ILS GS below charted altitudes?

[username]

I am not personally aware of any PD's being issued. However, the subject is actively being reviewed by the Aeronautical Charting Forum - Instrument Procedures Group. You can find the current status of Issue 09-01-283 at: http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/afs420/acfipg/open/

Good call, John. The direct link to 09-01-283 is http://www.faa.gov/about/office_org...0/afs420/acfipg/open/media/Hist 09-01-283.pdf

There is a related discussion at http://www.faa.gov/about/office_org...0/afs420/acfipg/open/media/Hist 09-01-282.pdf

Both reference the LAX ILS 25L, which has become notorious.

FWIW, I know of a crew that received letters after ignoring the stepdown fixes on the LAX ILS 25L, but I don't know either of them personally nor do I know the final disposition of the investigation.

Thanks,
buzz

 

[John Collins]

Yes, that is essentially what I said in post #38 that you quoted in bold.

As of this time, however, it is an unresolved issue. It seems to me that at a minimum, if it were clarified to be the consensus view, that the AIM and 7110.65 should both be updated to reflect the view.

I have not seen any instructions in 7110.65 that provide wording to support an ATC communication to authorize following the GS as indicated on the chart, have you?

 

[Henning]

If the question is a simple one of "If someone intercepts the GS on the localizer outside the FAF can they hit something?" I would say it's highly unlikely since about the only thing that could get you is a mountain, and a GS won't broadcast through a mountain.

 

[gismo]

If the question is a simple one of "If someone intercepts the GS on the localizer outside the FAF can they hit something?" I would say it's highly unlikely since about the only thing that could get you is a mountain, and a GS won't broadcast through a mountain.

A GS signal will follow the curvature of the earth somewhat and it will also easily pass through trees (especially without leaves) so I wouldn't count on signal reception as an infallible indication of safe passage.

 

[gismo]

Yes, that is essentially what I said in post #38 that you quoted in bold.

As of this time, however, it is an unresolved issue. It seems to me that at a minimum, if it were clarified to be the consensus view, that the AIM and 7110.65 should both be updated to reflect the view.

I have not seen any instructions in 7110.65 that provide wording to support an ATC communication to authorize following the GS as indicated on the chart, have you?

Given the noted optional GS intercept altitudes on the SEA chart, I'd hope there was a way to obtain such a clearance.

 

[username]

Note: In order to placate the occasional persnickety instructor and examiner that you may encounter, keep in mind the fact that an ILS approach has no final approach fix (FAF). The final approach SEGMENT (FAS) of an ILS begins at the point at which the glideslope intersects the charted minimum glide slope intercept altitude (GSIA).

I was incorrect: The point at which the glideslope intersects the GSIA is now referred to as the Precision Final Approach Fix. Mea culpa.

buzz

 

[username]

If the question is a simple one of "If someone intercepts the GS on the localizer outside the FAF can they hit something?" I would say it's highly unlikely since about the only thing that could get you is a mountain, and a GS won't broadcast through a mountain.

Highly unlikely, yes, unless the definition of "something" includes other aircraft.

buzz

 

[John Collins]

Given the noted optional GS intercept altitudes on the SEA chart, I'd hope there was a way to obtain such a clearance.

I would hope so as well, but what would it be?

 

[Henning]

A GS signal will follow the curvature of the earth somewhat and it will also easily pass through trees (especially without leaves) so I wouldn't count on signal reception as an infallible indication of safe passage.

I guess but it seems like a real outside chance that if you are on a glideslope & localizer that you're going to hit something. Be interesting to find out where/if the condition exists....

 

[pstan]

Wow, quite the replies. As the Original Poster, I'm sorry I was away the last 3 days, and am only now able to sift through this great topic - great because of the replies from you all!!

I might point out the following;

1. I was asking whether the ils GS would ALWAYS keep you ABOVE the charted altituded, not whether flying the GS may make you bust a mandatory altitude which may be below it. Hence the ILS 06 TEB with Dandy mandatory altitude below the GS altitude was not what I was wondering about. I am aware they exist in places.

2. Chucky, if max 500 feet per nm for the intial and intermediate segment is correct, then it seems possible that an ils meeting this criteria is possible. I thought 300 feet to be the maximum, but I bow to your greater knowledge

3. Scott, very interesting email. It says though"failure to meet altitude requirements", perhaps as per TEB ils 06 above. Not necessarily that flying the GS will take you BELOW charted altitudes.

