Airline Pilot Forums

Hi! If I am at the PFAF at a higher altitude (above GSIA at the charted distance, and lets say there is no OM and in IMC) What is the lowest altitude to intercept the GS???

I don't know what your company FOM says, but wouldn't that constitute an unstabilized approach?

whats with all the acronyms

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whats with all the acronyms

***:D

Generally, I don't think it's a legality issue. Unless it's covered in your company's Operations Specifications (wanted to avoid another acronym :D), it's one of the fairly rare approach charts with mandatory or maximum altitudes, or ATC gives you a maximum altitude restriction, the altitudes on approach plates are minimums. I don't think there's anything prohibiting a pilot from never intercepting the glideslope completely or capturing it on the way down.

As Ziggy suggested, how high you are and how steeply you dive down to meet it might implicate some other issues.

(caveat first: I'm an IFR student... so not yet qualified. I hope I'm not totally putting my foot in my mouth here).

In my IFR ground school, I was taught that you *always* intercept the ILS glide slope from below for a very particular reason:

The way the radio signals of the glideslope are broadcast, there are false glide slopes above the real one.
And because of the phases of these false glide slopes, every alternate one is reverse-sensing.
This is a phenomena that exists only above the true GS.

So, the only way to be sure you have the proper slope, and one that is "normal" sensing, is to intercept the GS from below, never from above.

If you do intercept it from above, you will need to verify that you are on the correct GS by cross-checking FAF altitude when you get there (this assumes there is an OM, DME, or you have RNAV).

Like others have said, probably not conducive to a stable approach...I think the first false GS is pretty far above the real one.

This is a little off-topic, but from an ATC perspective, I've been under the impression that you don't necessarily have to be at the glideslope intercept altitude until the FAF. I had a SWA pilot refuse an approach clearance because he projected that he couldn't intercept the g/s until past the FAF, but I've had a couple accept approach clearances above the glideslope farther out.

My question is, do you immediately descend to the minimum altitude on that segment of the approach, or do you hit the glideslope and "ride the slide" from there? I've had some issues with giving an altitude to maintain until established on the localizer, but some airplanes seem to intercept the g/s and follow that down, which is usually a slower rate of descent.

My question is, do you immediately descend to the minimum altitude on that segment of the approach, or do you hit the glideslope and "ride the slide" from there?

If you're ATC, get ahold of the ORD TRACON and ask them about the issues they had last summer concerning GRABL on the ILS 27L. There was a whole print out of data that included the points of where guys intercepted the GS corresponding to the alt settings and temperatures for each data point. The problem was, the controller would say "cross GRABL at 7000 (or whatever it is), you're cleared for the ILS 27L"

Also, ask how many pilots got dinged from a certain airline that flies in and out of there. When the controller gives you an approach clearance, but gives you an altitude restriction TILL a certain fix, you have to maintain that altitude. In other words, you can't just "ride the slide" all the way down. As a bunch of guys from a certain airline found out the hard way. In the winter, it's usually not a big deal. In the summer, the specific approach could have you crossing GRABL BELOW the 7000' (or whatever it is) restriction.

In the above cases, maintaining 7000 (or whatever it is) to GRABL results in being above the GS as it's crossed. Not by much, but still above.

If you're ATC, get ahold of the ORD TRACON and ask them about the issues they had last summer concerning GRABL on the ILS 27L. There was a whole print out of data that included the points of where guys intercepted the GS corresponding to the alt settings and temperatures for each data point. The problem was, the controller would say "cross GRABL at 7000 (or whatever it is), you're cleared for the ILS 27L"

Also, ask how many pilots got dinged from a certain airline that flies in and out of there. When the controller gives you an approach clearance, but gives you an altitude restriction TILL a certain fix, you have to maintain that altitude. In other words, you can't just "ride the slide" all the way down. As a bunch of guys from a certain airline found out the hard way. In the winter, it's usually not a big deal. In the summer, the specific approach could have you crossing GRABL BELOW the 7000' (or whatever it is) restriction.

In the above cases, maintaining 7000 (or whatever it is) to GRABL results in being above the GS as it's crossed. Not by much, but still above.


The way these approaches were designed, you should be on GS at the correct altitude at the fix.

