Glideslope Intercept

[Ryan F.]

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...

No, it's for all aircraft, not just "large" aircraft.

InFOs apply to everyone. Helicopters, GA, airlines, 135, 91, you name it. Scroll around through the last few years (https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/info/all_infos/) and you'll find topics which address just about every segment of aviation.

Regardless, the size of the aircraft matters not given the subject matter. A C-172 presents just as much hazard as a 737 when it descends into airspace into which it does not belong.

 

[Ryan F.]

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...

Also, it's directly relevant - the OP's question is precisely addressed with the guidance. In his post he stated he captured prior to the altitude depicted at the FAF (glideslope intercept), descended and crossed the "FAF" (which is actually GS intercept on an ILS). That's pretty much the problem posited by the InFO.

 

[IK04]

Also, it's directly relevant - the OP's question is precisely addressed with the guidance. In his post he stated he captured prior to the altitude depicted at the FAF (glideslope intercept), descended and crossed the "FAF" (which is actually GS intercept on an ILS). That's pretty much the problem posited by the InFO.

OK, I was thinking Intermediate approach segment. That would be dumb.

 

[John Collins]

Also, it's directly relevant - the OP's question is precisely addressed with the guidance. In his post he stated he captured prior to the altitude depicted at the FAF (glideslope intercept), descended and crossed the "FAF" (which is actually GS intercept on an ILS). That's pretty much the problem posited by the InFO.

Ryan,

The InFO is close to the OP's question, but deals with a related, but different topic. If there are no step downs between the IF and the FAF, the issue described in the InFO does not apply because the IF is already passed at the minimum altitude and not by reference to the GS. The IF has a minimum altitude and at that altitude is well below the GS. The OP discusses intercepting the GS outside of the FAF, but at the higher IF altitude. That is different than intercepting the GS outside of the FAF and using the GS to manage altitude to comply with crossing minimums for step down fixes, which is what is discussed in the InFO, because the GS may be lower than an indicated MSL altitude minimum specified for a step down fix on a hot day. So crossing the IF (last step down) at the indicated MSL minimum and then intercepting the GS on this final segment, does not conflict with the guidance in the InFO which states:

operators of aircraft should ensure that aircraft under their control, when cleared for an ILS approach, do not descend below published step-down altitudes on an ILS final approach course, while outside the Final Approach Segment.

The OP did not descend below the IF on an ILS final approach course, he maintained it until intercepting the GS.

 

[RussR]

OK, I was thinking Intermediate approach segment. That would be dumb.

Not necessarily. There are numerous examples of fixes on glidepath outside the IF. For a while it was a "thing" where the chart would say "when directed by ATC, intercept glideslope at 3000 or 4000 or 5000" where corresponding fixes were at those altitudes on glidepath. These notes have since been removed but in many cases the fixes are still on glidepath. See KSEA ILS OR LOC RWY 17L for one example (any time you see a distance of 3.1 or 3.2 nm (due to rounding) between fixes 1000 feet apart vertically, they're probably on glidepath):

https://skyvector.com/files/tpp/1905/pdf/00582IL16L.PDF

Runways 16C and 16R are the same way.

Similarly, DTW has the same thing on many of its approaches. For example, ILS OR LOC RWY 21L:

https://skyvector.com/files/tpp/1905/pdf/00119IL21L.PDF

So on these, you could easily intercept glideslope outside the IF and fly it down, just as long as you are sure not to dip below any altitudes prior to the FAF, which can happen on hot days.

 

[drjcustis]

Great discussion and I’ve learned a lot.

Sorry, I haven’t been a very good OP, been reading but haven’t had much time to respond.

My only other thought or concern is with the guaranteed reliability of the glide slope outside of the FAF. Looks from the discussion that one would have to watch and be sure not to bust any of the step down fixes and that obstacle clearance is only guaranteed from the FAF in.

Is there any scenario, or concern, that the glide slope would be unreliable beyond the FAF at a higher altitude?


Sent from my iPad using Tapatalk

 

[Ryan F.]

The InFO is close to the OP's question, but deals with a related, but different topic. If there are no step downs between the IF and the FAF, the issue described in the InFO does not apply because the IF is already passed at the minimum altitude and not by reference to the GS. The IF has a minimum altitude and at that altitude is well below the GS. The OP discusses intercepting the GS outside of the FAF, but at the higher IF altitude. That is different than intercepting the GS outside of the FAF and using the GS to manage altitude to comply with crossing minimums for step down fixes, which is what is discussed in the InFO, because the GS may be lower than an indicated MSL altitude minimum specified for a step down fix on a hot day.

Thank you for the reply, John. I respectfully disagree. The InFO states:

"Purpose: This InFO emphasizes the requirement for operators to comply with all altitude restrictions (i.e. stepdown altitudes) prior to the Final Approach Segment when cleared for an ILS approach."
​

The AIM (5-4-5) also cautions that...

"2. The ILS glide slope is intended to be intercepted at the published glide slope intercept altitude. This point marks the PFAF and is depicted by the ”lightning bolt” symbol on U.S. Government charts. Intercepting the glide slope at this altitude marks the beginning of the final approach segment and ensures required obstacle clearance during descent from the glide slope intercept altitude to the lowest published decision altitude for the approach. Interception and tracking of the glide slope prior to the published glide slope interception altitude does not necessarily ensure that minimum, maximum, and/or mandatory altitudes published for any preceding fixes will be complied with during the descent."​

This language was adopted in part thanks to an NBAA request for the insertion (2009 Aeronautical Charting Forum, Instrument Procedures Group).

