# ILS - when to descend?

Ronald Stevens and I did a bunch of practice ILS approaches into Tamiami today - by the end he was doing a pretty good job and should be ready to dazzle the examiner in short order.

One question came up in my mind. Imagine this scenario (a .pdf of the approach is attached):

You’re a few miles northwest of QEEZY at 1,700’. ATC says “N415GF, you’re 3 from QEEZY - turn left heading 120, cleared for the ILS RWY 9R approach. Maintain 1,700’ until established on the localizer, contact tower 118.9.”

Of course, we maintained 1,700’ until established on the localizer. So far so good.

My question is:

When the localizer needle centered would you immediately descend to 1,300’? My inclination was to remain at 1,700’ until the g/s needle centered, then begin descent, following the g/s and crossing QEEZY at 1,230 as depicted. My logic is that this method gives even more time to get squared away on the approach.

Since the 1,300’ crossing altitude at QEEZY is a MINIMUM, I’d think you could legally do it either way. Just wondering how other CFII’s are teaching it and/or how other pilots are flying it.

BTW, I just see I goofed and put this in the Public forum. No harm done, I guess if the “Public” sees it!
1-93017-05349I9R.pdf (209 KB)

When the localizer needle centered would you immediately descend to 1,300’? My inclination was to remain at 1,700’ until the g/s needle centered, then begin descent, following the g/s and crossing QEEZY at 1,230 as depicted. My logic is that this method gives even more time to get squared away on the approach.

Ed,

The Glide Slope Intercept Altitude (GSIA) is a mandatory altitude. You cannot intercept the GS anywhere else. So the proper course of action is to maintain the last assigned altitude until you are established on the localizer (whatever that means) and then you begin a descent down to 1,300 without using the GS as a primary means to descend, thus intercepting the GS at 1,300.

This is true if you are required to do the PT or are VTF.

“N415GF, you’re 3 from QEEZY - turn left heading 120, cleared for the ILS RWY 9R approach. Maintain 1,700’ until established on the localizer, contact tower 118.9.” Of course, we maintained 1,700’ until established on the localizer. So far so good.

My question is:

When the localizer needle centered would you immediately descend to 1,300’?

Ed: To approach this slightly differently, I believe the test standard for an IAP is 100’ above and 0’ below. Therefore, I would darn sure be between 1,300’ and 1,400’ at QEEZY. To do this you may have to begin descent prior to the needle centering.
I have also been instructed that you are clear to descend once “10 - 10 & Clear.” Meaning within 10 miles, 10 degrees and cleared for the approach. Furthermore, I would define “established” when used in, “maintain x,xxx’ until established…,” to mean that as long as you were within 10 miles and had a good indication on the CDI.

Long and short, in your case, I would start down as indicated by the GS as soon as cleared.

To extend the question, when “cleared for the visual,” can you begin descent at pilot’s discretion immediately? I believe the answer is yes.

<<When the localizer needle centered would you immediately descend to 1,300’?>>

Would I? probably not. Am I legally allowed to, yes.

<<My inclination was to remain at 1,700’ until the g/s needle centered, then begin descent, following the g/s and crossing QEEZY at 1,230 as depicted. >>

That’s how I normally do it, also.

Depends, though. Sometimes, on a nice day, with a rookie controller, I will dive down as fast as I can to see how they handle their altitude alert.

Jerry

Ok, lets change this to a non-precision approach.

What are the official rules for the RATE of decent when going from one step down fix to another?

Jerry

Scott,

Your response got me looking further for sources. I first looked at the TERPS legend, that seems to support you, though it’s not all that clear. What’s confusing is that an underline normally indicates a minimum altitude.

I then tried the old standyby - GOOGLE. Via a Microsoft Flight Simulator page here I read the following from Air Force Manual 11-217:

“14.1.2.3.2. Descent. When on the localizer course, maintain glide slope intercept altitude (published or assigned) until intercepting the glide slope. Published glide slope intercept altitudes may be minimum, maximum, mandatory, or recommended altitudes and are identified by a lightning bolt . When the glide slope intercept altitude is a recommended altitude, you must only comply with other IAP altitudes (FAF altitude for example) until established on the glide slope.”

