ILS intercept altitudes

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mxalix258

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mxalix258
Hi everyone, another quick question regarding glideslope intercepting altitude. First, if you are at a higher altitude and have step down fixes before glide slope intercept altitude, do you intercept the gs at the higher altitude and follow it down to DA or do you do the step downs to the gs intercept altitude and follow it from there?

Also, say I am coming in at 5000 and the controller says "cessna 123 fly heading 010 maintain 3000 until established, cleared ils xyz".

Can I immediately start down to 3000?

Is it my discretion to descend from 5000 to 3000?

Thanks!
 
mxalix258 said:
First, if you are at a higher altitude and have step down fixes before glide slope intercept altitude, do you intercept the gs at the higher altitude and follow it down to DA or do you do the step downs to the gs intercept altitude and follow it from there?
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Depends on what it says on the chart, but generally speaking, you are expected (but not required) to make the stepdowns -- in essence, it's pilot's discretion as long as you obey any other restrictions once cleared for the approach. Only when the intermediate altitudes have overlines on them are you required to descend to (or below) those altitudes.

Also, say I am coming in at 5000 and the controller says "cessna 123 fly heading 010 maintain 3000 until established, cleared ils xyz".

Can I immediately start down to 3000?
Click to expand...
Yes. In fact, you must.

Is it my discretion to descend from 5000 to 3000?
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No. When the controller says to "maintain" an altitude, you are required to begin your descent to that altitude promptly.
 
What would you say is better practice? To follow the stepdowns to GS intercept altitude? or intercept and follow the GS down?
 
mxalix258 said:
What would you say is better practice? To follow the stepdowns to GS intercept altitude? or intercept and follow the GS down?
Click to expand...

For me, I like step downs to GS intercept like it shows on the chart. Those who like the high GS intercept method will correctly point out that eliminating the step downs reduces the workload.

Ryan
 
Is it my discretion to descend from 5000 to 3000?
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If ATC uses the phrase "at pilot's discretion" in an altitude clearance then yes. Otherwise no. AIM 4-4-10.
 
mxalix258 said:
What would you say is better practice? To follow the stepdowns to GS intercept altitude? or intercept and follow the GS down?
Click to expand...
Following the stepdowns always keeps you legal, following the GS may or may not and it's up to the pilot to determine when going down the GS won't work.
 
mxalix258 said:
Also, say I am coming in at 5000 and the controller says "cessna 123 fly heading 010 maintain 3000 until established, cleared ils xyz".



Thanks!
Click to expand...


Would this not indicate some confusion or lack of clarity by the controller if you were at 5000. Would you not expect him to say "Descend" and maintain 3000? If he doesn't say "decend", then would that indicate that he thinks you are already at 3000?
 
skidoo said:
Would this not indicate some confusion or lack of clarity by the controller if you were at 5000. Would you not expect him to say "Descend" and maintain 3000? If he doesn't say "decend", then would that indicate that he thinks you are already at 3000?
Click to expand...

Leaving 5000 for 3000
 
skidoo said:
Would this not indicate some confusion or lack of clarity by the controller if you were at 5000. Would you not expect him to say "Descend" and maintain 3000? If he doesn't say "decend", then would that indicate that he thinks you are already at 3000?
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I thought that, too, the first time I got an instruction like that, but since my checkride I've gotten that as often as the explicit "descend and maintain". I think descend and maintain is implied in that context.
 
Ron Levy said:
Depends on what it says on the chart, but generally speaking, you are expected (but not required) to make the stepdowns -- in essence, it's pilot's discretion as long as you obey any other restrictions once cleared for the approach.
Click to expand...

Where did you get that idea? Step-down fixes control, not the G/S prior to the G/S intercept point. Pilots have been violated for following the G/S on hot days and going below step-down fixes outside the P-FAF. This happened often in the Ontario area on the LAX ILS IAPs.

Flying Baro VNAV to the P-FAF does resolve the issue, though.
 
Normally with radar vectors to final, step downs will not be an issue. However, once you join the localizer course, and cleared for the approach, you are required to follow any approach chart minimum altitudes from the point of interception. If the vector was intercept and track the localizer and there are step downs between your point of interception and the GS intercept altitude, you have to abide by the step down minimums. Following the GS will not assure you will clear the step down fixes at or above the minimum altitudes and can result in a pilot deviation.
 
aterpster said:
Where did you get that idea? Step-down fixes control, not the G/S prior to the G/S intercept point. Pilots have been violated for following the G/S on hot days and going below step-down fixes outside the P-FAF. This happened often in the Ontario area on the LAX ILS IAPs.
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Inside the P-FAF, not an issue? That has been my understanding but I would like to confirm (or reconfirm). On the plates I've seen around here, most such stepdowns on vertically guided approaches are marked with an asterisk referring to a note that says LOC only or LNAV only.
 
azure said:
Inside the P-FAF, not an issue? That has been my understanding but I would like to confirm (or reconfirm). On the plates I've seen around here, most such stepdowns on vertically guided approaches are marked with an asterisk referring to a note that says LOC only or LNAV only.
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Inside the P-FAF those fixes only apply to the LOC procedure. (or LNAV on an LPV and or LNAV/VNAV IAP.)
 