It's an AIM recommendation, and not a regulatory requirement to intercept the glideslope at the "published glide slope intercept altitude" (absent a a mandatory altitude) but there are countless other recommendations within the AIM, especially w/r/t instrument approach procedures which are the instrument pilot's bread and butter of "best practices." In my opinion the best practice on an ILS is only intercept the glideslope at the published glideslope intercept altitude.

While in the case of the specific approach mentioned there are no stepdown fixes between the IF and the PFAF, the question posed by drjcurtis didn't reference a specific approach at the start of the discussion. As a general rule, the pilot should meet the stepdown fix requirements and then intercept the glide slope per the AIM's recommendation. That means always descending to the glide slope intercept altitude, not tracking the glideslope down from an earlier point in the approach.

So crossing the IF (last step down) at the indicated MSL minimum and then intercepting the GS on this final segment, does not conflict with the guidance in the InFO which states:

I believe it does conflict with the guidance of the InFO given the first sentence in its Purpose statement. That's my personal/professional opinion, but it is only opinion. However, it is clearly at odds with the AIM's guidance.

 

[John Collins]

I do respectfully disagree as the pilot complied with the altitude restriction at the IF. If there was a step down between the IF and the FAF, then the InFO is applicable, but the pilot identified the approach involved later in the thread.

The AIM wording does state: "Interception and tracking of the glide slope prior to the published glide slope interception altitude does not necessarily ensure that minimum, maximum, and/or mandatory altitudes published for any preceding fixes will be complied with during the descent." In this case, there are no preceding fixes from the point of the GS interception to the PFAF, so at all times an aircraft intercepting the GS is at or above any minimum altitude and although it is technically possible for a PFAF to be a mandatory altitude, it is an exception to TERPS. I don't know off hand of any such cases, but that would mandate the GS intercept altitude.

For operational reasons and simplicity during a high work load point in the approach, particularly when operating a complex aircraft single pilot, there is more room for error by using a single point of descent on the IF leg. Instead of making multiple configuration changes and power changes during the IF leg, one can make a single power/configuration change to intercept and fly the ILS or LPV. Compare these two sequences: 1) making a power change to descend to the GS intercept altitude, then level off at the intercept altitude with another power change to reverse the descent, then make a final configuration change such as lowering the landing gear and setting approach power, verses 2) maintain existing and stable altitude and when intercepting the GS and then lowering the landing gear and setting approach power. It provides for fewer power changes and in some cases configuration changes, eliminates a step down, and provides additional time to track the GS. Most GA aircraft are operated single pilot and don't have baro-VNAV or VNV capability. One needs to descend to the GS intercept altitude anyway, and accomplishing this without a level off is both easier and gets one to the same PFAF at the same altitude.

Here is a direct link to the IPG case Intermediate Fix Altitudes & ILS Glide Slope and a link to the IPG site, yours was stale

FYI, I regularly attend the ACF/IPG meetings.

 

[1RTK1]

Would you bust someone on a checkride who intercepts outside the PFAF? Why or why not? Not trying to be argumentative here just curious.

I imagine if they went below published altitude restriction before the FAF it's a bust otherwise no...??????????

 

[luvflyin]

Some more thoughts on this. Definition of Glideslope Intercept Altitude. Note a couple of things that I have highlighted. I added the 'if' before ATC. I think they had an editorial error.

GLIDESLOPE INTERCEPT ALTITUDE− The
published minimum altitude to intercept the
glideslope in the intermediate segment of an
instrument approach. Government charts use the
lightning bolt symbol to identify this intercept point.
This intersection is called the Precise Final Approach
fix (PFAF). [if] ATC directs a higher altitude, the
resultant intercept becomes the PFAF.
(See FINAL APPROACH FIX.)
(See SEGMENTS OF AN INSTRUMENT
APPROACH PROCEDURE.)

The directives on this subject such as the InFO thing above, do not use words and acronyms such as FAF, PFAF. They say outside the "Final Approach Segment." I'm thinking that they were very deliberate about using 'final approach segment' instead of PFAF or FAF. However, the explanations given after the definition above do create a question. What if ATC does in fact 'direct a higher altitude' and the 'resultant intercept' becomes the PFAF. Is that now the beginning of the Final Approach Segment??? What if there are now step down fixes between there and the 'depicted by lightning bolt' glideslope intercept on the Chart. Someday maybe all the folk writing definitions, AIMs, InFO's, AC's etc will communicate better. The dudes writing the Glossary definition Glideslope Intercept Altitude I think should have let it go at that, the definition, and not added the last sentence. It was unnecessary and not quite accurate.

 

[midlifeflyer]

I believe it does conflict with the guidance of the InFO given the first sentence in its Purpose statement. That's my personal/professional opinion, but it is only opinion. However, it is clearly at odds with the AIM's guidance.