Where the problem has come up, and this have been a big deal in 121 lately, is that we correct out altimeters for non-standard barometric pressure, but we do not correct them for non-standard temperature.

The GS is always in the same place AGL, and generally matches up with the published altitudes on a standard day. But the intercept altitudes move up and down with non-standard temp and pressure.

On a standard day you can fly the GS and make all the restrictions.

But on a hot day, the barometric altitudes (which is what we use below 18,000) actually move up from the ground. So if you are the GS, the correct barometric altitude on that can be above you. I actually flew a flight which was tasked with evaluating this on a hot day at LAX. We had to turn to AP off to avoid following the GS below the restriction.

This creates a problem because low altitude VFR traffic below the Bravo is going to be using the correct barometric altitudes, which are higher than on a standard day. That could take a big chunk out of 500' separation.

My understanding is that the FAA is going to adjust all GS referenced altitudes to account for non-standard days to prevent this. Instead of being referenced to a standard day, they will be referenced to a reasonably hot day for that location. It won't matter for a stabilized approach since the GS doesn't move, it's just the crossing altitude will indicate high on a cold day.

Ziggy...it does if not properly stabilized at 1000'AGL, but the FOM doesnt state anything else. Thanks anyway!

whats with all the acronyms

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HAHAHA Isnt aviation the mother of acronyms!!! cheers

The way these approaches were designed, you should be on GS at the correct altitude at the fix.

Where the problem has come up, and this have been a big deal in 121 lately, is that we correct out altimeters for non-standard barometric pressure, but we do not correct them for non-standard temperature.

The GS is always in the same place AGL, and generally matches up with the published altitudes on a standard day. But the intercept altitudes move up and down with non-standard temp and pressure.

On a standard day you can fly the GS and make all the restrictions.

But on a hot day, the barometric altitudes (which is what we use below 18,000) actually move up from the ground. So if you are the GS, the correct barometric altitude on that can be above you. I actually flew a flight which was tasked with evaluating this on a hot day at LAX. We had to turn to AP off to avoid following the GS below the restriction.

This creates a problem because low altitude VFR traffic below the Bravo is going to be using the correct barometric altitudes, which are higher than on a standard day. That could take a big chunk out of 500' separation.

My understanding is that the FAA is going to adjust all GS referenced altitudes to account for non-standard days to prevent this. Instead of being referenced to a standard day, they will be referenced to a reasonably hot day for that location. It won't matter for a stabilized approach since the GS doesn't move, it's just the crossing altitude will indicate high on a cold day.

You are right on buddy! I was reading about this somewhere else!!! Scary to know that following the GS you are not necessarily crossing at or above any or all altitude restrictions...so now I know I have to be even more carefull!

Thanks for your replies... I recently had to go around at 2 special airports, in both cases the AutoPilot was on, the first time the proper ILS frequency was tuned but I had the dme hold feature on, and apparently the aircraft wouldnt let APProach capture the ILS so I reverted to VOR/ LOC and tried to fix it, when I disengaged the AP it was too late to intercept the GS. Second time at a high altitude airport, after having the aircraft configured for landing and following a turn for the inbound course(LNAV/VNAV), when LOC was captured, the GS was down just past the limit for automatic capture criteria. Same thing...I wasnt able to intercept the GS before 1000AGL and had to Go around. Please dont think this are the only two times these happens to me!!! I have been successful every time before correcting the situation!!! But the difference was both these times I wasnt expecting or particularly worried this could happen. Most times it happened when flying direct to the PFAF at a higher assigned altitude. Great to know you can learn a lot in any situation...I just learned a few things!!! best regards to you all....

:D

Generally, I don't think it's a legality issue. Unless it's covered in your company's Operations Specifications (wanted to avoid another acronym :D), it's one of the fairly rare approach charts with mandatory or maximum altitudes, or ATC gives you a maximum altitude restriction, the altitudes on approach plates are minimums. I don't think there's anything prohibiting a pilot from never intercepting the glideslope completely or capturing it on the way down.

As Ziggy suggested, how high you are and how steeply you dive down to meet it might implicate some other issues.

thank you!

