ILS GS below charted altitudes?

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pstan

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Stan
Looking for some help from the group. Does there exist any ils approach where you can be established on a portion of the instrument approach procedure that is on the extended centreline, and that it is possible that following the glideslope will not ensure you remain above the step down fix altitudes?
Hope my question is clear; I'm meaning if you were to follow the glideslope before the feather depicted on the profile view, but laterally on the instrument procedure and on the final approach course, is it possible to be below charted altitudes?

Any replies appreciated.

Stan
 
Let me rephrase your question so I'm sure I understand it ("let's see if the symbols you sent and I received actually communicated the ideas you intended"). I think you're asking:

Are there any ILS approaches where intercepting the localizer and glideslope well outside the final approach fix and following their guidance from that point inbound would put you below what would otherwise be a minimum altitude if you were only on the localizer and the glideslope was inoperative?

or

If I get established on the localizer and glideslope outside the FAF, and follow the glideslope for vertical guidance, am I gonna hit anything?
 
pstan said:
the glideslope before the feather depicted on the profile view,
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I think this "before the feather" is the key here.
But then I don't know why you are asking this question - the glideslope is really supposed to be flown from the FAF (or maybe slightly before that) so I don't understand why you would hope it is going to protect you from whatever altitude restrictions proceed the FAF. I just don't see any relevance here.
 
"Are there any ILS approaches where intercepting the localizer and glideslope well outside the final approach fix and following their guidance from that point inbound would put you below what would otherwise be a minimum altitude if you were only on the localizer and the glideslope was inoperative?"

Very well put Tim, on re reading my post, I realize what I wrote was not quite what I meant. So...I like the way you worded it, however I would change what you wrote to say "...would put you below the required obstacle clearance ROC of an ils approach" So...let me re state it

"Are there any ILS approaches where intercepting the localizer and glideslope well outside the final approach fix and following their guidance from that point inbound would put you below the required obstacle clearance ROC of an ils approach". With the caveat that I don't mean out past the distance to which the final approach is drawn, I mean for portions of the charted final approach.

Basically, I'm trying to determine a case where following the glideslope is possibly going to get you into trouble.
 
Olasek, I'm not hoping it's going to protect me. On the contrary, if it doesn't then I'm trying to show others that it may not protect them. But I need some facts/references to that effect. That's the relevance.

If there's some relevance to the feather, I'd love to read it, pass it on to others. Do you have some references? I'd like to show that following the glideslope while still on the approach laterally has some perils. But I'm not sure.

Info anyone?

thanks
 
Stan, I know what you are after but I don't think you will have much luck in finding such examples. And the reason is that the only way such situation could arise if the intermediate portion of the approach was steeper than the final approach. This could happen only in some peculiarly challenging mountainous terrain but even there I don't think you will see it. Because designers of the approach always have more flexibility with the intermediate segment - they can bent it (DME arcs), position it an angle, etc. there are various techniques to make sure this part is not very steep. It is the final segment that is usually more challenging, hence I wouldn't spent sleepless nights sifting through thousands of approaches trying to find a suitable example since it may not exist.
 
pstan said:
Basically, I'm trying to determine a case where following the glideslope is possibly going to get you into trouble.
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I don't think you're going to find that. :no:

FWIW, I don't think the "feather" on the profile view means anything, distance-wise. The thing that's most important is the service volume of a glideslope. IIRC, standard service volume of a localizer within 10 degrees is 18nm and within 30 degrees is 10nm, and standard. service volume of a glideslope when on the localizer it 10nm. Now I'm gonna go look that up and see if I'm right.

EDIT: I was almost right. The localizer is good out to 10nm up to 35 degrees from centerline.
 
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olasek said:
the glideslope is really supposed to be flown from the FAF (or maybe slightly before that)
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Standard service volume on a glideslope is 10nm which is usually twice as far as the FAF, so there's nothing wrong with intercepting it farther out. In fact, some of the larger/busier airports such as ORD have glideslopes with extended service volumes and they bring in long lines of planes down an extended final, on the localizer and glideslope from 8 or 10 thousand feet. It's not at all unusual to see at least a handful of planes on the glideslope at once coming into ORD (kinda neat to see out over Lake Michigan at night) or even SDF at night when all the UPS planes are coming in. At 3 miles in trail... Do the math.

But yeah, us baby birds usually don't get to do that.
 