You lost me on that one, but with all the thread drift, I may have misunderstood. To use a specific example

it seems to me we have been discussing two options:

(a) remain at 4,000 after crossing PECIT until intercepting the glideslope, and make a single configuration change from level to glidepath;
(b) make a configuration change at PECIT to dive down to 2,400, then make a second configuration change to level at 2,400 until intercepting the glideslope and then make a third configuration change to when intercepting the glideslope.​

Are you really saying that (a) conflicts with the guidance to "maintain all altitude restrictions"? If so, exactly what altitude restriction does it fail to maintain?

 

[John Collins]

Luvflyin,

Good point. There are always things missed and there are constant revisions to the AIM. I think the distinction being made in the InFO is between step down fixes inside and outside the final approach segment. The ones inside don't apply to a vertically guided approach, but those outside do. The TERPS definition of the Final Approach Segment is:

Final approach segment (FAS). The segment of an instrument approach procedure that begins at the PFAF and ends at the MAP or LTP/FTP, whichever is encountered last.

The PFAF is defined as:

Precise final approach fix (PFAF). The PFAF is a calculated WGS84 geographic position located on the final approach course where the designed vertical path (NPA procedures) or glidepath (APV and PA procedures) intercepts the intermediate segment altitude (glidepath intercept altitude). The PFAF marks the beginning of the FAS. The calculation of the distance from LTP to PFAF includes the earth curvature.

So the Final Approach Segment is slightly different based on whether on is flying a vertically guided procedure or an NPA procedure, but for all practical purposes it is from the charted location of the FAF if there is one. There are limitations in the TERPS for placing a step down inside the IF, I believe only one is allowed.

 

[IK04]

It seems the 11009.pdf document was intended to draw attention to air carriers who were operating below the stepdown leg altitudes at large airports (LAX, ORD, ATL, SLC) in order to get a better signal from weak glideslope signals while they were way out on a more distant than usual FAC. The temperature/altitude conundrum is referenced also, as another possible reason for intentional deviations by those air carriers.

Since the title is :"Failure to comply..." it is a wordy way to say "Hey! Knock it off!."

 

[dmspilot]

It seems the 11009.pdf document was intended to draw attention to air carriers who were operating below the stepdown leg altitudes at large airports (LAX, ORD, ATL, SLC) in order to get a better signal from weak glideslope signals while they were way out on a more distant than usual FAC. The temperature/altitude conundrum is referenced also, as another possible reason for intentional deviations by those air carriers.

Since the title is :"Failure to comply..." it is a wordy way to say "Hey! Knock it off!."

They're violating it in purpose because you get better glideslope reception the lower you go. That's seriously what you're saying? What are you on lately?

 

[Ryan F.]

I do respectfully disagree as the pilot complied with the altitude restriction at the IF. If there was a step down between the IF and the FAF, then the InFO is applicable, but the pilot identified the approach involved later in the thread.

I understand. In the case in which there are no fixes between the IF and the FAF, I agree that in practice there should be no issue with intercepting the glideslope at the higher altitude. However, the AIM wording (a recommendation) remains clear - sorry for the repetition, but again, "... The ILS glide slope is intended to be intercepted at the published glide slope intercept altitude." There's no wiggle room here, no "if no stepdown fixes this does not apply" criteria, it's just the guidance, all on its own. We can choose to disregard it, or not.

The AIM wording does state: "Interception and tracking of the glide slope prior to the published glide slope interception altitude does not necessarily ensure that minimum, maximum, and/or mandatory altitudes published for any preceding fixes will be complied with during the descent." In this case, there are no preceding fixes from the point of the GS interception to the PFAF, so at all times an aircraft intercepting the GS is at or above any minimum altitude and although it is technically possible for a PFAF to be a mandatory altitude, it is an exception to TERPS. I don't know off hand of any such cases, but that would mandate the GS intercept altitude.

Yes, I understand. I don't think that changes my position or yours, though. In fact, I think it points out a particularly worrisome possibility - of the pilot intentionally having to destabilize the approach in order to remain above/at minimum/mandatory altitudes. Not a good way to fly, particularly in single pilot aircraft in which the pilot would be reverting to lower forms of automation or hand flying the aircraft in order to deviate from the glideslope and then descend back to it.

For operational reasons and simplicity during a high work load point in the approach, particularly when operating a complex aircraft single pilot, there is more room for error by using a single point of descent on the IF leg. Instead of making multiple configuration changes and power changes during the IF leg, one can make a single power/configuration change to intercept and fly the ILS or LPV. Compare these two sequences: 1) making a power change to descend to the GS intercept altitude, then level off at the intercept altitude with another power change to reverse the descent, then make a final configuration change such as lowering the landing gear and setting approach power, verses 2) maintain existing and stable altitude and when intercepting the GS and then lowering the landing gear and setting approach power. It provides for fewer power changes and in some cases configuration changes, eliminates a step down, and provides additional time to track the GS. Most GA aircraft are operated single pilot and don't have baro-VNAV or VNV capability. One needs to descend to the GS intercept altitude anyway, and accomplishing this without a level off is both easier and gets one to the same PFAF at the same altitude.

I'm aware of the issues you're describing and conceptually speaking, I certainly agree. (Although I think you meant to say there is less room for error by using a single point... not more.) These issues drove CDFA in the non-precision approach category.

