inbound is when the 430 auto selects to VLOC. Does this meet the letter of the AC 90-108?

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