Tag: ILS

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

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!


Aviation, if you haven’t noticed, is loaded with abbreviations and acronyms. There are FARs, the AIM, MAPs (not to be confused with maps), ILSs, and DHs, METARS, TAFs, and NOTAMS. Airspace used to have TRSAs, ARSAS, and TCAs…and I haven’t even mentioned NASA, which speaks acronym-ese. Speaking of NASA, all pilots are able to participate in the ASRS program, which brings me to ASAP.

ASAP stands for Aviation Action Safety Program, and it is commonly in use at Part 121 air carriers. At first glance, it appears to be just like the ASRS program, and it is…sort of. Like the ASRS, pilots and other participating employees can self-disclose when they make a mistake. For example, if a pilot taxis two feet over the hold-short line for a runway, he is guilty of a committing a runway incursion. Now, it is possible that the controller never saw it because of the vantage point of the tower. It’s also possible that no other aircraft was affected.

But the pilot still made a mistake and inadvertently violated a federal aviation regulation. Worse, it’s an area in which the FAA has been aggressive in the last few years to change pilot behavior because the safety ramifications and some accidents.

With ASAP, the pilot is able to fill out a form (usually online) and explain what happened, and if possible, why. In this case, it might be something simple (“I just screwed up.”) or it might be a previously unknown safety issue (“The paint was difficult to see, especially at night,” or “The flashing lights were not working”). The report then goes to a central data base where it is reviewed by a committee.

The committee can be called any number of things, but what’s important is who is on it. At unionized carriers (which is almost all of them), the committee consists of a representative from the FAA, the airline’s safety department, and the safety committee of the union. You typically won’t see chief pilots or anybody who can impose discipline on the pilots. The reason is that the program is built on trust and confidentiality.

Once the report is opened by the committee, they discuss it in detail, and decide how to act on it. If, in the above case, they agree that the incursion was simply inadvertent, they may close the report. Or, if they suspect something else may have been involved, such as fatigue or poor judgment, they may call one or both pilots in for questioning to see what might be done to prevent similar problems in the future. If the problem is poor paint or broken lights or a bad airport diagram, then the information is forwarded to the appropriate people as quickly as possible.

The only time a pilot can face discipline is if the committee agrees that the pilot deliberately violated a FAR or exercised poor judgment, or if the infraction was reported from someone other than the pilot (the assumption then is that the pilot may have gotten caught anyway, even without participation in the ASAP program). Acknowledging that you crossed a hold-short line because you were discussing impact of artificial turf on the lifetime batting stats of career designated hitters is bound to get you not only called in, but also may lead to a Letter of Warning from the FAA.

The overwhelming number of reports in ASAP files fall under “Oops!” banner, but many go deeper than that. Pilots can also report on any aspect of their company or FAA operation that they feel needs to be addressed. Examples run the gamut: Poorly designed approach and arrival procedures have been flushed out; better operational practices have been developed; charting errors have been corrected more quickly; and most importantly, better training has occurred because of the ASAP umbrella. ASAP is not a get-out-of-jail free card. It’s a tool that is used by the airline, the FAA, and the pilot community to maintain the highest level of safety.

There is much more to the program than what I have described, and it goes beyond pilots. Mechanics, dispatchers, and flight attendants can also craft ASAP programs, and the air traffic controllers also have their own. There have been a few instances when ASAP programs have been shut down by a participating group, and when that happens it almost always comes down to the suspicion that the necessary level of trust has been breached. Airlines and labor don’t always get along, but with ASAP, the level of trust is high, and the real beneficiary is not the participants, but the traveling public.—By Chip Wright

Truly rare events

For more than 20 years I’ve been flying airplanes, 15 of them for an airline. As a student and a flight instructor, one skill I practiced repeatedly was the missed approach (or go-around for VFR flying). It’s a critical task to be able to do in any possible landing configuration in any airplane. If you want proof, try doing one in a heavy Cessna 150 with 40 degrees of flaps on a hot day. It’s a challenge, to say the least, and it requires a fair amount of finesse to do well. As an instructor, I probably did no fewer than five go-arounds or missed approaches on a given day. Any that were done for real were almost always done because the preceding traffic was still on the runway.

In airline flying, however, they are exceedingly rare. Controllers handle the spacing, and even when there is a snafu, the missed approach is almost always done in visual meteorological conditions, and it almost always starts from an altitude of 500 feet or above.

What is even more rare, though, is to do one because of weather. I can recall doing fewer than 10 for wind shear or a microburst that beat me to the airport. And I have only done two—two—because we could not see the runway. The first one was an ILS in low ceilings but with good visibility beneath in Charleston, West Virginia. In fact, we saw the runway on the go-around. We came back around and landed on the second try.

The most recently was in January 2012. I was flying the right seat with a simulator instructor in the left. Both of us were slightly out of our element. The weather everywhere that day had been lousy, and we’d already done three ILS approaches to minimums, including our previous leg into Cincinnati that morning. The weather was down to a ceiling of 100 feet and a reported visibility on Runway 18L of a half-mile with the runway visual range hovering at 2,400 feet. The previous aircraft got in, but the crew reported that it was awfully close. We quickly reviewed the missed approach procedure again and went over the calls.

Since I was the nonflying pilot on this leg, it was up to me to make the altitude calls, and it would be up to me to call for the missed approach if the captain did not see the approach lights or the runway. One thing that experience teaches you is that the color of the clouds changes fairly dramatically when you near the base and will break out. These clouds stayed battleship gray. Further, we were flying into the sun, which did not help. The radar altimeter told us when we were crossing the Ohio River. The river and its deep valley have a stark impact on the local weather, and today was no exception. There was no sign of the runway at 500 feet above the minimum descent altitude. No sign of it either at 200 feet or at 100 feet. At the MDA/decision height, I called missed approach. We never once saw the ground.

Normally, an airline crew going missed is all thumbs because of the lack of practice. This one, though, was right out of the book. We had to deal with a flurry of activity in the ensuing minutes as we planned our next course of action. The flight behind us also went missed, and the controllers immediately turned the airport around, which opened the Category II approach to Runway 36R; the RVR was down to 1,600 feet, and the ceiling was holding steady. Our immediate concern was whether or not we had the fuel to do another approach and still safely divert. A call to our dispatcher confirmed that several airports within close range had VFR weather. His fuel computations matched ours, so we decided to try it again.

They only procedure that is on par for me as an unseen runway is a Category II ILS. This was only my fourth one, and it was my first from the right seat (we fly the CAT II as a captain’s-only maneuver). Being two miles from the top of the river valley made all the difference. We got the lights in sight just before reaching our DH, and in the blink of an eye, we had the runway in sight.

There is a lot of satisfaction in executing a difficult maneuver correctly, and a go-around can certainly qualify, especially with tail-mounted engines and wings with no slats. But the training and practice pay off, and that is critical when you are starting the maneuver so close to the ground that you might actually bounce. I haven’t done that yet, so I can’t help but wonder what it’s like…—Chip Wright