As a nitpick, the issue has nothing to do with LPV; those approaches are immune to nonstandard pressure and temperature challenges faced by ILS approaches (which necessitated the need for the discussion).

However, the AIM's guidance remains clear for the reasons discussed above, and my recommendations remain unchanged as well.

 

[IK04]

They're violating it in purpose because you get better glideslope reception the lower you go. That's seriously what you're saying? What are you on lately?

It has been referred to as a "false glideslope" early on in this thread.

 

[Ryan F.]

You lost me on that one, but with all the thread drift, I may have misunderstood. To use a specific example
View attachment 74290
it seems to me we have been discussing two options:

(a) remain at 4,000 after crossing PECIT until intercepting the glideslope, and make a single configuration change from level to glidepath;
(b) make a configuration change at PECIT to dive down to 2,400, then make a second configuration change to level at 2,400 until intercepting the glideslope and then make a third configuration change to when intercepting the glideslope.​

​

Neither. Capture the glideslope at the published glideslope intercept altitude (per the AIM) and configure once, although I'm speaking generically and some aircraft may require different techniques. Most likely, the majority of the participants in this thread are flying light piston airplanes.

Are you really saying that (a) conflicts with the guidance to "maintain all altitude restrictions"? If so, exactly what altitude restriction does it fail to maintain?

No. I'm not saying that. I am suggesting that in its entirety, it's pretty clear that if you're going to intercept the glideslope at the published glideslope intercept altitude, you're going to have to comply with the published minimum altitude on that segment of the approach. Again, it's just my opinion.

 

[dmspilot]

It has been referred to as a "false glideslope" early on in this thread.

What "it" are you referring to?

 

[Ryan F.]

Some more thoughts on this. Definition of Glideslope Intercept Altitude. Note a couple of things that I have highlighted. I added the 'if' before ATC. I think they had an editorial error.

GLIDESLOPE INTERCEPT ALTITUDE− The
published minimum altitude to intercept the
glideslope in the intermediate segment of an
instrument approach. Government charts use the
lightning bolt symbol to identify this intercept point.
This intersection is called the Precise Final Approach
fix (PFAF). [if] ATC directs a higher altitude, the
resultant intercept becomes the PFAF.
(See FINAL APPROACH FIX.)
(See SEGMENTS OF AN INSTRUMENT
APPROACH PROCEDURE.)

Yes, this is consistent with all of the guidance we have discussed.

It is the "minimum" altitude vs. an ATC-directed higher altitude.

 

[BrianNC]

(a) remain at 4,000 after crossing PECIT until intercepting the glideslope, and make a single configuration change from level to glidepath;

This is how I was taught to do it. Just one configuration change. Then at PURME check and see if my altitude is at 2400 as a cross check.

Not trying to derail, just asking you this personally, how do you feel about not using flaps because that is introducing more changes? I did an IPC where the instructor didn't use flaps. He felt it just introduced more action into a process that can be stressful, the final approach. I really didn't notice any difference when landing. I had never considered that. I actually liked it. Since then I've seen that people do advocate both sides. I had never considered the no flaps before.

 

[Ryan F.]

Would you bust someone on a checkride who intercepts outside the PFAF? Why or why not? Not trying to be argumentative here just curious.

No. Capturing the GS outside the FAF is not prohibited.

Exceeding ACS tolerances is a different matter, regardless of the conditions leading up to the situation.

 

[mondtster]

Not trying to derail, just asking you this personally, how do you feel about not using flaps because that is introducing more changes? I did an IPC where the instructor didn't use flaps. He felt it just introduced more action into a process that can be stressful, the final approach. I really didn't notice any difference when landing. I had never considered that. I actually liked it. Since then I've seen that people do advocate both sides. I had never considered the no flaps before.

I know you're not asking me, but I'll give you an opinion anyway. In most airplanes I don't use any flaps nor do I teach to use them. Like the instructor you flew with, I think this can add unnecessary complexity to the approach, especially when someone is trying to learn to fly approaches. Plus, you haven't committed to land yet, you can choose to add flaps once you break out and decide to continue to a landing.

 

[John Collins]

Yes, I understand. I don't think that changes my position or yours, though. In fact, I think it points out a particularly worrisome possibility - of the pilot intentionally having to destabilize the approach in order to remain above/at minimum/mandatory altitudes. Not a good way to fly, particularly in single pilot aircraft in which the pilot would be reverting to lower forms of automation or hand flying the aircraft in order to deviate from the glideslope and then descend back to it.

Yes, if one was to destabilize the approach in order to remain at or above a step down, it is not a particularly good way to fly the approach. In a similar way, one who is flying an LNAV or LP procedure and using advisory vertical guidance to the MDA will find they may need to destabilize the CDFA at and below the MDA when the note "Visual Segment - Obstacles" is charted. Pilots are not permitted to use the advisory glidepath below the MDA.

I'm aware of the issues you're describing and conceptually speaking, I certainly agree. (Although I think you meant to say there is less room for error by using a single point... not more.) These issues drove CDFA in the non-precision approach category.

As a nitpick, the issue has nothing to do with LPV; those approaches are immune to nonstandard pressure and temperature challenges faced by ILS approaches (which necessitated the need for the discussion).

My wording was not the clearest and agree with your edit.