This is a little off-topic, but from an ATC perspective, I've been under the impression that you don't necessarily have to be at the glideslope intercept altitude until the FAF. I had a SWA pilot refuse an approach clearance because he projected that he couldn't intercept the g/s until past the FAF, but I've had a couple accept approach clearances above the glideslope farther out.

My question is, do you immediately descend to the minimum altitude on that segment of the approach, or do you hit the glideslope and "ride the slide" from there? I've had some issues with giving an altitude to maintain until established on the localizer, but some airplanes seem to intercept the g/s and follow that down, which is usually a slower rate of descent.

Thanks! as I wrote before, I just recently learned that the FAF on a precision approach is not necessarily the GS intercept altitude fix, it could be any given point past this where you actually intercept the GS, I learned this after posting the question, I read it somewhere else. Very interesting topic. All I have to say is that although you can ride the slide( convenient for fuel efficiency), most ILS only assure signal and a safe area within 18NM on the LOC/ LLZ and 10NM on the GS. I guess depending on where you are it is ok the ride the slide. After all I think the ILS is not as SAFE as I thought, SO I will be crosschecking even more from now on!!

Thanks! as I wrote before, I just recently learned that the FAF on a precision approach is not necessarily the GS intercept altitude fix, it could be any given point past this where you actually intercept the GS, I learned this after posting the question, I read it somewhere else. Very interesting topic. All I have to say is that although you can ride the slide( convenient for fuel efficiency), most ILS only assure signal and a safe area within 18NM on the LOC/ LLZ and 10NM on the GS. I guess depending on where you are it is ok the ride the slide. After all I think the ILS is not as SAFE as I thought, SO I will be crosschecking even more from now on!!
All domestic ILSs have this service volume unless there has been a request for an extended service volume (ESV); and btw, the 18 nm service volume on the Localizer is only within 10 degs of centerline. In most circumstances the ILSs are all very safe, but you should always keep your guard up and be crosschecking your instruments along the way.
The more I learn about instrument approaches and the technicalities behind the curtain, the more I am amazed that they work at all! Smoke and mirrors with some black magic thrown in for good measure!

USMCFLYR

Thanks USMCFLYR...I am not a US pilot, but I do fly to a few destinations in the US!!! Its great to have all these people sharing good advise and new things. Cheers

(caveat first: I'm an IFR student... so not yet qualified. I hope I'm not totally putting my foot in my mouth here).

In my IFR ground school, I was taught that you *always* intercept the ILS glide slope from below for a very particular reason:

The way the radio signals of the glideslope are broadcast, there are false glide slopes above the real one.
And because of the phases of these false glide slopes, every alternate one is reverse-sensing.
This is a phenomena that exists only above the true GS.

So, the only way to be sure you have the proper slope, and one that is "normal" sensing, is to intercept the GS from below, never from above.

I thought this was true before...turns out its not! Although its safer, remember the Localizer might have false signals as well, this doesnt mean you must intercept straight ahead....as some fellows suggest, you have to be vigilant of your instruments, and cross check with charts, dme etc.
I guess its not that often that you get this false gs signals because they a 6-9 degress higher. The good thing about a stabilized approach is that you are not allowed to continue with high descent rates typicall of these false signals. Best regards

The way these approaches were designed, you should be on GS at the correct altitude at the fix.

Where the problem has come up, and this have been a big deal in 121 lately, is that we correct out altimeters for non-standard barometric pressure, but we do not correct them for non-standard temperature.

The GS is always in the same place AGL, and generally matches up with the published altitudes on a standard day. But the intercept altitudes move up and down with non-standard temp and pressure.

On a standard day you can fly the GS and make all the restrictions.

But on a hot day, the barometric altitudes (which is what we use below 18,000) actually move up from the ground. So if you are the GS, the correct barometric altitude on that can be above you. I actually flew a flight which was tasked with evaluating this on a hot day at LAX. We had to turn to AP off to avoid following the GS below the restriction.

This creates a problem because low altitude VFR traffic below the Bravo is going to be using the correct barometric altitudes, which are higher than on a standard day. That could take a big chunk out of 500' separation.

My understanding is that the FAA is going to adjust all GS referenced altitudes to account for non-standard days to prevent this. Instead of being referenced to a standard day, they will be referenced to a reasonably hot day for that location. It won't matter for a stabilized approach since the GS doesn't move, it's just the crossing altitude will indicate high on a cold day.