Taking a look at the TERPs manual (with the usual caveat that I don't know what I'm talking about), it does seem possible to construct an approach that adheres to the TERPs criteria and has a situation where the glideslope is below an intermediate segment minimum altitude. The 'optimum' decent gradient for an initial or intermediate segment is 250ft/nm, which is shallower than 3 degrees, so in any approach where the optimum decent gradients are adhered to, the glideslope will be above the minimum altitudes. The maximum allowable gradient, however is 500ft/nm, which is closer to 5 degrees, so in theory, you could have a very short intermediate segment (there's no minimum length) with the maximal allowable decent gradient that would start you above the glideslope and then drop you below it. In order to have a situation where the glideslope actually put you below the intermediate segment obstacle clearance surface, you'd need to have a segment where you loose 2500ft or more at the maximum gradient (that would be 5nm for 2500ft). Glideslopes are only protected to the glideslope intercept point.

But all this is academic - I'm not aware of any actual examples of an approach that would do this.
 
Yep - the southward facing ILS approaches into SEA require decent gradients steeper than 3 degrees to get to the final approach fix, but you can intercept the ILS up higher, so there's clearly no obstacle problem in doing so. Which approach at LAX, though? They all looked shallower than 3 degrees to me.
 
chucky said:
Yep - the southward facing ILS approaches into SEA require decent gradients steeper than 3 degrees to get to the final approach fix, but you can intercept the ILS up higher, so there's clearly no obstacle problem in doing so. Which approach at LAX, though? They all looked shallower than 3 degrees to me.
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I have heard (not looked at it closely enough though) that one of the ILS approaches at LAX will put you below the B airspace if you are on the glideslope... Think 'airspeed violation' for the jets.
 
Speed said:
I have heard (not looked at it closely enough though) that one of the ILS approaches at LAX will put you below the B airspace if you are on the glideslope... Think 'airspeed violation' for the jets.
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Apparently that's a common problem at several Class Bs and being addressed one by one at the FAA. The recent changes at ORD and MSP were driven by this issue.
 
olasek said:
the glideslope is really supposed to be flown from the FAF (or maybe slightly before that).
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Any reference to this available? I always thought the "feather" had a significance, and that one was not to trust the GS before that point, but popular opinions seems to be that this is bogus. I'd like to find some direction on how one flies an ils approach when it comes to using the "feather" or not.

Even a reference to NOT use the GS outside it's service volume would be helpful, as I've been told that the angle and distances are a minimum you should expect, and if you have a good GS signal before that you can use it.

If I don't, the naysayers will keep telling me to "just fly the GS once you get it" and I will have no argument to counter that with.

Re Seattle and LAX, using 318 feet per nm these do NOT seem greater than the 3 deg ils GS, so those do not seem to be examples.
 
pstan said:
Re Seattle and LAX, using 318 feet per nm these do NOT seem greater than the 3 deg ils GS, so those do not seem to be examples.
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GS angle of elevation can be found in the profile view of the approach plate. All of the ILSs at KLAX are 3.00 degrees. The ILS RWY 34R at KSEA is 2.75 degrees, the rest are 3.00 degrees.
 
Lots of Class B airports have ILS's with stepdown fixes outside the precision approach FAF and a note that says "when assigned by ATC, intercept glidepath at xxxx and xxxx and ..." where the xxxx are the stepdown fix minimum altitudes. Being barometric altitudes, their vertical position relative to the GS varies according to the temperature relative to standard, so it's quite possible for the GS to cross below the stepdown fixes on a warmer than standard day. There's a thread over on the Red Board where someone reported that pilots at LAX are being given deviation citations for intercepting the GS outside the p-FAF (presumably without being specifically assigned an intercept altitude but only being given a "maintain until" altitude and cleared for the approach) and busting the stepdown altitudes.
 
Azure:

azure said:
when assigned by ATC, intercept glidepath at xxxx
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OK I've seen that on plates, "intecept glidepath at 5000..." etc. But sometimes this is outside the "feather". So will the GS have been verified for quality/reliability outside of the "feather". And if so, what does the feather really mean then?

azure said:
so it's quite possible for the GS to cross below the stepdown fixes on a warmer than standard day. There's a thread over on the Red Board
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I understand the concept, but is there a single approach anywhere you know of that this condition exits?

Red board, can you give me a link?

thanks, Stan
 
scottd said:
When the approach is published, there's no requirement to verify the quality/reliability of the signal outside of the GSIA.
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thanks Scott. I presume the GS has been verified at those airports that state words like "intercept GS at 6000 feet"?

Do you know a pub available online that states that the GS is not verified prior to the GSIA?
 
@Stan: My understanding is that this condition has led to loss of separation and hence PD citations at LAX at least.

Also, Steven (roncachamp) is the authority on this, but my understanding is that the GS on these approaches has an extended service volume. I'm actually not sure what the GS feather in the profile view means and would be interested in the answer.