However, I can't agree with your nitpick, both an ILS GS and an LPV GP are straight lines in space. For example, at KORD, the ILS or RNAV (GPS) using LPV to RWY 10C, the GS/GP at the SHARN stepdown is nominally at a true altitude of 5151 feet, but on a hot day of 40C, the indicated MSL altitude will be at 4783 feet on the GS, yet the crossing minimum is 5000 feet MSL. In this case, it is not that the GS or GP moved, what moved was the MSL altitude indication, or to be more precise, the pressure altitude reported by the transponder, because that is what reports the aircraft altitude to the snitch. The vertical path for the ILS and LPV are identical and they do not move as a function of temperature, but that darned atmosphere the and altimeter indications that are not corrected for temperature allow the baro altimeter to read lower than you actually are on a hot day and higher than you are on a cold day. You won't hit the rocks on a hot day following the GS, but you may lose vertical separation from traffic below you. The DA will be affected by temperature as it is indicated on the baro altimeter. Interestingly enough, if you fly the RNAV using Baro-VNAV for the vertical to fly the LNAV/VNAV, and the Baro-VNAV is not temperature corrected, the vertical GP will be affected by the temperature to the same degree as the altimeter is affected, so this equipment will keep you above the step down altitudes. The Baro-VNAV path is not a straight line in space and will fly a higher true path on a hot day.

 

[BrianNC]

I know you're not asking me, but I'll give you an opinion anyway. In most airplanes I don't use any flaps nor do I teach to use them. Like the instructor you flew with, I think this can add unnecessary complexity to the approach, especially when someone is trying to learn to fly approaches. Plus, you haven't committed to land yet, you can choose to add flaps once you break out and decide to continue to a landing.

Thanks for the reply. Good points. I had never run across it before until the last IPC. It was one less thing to think about.

 

[Ryan F.]

My wording was not the clearest and agree with your edit.

However, I can't agree with your nitpick, both an ILS GS and an LPV GP are straight lines in space.

I read through your post with interest. Everything you stated was accurate, except the predication of my LPV example - the LNAV/VNAV function on a WAAS GPS, used to cross the stepdown fixes, will do so at the true altitude of the fixes, and will be immune to nonstandard temperature or pressure. It is true that Baro-VNAV units function differently and require temperature compensation to cross those fixes at the correct altitude.

What I am stating, and have stated, is that an aircraft capable of flying an LPV approach with a WAAS Navigator will generally (almost always) be equipped to be able to follow vertical guidance from an IF or IAF and cross all stepdown fixes at the correct true altitude regardless of temperature and pressure. The unit will then switch to LPV just prior to reaching the FAF. In this manner, such-equipped airplanes flying LPV approaches will be impervious to the temperature and pressure issues which plague the ILS.

Good discussion,

 

[luvflyin]

Luvflyin,

Good point. There are always things missed and there are constant revisions to the AIM. I think the distinction being made in the InFO is between step down fixes inside and outside the final approach segment. The ones inside don't apply to a vertically guided approach, but those outside do. The TERPS definition of the Final Approach Segment is:



The PFAF is defined as:



So the Final Approach Segment is slightly different based on whether on is flying a vertically guided procedure or an NPA procedure, but for all practical purposes it is from the charted location of the FAF if there is one. There are limitations in the TERPS for placing a step down inside the IF, I believe only one is allowed.

Yeah. Precise Final Approach Fix has only been around a couple years. It’s not really something pilots get involved in. But Precision Final Approach Fix is. Both have the same Acronym, PFAF. I think they should have come up with something different like FSCP, Final Segment Calculation Point. I know you know the difference between them but just getting it out there. Precise Final Approach Fix is where the TERPS guys designing approaches begin applying ‘Final Segment’ criteria to separate planes from rocks and obstructions. Has nothing to do with separating airplanes from other airplanes. We’re all familiar with all the controversies about what is FAF, what is PFAF, what’s the ‘lightning bolt,’ what is Glideslope Intercept Altitude. On it’s own, Glideslope Intercept Altitude can sound like a ‘hard altitude,’ not a minimum altitude, which is what it is. This seems to lead to many pilots thinking they must not actually ‘intercept’ the Glideslope until then, that they must ‘dive and drive’ so to speak. Not true, you can have the Glideslope needle centered long before the PFAF, be it precise or precision. But, ‘though shalt not descend below a minimum altitude on the Chart at any Fix beyond the Final Approach Segment.’

 

[midlifeflyer]

​

Neither.

I don't see how it can be neither. What is the third option arriving at PECIT at 4000?

 

[midlifeflyer]

This is how I was taught to do it. Just one configuration change. Then at PURME check and see if my altitude is at 2400 as a cross check.

Not trying to derail, just asking you this personally, how do you feel about not using flaps because that is introducing more changes? I did an IPC where the instructor didn't use flaps. He felt it just introduced more action into a process that can be stressful, the final approach. I really didn't notice any difference when landing. I had never considered that. I actually liked it. Since then I've seen that people do advocate both sides. I had never considered the no flaps before.

Personally answering, it's a question of personal technique. The CFII I trained with in a 172 attempted in vain to get me to use approach flaps. I just didn't see the value. Of the 30 or so models of singles I have flown I've only used approach flaps in three, and I'm inconsistent in two of them.