Yep, all that information was contained in the report. But until they get it fixed, that doesn't mean that when a controller clears you for an approach that you're cleared "ride the slide" all the way down, disregarding altitudes at fixes outside the lowest published GSIA altitude.

Which was the intent of my post.

I just recently learned that the FAF on a precision approach is not necessarily the GS intercept altitude fix, it could be any given point past this where you actually intercept the GS

It's at glide slope intercept AT THE LOWEST PUBLISHED GS INTERCEPT ALTITUDE.

It's a pretty standard job interview question, usually in relation to the question of "you're here (insert wherever you wish on an ILS plate) and the weather goes below the mins for the approach. Can you continue?"

The way these approaches were designed, you should be on GS at the correct altitude at the fix.

Where the problem has come up, and this have been a big deal in 121 lately, is that we correct out altimeters for non-standard barometric pressure, but we do not correct them for non-standard temperature.

The GS is always in the same place AGL, and generally matches up with the published altitudes on a standard day. But the intercept altitudes move up and down with non-standard temp and pressure.

On a standard day you can fly the GS and make all the restrictions.

But on a cold day, the barometric altitudes (which is what we use below 18,000) actually move up from the ground. So if you are the GS, the correct barometric altitude on that can be above you. I actually flew a flight which was tasked with evaluating this on a cold day at LAX. We had to turn to AP off to avoid following the GS below the restriction.

This creates a problem because low altitude VFR traffic below the Bravo is going to be using the correct barometric altitudes, which are higher than on a standard day. That could take a big chunk out of 500' separation.

My understanding is that the FAA is going to adjust all GS referenced altitudes to account for cold days to prevent this. Instead of being referenced to a standard day, they will be referenced to a reasonably cold day for that location. It won't matter for a stabilized approach since the GS doesn't move, it's just the crossing altitude will indicate high on a cold day.

I get what you are trying to say, but on a cold day, barometric altitudes are lower than they are on a standard day, not higher. When it is colder than standard, you run the risk of hitting high terrain that your altimeter says you will clear. I can clearly remember flying west of Nome on a cold day at 4,500 feet and noticing I would hit the mountain next to me charted at 3,870. That is a big difference, but it was 20 or 30 below.
The risk of VFR aircraft striking IFR aircraft on an approach is on hot days, when the VFR guy is at a higher AGL altitude than his indicated altitude. The IFR guy is on a fixed glideslope that doesn't care what the temperature is, therefore the VFR guy will be closer to him when it is hot, and further away when it is cold.
Good discussion; raises some points to consider.

I get what you are trying to say, but on a cold day, barometric altitudes are lower than they are on a standard day, not higher. When it is colder than standard, you run the risk of hitting high terrain that your altimeter says you will clear. I can clearly remember flying west of Nome on a cold day at 4,500 feet and noticing I would hit the mountain next to me charted at 3,870. That is a big difference, but it was 20 or 30 below.
The risk of VFR aircraft striking IFR aircraft on an approach is on hot days, when the VFR guy is at a higher AGL altitude than his indicated altitude. The IFR guy is on a fixed glideslope that doesn't care what the temperature is, therefore the VFR guy will be closer to him when it is hot, and further away when it is cold.
Good discussion; raises some points to consider.

I think you are right. Your QNH will read high on GS on a cold day, and low on GS on a hotter than standard day, so you are at risk of missing the at-or-above crossing restriction on a hot day. A hard restriction would be a problem on any significantly non-standard day.

and low on GS on a hotter than standard day, so you are at risk of missing the at-or-above crossing restriction on a hot day. A hard restriction would be a problem on any significantly non-standard day.

Hence the reason the report was compiled last summer. And if I remember correctly from the data points, it doesn't necessarily happen on "significantly" non-standard days. Not at least for ORD, maybe others.

All domestic ILSs have this service volume unless there has been a request for an extended service volume (ESV); and btw, the 18 nm service volume on the Localizer is only within 10 degs of centerline. In most circumstances the ILSs are all very safe, but you should always keep your guard up and be crosschecking your instruments along the way.
The more I learn about instrument approaches and the technicalities behind the curtain, the more I am amazed that they work at all! Smoke and mirrors with some black magic thrown in for good measure!