I see palmpilot has already given you a link to the thread on the red board.
 
flyingcheesehead said:
You shouldn't use ANY navaid outside its service volume. :no: Or did you mean outside the FAF/GSIA/something else?
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There are some airways that use VORs outside their service volumes, but in those cases they would have been flight-checked. In Nevada, V113 north of Mustang VORTAC is an example.
 
scottd said:
I've talked to several pilots who flight check the approaches for the FAA and they have no requirement to flight check the GS above the GSIA for an ILS. Here's one of the many e-mails I received from him on this subject...

The glide slope is routinely flight checked within the operational service volume to ensure it supports the intended use procedurally on the Standard Instrument Approach Procedure (SIAP). Among other checks, on-path evaluations are conducted from GSIA inbound and checks beyond the published GSI are evaluated below the GS for adequate fly-up indication. The GS is NOT checked for crossing restriction compliance at fixes outside the published GSI. Intercepts of the GS at altitudes above the published GSIA may result in deviations from procedural requirements and failure to meet altitude requirements at fixes outside the published point of GS intercept. [emphasis added]
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This brings up a problem with the way glide slope intercept altitudes are depicted on FAA approach charts. They are shown as minimum altitudes, not mandatory altitudes, and no maximum intercept altitudes are given.

Edit: You can look at US Standard Flight Inspection Manual, FAA Order 8200.1B (or whatever the latest version is). There may be some information in there. But I'm not sure.
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Verson C appears to be current.

http://www.faa.gov/documentLibrary/media/Order/8200.1C.pdf
 
Palmpilot said:
There are some airways that use VORs outside their service volumes, but in those cases they would have been flight-checked. In Nevada, V113 north of Mustang VORTAC is an example.
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That is an interesting example - I'm somewhat surprised that the changeover point is where it is, much closer to the high-altitude VOR (FMG) than the low-altitude VOR (SDO).

However, as you note, on an airway the navaid has been tested so it's OK to use it outside the *standard* service volume, as its particular service volume is effectively extended. That's somewhat similar to the extended glideslopes I mentioned before - They've been checked, so their service volume is effectively extended.
 
pstan said:
"Are there any ILS approaches where intercepting the localizer and glideslope well outside the final approach fix and following their guidance from that point inbound would put you below the required obstacle clearance ROC of an ils approach". With the caveat that I don't mean out past the distance to which the final approach is drawn, I mean for portions of the charted final approach.
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Without access to the survey(s) used to construct the ILS approach(es) in question, it would seem impossible to answer this inquiry. ROC is only one of many reasons why a stepdown altitude may be established outside the charted glide slope intercept altitude (GSIA)

Basically, I'm trying to determine a case where following the glideslope is possibly going to get you into trouble.
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Basically, yes: Any time you are operating on an IFR clearance, descent below a charted minimum or mandatory altitude can possibly get you into trouble. As noted in previous posts, a number of pilots have been subject to FAA enforcement action for descending below charted step-down altitudes while established on the localizer and glideslope of an ILS approach.

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).

Hold the presses--I'm living in the past. Apparently this has changed and the point at which the glideslope intersects the GSIA has been given a shiny new moniker: Precision Final Approach Fix (PFAF). Apologies for having posted bad info.


Hope this helps.

buzz
 
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username said:
As noted in previous posts, a number of pilots have been subject to FAA enforcement action for descending below charted step-down altitudes while established on the localizer and glideslope of an ILS approach.
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Are you talking about flight on the GS past (towards the runway from) glide slope intercept at the published GSIA and prior to reaching DA. And if so could you provide an example or two. Or was this about descending on the GS prior to the published intercept where there might be an underlying segment altitude that's higher than the GS at that point?
 
username said:
As noted in previous posts, a number of pilots have been subject to FAA enforcement action for descending below charted step-down altitudes while established on the localizer and glideslope of an ILS approach.
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Somehow I fail to see such posts. Could you elaborate or give examples.
 
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gismo said:
Post 21 indirectly refers to an example of ILS 16 at SEA. On that approach the 3 degree GS runs below the beginning of each intermediate segment because the straight line path through those points has a slightly steeper approach angle (3.18-3.29°).

http://aeronav.faa.gov/d-tpp/1011/00582IL16L.PDF
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Although someone pointed out that the altitudes on the GS don't take into account the curvature of the Earth.
 
PilotAlan said:
Although someone pointed out that the altitudes on the GS don't take into account the curvature of the Earth.
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The curvature of the earth out at 15 NM from the antenna, can be more than 200 feet. The equation to determine the adjusted MSL altitude can be derived from high school geometry. It is

Z = ((r+LTP elev +TCH)*cos(θ*π/180))/cos(D/r+θ*π/180)-r

Where Z is the MSL altitude of the GS at a distance D feet from the threshold. LTP is the Landing Threshold Point elevation and is feet MSL. LTP elevation is from the survey, but you can substitute the TDZE (Touchdown Zone Elevation) which will usually be within a few feet of the survey value. The parameter r is the radius in feet of the earth (20,890,537). θ is the GS angle in degrees. TCH is the Threshold Crossing Height in feet. All of the parameters can be obtained from the approach chart.