John Collins said:
Normally with radar vectors to final, step downs will not be an issue. However, once you join the localizer course, and cleared for the approach, you are required to follow any approach chart minimum altitudes from the point of interception. If the vector was intercept and track the localizer and there are step downs between your point of interception and the GS intercept altitude, you have to abide by the step down minimums. Following the GS will not assure you will clear the step down fixes at or above the minimum altitudes and can result in a pilot deviation.
Click to expand...

More and more there are ILS approaches where vectors are not involved, rather a STAR ties to the ILS. The west ILS IAPs at LAX are a good example.

What does happen at a place like LAX, you can be told to expect 25L, for example, you program that, then when you are handed off to approach control they say, "Turn right heading 260 to intercept the 24R ILS, cleared for the approach." And, this is early on with several step-down fixes remaining.
 
I've found, at least lately, that they'll get you do the altitude they want, 5,400 for example. Then even though the ILS intercept altitude is 5,000 they'll say "Maintain 5,400 until established cleared ILS 32." The last approach I flew about a week ago was at RAP and that's what it sounded like. Notice you're well above 5,500 even if you're not getting vectors to final, but they put you at 5,400 before clearing you on the approach. I think I was cleared a couple miles from RANCH. Glide slope came in like it was supposed to.. heck even saw the marker beacons, never shot an ILS with operative beacons before.

http://155.178.201.160/d-tpp/1306/00877IL32.PDF
 
mxalix258 said:
so if I'm doing the ILS at Winchester, and I'm coming in at 3700...I'd stay at 3700 until intercepting the localizer then I'd drop down to 3100 until past Cedar, then down to 2200 to intercept GS?

http://155.178.201.160/d-tpp/1306/05251IL32.PDF
Click to expand...

It would depend on if you were on vectors or one of the feeder routes. With a GPS, now you could also be cleared direct to CLADD or CEDAR. With vectors, it would depend on where you intercepted the localizer. Outside of CLADD, you would remain at 3700 until CLADD, then 3100 to CEDAR, then 2200 until GS intercept.
 
Glideslope may be unreliable if you are far out from the Runway (usual service volumes 10 but can be-18NM)..so I there I would caution to follow the step downs to maintain terrain clearance and airspace compliance. Per above in Bravo airspace many ILS approaches have step downs that keep you out of conflicting airspace.

Always a good idea to at least make sure that the Glide-Slope "checks" at the step down fixes if you are going to follow it.

I usually follow the GS if within a reasonable distance and with the points above in mind.
 
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mxalix258 said:
so if I'm doing the ILS at Winchester, and I'm coming in at 3700...I'd stay at 3700 until intercepting the localizer then I'd drop down to 3100 until past Cedar, then down to 2200 to intercept GS?

http://155.178.201.160/d-tpp/1306/05251IL32.PDF
Click to expand...


perfectly fine,
Obviously the GS should track through these waypoints anyway. If you choose to follow the GS outside the FAF think "for advisory purposes only" until you get to the FAF.
 
Sidebar question- the links to the approach plates (as in the above examples) where do you find them?
 
taters said:
perfectly fine,
Obviously the GS should track through these waypoints anyway. If you choose to follow the GS outside the FAF think "for advisory purposes only" until you get to the FAF.
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Well, not really obvious is it? The glideslope on an ILS isn't adjusted based on temperature AFAIK, and those altitudes being barometric definitely depend on temperature. The higher your altitude above the station from which the altimeter setting was obtained, the greater the difference if the conditions are significantly non-standard.