USMCFLYR

Thanks USMCFLYR...I am not a US pilot, but I do fly to a few destinations in the US!!! Its great to have all these people sharing good advise and new things. Cheers
CRPilot -

Ran across the ICAO standard sevice volumes discussed in our previous post. GS = 17 nm and the Localizer goes out to 25 nm. Just for clarification.

USMCFLYR

Hence the reason the report was compiled last summer. And if I remember correctly from the data points, it doesn't necessarily happen on "significantly" non-standard days. Not at least for ORD, maybe others.

I don't think ATC would notice if you were ISA +5 at a fairly low AGL alt (like the FAF).

The error is compounded at higher altitudes because the temp error is a percentage of altitude measured (unlike barometric error which is just 10' per .01 Hg).

I don't think ATC would notice if you were ISA +5 at a fairly low AGL alt (like the FAF).

The error is compounded at higher altitudes because the temp error is a percentage of altitude measured (unlike barometric error which is just 10' per .01 Hg).

I'm not talking about the FAF, never was. I'm talking about the specific part of the ILS in ORD where they may be running into a separation problem with traffic below.

It's at glide slope intercept AT THE LOWEST PUBLISHED GS INTERCEPT ALTITUDE.

It's a pretty standard job interview question, usually in relation to the question of "you're here (insert wherever you wish on an ILS plate) and the weather goes below the mins for the approach. Can you continue?"

According to the Pilot/Controller Glossary:
"FINAL APPROACH FIX- The fix from which the final approach (IFR) to an airport is executed and which identifies the beginning of the final approach segment. It is designated on Government charts by the Maltese Cross symbol for nonprecision approaches and the lightning bolt symbol for precision approaches; or when ATC directs a lower-than-published glideslope/path intercept altitude, it is the resultant actual point of the glideslope/path intercept.

Also I found a thread on this forum with interesting things discussed here...its under Flight Training....Glide Slope Cheers

Guys I just read this... Resources - Notices - FAA - FAASTeam - FAASafety.gov (http://www.faasafety.gov/spans/noticeView.aspx?nid=2917) and I am back to square 1!!! Now I dont know if its ok to intercept the GS beyond the PFAF....any comments???
Last paragraph :

ILS Altitudes

A Precision Final Approach Fix (PFAF) and/or a Glideslope Intercept Point defines the final approach segment (the end of the “feather”) as depicted in the Profile View on the approach plate. From the PFAF or Glideslope Intercept Point to the runway, use of the approach mode (APP) is the proper way to navigate the ILS. Without explicit guidance otherwise, there is no provision for capturing the glideslope beyond the PFAF or Glideslope Intercept Point and all altitude constraints must be met. Published altitudes at fixes outside of the Precision Final Approach Fix are part of the initial or intermediate segments of the approach and provide vertical separation from obstructions or other aircraft. An extension of the glideslope may not satisfy the minimum altitudes published outside the PFAF

According to the Pilot/Controller Glossary:
"FINAL APPROACH FIX- The fix from which the final approach (IFR) to an airport is executed and which identifies the beginning of the final approach segment. It is designated on Government charts by the Maltese Cross symbol for nonprecision approaches and the lightning bolt symbol for precision approaches; or when ATC directs a lower-than-published glideslope/path intercept altitude, it is the resultant actual point of the glideslope/path intercept.

I'm guessing what they mean in the bold part is ONLY when the GS is intercepted below the lowest published GSIA. Possible of the controller had you at the MVA, which may be below the lowest published GSIA. But NOT when you intercepted the GS somewhere above the lowest published GSIA.

But if we're to assume that's NOT the case, this is a question for YOU specifically, NOT everybody else. I'm quoting myself below to help reinforce the point trying to be made. What is the FAF on an ILS approach? If you intercepted the GS outside and above the LOWEST PUBLISHED GSIA on an ILS and the weather goes below the mins for the approach, what are you going to do? Like I said, it's a question for you, as it relates to what is ACTUALLY the FAF on CAT I ILS involving air carrier type operations.

It's a pretty standard job interview question, usually in relation to the question of "you're here (insert wherever you wish on an ILS plate) and the weather goes below the mins for the approach. Can you continue?"

Moving on.......


