Glideslope Intercept

[drjcustis]

I was flying with a friend yesterday. I was the safety pilot, he’s doing hid IFR training. An interesting discussion occurred and I’m curious about others thoughts. I did a search for a related topic but found none.

We were flying an ILS approach. At the IAF, he descended to the published altitude (which was the assigned altitude) and we were cleared for the approach. He maintained that altitude through the IF until he captured the glideslope and then began his descent, through the FAF to minimums and landed. He crossed the FAF at the published altitude. This was based upon instruction from his CFII.

I was taught to cross the FAF and descend to the published altitude at the IF and then descend to the published altitude at the FAF, capture the glideslope and descend.

Procedure 1 (capture high) vs. Procedure 2 (capture at published).

Which do you use? Why one versus the other? Any thoughts on benefits or detriments of either way?


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[drjcustis]

And, before someone raising the issue, we discussed the possibilities of a false glideslope being high. With Procedure 1, you cross the FAF at the published altitude which is a cross check to make sure you are on the true glideslope.


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[dmspilot]

What approach was it, so we can look at the chart.

 

[midlifeflyer]

The reason for descending to the published intercept on an ILS is to avoid the potential for a false glideslope. You can do the math and figure out the chances if that happening on the typical ILS are fairly remote. It's not even an issue with an LPV.

So, it's largely a matter of technique. Those who favor it point to its stability. Descent to intercept altitude requires multiple configuration changes. Level to descent to level to descent. Remaining at the earlier higher altitude only requires one.

 

[MauleSkinner]

Most GS intercept altitudes are “at or above” altitudes, so it comes down to operational convenience.

It becomes more than “trivia” when it becomes operationally convenient to stay out of Icing or other hazards that may exist at, but not above, the published GS intercept altitude.

 

[midlifeflyer]

And, before someone raising the issue, we discussed the possibilities of a false glideslope being high. With Procedure 1, you cross the FAF at the published altitude which is a cross check to make sure you are on the true glideslope.


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You can do the cross check using either procedure.

 

[Larry in TN]

The altitudes on the approach are usually minimum altitudes. If they are mandatory, or separate maximum, altitudes, it will be so marked.

As long as the approach, and your clearance, allows you to stay at your current altitude until G/S intercept, doing it that ways is simpler as it eliminates a separate descent and level-off prior to G/S capture. All else being equal, that is what I would do.

Sometimes the weather might make it desirable to descent earlier to get under the bases, out of turbulence, etc. The minimum altitudes are available in those situations.

As long as you're starting out below the G/S, you aren't going to run into a higher false-G/S before you intercept the correct one.

 

[drjcustis]

What approach was it, so we can look at the chart.

We were doing the ILS 27 approach into KJFX.

(I don’t know how to post the plate. Sorry)


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[benyflyguy]

I like the opportunity to intercept the gs asap to allow for a more stabilized less busy approach. That can happen pretty far away on an LPV. As long as I’m not busting what I was cleared for and it makes sense. When the GS comes in it usually is higher then you are anyway.

 

[midlifeflyer]

We were doing the ILS 27 approach into KJFX.

(I don’t know how to post the plate. Sorry)


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https://skyvector.com/files/tpp/1905/pdf/06803ILD27.PDF

 

[dmspilot]

We were doing the ILS 27 approach into KJFX.

(I don’t know how to post the plate. Sorry)

As others have said it is more of a personal preference. Doing the full approach I would descend to 2200 and intercept the glideslope at the lightning bolt. Getting vectors I would remain at the last assigned altitude and intercept the glideslope (unless I was above it).

 

[benyflyguy]

We were doing the ILS 27 approach into KJFX.

(I don’t know how to post the plate. Sorry)


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Well you have 8.7 miles to lose 600 ft. In this case probably would just drop to 2200 and wait for it.

 

[midlifeflyer]

Well you have 8.7 miles to lose 600 ft. In this case probably would just drop to 2200 and wait for it.

I know there are folks here who can do the math quickly.

To those, please help to illustrate where in this approach one would intercept the GS if maintaining 2800.