Which I guess begs the question, when crossing altitudes along a glideslope are charted, what conditions are assumed? Standard would seem the obvious answer, but I really don't know. :dunno:
 
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mxalix258 said:
What would you say is better practice? To follow the stepdowns to GS intercept altitude? or intercept and follow the GS down?
Click to expand...
I wouldn't. I consider it technique, and leave it up to the individual to decide what works best for him/her.
 
gismo said:
Following the stepdowns always keeps you legal, following the GS may or may not and it's up to the pilot to determine when going down the GS won't work.
Click to expand...
Absent a hardline altitude between the IAF and the FAF, do you know of any case where following the GS from the last assigned altitude would not work?
 
aterpster said:
Where did you get that idea? Step-down fixes control, not the G/S prior to the G/S intercept point. Pilots have been violated for following the G/S on hot days and going below step-down fixes outside the P-FAF. This happened often in the Ontario area on the LAX ILS IAPs.
Click to expand...
Did you miss the part in my post about "other restrictions"? There simply isn't anything requiring you to drop to the step-down rather than following the GS down as long as there are no hardline restrictions that following the GS would violate. So, for example, on the ILS 4 at KESN, after passing RIKME, you can:
  1. stay at 2000, intercept the GS, and follow it all the way to DA, or
  2. drop to 1600 and intercept it near WEGRO.
Your choice, and completely legal either way.
 
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Ron Levy said:
Did you miss the part in my post about "other restrictions"? There simply isn't anything requiring you to drop to the step-down rather than following the GS down as long as there are no hardline restrictions that following the GS would violate. So, for example, on the ILS 4 at KESN, after passing RIKME, you can:
  1. stay at 2000, intercept the GS, and follow it all the way to DA, or
  2. drop to 1600 and intercept it near WEGRO.
Your choice, and completely legal either way.
Click to expand...

I commented based on your first statement in Post 2:

Depends on what it says on the chart, but generally speaking, you are expected (but not required) to make the stepdowns -- in essence, it's pilot's discretion as long as you obey any other restrictions once cleared for the approach. Only when the intermediate altitudes have overlines on them are you required to descend to (or below) those altitudes.
Click to expand...
 
Ron Levy said:
And I'll stand by exactly what I said in that quotation -- word for word.
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You gave a simple example so I'll give you a complex, but real world example where pilot deviations have occurred. Attached is LAX ILS 24R.

If I have a good stable G/S out around SKOLL can I disregard the step-downs between SKOLL and MERCE so long as I follow the G/S?
 

Attachments

  • Jepp KLAX ILS 24R.pdf
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aterpster said:
You gave a simple example so I'll give you a complex, but real world example where pilot deviations have occurred. Attached is LAX ILS 24R.

If I have a good stable G/S out around SKOLL can I disregard the step-downs between SKOLL and MERCE so long as I follow the G/S?
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No -- you cannot disregard those restrictions. And that is entirely consistent with what I said.
...in essence, it's pilot's discretion as long as you obey any other restrictions once cleared for the approach.
Click to expand...
 
Ron Levy said:
No -- you cannot disregard those restrictions. And that is entirely consistent with what I said.
Click to expand...

Happy to hear you say that. I didn't understand you the first time around.

Thus, the limitation of this medium. :yes:
 
azure said:
Which I guess begs the question, when crossing altitudes along a glideslope are charted, what conditions are assumed? Standard would seem the obvious answer, but I really don't know. :dunno:
Click to expand...

My guess is that the stepdowns are selected to ensure that sufficient terrain clearance exists over a specified range of altimeter settings, and that their relationship to the glideslope is not necessarily considered, but Wally probably knows for sure.
 
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aterpster said:
Happy to hear you say that. I didn't understand you the first time around.

Thus, the limitation of this medium. :yes:
Click to expand...

What makes those step owns different (in terms of ability to follow GS) ?And on that chart, would ATC be advising you when you are at specific fixes to descend?
 
Palmpilot said:
My guess is that the stepdowns are selected to ensure that sufficient terrain clearance exists over a specified range of altimeter settings, and that their relationship to the glideslope is not necessarily considered, but Wally probably knows for sure.
Click to expand...

Some of the step downs were originally too close to the GS altitude meaning that any ISA temperatures above standard would make the step down MSL altitude be below the GS. An example was the 25L at LAX. The nominal GS altitude at the 5000 step down fix was 4993 MSL. On a hot day, for example at 95 F, since the GS remains fixed in space but the altimiter would read more than 300 feet lower at this point. This would be detected as a deviation. The 5000 foot minimum was at 16.6 DME. It was changed to be at 17.2 DME. This moved the nominal GS crossing altitude to 5200 feet, so following the GS now under the same conditions would put the aircraft at 4900 feet. It is below the 5000 foot minimum, but within IFR permitted tolerances and would not generate a deviation.
 
John Collins said:
Some of the step downs were originally too close to the GS altitude meaning that any ISA temperatures above standard would make the step down MSL altitude be below the GS. An example was the 25L at LAX. The nominal GS altitude at the 5000 step down fix was 4993 MSL. On a hot day, for example at 95 F, since the GS remains fixed in space but the altimiter would read more than 300 feet lower at this point. This would be detected as a deviation...
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Is that 95 deg. F at 5000 feet?