ILS Altitudes

A Precision Final Approach Fix (PFAF) and/or a Glideslope Intercept Point defines the final approach segment (the end of the “feather”) as depicted in the Profile View on the approach plate. From the PFAF or Glideslope Intercept Point to the runway, use of the approach mode (APP) is the proper way to navigate the ILS. Without explicit guidance otherwise, there is no provision for capturing the glideslope beyond the PFAF or Glideslope Intercept Point and all altitude constraints must be met. Published altitudes at fixes outside of the Precision Final Approach Fix are part of the initial or intermediate segments of the approach and provide vertical separation from obstructions or other aircraft. An extension of the glideslope may not satisfy the minimum altitudes published outside the PFAF

And that's what I've been trying to get at the whole time. For example purposes being cleared for an approach at say 20 miles out and 6000ft does NOT allow you to "ride the slide" all the way down. Altitude constraints at fixes outside the LOWEST PUBLISHED GSIA/PFAF have to be met.

And I'll repeat it, some legacy pilots flying in and out of ORD found out the hard way last summer.

I'm guessing what they mean in the bold part is ONLY when the GS is intercepted below the lowest published GSIA. Possible of the controller had you at the MVA, which may be below the lowest published GSIA. But NOT when you intercepted the GS somewhere above the lowest published GSIA.

But if we're to assume that's NOT the case, this is a question for YOU specifically, NOT everybody else. I'm quoting myself below to help reinforce the point trying to be made. What is the FAF on an ILS approach? If you intercepted the GS outside and above the LOWEST PUBLISHED GSIA on an ILS and the weather goes below the mins for the approach, what are you going to do? Like I said, it's a question for you, as it relates to what is ACTUALLY the FAF on CAT I ILS involving air carrier type operations.



Moving on.......




And that's what I've been trying to get at the whole time. For example purposes being cleared for an approach at say 20 miles out and 6000ft does NOT allow you to "ride the slide" all the way down. Altitude constraints at fixes outside the LOWEST PUBLISHED GSIA/PFAF have to be met.

And I'll repeat it, some legacy pilots flying in and out of ORD found out the hard way last summer.


Well it was my understanding, and I am trying to find where I read it, that it didnt matter if you intercept above or below, the point is that if you intercept beyond(I mean after passing the charted PFAF) the point where you intercept becomes your FAF. To answer you question, you can continue on the ILS CAT I only if you have passed the FAF. But what happens if vectored on a mva and cleared for the approach, if weather goes below minimums and I am beyond the charted PFAF, but still not established on GS? I am guessing I have to go around...not sure, but I will do that and find out later!! Good thing I am probably going to know now!!!
I am a holder of an ICAO license, PANS OPS and TERPS probably have some differences. I had never seen this FAASTeam Notice, it is recent, but it screws with my paradigms!! So this notice is answering my origianal question??? is it really telling me that if I am high coming in to intercept the GS at GSIA, I shouldnt bother trying to chase it and just go around? If I were to chase it, would I be in trouble??!!!

after reading this thread I am amazed I have even flown an ILS safely to the deck for 20 years.....

didn't know keeping the needles crossed was so complicated

I'm pretty sure that the bolded part refers to the principle that outside the PFAF, you must you must meet any step-down altitudes along the way. The glideslope can be captured outside the PFAF but it is not an official altitude reference until inside the PFAF.

IOW, If there is a step-down outside the FAF but the captured-outside-the FAF glideslope would bring you below it, you stop descent until passing beyond the step-down fix.

They are not that common but those types of approaches are there.

I'm still a bit shaky on how all this might play out in a class Bravo, so I'll offer some theory and make two scenarios.

“High to low, lookout below; Low to high, look to the sky”. Applies to both temperature and pressure
variations in the ambient air.

Ex 1. Ambient temp (or pressure) goes down, look below because you are lower than your altimeter shows, it is reading too high.
Ex 2. Ambient temp (or pressure) goes up, look to the sky because you are higher than your altimeter shows, it is reading too low.

Ok, here's a scenario: you are an airliner assigned to stay above some minimum altitude waiting for your ILS to arrive, and ATC is trying to keep you separated from a Piper Cub flying under the typical class Bravo outer shelf.

