Approach question

[Tommy Boy]

When inbound on an ILS approach and there are several stepdown fixes outside of the FAF, and your clearance is "Maintain xxthousand until passing the FAF, cleared for the ILS xx", are you REQUIRED to cross all those points at there specified altitudes or can you stay where you are at and wait for the glide slope and follow it down?

 

[PA31Ho]

i would assume that if he says maintain xx until the FAF, i would they they would say maintain xx, clearn for ils xx approach... you would stay there until you intercept the g/s.... what else could you do right?

also, those altitudes on the fixes are at and above right? unless they have the bars under and over the altitdue.

____
4000
____

something like that

 

[dhc8fo]

Maintain the assigned altitude until the FAF, as you were told. the step downs would be for a full procedure, the LOC, or maybe an ILS without radar contact or something....

 

[flyinghunter]

Are the altitudes listed on the plate required altitudes or just minimum altitudes? Normally the approach controller says maintain xx altitude until established on a portion of the final approach course, cleared ILS X. If that's the case, then you can maintain your altitude, as far as i know, until the glideslope comes in.

 

[Tommy Boy]

Sorry folks, I worded my question wrong. Same scenario only the controller says "Cross the FAF at XX thousand, cleared for the ILS xx"

Now do you have to cross those previous step down fixes or can you just intercept the Glideslope?

 

[avbug]

The clearance as you have provided it is nonsensical. On a precision approach, the FAF is the glideslope intercept altitude, or an altitude that is lower when provided vectors to glideslope intercept at a lower altitude.

Accordingly, ATC cannot say maintain XXX until the FAF...because the FAA occurs at the published glide slope intercept altitude (GSIA). If you're assigned XXX altitude "prior to the FAF," and that altitude is higher than GSIA, you'll never reach the FAF because you'll have to descent to get there...FAF dosn't exist on the glideslope until passing through GSIA.

I believe I get the gist of your question, but the wording is still incorrect, and multiple answers apply depending on your wording.

Generally an apporoach clearance will contain an altitude to maintain until established. For example, one might be told "Three miles from KNOLE, maintain three thousand until established, cleared the Jahooliwooli ILS 23C." In this case, you maintain three thousand feet until you're established on the approach. One established, if lower published altitudes apply, such as step downfixes, you may follow them prior to glideslope intercept, or you may wait for the glidesope.

You do not have glideslope gaurantees above or beyond the published GSIA, and for you the glideslope is advisory-only until passing the GSIA...but you can intercept it and follow it down instead of stepping down with stepdown fixes...so long as you take care to ensure that you're meeting the altitude restrictions of each stepdown as you continue down the glideslope.

I watched an experienced captain in a light jet some years ago intercept a false glideslope and attempt to follow it down. I alerted him repeatedly to the fact that he was too far out and that his descent did not conform to the step down fixes; he was below minimum IFR altitudes for that approach segment. He felt that because he had crossed needles that he was okay, and he wasn't. False signals should occur above the glideslope, not below it where they did...but they were viable signals, the aircraft was coupled, and despite valid information showing him he was in the wrong place, he would have followed it into the ground.

Many pilots don't bother to verify that their descent is going according to plan; once the needles are crossed, they blindly follow the signal down, never verifying that they are indeed meeting their minimum altitude-distances at each fix they pass, on the way down. I'm very big on that, and outside the published FAF and before GSIA, I put each altitude in the alerter in succession as we pass each step down fix...even though we're coupled on the glideslope above GSIA.

I know a lot of folks don't do this, and many would consider it overkill, but I've seen what can happen when you don't. While not entirely the same, the GIII that was picking up former president Bush in November 2004 would have benifitted from this same practice. Both pilots of that aircraft were experienced pilots, one a former chief pilot for the company, and the other the current chief pilot. They flew into the ground.

Without pounding the soapbox too much, you maintain your assigned altitude until established. At that point, you may descend using the published stepdown fixes, or you may intercept the glidesope, if appropriate. If you do intercept the glideslope above GSIA, you're still responsible for ensuring that you meet each published minimum altitude until you're past the fix to which that altitude applies, during your descent on the glideslope. Doing so, while unfortunately not a common practice, ensures proceedure compliance, terrain and obstacle separation, and is acting in the best interests of safety...as well as ensures that your legal obligations in executing the approach are met with respect to altitude. It may just save your bacon.

 

[100LL... Again!]

ATC can't require you to cross the FAF at some arbitrary higher altitude. They COULD descend you to the normal inbound altitude, which makes that altitude in essence mandatory, since you can't go below until the FAF.

That said, there are differences of opinion on the issue of fixes prior to the GS interecept.

If the GSIA is 3700, fo example, and you decide to remain at 4000 (as cleared) until GS interecept, no one cares, and it is legal to do so.

Bear in mind, of course, that you will intercept roughly one mile sooner for every 300' higher than the GSIA.

