ILS approach

[Movin' on up!]

So there I am being vectored to the ILS approach into MHR. It is a pretty long approach course with several step downs. While I am at 3000 feet and on the base leg for the localizer, I see the GS is starting to come alive. I am told to intercept the localizer 8 miles out and 5 miles away from the FAF. The next step down is to 2500 feet but I already have the GS alive. I am then cleared for the apporach. My question is: Can I stay at 3000 and intercept and follow the GS or must I comply with the step down and intercept the GS at the intercept altitude?

 

[nosehair]

Start the Approach

Yes, you may intercept the glideslope and continue on your approach. The glideslope intercept altitude is a minimum altitude to hold until intercept. If the intercept happens early, I say get established on it to avoid the extra task of re-establishing the glideslope at Outer Marker, making Frequency Change & tower Contact.
Some folks will say you could intercept a false glideslope if you are high. I have heard of that false glideslope, and I think I remember seeing a flash of a false glideslope movement once when holding over the OM at about 6000'. The scenario you describe, which is a common one, cannot cause a "false" indication. Perhaps if you were twice as high above the glideslope as is shown, then maybe you might be concerned, but the false glidslope will bring you screaming down at such rediculously high descent rates, it would be obvious.
I teach the technique of trying to establish the glideslope early, so that you don't have that "G/S-intercept-Gear-Down/Power-Back, Change-the-Radio/Call-the-Tower" thing all going on at the same time there.

 

[Movin' on up!]

ILS Approach

I thought I could intercept the GS but wasn't sure. Thanks for the info.

 

[A Squared]

To expand a little on what Nosehair said:

You see a lot of trees killed and ink spilled on the subject of "false glideslopes". Most of it is bunk. There are indeed false glideslopes. they are at even multiples of the glideslope. The first "false glideslope" will be at approximately 6 degrees. It will be reverse sensing. If you were to intercept this one you might be tipped off by the fact that you have to fly opposite the needle indications to maintain the glideslope, and you need double the descent rate to stay on it. The second one will be at about 9 degrees. this one will be "correct" sensing, but you'll have to triple your normal descent rate to stay on it.

As for "false glideslopes" below the true glideslope; there aren't any. Think about it, if it were possible to have a false glideslope below the true one, you'd intecept the false glideslope every time, 'cause you'd always encounter that one before the true slope. An ILS system that produced false glideslopes below the correct one would be result in more crashes than sucessful landings. ok that's overstating it a bit, but you get the idea. When they flight check an ILS, they only flight check the glideslope itself for accuracy out to about the outer marker (and a little beyond) However they flightcheck the area below the the GS for correct fly-up indication out a long way (I forget how far, exactly) The purpose of this is to ensure that there is no "false glideslope" below the true GS.

Now 2 things to be careful of:

1) you may have intermediate altitudes which are *mandatory* there are a few ILS approaches like this around the country. Mandatory means just that, mandatory. You have to be at that altitude, not on the glideslope. these will be denoted on hte plate as mandatory though, so you should see that if you're paying attention.

2) in some few cases, the glideslope may take you below a minimum stepdown altitude. Again, uncommon, but it happens. Be alert for this, just because you're on the GS doesn't mean that you can ignore minimum altitudes. outside of the outer marker, use the GS for a descend *aid* and use your altimiter and published minimum altitudes as *primary* descent information.

 

[avbug]

The legal usability of the glideslope extends as far as the outer limit or fix defining the approach. Typically this may be the outermarker, but may also be the published GSIA. Beyond that, technically the glide slope is not approved for guidance.

However, it's acceptable to follow the glideslop so long as it doesn't conflict with any intermediate fixes or step down retrictions on the localizer.

An example is use of the ILS to 24 L/R or 25/R at LAX. The approach is often flown from 26 miles out, though the glideslope isn't legal for use at that distance. Everyone flies the glideslope all the way down all the same, as it provides a stablized consistent descent angle; it provides a descent that meets each crossing restriction and step down fix. So long as it doesn't interfere, you can do that, as the glideslope is really only providing advisory information at that point.

I always put the stepdown fixes in the altitude alerter all the way down, anyway, and monitor them, regardless of the presence of the glideslope. I believe it's a good habit to get into.

 

[SSDD]

I may have read your question a little different than everyone else, so I just want to add that you should make sure you are established on the localizer before starting your descent.

 

[CaSyndrm]

I thought the answer was yes. Then I saw all those lengthy posts and could only say "wow" whatever happened to the KISS method. Just make sure your altitude will match up correctly at the glideslope intercept fix or outer marker, often the same place.

 

[100LL... Again!]

One more caution.

It is POSSIBLE (though not probable) that following the slope down from a higher alt could cause you to be somewhere ATC does not expect you to be.

For ex: suppose that stepping down kept you below a class B floor, where staying up high and joining the slope would take you through it. If ATC expects you to hit all the altitudes prior to the marker, you could end up messing with other traffic. This happened to someone I know. Forget the airport, though, but it was a feeder under a very busy class B.