BTW never more than approach flaps. First notch or the equivalent.

And I don't try to switch a pilot during an IPC. I'm agnostic.

 

[John Collins]

What I am stating, and have stated, is that an aircraft capable of flying an LPV approach with a WAAS Navigator will generally (almost always) be equipped to be able to follow vertical guidance from an IF or IAF and cross all stepdown fixes at the correct true altitude regardless of temperature and pressure. The unit will then switch to LPV just prior to reaching the FAF. In this manner, such-equipped airplanes flying LPV approaches will be impervious to the temperature and pressure issues which plague the ILS.

Although the capability exists to fly step downs in GA aircraft,(G1000, GTN with GFC500 and G500/600, it is very high end avionics and not almost always. Those systems have VNV available, but they are a minority of the GA aircraft. I have a Bonanza with an Stec 60-2, G500TXi. GTN750 and it is not capable of automatically doing the step downs, although I did enable the VNV functions in the GTN. Still well over 100,000 WAAS GNS430 in the field and they don't have VNV. I expect the vast majority of GTN installations are not installed in aircraft with a GFC500 autopilot.

If one is using a WAAS Navigator and flies the LPV as you indicated, I would agree. But in order to get the LPV GP available for use outside of the step down fix that connects to the FAF, pilots use VTF. In that case, flying the LPV at KORD, the RNAV (GPS) RWY 10C, the same issue as flying the ILS. For too many GA pilots, VTF is all they know.

 

[apr911]

It's an AIM recommendation, and not a regulatory requirement to intercept the glideslope at the "published glide slope intercept altitude" (absent a a mandatory altitude) but there are countless other recommendations within the AIM, especially w/r/t instrument approach procedures which are the instrument pilot's bread and butter of "best practices." In my opinion the best practice on an ILS is only intercept the glideslope at the published glideslope intercept altitude.

I dont view the paragraph outlined from the AIM as a "recommendation." Nowhere in the paragraph does it use the language "recommend." It is merely a statement of intended use of the glideslope's vertical guidance.

No. I'm not saying that. I am suggesting that in its entirety, it's pretty clear that if you're going to intercept the glideslope at the published glideslope intercept altitude, you're going to have to comply with the published minimum altitude on that segment of the approach. Again, it's just my opinion.

My interpretation of what the AIM is saying is that if you intercept the glideslope before the final approach segment, you still have to comply with the altitudes published for any fixes between your intercept point and the PFAF where the glideslope is intended for use in the descent. What the AIM guindance seems intended to address is a situation where following the glideslope outside the PFAF/FAS could cause you to get below the published minimum for that portion of the approach and fix. The FAA only clears the ILS glideslope out to the FAF so there could easily be a tower just past the FAS that extends above the glide slope and for which a higher fix altitude is required.

The AIM guidance does not to me suggest that you have to be at altitude when intercepting the glideslope, just that you cannot necessarily follow the glideslope down if you intercept prior to the PFAF because you may violate an altitude restriction.

Lets take the approach provided by @midlifeflyer as an example.

If I maintain 4000' after crossing PECIT, I can expect glideslope intercept at approximately 12.5NM DME. That's 1NM beyond PECIT and 5NM before PURME. Great. Since there are no fixes between PECIT and PURME, I can fly the glideslope as the AIM indicates because the next altitude restriction of 2400ft is coincident with my PFAF and I therefore have no risk of descending below the minimum altitude for the segment between PECIT and PURME by following the glideslope.

But lets say I have a tower between PECIT and PURME. That tower is a tall tower at 1900' and sitting on top of a hill that is 200ft above the surrounding terrain of 700'. That puts the tower right at 2800 ft so I now need a fix at a higher altitude between PECIT and PURME, so the FAA creates a fix called TOWER with a minimum altitude of 3,000ft at DME 9.0, just 1.5NM before PURME. If I were to follow the glideslope from 4000' at my intercept point at 12.5NM, 3.5NM later I would be between 2850 and 2950' and below the minimum altitude of my new fix TOWER and dangerously close to the tower at 2800'.

This also works in reverse with overlying airspace. If I intercept at 4000 and 12.5NM but have a Class B shelf that extends down to 3000' at DME 10NM and therefore have a fix with a maximum altitude of 3000' ft at DME 10NM, following the glideslope down would have me bust the maximum altitude and the overlying Bravo by 250ft.

The FAA cleared the remaining 7.5NM of the final approach course inside the PFAF when they surveyed the approach so that a 3degree glide slope would work; minimum altitudes at fixes inside the FAF do not apply as long as you are on the glideslope but outside the PFAF, as I just illustrated its possible that fixes extend above or below the glideslope and this area has not been specifically surveyed for compliance with the glideslope.

That to me is what the AIM means when they say "Interception and tracking of the glide slope prior to the published glide slope interception altitude does not necessarily ensure that minimum, maximum, and/or mandatory altitudes published for any preceding fixes will be complied with during the descent."

It is therefore on you as the pilot to ensure compliance with minimum/maximum/mandatory altitudes at fixes outside the PFAF if you choose to intercept and track the glideslope prior to the PFAF. Notably the AIM does not suggest that you cannot or should not intercept prior to the PFAF, only that the glideslope is not intended to be used for vertical guidance before the PFAF and therefore you may find yourself in conflict with fix altitudes between the point of intercept and the PFAF.