Cold day example: same deal as above and Ex. 1 says your altimeter is reading too high. You and the Piper Cub are very close because assuming he has the correct setting based perhaps on a local ground setting and he is where he thinks he is, you are likely a good bit lower than you think you are and separation could be lost in this scenario. This is where the pilot gets a call from an angry controller.

Warm day example: Ex. 2 says your altimeter is reading too low. So you are actually too high. All is well. You may have to dive to find the glideslope, but separation is not lost below. It may be lost above but that was not the scenario.

Have I got it right?

By the way, when I was taking the instrument written a few months back for my CFII, it had that old question about what does the altimeter do if you turn the baseline (Kohlsman setting) up or you turn it down. I always got wound about the axle thinking to think through this, so I came up with a saying to eliminate the thought process:

"Turn it down, read it down. Turn it up, read it up."

I'm still a bit shaky on how all this might play out in a class Bravo, so I'll offer some theory and make two scenarios.

“High to low, lookout below; Low to high, look to the sky”. Applies to both temperature and pressure
variations in the ambient air.

Ex 1. Ambient temp (or pressure) goes down, look below because you are lower than your altimeter shows, it is reading too high.
Ex 2. Ambient temp (or pressure) goes up, look to the sky because you are higher than your altimeter shows, it is reading too low.

Ok, here's a scenario: you are an airliner assigned to stay above some minimum altitude waiting for your ILS to arrive, and ATC is trying to keep you separated from a Piper Cub flying under the typical class Bravo outer shelf.

Cold day example: same deal as above and Ex. 1 says your altimeter is reading too high. You and the Piper Cub are very close because assuming he has the correct setting based perhaps on a local ground setting and he is where he thinks he is, you are likely a good bit lower than you think you are and separation could be lost in this scenario. This is where the pilot gets a call from an angry controller.

Warm day example: Ex. 2 says your altimeter is reading too low. So you are actually too high. All is well. You may have to dive to find the glideslope, but separation is not lost below. It may be lost above but that was not the scenario.

Have I got it right?

By the way, when I was taking the instrument written a few months back for my CFII, it had that old question about what does the altimeter do if you turn the baseline (Kohlsman setting) up or you turn it down. I always got wound about the axle thinking to think through this, so I came up with a saying to eliminate the thought process:

"Turn it down, read it down. Turn it up, read it up."

It works backwards if you are on the GS, since your airplane is at a specific AGL altitude for his DME, and is not chasing the temp error. Actually the OTHER airplane is chasing the temp error, but that is what ATC is expecting everyone to do.

Try it sometime on a non-standard day with an accurate altimeter at a FAF which has DME to identify it.

InFO11009 from the FAA was all about this topic. Here's a link http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/info/all_infos/media/2011/InFO11009.pdf

InFO11009 from the FAA was all about this topic. Here's a link http://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/info/all_infos/media/2011/InFO11009.pdf

Well ain't that come crap?

That's what I was getting at all along. If the procedure has altitude restrictions BEFORE the lowest GSIA alt, you STILL have to comply with them. Regardless of what your altimeter says when crossing those fixes if you choose to "ride the slide" before the lowest published GSIA.

pilots are cautioned to adhere to published step-down fixes located outside the Final Approach Segment on an ILS approach. If a pilot elects to follow the glide slope while outside the Final Approach Segment he should be fully aware that this technique needs to be closely monitored and, if necessary, action must be taken to meet all stepdown altitudes. Examples of airports where multiple altitude deviations have occurred include, but are not limited to; LAX, ORD, ATL, SLC.

I'm pretty sure that the bolded part refers to the principle that outside the PFAF, you must you must meet any step-down altitudes along the way. The glideslope can be captured outside the PFAF but it is not an official altitude reference until inside the PFAF.

IOW, If there is a step-down outside the FAF but the captured-outside-the FAF glideslope would bring you below it, you stop descent until passing beyond the step-down fix.

They are not that common but those types of approaches are there.

Yep, the ILS in ORD is one of them.

Well it was my understanding, and I am trying to find where I read it, that it didnt matter if you intercept above or below, the point is that if you intercept beyond(I mean after passing the charted PFAF) the point where you intercept becomes your FAF.