That seems to imply that the Glide Slope Intercept Altitude is not always mandatory, but can be a minimum or maximum.

Doesn’t it seem like the underline in the profile view implies “minimum”?

I’m not being argumentative - I just like to be totally confident in what I’m teaching and why.

Your response got me looking further for sources. I first looked at the TERPS legend, that seems to support you, though it’s not all that clear. What’s confusing is that an underline normally indicates a minimum altitude.

Ed,
This is always unclear to most. The only altitude from which you should join the GS is the published GSIA. The approach plate references to this as the minimum GSIA are indeed correct; you donÂ’t want to be joining the GS below this altitude. However, this does not imply that it is acceptable to join the GS at an altitude above the published GSIA either. I really do believe that all GSIAs should be depicted as a mandatory altitude.
The published GSIA is the altitude at which you must join the GS to fly the final approach segment of the precision approach. When descending to fixes outside the published precision FAF, they are intended to be flown using the dive and drive, or non-precision method, not following or coupling to the GS as primary guidance.

From time to time you may see this altitude shown as a mandatory altitude (lines on top and bottom of the altitude listed). These mandatory altitudes in the profile section of the ILSs are to assure traffic separation for simultaneous approaches – usually.

There are cases where ATC can permit you to intercept the GS at an altitude lower than the published GSIA. See below for the ILS 10 at BWI.

When the temperature deviates greatly from standard temperatures, the actual altitude at which the GS crosses a given point will also vary. The GS is 1.4 degrees thick from full-scale fly up to full-scale fly down. The slightest deviation from the exact centerline of the GS results in a deviation in altitude. These variances are more greatly magnified the farther you are from the GS antenna.

There have been cases in some locations where pilots have intercepted the GS well outside the GSIA and have failed to comply with the minimum altitude at an intermediate fix on the approach.

To me this still doesn’t answer the question. As long as you meet other altitude requirements, if you intercept higher then in a sense that is no different than flying above the minimum descent altitude and then descending to the intercept altitude so as to intercept the glideslope at the exact place. It just merges the descent from the “maintain until established” altitude into the glideslope intercept. You just happened to use the glideslope as a descent aid until that point. Using the chart you showed, what is wrong with staying at 3,000 until approaching the glideslope and then descending to intercept at 2500? I read the 2500 as a minimum and not as mandatory. Oh yes, I did happen to use the glideslope as an aid to get to 2500 at the proper point. I can’t find a reference that says this is wrong.

I agree…seems to me that using the glideslope as an aid to descend to the intercept altitude and fix point, whereupon you confirm the altitude and fix, is harmless and couldn’t violate any rules. There is a risk, however, if you have interecepted something other than the true glidslope prior to the intercept point. I don’t recall the technical term, but I’m referring to the “ghostslopes” or “shadows” of the true glideslope that can appear above or below the true slope. I could see one intercepting one of these, convincing oneself that he had the glideslope, and failing to confirm the glideslope at the right altitude and proper fix. How one chooses to avoid that trap is up to the individual. Certainly, making a point NOT to pay any attention to the glideslope prior to the fix might work fine for some. Others might find it easier to make all the proper confirmations at the fix, if they have already done most of the work of getting established on the slope prior to hitting the fix.

I can’t find a reference that says this is wrong.

Paul,
It is a personal preference on your part. I wouldn’t use a screwdriver to pound a nail into a piece of wood. Would it work, probably? It may not work well and it may not work all of the time. The GS was not intended to be used outside the GSIA, period.

Just as long as you are aware that the GS has NEVER been officially flight checked outside of the GSIA and can be VERY erratic or non-existent. Just as long as you are aware of any stepdown fixes that may have to be observed. Just as long as you are aware of the potential for a false GS.

IMHO, it isn’t worth the risk. I prefer to dive down to the GSIA and couple or fly the GS from here. That’s the way the GS was meant to be used and I know this will work every time. I don’t ignore the GS. I don’t use it for primary guidance until the GSIA. Additionally, I would never couple the autopilot above the GSIA. Autopilots are not designed to handle erratic behaviour of the GS that may exist. There are no legal issues to contend with as long as you obey stepdown fixes and do not descend early below the GSIA (unless authorized to do so by ATC).