In all situations, I would say the definition provided by @luvflyin applies:

"GLIDESLOPE INTERCEPT ALTITUDE− The published minimum altitude to intercept the glideslope in the intermediate segment of an instrument approach"

Intercepting at a higher altitude above the minimum altitude further out in the intermediate segment is fine so long as you are aware of the AIM guidance which says you might not want to follow the glideslope once intercepted from further out because it might not comply with all altitude restrictions between the intercept point and the PFAF

 

[aterpster]

It's an AIM recommendation, and not a regulatory requirement to intercept the glideslope at the "published glide slope intercept altitude" (absent a a mandatory altitude) but there are countless other recommendations within the AIM, especially w/r/t instrument approach procedures which are the instrument pilot's bread and butter of "best practices." In my opinion the best practice on an ILS is only intercept the glideslope at the published glideslope intercept altitude.

It is a regulatory requirement to not be below the lowest altitude charted for the intermediate segment altitude at GS interception.

 

[midlifeflyer]

But lets say I have a tower between PECIT and PURME. That tower is a tall tower at 1900' and sitting on top of a hill that is 200ft above the surrounding terrain of 700'. That puts the tower right at 2800 ft so I now need a fix at a higher altitude between PECIT and PURME, so the FAA creates a fix called TOWER with a minimum altitude of 3,000ft at DME 9.0, just 1.5NM before PURME. If I were to follow the glideslope from 4000' at my intercept point at 12.5NM, 3.5NM later I would be 2950 and below the minimum altitude of my new fix TOWER.

If one reads the full AIM and InFO text on the subject rather than picking one sentence out of it, this is exactly what both are talking about. If you intercept the GS further out, it does not guarantee you will not bust minimums for published step-down fixes before reaching the (P)FAF.

The inFO even helps explain itself by mentioning four airports as examples. Doesn't take much to see what the concern is. Here's the profile for an ILS at one of them - the SLC ILS 16L.


I haven't gone through them all, but if you intercept the glidepath somewhere around IRRON and follow it, you will bust the 9,000 stepdown minimums at TOOME.

 

[Ryan F.]

It is a regulatory requirement to not be below the lowest altitude charted for the intermediate segment altitude at GS interception.

Agree 100%. And that is why the AIM guidance to capture the glideslope at the published glideslope intercept altitude exists in 5-4-5. But it is possible to meet the regulatory requirement you describe and still be on the glideslope (temperature dependent.) If you're suggesting this is an unwise practice, I agree with you.

 

[Ryan F.]

Although the capability exists to fly step downs in GA aircraft,(G1000, GTN with GFC500 and G500/600, it is very high end avionics and not almost always. Those systems have VNV available, but they are a minority of the GA aircraft. I have a Bonanza with an Stec 60-2, G500TXi. GTN750 and it is not capable of automatically doing the step downs, although I did enable the VNV functions in the GTN. Still well over 100,000 WAAS GNS430 in the field and they don't have VNV. I expect the vast majority of GTN installations are not installed in aircraft with a GFC500 autopilot.

If one is using a WAAS Navigator and flies the LPV as you indicated, I would agree. But in order to get the LPV GP available for use outside of the step down fix that connects to the FAF, pilots use VTF. In that case, flying the LPV at KORD, the RNAV (GPS) RWY 10C, the same issue as flying the ILS. For too many GA pilots, VTF is all they know.

I think at this point we have found agreement in all of our statements.

 

[Ryan F.]

I dont view the paragraph outlined from the AIM as a "recommendation." Nowhere in the paragraph does it use the language "recommend." It is merely a statement of intended use of the glideslope's vertical guidance.

You may view it that way if you like.

The AIM guidance does not to me suggest that you have to be at altitude when intercepting the glideslope, just that you cannot necessarily follow the glideslope down if you intercept prior to the PFAF because you may violate an altitude restriction.

It's the English language and one can derive whatever they like from whatever has been written, but the AIM's wording is black and white. "The ILS glide slope is intended to be intercepted at the published glide slope intercept altitude." That is a very explicit statement which is difficult to twist into a different meaning. The intention is for the user to intercept the glideslope at the published glideslope intercept altitude.

If I maintain 4000' after crossing PECIT, I can expect glideslope intercept at approximately 12.5NM DME. That's 1NM beyond PECIT and 5NM before PURME. Great. Since there are no fixes between PECIT and PURME, I can fly the glideslope as the AIM indicates because the next altitude restriction of 2400ft is coincident with my PFAF and I therefore have no risk of descending below the minimum altitude for the segment between PECIT and PURME by following the glideslope.

This is where I believe your understanding lapses into misunderstanding. There is still a restriction not to descend below the published glide slope intercept altitude prior to intercepting the glideslope. It is possible the user may do so if intercepting outside the PFAF, depending on the temperature and pressure of the day.

It is therefore on you as the pilot to ensure compliance with minimum/maximum/mandatory altitudes at fixes outside the PFAF if you choose to intercept and track the glideslope prior to the PFAF. Notably the AIM does not suggest that you cannot or should not intercept prior to the PFAF, only that the glideslope is not intended to be used for vertical guidance before the PFAF and therefore you may find yourself in conflict with fix altitudes between the point of intercept and the PFAF.