4. Those who sent a link to the red board, thanks, but I'm not a member and couldn't access

And the prize goes to...John Collins (no surprise there I guess)

You sealed it buddy!! That link to the charting forum, Intermediate Fix Altitudes and ILS GS at

http://www.faa.gov/about/office_org/...009-01-283.pdf


explains it all. I even did the math, using 318 feet per nm for a 3 deg GS, assumed a TCH of 50 feet, and used both the dme difference and the distances posted at the bottom of the profile view. The numbers I got while not exactly the FAA numbers in red on the link, were clearly BELOW the charted altitudes.

If anyone hasn't read the link, I'd recommend they do so...shows indeed that an ils GS can be below charted altitudes.

So I'm thinking my question has been answered, but hope the link can continue if those with further inquires so desire.

Thanks to all!! Stan

 

[Palmpilot]

On the red board someone said that the charting forum members calculated the glide slope heights incorrectly on their LAX 25R ILS example, and that the GS actually stays above the minimum stepdown altitudes unless the weather gets too hot.

The link you posted seems to be broken at the moment.

 

[gprellwitz]

The link you posted seems to be broken at the moment.

I think he meant this one: http://www.faa.gov/about/office_org...0/afs420/acfipg/open/media/Hist 09-01-283.pdf

 

[olasek]

If anyone hasn't read the link, I'd recommend they do so...shows indeed that an ils GS can be below charted altitudes.

This is all very interesting but I don't think I will be making an ILS approach at LAX anytime soon or to any airport where step down fixes will have word RADAR prominently assigned to them. If this is the only example they have it is a bit academic from my vantage point. I think I am going to sleep well

 

[John Collins]

1. I was asking whether the ils GS would ALWAYS keep you ABOVE the charted altituded, not whether flying the GS may make you bust a mandatory altitude which may be below it. Hence the ILS 06 TEB with Dandy mandatory altitude below the GS altitude was not what I was wondering about. I am aware they exist in places.

...

4. Those who sent a link to the red board, thanks, but I'm not a member and couldn't access

...

I even did the math, using 318 feet per nm for a 3 deg GS, assumed a TCH of 50 feet, and used both the dme difference and the distances posted at the bottom of the profile view. The numbers I got while not exactly the FAA numbers in red on the link, were clearly BELOW the charted altitudes.

If anyone hasn't read the link, I'd recommend they do so...shows indeed that an ils GS can be below charted altitudes.

Stan,

I snipped a few paragraphs from your last post to comment on.

There is a difference between the calculated altitude of the GS at a fix and what a barometric altimeter will read if you are at the fix on the GS. The altimeter is not compensated for temperature, so on cold days the altimeter will read higher than you actually are and on hot days it will read lower than what you actually are. At the altitudes in question, this can result in the altimeter being hundreds of feet off of the actual altitude.

So, let us assume that the GS is in fact at or higher than the crossing fix minimum altitude. This doesn't change the fact that the altimeter can be off by hundreds of feet. The absolute altitude of the GS at the fix doesn't change, but your indicated altitude does when tracking the GS. On a hot day, you may be indicating 300 feet lower than you are, which means that when you cross the fix on the GS, your altimeter indicates you are below the minimum altitude. Since your encoding altimeter should match your baro altimeter, it will report the lower altitude and the snitch will record the event.

My point is that it isn't just whether the GS is above the step down fix minimum altitude, it has more to do with the barometric altimeter error introduced by temperature. In fact, for the LAX ILS RWY 25L approach, the referenced FAA report's altitudes for the GS are mathematically wrong, as the GS does cross the step down fixes ABOVE all of them, not below. I explained in my post #37 why this is true, the effect of the curvature of the earth is not included in the calculation. The equation I quoted is from the TERPS and is used to determine the true altitude of the GS at a fix on the ILS. You can download a calculator from my post #37 to verify this yourself. The simple 318 feet per mile will be off by a few hundred feet around 15 NM from the threshold, and shouldn't be used to determine the altitude of the GS.