 

[MauleSkinner]

This came up with me as well during an IPC several years. The CFII asked why I was making things so complicated by descending and intercepting from below. Well, that is what I thought you are supposed to do, intercept below as published is all I could say. He just laughed, said it's not wrong, but overly complicates the approach during a busy time.

I still intercept from below BTW.

I’m guessing you misunderstood your instructor...you should almost never intercept the glideslope from above. I assume he was just saying it’s easier to intercept the glideslope from below farther out in order to stabilize early.

 

[MauleSkinner]

I know there are folks here who can do the math quickly.

To those, please help to illustrate where in this approach one would intercept the GS if maintaining 2800.

2 miles further out than you would at 2200.

 

[luvflyin]

I know there are folks here who can do the math quickly.

To those, please help to illustrate where in this approach one would intercept the GS if maintaining 2800.

About 2 miles outside of MAYLE. A 3 degree glide slope is 319.xxxx feet per mile, I don’t remember the precise number. The FAF, MAYLE, is a DME fix or intersection on this Approach so the ‘lightning bolt’ is right at MAYLE, unlike some that are not right at the FAF because the FAF may be an OM or NDB that is on the ground where they could find a place to put the antenna. So you can do the math from MAYLE. 2800 minus 2200 is 600. At 319 feet per mile it’s pretty darn close to 2 miles outside MAYLE

 

[luvflyin]

This came up with me as well during an IPC several years. The CFII asked why I was making things so complicated by descending and intercepting from below. Well, that is what I thought you are supposed to do, intercept below as published is all I could say. He just laughed, said it's not wrong, but overly complicates the approach during a busy time.

I still intercept from below BTW.

Are you literally talking about intercepting from below as opposed to intercepting from above? In most situations you can intercept from below without ‘diving and driving’. You just intercept, from below, farther out.

 

[BrianNC]

I’m guessing you misunderstood your instructor...you should almost never intercept the glideslope from above. I assume he was just saying it’s easier to intercept the glideslope from below farther out in order to stabilize early.

Are you literally talking about intercepting from below as opposed to intercepting from above? In most situations you can intercept from below without ‘diving and driving’. You just intercept, from below, farther out.

Yeah, this sounds more like a case of intercepting from below higher, not actually intercepting from above. In this case I was taught like his friend, just stay and 2800 and intercept it from below higher.

 

[mondtster]

As has already been pointed out, either technique is acceptable. I generally just stay at my last assigned altitude until I intercept the glideslope unless there is some reason not to.

Based on my observations when providing instruction to already rated instrument pilots, almost everyone descends to the published altitude. When I ask why the answer is almost always “because I was taught that way” or “I thought I had to.” Nothing wrong with doing that, but I like to know the thought process behind why we’re doing it. I think much of the confusion on what to do stems from the fact that many of the ILS approach plates double as a localizer approach plate and some of the information on the plate is there so you can fly a localizer, not an ILS.

 

[midlifeflyer]

@MauleSkinner and @luvflyin, thanks. I know it's simple but I am severely math challenged without at least pencil and paper.

 

[midlifeflyer]

As has already been pointed out, either technique is acceptable. I generally just stay at my last assigned altitude until I intercept the glideslope unless there is some reason not to.

Based on my observations when providing instruction to already rated instrument pilots, almost everyone descends to the published altitude. When I ask why the answer is almost always “because I was taught that way” or “I thought I had to.” Nothing wrong with doing that, but I like to know the thought process behind why we’re doing it. I think much of the confusion on what to do stems from the fact that many of the ILS approach plates double as a localizer approach plate and some of the information on the plate is there so you can fly a localizer, not an ILS.

I think there are two pieces. One is the error of thinking intercepting higher means intercepting from above.

The other is indeed "because I was taught to" and the habit that training produces (primacy and exercise working together). I know better but unless I actively think about it, I still drop down, even on an LPV where false glideslope doesn't even exist.

 

[BrianNC]

I think there are two pieces. One is the error of thinking intercepting higher means intercepting from above.

Yeah, I think some hear 'intercept higher' and think 'intercept from above', which is not being said.

 

[mondtster]

I think there are two pieces. One is the error of thinking intercepting higher means intercepting from above.