I have attached a spreadsheet that you can use to determine the GS elevation at a given distance in NM (I do the conversion from NM to feet). For the SEA ILS or LOC RWY 16L approach, the following are the GS elevations corrected for the curvature of the earth at the named fixes, followed by the minimum crossing altitude:
DGLAS - 1933, 1900
KARFO - 3225, 3200
HELZR - 4030, 4000
KENMO - 5016, 5000
COROK -6018, 6000


 

Attachments

  • GS altitude.zip
    6.5 KB · Views: 8
The issue pointed out by the red board is that the minimum crossing altitudes must be adhered to on the approach for the step downs outside of the PFAF. The baro altimeter reads lower than the actual altitude when the temperatures are above ISA and above when colder. The GS is fixed in space. So on hot days, the altimeter will read below the step down altitude if you follow the GS. Following the GS will keep you the same height above any obstacle, regardless of temperature, but will provide less vertical separation from any traffic or airspace that is below the GS.

A clearance for the ILS would still require observance of the minimum altitudes as it is only a clearance to follow the procedure. I have not heard an ATC clearance to intercept and track the GS and no wording for such a clearance is included in the controller manual (7110.65T), but if it was issued, then one would think that the note authorizing intercept of the GS at higher altitudes when authorized by ATC would absolve the pilot of adhering to the step down minimums, but apparently they don't.

The issue is still unresolved. Hopefully, eventually there will be wording n the AIM and 7110.65 to deal with this, but until it does, don't descend below a step down altitude while following the GS and outside of the PFAF. Be particularly careful on hotter days.
 
John Collins said:
A clearance for the ILS would still require observance of the minimum altitudes as it is only a clearance to follow the procedure. I have not heard an ATC clearance to intercept and track the GS and no wording for such a clearance is included in the controller manual (7110.65T), but if it was issued, then one would think that the note authorizing intercept of the GS at higher altitudes when authorized by ATC would absolve the pilot of adhering to the step down minimums, but apparently they don't.
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Are you actually aware of any cases where a PD was issued for going down minimum step-down altitudes after being assigned an intercept altitude outside the GSIA/p-FAF? (as opposed to just being told to maintain an altitude until established and given an approach clearance) The approach plate seems to explicitly authorize doing that!

People raised this point on the Red Board and no one came in with an example. The consensus seemed to be that being cleared for the approach wasn't sufficient authorization to ignore the step-down minimum altitudes outside the GSIA, but being assigned a higher intercept altitude was. The OP did not clarify this point, but after rereading post #1 several times I thought the consensus was likely correct... but one counterexample would smash this idea.
 
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azure said:
Are you actually aware of any cases where a PD was issued for going down minimum step-down altitudes after being assigned an intercept altitude outside the GSIA/p-FAF? (as opposed to just being told to maintain an altitude until established and given an approach clearance) The approach plate seems to explicitly authorize doing that!

People raised this point on the Red Board and no one came in with an example. The consensus seemed to be that being cleared for the approach wasn't sufficient authorization to ignore the step-down minimum altitudes outside the GSIA, but being assigned a higher intercept altitude was. The OP did not clarify this point, but after rereading post #1 several times I thought the consensus was likely correct... but one counterexample would smash this idea.
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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/
 
Thanks. So wouldn't you say that this excerpt from that publication supports what I called the "consensus" interpretation from the Red Board?

While ATC may authorize a pilot to intercept and track the glide slope above the glide slope interception altitude in support of parallel simultaneous ILS approach operations, as noted on the approach chart, absent such authorization [emphasis mine] the pilot must meet all published altitudes for each fix in accordance with AIM 5-4-5b.

When cleared for the approach, quite often the pilot will intercept and track the ILS glide slope at the earliest opportunity perhaps believing that the glide slope will ensure that published fix crossing altitudes will be achieved. However, this practice has resulted in numerous pilot deviations of the mandatory 1,500 ft MSL crossing altitude published at DANDY intersection on the Teterboro, NJ ILS RWY 6 approach:
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Once the FAA figures out what they really want to do about this, they should add clear guidance to their publications. Until then, If I were king, the PDs that have been issued for this would be rescinded, because the glide slope intercept altitude is not depicted as a mandatory altitude on FAA approach charts or the Aeronautical Chart Users Guide, and it's not reasonable to publish charts that say you can intercept the glide slope at OR ABOVE a certain altitude, and then turn around and say that oh-by-the-way you can get in trouble for following a glide slope from an altitude that the chart says is OK to use for interception.
 
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