Where this issue gets interesting is where these stepdowns are used to keep aircraft on the invbound course below overlying airspace (i.e. approach path for a larger airport).

If it is only a couple of hundred feet, you are PROBABLY okay staying at last assigned.

If there are actual published fixes outside the FAF (as in your example), I would follow them.

I'll go have a look at my TERPS, but I do not recollect there being any guarantee that the glideslope will keep you above all fixes. There is a maximum gradient, which would probably achieve the same effect, but I can't say without reviewing.

 

[FL420]

You do not have glideslope gaurantees above or beyond the published GSIA, and for you the glideslope is advisory-only until passing the GSIA...but you can intercept it and follow it down instead of stepping down with stepdown fixes...so long as you take care to ensure that you're meeting the altitude restrictions of each stepdown as you continue down the glideslope.

I watched an experienced captain in a light jet some years ago intercept a false glideslope and attempt to follow it down. I alerted him repeatedly to the fact that he was too far out and that his descent did not conform to the step down fixes; he was below minimum IFR altitudes for that approach segment. He felt that because he had crossed needles that he was okay, and he wasn't. False signals should occur above the glideslope, not below it where they did...but they were viable signals, the aircraft was coupled, and despite valid information showing him he was in the wrong place, he would have followed it into the ground.

Many pilots don't bother to verify that their descent is going according to plan; once the needles are crossed, they blindly follow the signal down, never verifying that they are indeed meeting their minimum altitude-distances at each fix they pass, on the way down. I'm very big on that, and outside the published FAF and before GSIA, I put each altitude in the alerter in succession as we pass each step down fix...even though we're coupled on the glideslope above GSIA.

I know a lot of folks don't do this, and many would consider it overkill, but I've seen what can happen when you don't.

Safety literature is full of incidents where crews have followed, either manually or coupled, false glideslope indications. In almost every case, the crews set themselves up by either being unaware or were aware and still ignored the fact that instrument landing systems have certified service volumes.

I have flown with many experienced pilots who tune, identify and follow an ILS signal from many miles out and never give it another thought, not even to verify the reasonableness of the glideslope indication by using a gouge like 300 feet per NM.

Keep on preaching, 'bug. I'm a member of the choir but everyone needs to hear the sermon more than once.

Cutting to the chase, crews should keep a healthy skepticism about the quality of the localizer and glideslope signals until within 10 degrees of the localizer course and 10NM of the threshold at or below 4,500 feet HAT.

Here is what the AIM has to say:

1-1-9. Instrument Landing System (ILS)

b. Localizer

5. The localizer provides course guidance throughout the descent path to the runway threshold from a distance of 18 NM from the antenna between an altitude of 1,000 feet above the highest terrain along the course line and 4,500 feet above the elevation of the antenna site. Proper off-course indications are provided throughout the following angular areas of the operational service volume:

(a) To 10 degrees either side of the course along a radius of 18 NM from the antenna; and

(b) From 10 to 35 degrees either side of the course along a radius of 10 NM

6. Unreliable signals may be received outside these areas.

d. Glide Slope/Glide Path

3. The glide path projection angle is normally adjusted to 3 degrees above horizontal so that it intersects the MM at about 200 feet and the OM at about 1,400 feet above the runway elevation. The glide slope is normally usable to the distance of 10 NM. However, at some locations, the glide slope has been certified for an extended service volume which exceeds 10 NM.

4. Pilots must be alert when approaching the glidepath interception. False courses and reverse sensing will occur at angles considerably greater than the published path.

5. Make every effort to remain on the indicated glide path.

 

[avbug]

Someone PM'd me regarding a related issue to this. That is,when to start down, relative to what is considered "established." This individual had been taught ten degrees within ten miles is established, and therefore one may start down when within ten degrees of centered, and within ten miles.

I certainly can't speak for the masses, but for my own judgement, I'm established when the needle is centered. If you stick to the published proceedure, it's awfully hard to hit anything. Deviating, even though the course and/or glidelsope indications may be receivable outside of centered, is not appropriate. No doubt many will disagree,but with the needles centered and backed up by all available data, you're much more likely to reach terra firma at the appointed place, rather than short of it in a place you'd rather not be.

 

[A Squared]

I'll go have a look at my TERPS, but I do not recollect there being any guarantee that the glideslope will keep you above all fixes. There is a maximum gradient, which would probably achieve the same effect, but I can't say without reviewing.

You're right, there is no *guarantee*, but when the maximum gradient for an intermediate segment descent is considered, it would have to be a lower than standard glidepath (2.8 degrees) to take you below a stepdown fix on the intermediate segment. A 3 degree glideslope will keep you above stepdown fixes.


That is *not* to say it's safe to follow he glideslope without checking other references against your altitude. I'm just commenting on the mathematical reality.