Perhaps. I’ve never heard that reasoning from any student so it never really crossed my mind. If that is some folks belief, how is the glideslope needle being pegged at the top of the indicator prior to intercept rationalized?

 

[midlifeflyer]

Yeah, I think some hear 'intercept higher' and think 'intercept from above', which is not being said.

I wonder if there are any ILS approaches where, if one maintains the previous minimum altitude, one would intercept from above. Looking at one near me to illustrate, here's the RDU ILS 5L (https://aeronav.faa.gov/d-tpp/1905/00516IL5L.PDF).

Stay at the 3000' SCHOO crossing altitude instead of descending 900' to the 2100 WEPAS altitude and you still intercept the glideslope from below.

 

[midlifeflyer]

Perhaps. I’ve never heard that reasoning from any student so it never really crossed my mind. If that is some folks belief, how is the glideslope needle being pegged at the top of the indicator prior to intercept rationalized?

I've heard it. It's not rationalized. It was purely rote learning. "Go down to avoid intercepting from above." Don't assume understanding why and how.

 

[RussR]

I wonder if there are any ILS approaches where, if one maintains the previous minimum altitude, one would intercept from above.

Should not happen as long as by "previous altitude" we are talking about the altitude at the IF. Maximum descent gradient for the intermediate segment is 318 ft per nm, which is almost exactly 3 degrees. This is intentional, not a coincidence. It's possible (and not uncommon) that the altitude published at the IF is right on glideslope.

Fixes prior to the IF do not have that same protection.

 

[luvflyin]

I wonder if there are any ILS approaches where, if one maintains the previous minimum altitude, one would intercept from above. Looking at one near me to illustrate, here's the RDU ILS 5L (https://aeronav.faa.gov/d-tpp/1905/00516IL5L.PDF).

Stay at the 3000' SCHOO crossing altitude instead of descending 900' to the 2100 WEPAS altitude and you still intercept the glideslope from below.

Yeah. Glide slope should be around 4400 or so at SCHOO. There are situations where you must intercept from above though. You have to comply with the minimum altitudes on the charts for fixes outside of the FAF. You aren’t supposed to follow the glideslope through them if it takes you lower than the minimum altitude at that fix. This can happen on very hot days. Especially when there is a temperature inversion and it’s extra very hot up high.

 

[mondtster]

Yeah. Glide slope should be around 4400 or so at SCHOO. There are situations where you must intercept from above though. You have to comply with the minimum altitudes on the charts for fixes outside of the FAF. You aren’t supposed to follow the glideslope through them if it takes you lower than the minimum altitude at that fix. This can happen on very hot days. Especially when there is a temperature inversion and it’s extra very hot up high.

Can you provide an example approach? I'd like to have a look at it.

 

[luvflyin]

Can you provide an example approach? I'd like to have a look at it.

ILS or LOC RWY 24L at KLAX. You must comply with the minimum altitude at all those fixes up to BOUBY. On a very hot day you will be above the Glideslope doing that. After BOUBY you can get back down to the Glideslope. You will cross SUTIE below 2200 but that’s ok.

 

[RussR]

ILS or LOC RWY 24L at KLAX. You must comply with the minimum altitude at all those fixes up to BOUBY. On a very hot day you will be above the Glideslope doing that. After BOUBY you can get back down to the Glideslope. You will cross SUTIE below 2200 but that’s ok.

I don’t believe that’s correct any longer. If you look at the current chart, all the fixes prior to BOUBY appear to be a good amount below glide path. There is a level segment prior to BOUBY that forces them all to be underneath the glide path.

I remember there was a big problem with that some years ago, and this procedure may have been redesigned to solve the issue - by bringing all the published altitudes down so that aircraft would be less likely to bust them on hot days.

 

[apr911]

I feel like I've never actually encountered this issue. I cant think of a time where I've been cleared for a precision approach and not been told to maintain a heading and altitude until established. I view "until established" to apply to both altitude and heading so you "establish" on the localizer and modify your course to maintain the localizer until you "establish" on the glideslope and can begin to modify your altitude/rate of descent to maintain glideslope.

