Yes, no quarrel with any of the above. However, can I sound a note of caution? I was consulted over this issue when my company was writing SOPs for a new type. The Flight Manual for that also said the GP could be intercepted from above or below. On investigation it turned out that all the manufacturer meant was that the equipment was capable of intercepting a glidepath signal from above or below, not that it was recommended practice.
And although the advice above about "you'll know if you are on a false GP by Rate of Descent, Ht vs range, etc" is correct, it is better to avoid the false GP than having to recover from one. For this reason we use a short level segment.

OzExpat- Thankfully, I'm pretty sure (or maybe just hopefull) our fleet captain is open to a discussion on the procedure. I'm just not really for it (the level flight segment) because not only would it cause more noise/fuel burn it doesn't appear to have a GOOD enough reason- or maybe I'm just not seeing it.... Till then no doubt its the SOP all the way to the Glideslope intercept.

keithl

"The Flight Manual for that also said the GP could be intercepted from above or below. On investigation it turned out that all the manufacturer meant was that the equipment was capable of intercepting a glidepath signal from above or below, not that it was recommended practice."-- Fair enough. Agree with you on that. How about intercepting the Glide AT the Glide

:cool:

And although the advice above about "you'll know if you are on a false GP by Rate of Descent, Ht vs range, etc" is correct, it is better to avoid the false GP than having to recover from one. For this reason we use a short level segment.- Like I mentioned before keithl- you can't intercept a false glide till you are very high on the approach... so you'd already know/expect a false glide well before you came to the point of glideslope intercept. Again IMHO
:ok:

keithl... I couldn't agree with you more about your "note of caution". Despite being a procedure designer, I'm also a practicing pilot (and, man, do I ever need the practice! :O ) But, seriously, I like to have my level segment and our SOPs allow it.

FWW... Yes the noise and fuel burn issues are important considerations. They are going to be your best debate points with the Fleet Captain - especially the latter because every extra pound of fuel burned is an added cost on the operation. And this is normally at quite a low level, so the extra fuel burn could be considerable. Yiu may, however, need to develop some figures on that, to place before the Fleet Captain, so that he can see how much extra money it'll cost the airline over a year.

The only thing that I would draw to your attention is that the worst aspect of a "false GP" is that it provides premature descent below the actual GP. The amount of deviation below the real GP can easily exceed the protection that is provided to ILS procedures. I've done some research on this and it seems that rate of descent is likely to be excessive, though this isn't always going to be a good clue.

Personally, I like to use DME to check against my altitude. In my part of the world, ILS approach charts include a DME Dist Vs Altitude table, which makes the GP monitoring very easy. And, yes, because I've designed the approach, I've already worked out the DME Dist Vs Altitude table.

FlyingWithoutWings

Is this what you are looking for?

Airclues

Ozex, we seem to be agreed.
FWW, we're nearly agreed. Intercepting AT the GP would be OK. Any final SOP is a compromise, in this case between safety and economy. I personally believe you should give more weight to the safety side of this debate. I have (as an examiner) seen a false GP intercept - it was very obvious, and quite difficult to recover from. Certainly there was no possibility of 'adjusting' to the true GP. So, the Go-Round would be more expensive than a short level sector. Now, I admit that was a trainee, and far less likely to happen to a qualified crew. But don't many 'hot & high'accidents result from people losing spatial awareness and trying to get onto a normal approach path from too close in? I must leave it to you and your fleet captain to weigh the chances of this on your operation.

Captain A - direct hit! Thanks
FWW - apologies for all the TLA's:O

I'm yet to experience a "false GP" so my information on it is based on research. A lot of research to be sure, but nothing more than that. In a previous life, however, I was an Examiner (and before that an ATO), and have seen several flight test candidates trying to intercept a GP from above. None of those attempts were ever successful enough to give me enough confidence in them to grant a pass, mainly because of the excessive rate of descent that led to overcontrolling and, therefore, going up and down thru the GP all the way down.

Well, at least they knew that they'd blown the ride before I'd said anything!

I trust that my suggestions to FWW were interpreted as being based on the assumption that he and all his colleagues already understand the "false GP" phenomenom and know how to recognise it in good time to do something safe. I don't intercept the GP from above unless I really have to, so don't want anyone to think that I'm recommending it.

First of- many thanks to all for your valuable inputs.

Oz- You said "The only thing that I would draw to your attention is that the worst aspect of a "false GP" is that it provides premature descent below the actual GP.The amount of deviation below the real GP can easily exceed the protection that is provided to ILS procedures." I remember reading (my books are currently borrowed with a friend- hence I couldn't check right away) in the GSP that a false Glideslope ALWAYS occurs ABOVE the actual Glideslope- hence it can get you HIGH with high ROD on the approach... but not low.....:confused:

I agree with a lot of people here that have mentioned its a good idea to intercept the glide from below or AT.... the thing we try to do on our descents is to be AT or below the glidesope at intercept. The OPS CIRC we have out implies that a "2.5NM LEVEL segment is REQUIRED".- this is the real issue-- IS IT or NOT?????