These are my preferences based on my understanding and experiences of the systems involved. I believe that this is the safest and the least complicated way to fly a GS.

re: “false glideslopes”

That had occured to me as one reason to only intercept the g/s from a given altitude.

I think if you were to accidentally intercept one of the “higher” glideslopes it would be obvious by the much higher rate-of-descent required to maintain it. Certainly if the altitude at the OM was noticeably off, that would be a good hint something was wrong.

BTW, I hate to be one of those people that asks a question, then has someone nice enought to formulate an answer, and then seems to argue with the answer. Scott is always a patient and DEEP source of knowledge, and I appreciate his time. IOW, when I ask for sources of information, it’s not because I doubt the person, it’s just nice to be able to highlight a source to show to students whenever the question comes up in the future.

I prefer to dive down to the GSIA and couple or fly the GS fromt here. That’s the way the GS was meant to be used and I know this will work every time. I don’t ignore the GS. I don’t use it for primary guidance until the GSIA. Additionally, I would never couple the autopilot above the GSIA. Autopilots are not designed to handle erratic behaviour of the GS that may exist. There are no legal issues to contend with as long as you obey stepdown fixes and do not descend early below the GSIA (unless authorized to do so by ATC).

hmmm - that makes a lot of sense. It also makes some of the slam dunks we get here a tricky
prospect. The last two ILSes into POU have been a late turn on from above GSIA and the GS alive
before LOC capture. Both of them defeated the autopilot and in the heat of battle I never can remember
the magic button sequence to tell the 55x to capture now please.

What I have done both times is capture the GS at that point and fly it down, literally by the time
I’m on the localizer and ready to decend to GSIA the GS is already 2 dots high and beckoning
to be used.

I suppose having read this I should change that procedure, ignore the GS, dive for the GSIA and
then pick it up again at the proper altitude. I’ll bear that in mind.

I answered the way I did because you implied in your earlier answer that it was illegal (as opposed to poor technique) to intercept at anything other than GSIA. If weather is a bit bumpy I will sometimes pad the MDA for the leg up to intercept to be sure I don’t go below. I somehow doubt that being 150’ or 200’ high makes me illegal or that it is bad. The key is to intercept from below. That brings up the ATC slam dunk issues where I think it is sometime impossible or at least dangerous to do the dive required to get level and established prior to intercept. I do worry about intercepting from above because of ghosting of the glideslope.

Mabey it’s the word choice, but to me it just doesn’t seem rational to “dive down” while in the clouds relatively close to ground? That said, one benefit that nobody mentioned to going lower is that you may break out of the clouds making the approach safer.

-Todd

To extend the question, when “cleared for the visual,” can you begin descent at pilot’s discretion immediately? I believe the answer is yes.

This depends Marty. Here in Charm City, there is a noise abatement program that requires you to maintain 3000 until 10 DME coming into BWI. So if you were cleared for the visual 12 miles out, probably wouldn’t be a good idea to drop below 3000 ft until 10 DME.

Furthermore, I would define “established” when used in, “maintain x,xxx’ until established…,” to mean that as long as you were within 10 miles and had a good indication on the CDI.

The pilot/controller glossary does have a pretty good definition of what it means to be established. One of the things you left out here is continued positive course guidance (PCG). The underlying principle implicit in the TERPs in assuring vertical and lateral obstacle clearance is that PCG will be used with very few exceptions (that are spelled out in the procedure). Exceptions include limited dead reckoning initial approach segment, course-reversals and missed approach procedures that specify a heading rather than course (TERPs paragraphs 211 and 273).
In this example, I can be flying “across” the localizer at 90 degrees and have a good indication on the needle, but not be established since I don’t have any PCG. So I think there are two basic requirements.
The pilot knows with certainty that he/she has:

1. Passed the fix or point that marks the beginning of the segment entered, and
2. The PCG indication is on course and continued tracking of on course is assured.
I would say that on course means that you are maintaining the course within the instrument pilot practical test standards.