In essence, I agree with this statement.

In all situations, I would say the definition provided by @luvflyin applies:

"GLIDESLOPE INTERCEPT ALTITUDE− The published minimum altitude to intercept the glideslope in the intermediate segment of an instrument approach"

Intercepting at a higher altitude above the minimum altitude further out in the intermediate segment is fine so long as you are aware of the AIM guidance which says you might not want to follow the glideslope once intercepted from further out because it might not comply with all altitude restrictions between the intercept point and the PFAF

I disagree. The reason it's the published minimum altitude is in lieu of a higher ATC-assigned altitude (which is included in the definition, but you left it out). The guidance suggesting that the glideslope only be intercepted at the published glideslope intercept altitude makes it clear that in absence of the higher ATC-assigned glideslope intercept altitude, the intention is for the user to intercept the glideslope at the published glideslope intercept altitude. Nowhere else.

If we shift this conversation to legalities, I believe it is "legal" to intercept the glideslope at a higher altitude than the published glideslope intercept altitude, so long as the user complies with the minimum or mandatory altitudes on any stepdown fixes which lie between the altitude at which the glideslope was intercepted and the PFAF. But it is contrary to the AIM's guidance. That won't help the airman much if he has to defend his decision-making in case of some kind of airspace violation or other pilot deviation.

 

[Ryan F.]

If one reads the full AIM and InFO text on the subject rather than picking one sentence out of it, this is exactly what both are talking about. If you intercept the GS further out, it does not guarantee you will not bust minimums for published step-down fixes before reaching the (P)FAF.

The inFO even helps explain itself by mentioning four airports as examples. Doesn't take much to see what the concern is. Here's the profile for an ILS at one of them - the SLC ILS 16L.
View attachment 74309

I haven't gone through them all, but if you intercept the glidepath somewhere around IRRON and follow it, you will bust the 9,000 stepdown minimums at TOOME.

Agree with everything you've posted here.

 

[midlifeflyer]

Agree with everything you've posted here.

I know.

Where I think we (agree to) disagree is, you appear to be going a step beyond the goal of both the InFO and AIM by pulling one sentence out of the context of the rest of the document to create an additional regulatory requirement which does not exist.

 

[Ryan F.]

I know.

Where I think we (agree to) disagree is, you appear to be going a step beyond the goal of both the InFO and AIM by pulling one sentence out of the context of the rest of the document to create an additional regulatory requirement which does not exist.

How so?

 

[midlifeflyer]

How so?

You at least seem to be saying that remaining at 4000 after crossing (referring to my earlier profile) PECIT until intercepting the glideslope with no step-downs between it and PURME is contrary to the guidances.

I don't see that. You do, unless I misunderstood.

And yes, "regulatory requirement" as opposed to "best practice" was more than I meant. But I don't see a violation of and best practice in stabilizing in the final approach configuration - on airspeed and glidepath - before the PFAF.

 

[Ryan F.]

You at least seem to be saying that remaining at 4000 after crossing (referring to my earlier profile) PECIT until intercepting the glideslope with no step-downs between it and PURME is contrary to the guidances.

I don't see that. You do, unless I misunderstood.

Yes, that is contrary to the guidance - which is not a "regulatory requirement," and I never said it was, by the way.

The FAA Aeronautical Chart User's Guide describes the "lightning bolt symbol" as "glide slope intercept altitude."

As previously stated, AIM 5-4-5 contains the statement "The ILS glide slope is intended to be intercepted at the published glide slope intercept altitude. This point marks the PFAF and is depicted by the "lightning bolt" symbol on U.S. Government charts. Intercepting the glide slope at this altitude marks the beginning of the final approach segment and ensures required obstacle clearance during descent from the glide slope intercept altitude to the lowest published decision altitude for the approach."

In your example, the minimum altitude at PECIT is 4000. The published glideslope intercept altitude is 2400 (roughly coincident with the FAF, PURME.) Therefore, intercepting the glideslope at PECIT (or any altitude other than 2400, absent ATC instruction to the contrary) is directly at odds with the AIM's explicit guidance.

There are numerous other references which support the guidance. The FAA Instrument Procedures Handbook, Chapter 4, p. 4-19 states "The glideslope intercept altitude of a precision approach should also be included in the IAP briefing. Awareness of this altitude when intercepting the glideslope can ensure the flight crew that a “false glideslope” or other erroneous indication is not inadvertently followed." Page 4-49 states "The final approach segment for an approach with vertical guidance or a precision approach begins where the glideslope/glidepath intercepts the minimum glideslope/ glidepath intercept altitude shown on the approach chart. If ATC authorizes a lower intercept altitude, the final approach segment begins upon glideslope/glidepath interception at that altitude."

I understand your contention, which is that if there are no stepdown fixes between an IF (for example) and the PFAF, there should be no issue with simply following the glideslope from that point. However, the guidance doesn't contain that disclaimer and the IPH references the false glideslope consideration. Your real world analysis might end up suggesting that you're okay with tracking the glideslope down from PECIT in your example, but to be clear, that is contrary to the AIM's explicit guidance. If you can demonstrate how these various references are somehow in err or misleading in some way, I'm all ears.