Capt. AIR- Thanks :ok:

Erroneous Glide Path
 

Yes FWW, you're right. I should've been much more careful with my phraseology. The correct terminology for what I was trying to explain, now that I'm sober enough to think more clearly about it is : "Erroneous GP". The following links will give you more information on this and make it clear that I was referring to this phenomenon, in leading pilots into an unsafe, premature descent below the real GP.

NZ 60 Erroneous GP Capture

and

ILS Glide Path could be a serial killer

It is my understanding (and observation) that with certain GP transmiiters, a false GP lobe will exist at 6 degrees, and a weaker one at 9 and 12. The strength and stability of the various false lobes varies from installation.
(OzExp..From memory, Rwy 14 at POM had a noticeable false GP at 6 degrees, a point missed by the advocates of the higher-is-safer school of Instrument approach flying!).

Certainly, in specific instances, the 6 degree GP can be strong and steady enough to lure the dopey pilot into flying it. MInd you, one would think that being 6000' above aerodrome elevation at 10 miles should be a decent clue....but on a dark and stormy night, its never quite that easy is it?

Does anyone know if the new technology ILS installations have this problem beaten?

False GP's and Erroneous GP's, I think are not necessarily the same thing. False GP's are a fact of life and are an unfortunate side effect of GP design - but they are always steeper than than 3 degrees, occuring, as I remember, in mutiples of 3 degrees (bear with me - its been a long long time)

In a past life, I have observed that monitors have been disabled by the techies because of spurious warnings.

No problemo!!... until the day comes that the GP does actually drift off completely - as it will on a rainy night at 2 am. Since then, I have always been very careful to check GP against altimeter and distance to run on every ILS I fly...I cannot reccomend highly enough that everyone does his own personal GP integrity check at 10 miles.

Lucille- Right on the numbers. Great idea to make a personal cross check of GS relaibility a habit on the approaches. What view about the original post??????

lucille...

Yes, I believe that was true of the old installation. It may even be true of the new one but I never saw the commissioning data on it.


Very true, they are not the same at all. Let me quote the following excerpt from the June 2002 (I think) issue of of the New Zealand CAA’s magazine “Vector", in which they reported on the Erroneous ILS Indications at Faleolo...

"A false glideslope is a normal byproduct of the glide path. If it is intercepted, and if it can be followed, it will guide the aircraft to the source of the glide path.

An erroneous glideslope, on the other hand, is not well known. It is the result of a faulty or partial signal being transmitted. It will indicate to the aircraft that it is ‘on slope’ irrespective of where the aircraft is in space, and it will not lead the aircraft to the source of the glide path. Erroneous glideslope signals are occasionally transmitted for maintenance purposes."

This is a pretty general description of the differences, but should provide enough information so that everyone understands the difference. The information on the "false GS" is necessarily brief in the article, but it can be inferred that although it can guide the aircraft to the source of the GP, the descent rate will usually be very much steeper, as you say.

The "erroneous GS" situation is almost always the result of maintenance work, or lack of proper work practices by the navaid technicians. It will bite the unwary pilot very hard indeed but the same simple integrity checks that you describe will help to detect both problems.


Or they simply wind the monitor settings in until the LLZ or GP is so tight that it is impossible to fly. I've been caught by that one at POM... I ripped a new a-hole in the responsible person and, as far as I know, the problem hasn't recurred.

FWW : Original Post?? who ever heard of anyone keeping to the topic at hand on pprune?...sheeesh!.

In any case, I have yet to see any ICAO requirement for a mandatory 2.5nm level segment on the LLZ prior to GP intercept. It is remarkable for them to use a word as strong as: "mandatory". Rather, they are more usually so politically correct that the word of choice seems to be: "reccomended".

Mind you, they do like to surreptitiously change things from time to time.

If this requirement is indeed true, it will come as a rude shock to a lot of people (myself included) who like to fly constant descent approaches in the interest of smoothness (= passenger comfort) and reduced noise.

Consider just how pee'd off all all those voters, living about 2.6 n.m downwind from the nominal GP intercept point, will be. Can you imagine the political flak resulting from the noise of all those engines spooling up at 2:00 am....Hehehehe...

Nope, Sorry, I just cant see this idea working.

However, I await the final word on this topic with bated breath. Hopefully an ICAO guru will post the "final word".

I believe that FWW has had lots of confirmation, already, that this level segment is in the design criteria (2 NM only if there is no turn between the IAF and IF). Also that this is intended to allow the aircraft to become stablised in preparation to intercept the GP from below.

I believe that FWW's question was more aimed at the way in which other pilots interpret this. Certainly, from the procedure design point of view, I don't care whether the pilot follows this practice or not. I am also aware that pilots who use FMS will have VNAV capability that allows them to intercept the GP from TOPD, if they want - and, if I understand the situation correctly, that is precisely what they want.

But I think FWW asked about the operational use (or necessity) of this "level" segment and whether or not other operators apply it.