I think if you were to accidentally intercept one of the “higher” glideslopes it would be obvious by the much higher rate-of-descent required to maintain it. Certainly if the altitude at the OM was noticeably off, that would be a good hint something was wrong.

Ed,

False glide paths are a real issue. They occur at multiples of the commissioned angle and are alternately proper and reverse sensing. Therefore, the first proper sensing false path youÂ’d encounter for a 3 degree GS is at 9 degrees! Even if you tried to intercept it, it would pass from the top to the bottom of the GS indicator face so fast that youÂ’d have to struggle to join it. The 6 and 12 degree slopes are reverse sensing, so letÂ’s accept that none of us are going to be able to track those.

Scott is always a patient and DEEP source of knowledge, and I appreciate his time. IOW, when I ask for sources of information, it’s not because I doubt the person, it’s just nice to be able to highlight a source to show to students whenever the question comes up in the future.

Thanks Ed. I appreciate the kind words. You are doing the proper thing by asking for reference or clarification. I may not be able to always put my finger on it, but I usually try to. Always glad to help a fellow flight instructor!
Now, this issue of hitting the GS at a higher altitude and tracking it inbound is all a matter of preference. As long as these consequences are understood and monitored appropriately and you understand the potential problems, then it becomes a matter of choice. You are legal to descend to the GSIA once you are cleared and established on the localizer. You don’t have to descend, but in my opinion, descending is the safest and best option.

The concept of a “stabilized approach” is fairly well accepted as one of the safer ways to approach an airport; particularly in IFR conditions. As a result ILS approaches are considered safer than non-precision approaches.
If I am vectored onto a localizer 10 miles out from the airport, get “established” at 3000 feet and the GS intercept altitude is 2500 feet. At 10 miles out I will be below the glideslope. Keeping the stabilized approach concept in mind, would it not be better and safer to stay at 3000 and let you fly into the glideslope and then descend as opposed to descending lower to 2500 when already below the glideslope only to intercept closer in to the runway?
The latter method means 2 different descents and leaves you much lower to the ground at a point further from the airport.

Seems logical iof you are already below the glideslope when to get on the localizer, why go lower? You are already too low for the planned descent angle.

Brian,
I agree with you completely. Answering Ed’s initial question I would maintain the 1700 feet until I flew into the glideslope that (I’m assuming) was initially above me and then descend on the GS. ATC is always supposed to vector you so that when you intercept the localizer the GS is above you although I recognize that sometimes doesn’t happen in practice.
There are two reasons for maintaining 1700 until intercept. First, it makes capture easy for the AP (or manually if you’re hand flying) and allows for a smooth constant descent to the DH. Second it allows you to maintain altitude as long as possible. As long as the glideslope is already above you why go lower? If there is an engine problem or any other issue for that matter altitude is your friend. Don’t give it up unless you have to! The probability of intercepting a false glideslope 470 feet above where you are going to cross the marker when you’re still several miles from the marker is essentially zero.
For an interesting example of how the big boys do it look at the ILS 22R at ORD. There are several fixes along the localizer. GS crossing altitudes are given for FNUCH and NOLAN as well as the outer marker. If you listen to ATC you will often hear aircraft cleared for the approach when they are still NE of FNUCH. The clearance reads “5 from FNUCH, maintain 7000 until FNUCH, cleared ILS 22R approach. Speed 170 to RIDGE, Tower at RIDGE 126.9”.
Does anyone think that the typical 777 driver dives from 7000 at FNUCH to 5000 at NOLAN and then dives again to 2200 to intercept the GS just outside RIDGE? Of course not. You intercept the GS at 7000 at FNUCH and fly it down.
In my opinion Ed’s choice to maintain 1700 until GS intercept was correct, prudent and well within the spirit of the regulations. It was safe too and that’s really all that matters.

Jerry:
Could not agree more and I can assure you that ORD is not the only airport that handles arrivals in that manner. Almost every jet STAR has the notion of being vectored for a localizer far out from where the glidslope intercept point occus. You will not see airliners descend to that altitude and then flyinto the glideslope at a rediculously low altitude.
So why should we fly any different?