Although I'm a dive and drive type person for non-precision approaches, I feel like I would not change my procedure if ATC were just to clear me for the approach without instructions to maintain altitude until established either with 2 exceptions:

1) I was hard IMC at 2800ft and certain I'd be out of the clouds and VMC at 2200ft (I see little value to descending non-precision from VMC on-top to VMC below as I'd rather descend through that cloud layer on the precision glideslope and I see no value in descending non-precision from IMC to IMC or worse VMC on-top to IMC).
2) I was experiencing icing at 2800 ft and felt getting down to 2200ft quicker would help alleviate the issue though this would be real touchy as icing conditions could be the same or worse below me and I might want to keep the 2800ft in order provide additional altitude to help get myself out of trouble.

The altitudes published are minimums and while the ACS provides +/- 100 ft, most DPE's, instructors and pilots I know consider a deviation below an altitude to be more of a bust than being too high and as such typically treat the altitudes standard +100 and -0.

Regardless I will be at the designated altitude of 2200 ft by MAYLE whether I dive for it after passing SIPSY or wait for it until closer to MAYLE. By intercepting glideslope outside of MAYLE, I increase my time/distance to get the plane configured and applying the gear down to go down adage, I also increase the time/distance to stabilize my approach by 2NM.

Now that's not to say I would do this without some caveats... I would have certain expectations as to how far out I am going to intercept glideslope at 2800 feet and how far out I am going to begin my descent to 2200 whether I have it or not. With a 3 degree glide slope, I would anticipate intercept approx 2 miles from MAYLE or 8.1 DME from I-JFX; I'd give a 25% margin or 0.5NM in "error" in either direction so I would accept my descent down to 2200 for MAYLE as early as 8.6NM DME or 2.5NM from MAYLE and I would begin my descent down to 2200 for MAYLE no later than 7.6NM DME or 1.5NM from MAYLE.

While perfectly valid, the dive and drive method to FAF creates more workload as altitude maintenance is more critical since you have "no" margin for error below 2200 ft, you also have to transition from level flight to descent, descend likely picking up some airspeed in the process (which then has to be managed later and also reduces total time between SIPSY and MAYLE) and then transition back to level before slowing and dirtying up the aircraft to then transition back into a descent following the glideslope to the DA for a total of 3 transitions.

You create more margin for error by staying at 2800ft and have just 1 transition from level to descending to manage and you have a single continuous descent to DA, you also are already likely trimmed out with power set for your airspeed and altitude and therefore less likely to lose altitude or take on additional airspeed ahead of the transition and therefore will have an easier time reducing your speed and dirtying up the aircraft as well as setting up the trim and power for your final approach speed/altitude.

 

[apr911]

@MauleSkinner and @luvflyin, thanks. I know it's simple but I am severely math challenged without at least pencil and paper.

60-to-1 rule.

The distance "off-course" for every 1 degree is equal to 1 unit of measure for every 60 units of measure you are from the reference point.

In the case of an ILS, its easier to talk in terms of feet.

For every 6000ft you are from the ILS, you have 100ft (6000/60 = 100) in altitude for every 1 degree. So for 3 degree glideslope you have 300 ft to descend for every 6000ft horizontally. Notice 6000ft horizontally is pretty darn close to 6076ft which is the distance in a nautical mile... Thus the 60-to-1 rule can be simplified to be 100ft per 1 NM per degree of glide slipe.

Since everything is "over 1" we can quickly find the Glideslope intercept at any point on the ILS by recognizing the "over 1" is just to cancel out the unit of measure...

So if we wanted to know what altitude we should be at to be on glideslope at 2.5 DME, we'd have:
100ft * 2.5 NM * 3 degrees glideslope = approximately 750 ft.

There is some error in this equation because a nautical mile is not exactly 6000ft but its close enough to be a good rule of thumb.

Its the same equation that many use for Top-of-Descent planning. 3 times the altitude to lose in thousands of feet equals how far out in NM you should begin your descent.

 

[Dean V]

I never fly below the published altitudes. Maintain 50-75 ft high and descend upon intercept.

 

[luvflyin]

I don’t believe that’s correct any longer. If you look at the current chart, all the fixes prior to BOUBY appear to be a good amount below glide path. There is a level segment prior to BOUBY that forces them all to be underneath the glide path.