So you mean if I intercept the GS 20nm out & my company's profile says to get configured at the FAF, that I'm going to drop the gear & flaps 20nm out? I disagree. The lowest published altitude is where it's at. IMHO, one thing against riding the slide is that it might be difficult to decelerate to Vlo/Vle. Following the step downs prior to the published GS int altitude not only ensures that we hit each point along the approach at the required altitude, but helps with being on speed (certain a/c more than others).

The Precision FAF is, by definition, where the glide slope intercepts the G/S Intercept altitude, not where you intercept the glide slope.

GF

This is from Jeppesen Glossary...
GLIDE SLOPE / GLIDE PATH INTERCEPT ALTITUDE
— The minimum altitude to intercept the glide
slope/path on a precision approach. The intersection
of the published intercept altitude with the glide
slope/path, designated on Jeppesen Terminal charts
by the start of the glide slope/path symbol, is the precision
FAF; however, when ATC directs a lower altitude,
the resultant lower intercept position is then the
FAF

No...I meant if you intercept the GS after the GSIA, thats your actual PFAF.

Just use VNAV while outside the FAF. Of course the FMS must be loaded with the correct approach and all the altitude restrictions. Once inside the last Altitude Restriction and before the FAF / GSIA, hit the ILS arming button applicable to your airframe.

Problem solved.

Just use VNAV while outside the FAF. Of course the FMS must be loaded with the correct approach and all the altitude restrictions. Once inside the last Altitude Restriction and before the FAF / GSIA, hit the ILS arming button applicable to your airframe.

Problem solved.

That would be the easiest way to do it, but only on aircraft equipped with VNAV capability, as opposed to VNAV advisory ONLY.

That would be the easiest way to do it, but only on aircraft equipped with VNAV capability, as opposed to VNAV advisory ONLY.

Bingo. With an aircraft that can't couple VNAV either "dive and drive" or just use your penultimate professionalism and carve a vertical speed path respecting the crossings until your intercept altitude.

This seems to be more of a deal in recent years... it makes you wonder who decided that this is an issue.

The ATL issue just became known in recurrent less than a year ago. None of the crossings or airspace structure have changed.

Bingo. With an aircraft that can't couple VNAV either "dive and drive" or just use your penultimate professionalism and carve a vertical speed path respecting the crossings until your intercept altitude.

Which if you read back in the thread is ALL I was getting at anyway. Still amazes me how many guys seem to think that it's OK intercepting the GS at 7000 on the ILS into ORD as well as "ride the slide" all the way down.

A few weeks ago, temp was in the low 30's and alt setting over 30.20. On the ILS with grable, EVERY single fix outside the FAF would have been crossed 3-400 BELOW the published altitudes.

Which if you read back in the thread is ALL I was getting at anyway. Still amazes me how many guys seem to think that it's OK intercepting the GS at 7000 on the ILS into ORD as well as "ride the slide" all the way down.

A few weeks ago, temp was in the low 30's and alt setting over 30.20. On the ILS with grable, EVERY single fix outside the FAF would have been crossed 3-400 BELOW the published altitudes.

I didn't read back in the thread to be completely honest. Kudos again on excellent airmanship on your part!

The planes that I fly now can couple barometric VNAV to the autopilot, but the key is to respect the step downs until the legal GS altitudes which may vary widely. It never was really a deal in times past but the FAA keeps making a bigger deal about it.

Well it was my understanding, and I am trying to find where I read it, that it didnt matter if you intercept above or below, the point is that if you intercept beyond(I mean after passing the charted PFAF) the point where you intercept becomes your FAF.

[QUOTE=TheFly;1019901]So you mean if I intercept the GS 20nm out & my company's profile says to get configured at the FAF, that I'm going to drop the gear & flaps 20nm out? I disagree.

The Precision FAF is, by definition, where the glide slope intercepts the G/S Intercept altitude, not where you intercept the glide slope.

GF
Thought I would bring this old thread back to life just to put a *definition* to the PFAF.
Ran across this while going through some TERPS slides in training.
[QUOTE]
(PFAF): Applicable to all PA procedures. A 2D point located on the final appraoch course at a distance from the LTP/FTP where the GPA intersects the internediate segment altitude (glidepath intercept altitude). The PFAF marks the outer edge of the PA final segment.

USMCFLYR