 

[chucky]

2. Chucky, if max 500 feet per nm for the intial and intermediate segment is correct, then it seems possible that an ils meeting this criteria is possible. I thought 300 feet to be the maximum, but I bow to your greater knowledge

Nope, I'm about the least experienced and knowledgeable person here. I just read it out of the TERPs. Like you, I'm learning things in this thread!

 

[olasek]

it has more to do with the barometric altimeter error introduced by temperature.

Correct but nevertheless per my understanding we pilots are not allowed to make temperature corrections to barometric altitude unless the approach plate specifically authorizes it. So, I guess, on those rare very hot days on the approach in question we are supposed to be above the glide slope at some of the intermediate fixes (of course shooting such ILS approach at high temperatures is a bit of a contradiction in itself).

 

[John Collins]

Correct but nevertheless per my understanding we pilots are not allowed to make temperature corrections to barometric altitude unless the approach plate specifically authorizes it. So, I guess, on those rare very hot days on the approach in question we are supposed to be above the glide slope at some of the intermediate fixes (of course shooting such ILS approach at high temperatures is a bit of a contradiction in itself).

During the summer, I don't think the condition would be that rare.

The airlines often follow the procedure, regardless of conditions being IMC or not.

 

[pstan]

the referenced FAA report's altitudes for the GS are mathematically wrong, as the GS does cross the step down fixes ABOVE all of them, not below.

John, I used your xls with my own numbers and found indeed that within 5 feet, the GS does cross the step down fixes above them. I didn't have a value for TCH so I used 50 feet. Plus, I'm not sure how much the distances on the chart are rounded off - they are only one decimal point in distance, nautical miles I'm presuming.

I did understand about high temperatures affecting the baro altitude/GS points. So...it appears that the ils GS is always above step down fixes for isa conditions. I think that clears it all up.

 

[Palmpilot]

I didn't have a value for TCH so I used 50 feet.

TCH is stated in the profile area on FAA charts.

 

[HeavyDriver]

In fact, for the LAX ILS RWY 25L approach, the referenced FAA report's altitudes for the GS are mathematically wrong, as the GS does cross the step down fixes ABOVE all of them, not below. I explained in my post #37 why this is true, the effect of the curvature of the earth is not included in the calculation. The equation I quoted is from the TERPS and is used to determine the true altitude of the GS at a fix on the ILS. You can download a calculator from my post #37 to verify this yourself. The simple 318 feet per mile will be off by a few hundred feet around 15 NM from the threshold, and shouldn't be used to determine the altitude of the GS.

John,

I came across this thread yesterday because I had this same question.

[As a side note, the ILS to 25L at LAX has changed a little, but the changes don't affect the discussion, so I'll use the numbers from the old approach, i.e., the one referenced in the FAA discussion titled 09-01-283]

Regarding the FAA discussion (09-01-283): I computed the glideslope altitudes for both GAATE and HUNDA using the profile view shown. The glideslope altitudes of both fixes are above the minimum altitudes depicted on both approaches, just like you said.

I have two concerns/questions,

1) Do you have a reference for the formula you used? My numbers are slightly different, though higher, still, than the published minimums. The formula's I used are from FAA Order 8260.3B (specifically, Volume 1, Chapter 2, Sections 4 and 5).
Just for reference, I got 5062 for GAATE and 3771 for HUNDA (again, using the old approach referenced in the FAA discussion).


2) How did the FAA folks get their numbers? I tried using tan (angle) = Alt/Dist and I 'still' get both altitudes higher than they did. If they didn't account for the curve of the earth, that'd be on thing (wrong, but at least I'd understand why)... but their numbers are even lower than that. Using flat/euclidean geometry, I show the altitudes at 3620 for HUNDA and 4910 for GAATE; but the FAA somehow got 3570 and 4771.

I am hoping my math is wrong, or that I am missing something. This is why I was hoping you could tell me where you got your formula, and perhaps demonstrate how the FAA got their numbers in this example.