 

[Singlecoil]

I may have read your question a little different than everyone else, so I just want to add that you should make sure you are established on the localizer before starting your descent.

I would like to second that statement. The original question was "Can you intercept the glideslope while on base?" The answer is, absolutely not.

 

[A Squared]

I would like to second that statement. The original question was "Can you intercept the glideslope while on base?" The answer is, absolutely not.

Yeah, good point. In re-reading the question it could be interpreted a couple of ways.

In the approach clearence, you should be instructed "maintain XXXX until established." You absolutlely may not descend below this altitude until you are established on the localizer (or other published portion of the approach) regardless of what the GS is doing.

Now if the question is: "once established on the localizer, may I follow the gs or must I descend to the GS intercept altitude then imtercept.?" : then I stand by the answer gven by Nosehair, myself and avbug (we all said essentially the same thing)

As for CAsyndrm: read the question. it was multiple choice, not yes/no. So tell me, when you say the answer is "yes", does that mean "yes, you may stay at 3000 and intercept and follow the GS " or "yes you must comply with the step down and intercept the GS at the intercept altitude"?

In actuality, neither "yes you can follow the gs above GSIA" nor "no, you may not follow the GS before GSIA" are completely correct and in order to be meaningful an answer needs to address the considerations.

Simplification is nice right up to the point that it becomes so simple as to be meaningless.

 

[DrewBlows]

The legal usability of the glideslope extends as far as the outer limit or fix defining the approach. Typically this may be the outermarker, but may also be the published GSIA. Beyond that, technically the glide slope is not approved for guidance.

However, it's acceptable to follow the glideslop so long as it doesn't conflict with any intermediate fixes or step down retrictions on the localizer.

I think you may be mistaken about the legal usability of the glideslope. Firstoff the FAR's give no "legal" guidelines to glideslopes (or any other types of navigation for that matter), so we have to rely on the trusty AIM for service volume infomation which says, "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." So legally speaking I think your second paragraph is more correct.

Thoughts...

 

[DrewBlows]

2) in some few cases, the glideslope may take you below a minimum stepdown altitude. Again, uncommon, but it happens. Be alert for this, just because you're on the GS doesn't mean that you can ignore minimum altitudes. outside of the outer marker, use the GS for a descend *aid* and use your altimiter and published minimum altitudes as *primary* descent information.

Does anyone know of any specific approaches that have a minimum stepdown altitude below the glideslope. Do the plates have some sort of cautionary (sp?) statement in the notes or planview section, or do they leave it up to us to figure it out when we see the flashing beacon of a cell tower through the clouds.

 

[A Squared]

Does anyone know of any specific approaches that have a minimum stepdown altitude below the glideslope. Do the plates have some sort of cautionary (sp?) statement in the notes or planview section, or do they leave it up to us to figure it out when we see the flashing beacon of a cell tower through the clouds.

I assume you meant a minimum stepdown altitude *above* the glideslope.

I don't know of any in real life. In fact, it would be mathematically impossible on a 3 degreee GS if it were designed in accordance with the TERPS. The TERPS specifies a maximum design descent gradient on the intermediate segment of 300 ft/nm. A 3 degree glideslope descends at 318 ft/nm. If an intermediate minimum altitude took you above the GS, the approach next stepdown wouldn't be enough to get you back underneathe hte glideslope. It would be possible on any GS less than 2,8 degrees. They can be designed as flat as 2.5 degrees.


There are, however, ILS approaches with MANDATORY altitudes before the GS intercept. I believe that there is one in Houston. The altitude is depicted on the NACO plates with a solid line above and below.

 

[100LL... Again!]

A squared is correct. would be basically impossible.

 

[Tinstaafl]

There is one possibility under PANS-OPS (not sure about TERPS): If the approach plate charts two different type of approaches ie an ILS and a LOC or LOC/DME. They're two different approaches, with separte obstacle clearance criteria.

It can happen - theoretically, I can't think of an example - that an obstacle that doesn't impinge on an ILS obstacle free gradient can still impinge the obstacle free area required to be considered for the non-precision approach.

 

[SkywayFO]

I didn't read all the posts so sorry if someone said this already. But watch out for manditory step downs. There are someplaces that have them do to airspace constraints for airports near by.

 

[avbug]

think you may be mistaken about the legal usability of the glideslope. Firstoff the FAR's give no "legal" guidelines to glideslopes (or any other types of navigation for that matter), so we have to rely on the trusty AIM for service volume infomation which says, "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." So legally speaking I think your second paragraph is more correct.

"Service volume" is irrelevant for the use of the glideslope, as it pertains ONLY to the approach. You may be able to receive it at farther distances, but you are NOT guranteed obstacle protection or signal integrity beyond that which is charted. You are not gauranteed anything beyond that for which the approach has been flightchecked, and that's going to be the published portion of the approach only. Typically to the published GSIA.

The "FAR's" give no legal guidelines, do they? Are you not familiar with Part 97?

Have you not viewed TERPs?