I remember there was a big problem with that some years ago, and this procedure may have been redesigned to solve the issue - by bringing all the published altitudes down so that aircraft would be less likely to bust them on hot days.

That would make sense. The notes are still in the AIM warning pilots not to bust the minimum altitudes just because they are on Glideslope. But yeah, it probably doesn’t come up as much anymore as Approaches are redesigned to deal with the problem. FWIW the AIM reference is 5-4-5 b. I know you know this, just getting it out there for anyone who cares. FWIW a little history on the subject. Los Angeles is a place where this became a problem. Pilots following the Glideslope down would lose vertical separation from traffic underneath them that Approach Control had there, 1000 feet underneath them, based on the altitude that the airplane above should be at based on the charted altitude at that fix. But on very hot days they would be below that actual altitude following the Glideslope which is fixed and unaffected by temperature. The airplane underneath was at their assigned altitude, by their altimeter, which is affected by temperature. Now that had been going on for years. The ‘losses’ of approved vertical separation were not all that significant and attracted no attention. Enter ‘squeal a deal,’ which is a computer program that detects losses of approved separation. ‘Deal’ is what controllers call an Operational Error, the official term for loss of separation. Squeal a Deal had been in Centers for years. Then it got put in the Approach Controls. Then these not all that really significant losses of approved separation were being ‘officially’ detected. Most of them were classified as ‘pilot deviations’ not ‘operational errors,’ because the pilot had busted the charted altitude by following the Glideslope.

 

[luvflyin]

ILS or LOC RWY 24L at KLAX. You must comply with the minimum altitude at all those fixes up to BOUBY. On a very hot day you will be above the Glideslope doing that. After BOUBY you can get back down to the Glideslope. You will cross SUTIE below 2200 but that’s ok.

Replying to my own post so I can attract @mondtster ’s attention. See @RussR ’s post #32. More stuff on the subject in my post #36 just above. Hope it’s useful to you.

 

[Larry in TN]

I cant think of a time where I've been cleared for a precision approach and not been told to maintain a heading and altitude until established. I view "until established" to apply to both altitude and heading so you "establish" on the localizer and modify your course to maintain the localizer until you "establish" on the glideslope and can begin to modify your altitude/rate of descent to maintain glideslope.

Established means established on a segment of an approach. Each approach segment has a published minimum altitude (and sometimes mandatory or maximum altitudes). Once you are established on a segment of the approach, you can descend within the restrictions of the published approach altitudes.

 

[John Collins]

I wonder if there are any ILS approaches where, if one maintains the previous minimum altitude, one would intercept from above. Looking at one near me to illustrate, here's the RDU ILS 5L (https://aeronav.faa.gov/d-tpp/1905/00516IL5L.PDF).

Stay at the 3000' SCHOO crossing altitude instead of descending 900' to the 2100 WEPAS altitude and you still intercept the glideslope from below.

I think it would be hard to do between the IF and FAF. The TERPS recommends that this segment be optimally 150 ft/NM and no more than 318 feet/NM. 318 feet/NM is the gradient for a 3 degree GS assuming the earth is flat. The GS is a straight line in space, but appears to curve upwards from the point of view of an observer on the earth surface, because of the curvature of the earth. In the case of the ILS or LOC RWT 5L at KRDU, the GS crosses SCHOO at an MSL altitude on a standard day at 4526 ft. If the earth were flat, the crossing altitude of the GS would be 4390 feet. So crossing SCHOO at 3000 feet is 1526 feet below the GS and no chance of getting a false glideslope.

 

[Ryan F.]

I don't believe I saw this posted. Here's an InFO from 2011 on this topic.

https://www.faa.gov/other_visit/avi...afety/info/all_infos/media/2011/InFO11009.pdf

 

[IK04]

I don't believe I saw this posted. Here's an InFO from 2011 on this topic.

https://www.faa.gov/other_visit/avi...afety/info/all_infos/media/2011/InFO11009.pdf

That document refers to large aircraft intercepting the glideslope OUTSIDE the final approach segment. That would be dumb, for the reasons stated and not really the OPs question...

 

[IK04]

Oh yeah, to answer the OPs question: I always hold altitude and intercept ASAP. Why make unnecessary altitude changes?