Also, I attempted to attach the FAA discussion and the 8260.3B but no luck.
The discussion is still attached above in post #54.
The 8260.3 in at http://www.faa.gov/documentLibrary/media/Order/8260.3B Chgs 1-24.pdf

Thanks in advance,

HeavyDriver

 

[HeavyDriver]

In fact, for the LAX ILS RWY 25L approach, the referenced FAA report's altitudes for the GS are mathematically wrong, as the GS does cross the step down fixes ABOVE all of them, not below. I explained in my post #37 why this is true, the effect of the curvature of the earth is not included in the calculation. The equation I quoted is from the TERPS and is used to determine the true altitude of the GS at a fix on the ILS. You can download a calculator from my post #37 to verify this yourself. The simple 318 feet per mile will be off by a few hundred feet around 15 NM from the threshold, and shouldn't be used to determine the altitude of the GS.

John,

I came across this thread yesterday because I had this same question.

[As a side note, the ILS to 25L at LAX has changed a little, but the changes don't affect the discussion, so I'll use the numbers from the old approach, i.e., the one referenced in the FAA discussion titled 09-01-283]

Regarding the FAA discussion (09-01-283): I computed the glideslope altitudes for both GAATE and HUNDA using the profile view shown. The glideslope altitudes of both fixes are above the minimum altitudes depicted on both approaches, just like you said.

I have two concerns/questions,

1) Do you have a reference for the formula you used? My numbers are slightly different, though higher, still, than the published minimums. The formula's I used are from FAA Order 8260.3B (specifically, Volume 1, Chapter 2, Sections 4 and 5).
Just for reference, I got 5062 for GAATE and 3771 for HUNDA (again, using the old approach referenced in the FAA discussion).


2) How did the FAA folks get their numbers? I tried using tan (angle) = Alt/Dist and I 'still' get both altitudes higher than they did. If they didn't account for the curve of the earth, that'd be on thing (wrong, but at least I'd understand why)... but their numbers are even lower than that. Using flat/euclidean geometry, I show the altitudes at 3620 for HUNDA and 4910 for GAATE; but the FAA somehow got 3570 and 4771.

I am hoping my math is wrong, or that I am missing something. This is why I was hoping you could tell me where you got your formula, and perhaps demonstrate how the FAA got their numbers in this example.

Also, I attempted to attach the FAA discussion and the 8260.3B but no luck.
The discussion is still attached above in post #54.
The 8260.3 in at http://www.faa.gov/documentLibrary/media/Order/8260.3B Chgs 1-24.pdf

Thanks in advance,

HD

 

[John Collins]

On the 3-8-2012 approach plate cycle, most of the approaches that had the note "WHEN ASSIGNED BY ATC, INTERCEPT GLIDEPATH AT ...", had the note removed. An example is the KCLT ILS 18L. The rest of the approaches that had a similar note but did not get updated, had a NOTAM issued such as the following NOTAMs:


!FDC 2/4169 (KLAX A0199/12) LAX FI/T IAP LOS ANGELES INTL, LOS ANGELES, CA. ILS OR LOC RWY 25L, AMDT 12... DELETE NOTE: LOC PROCEDURE NA DURING SIMULTANEOUS OPERATIONS. DELETE PROFILE NOTE: WHEN ASSIGNED BY ATC, INTERCEPT GLIDEPATH AT 3600 OR 5000.

!FDC 2/4168 (KLAX A0198/12) LAX FI/T IAP LOS ANGELES INTL, LOS ANGELES, CA. ILS OR LOC RWY 24L, AMDT 26... DELETE NOTE: LOC PROCEDURE NA DURING SIMULTANEOUS OPERATIONS. DELETE PROFILE NOTE: WHEN ASSIGNED BY ATC, INTERCEPT GLIDEPATH AT 4000.


!FDC 2/4167 (KLAX A0197/12) LAX FI/T IAP LOS ANGELES INTL, LOS ANGELES, CA. ILS OR LOC RWY 25R, AMDT 17A... DELETE NOTE: LOC PROCEDURE NA DURING SIMULTANEOUS OPERATIONS. DELETE PROFILE NOTE: WHEN ASSIGNED BY ATC, INTERCEPT GLIDEPATH AT 3700 OR 5000.


!FDC 2/4166 (KLAX A0196/12) LAX FI/T IAP LOS ANGELES INTL, LOS ANGELES, CA. ILS OR LOC RWY 24R, AMDT 24... DELETE NOTE: LOC PROCEDURE NA DURING SIMULTANEOUS OPERATIONS. DELETE PROFILE NOTE: WHEN ASSIGNED BY ATC, INTERCEPT GLIDEPATH AT 4000