...The 5000 foot minimum was at 16.6 DME. It was changed to be at 17.2 DME. This moved the nominal GS crossing altitude to 5200 feet, so following the GS now under the same conditions would put the aircraft at 4900 feet. It is below the 5000 foot minimum, but within IFR permitted tolerances and would not generate a deviation.
Click to expand...

When an altitude is depicted as a minimum, I thought the IFR permitted tolerance was +100/-0. At least that's what I remember from the PTS.

http://download.aopa.org/ustprocs/current/SW-3/lax_ils_or_loc_rwy_25l.pdf
 
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mxalix258 said:
What makes those step owns different (in terms of ability to follow GS) ?And on that chart, would ATC be advising you when you are at specific fixes to descend?
Click to expand...

They are no different. With an ILS with one or more step-down fixes prior the G/S intercept point, the step-downs are the controlling altitudes. The G/S is there but if you decide to use it the step-down fixes still control.

At LAX it isn't unusual to get cleared for one of those west ILS IAPs from 30 miles out. Once the approach clearance is issued the fixes control, not ATC. ATC can intervene of course but they don't unless a traffic conflict comes up after they have issued the approach clearance.

Charting requirements require the profile view begin not later than the intermediate fix. Note at LAX they extended the profile well east of the IFs in an attempt to increase pilot awareness.
 
Palmpilot said:
Is that 95 deg. F at 5000 feet?



When an altitude is depicted as a minimum, I thought the IFR permitted tolerance was +100/-0. At least that's what I remember from the PTS.

http://download.aopa.org/ustprocs/current/SW-3/lax_ils_or_loc_rwy_25l.pdf
Click to expand...

The 95 degrees F is at the airport. Standard lapse rate is used from there. In LA an inversion is common, so the real altitude deviation can be worse.

From the Instrument PTS:

9. Maintains, prior to beginning the final approach segment,
altitude within ±100 feet, heading within ±10° and allows
less than ¾-scale deflection of the CDI or within ±10° in
the case of an RMI, and maintains airspeed within ±10
knots.
10. ...
11. ...
12. ....
13. Maintains the MDA, when reached, within +100 feet, −0 feet
to the MAP.
Click to expand...
 
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Palmpilot said:
My guess is that the stepdowns are selected to ensure that sufficient terrain clearance exists over a specified range of altimeter settings, and that their relationship to the glideslope is not necessarily considered, but Wally probably knows for sure.
Click to expand...

Step-downs are usually, but not always, for terrain clearance. For the LAX west ILS IAPs in the POM area they are to assure vertical separation over the Empire area of SoCal; i.e., effectively two different approach control facilities. The step-down fixes are also design to remain within the Class B airspace.

Keep in mind these LAX profiles are for the LOC as well as the ILS.
 
John Collins said:
Some of the step downs were originally too close to the GS altitude meaning that any ISA temperatures above standard would make the step down MSL altitude be below the GS make the GS be below the step down MSL altitude. An example was the 25L at LAX. The nominal GS altitude at the 5000 step down fix was 4993 MSL. On a hot day, for example at 95 F, since the GS remains fixed in space but the altimiter would read more than 300 feet lower at this point. This would be detected as a deviation. The 5000 foot minimum was at 16.6 DME. It was changed to be at 17.2 DME. This moved the nominal GS crossing altitude to 5200 feet, so following the GS now under the same conditions would put the aircraft at 4900 feet. It is below the 5000 foot minimum, but within IFR permitted tolerances and would not generate a deviation.
Click to expand...
FTFY.
 
John Collins said:
The 95 degrees F is at the airport. Standard lapse rate is used from there. In LA an inversion is common, so the real altitude deviation can be worse.

From the Instrument PTS:
Click to expand...

On a hot summer day it is both inversion and in a sense a different atmosphere at Pomona/Ontario than at LAX. On one of those hot summer days the surface temperatures at KONT, KPOC, KCNO, KEMT, etc, can be well over 100 degrees when it is 72 at KLAX.

A similar example is driving from KSMO over the hills to KVNY. The temperature can increase 30 to 35 F on that drive in late July or August.

That was the case before global warming came along. :lol:

As to the instrument rating PTS, the only issue that matters to So Cal is when the snitch goes off because the computer has detected less than 3 miles horizontal and 1,000 feet vertical. Many pilots have busted the step-downs arriving at LAX from the east without conflicting IFR traffic below them, so it was a non-event. And, as you point out, the ILS profiles that overlie the Empire sector of SoCal have been redesigned to make it less likely to happen. But it can still happen because the LAX glideslopes have not been changed.
 
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