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FlyinWithoutWings
31st Oct 2003, 22:11
Recently heard that as per some ICAO DOC, a 2.5nm LEVEL flight segment is MANDATORY on the localizer prior to intercepting the glidepath.

So far we'd try and arrange our descent to be AT glideslope intercept altitude AT the point of glideslose capture- prefrebably extending flap and gear so that there is minimum thrust lever change. Also noticed that the FMC builds a 2.5nm level flight segment between the G/S anf CF(localizer intercept point).

Any views/location of the relevant ICAO Doc would be much appreciated.

Thanks
:confused:

radar707
31st Oct 2003, 22:29
From an ATC point of view, the college of knowledge teaches us to try and give you 1/2 to 1 mile of level flight before intercepting the glide, however, I've noticed that most crews when given descent prior to localiser intercept, seem to delay it and go for the continuous descent on the ILS

quid
1st Nov 2003, 00:49
FWW-

Do you mean 2.5 nm in the procedure design, or 2.5 when flying the approach? In the US, I've not heard of this. ???

Hudson
1st Nov 2003, 06:45
Radar 707. Re level segments. Boeing (in the 737 anyway)recommend that initial flap selection on approach be carried out in level flight meaning that the flaps should not be used as speed brakes. This recommendation is commonly ignored. Flaps 1 through to flaps 5 are selected while the aircraft is going downhill at speeds sometimes well in excess of the recommended especially if the aircraft is closing in towards the localiser during en-route descent. Depends on the pilot's skill I suppose.

West Coast
1st Nov 2003, 11:45
I like the idea of a decel portion built into the approach/clearance, especially if things are getting a bit sideways.

FlyinWithoutWings
1st Nov 2003, 13:49
Quid-
"Do you mean 2.5 nm in the procedure design, or 2.5 when flying the approach? In the US, I've not heard of this. ???"

2.5nm when flying the approach....

Hudson-
"Boeing (in the 737 anyway)recommend that initial flap selection on approach be carried out in level flight "

Can't find any such recomendation in the FCTM- atleast the new version- though I do remember reading about "not using flaps as speedbrakes". Even on a descent if you would be extending flap to say 5 from UP, and were in say LVL Change mode, the ROD would reduce to about 500fpm and the speed would reduce down to the selected maneuvering speed. So I dont think extending Flap during a descent should be construed as "using Flap as Speedbrakes".

Thanks all for sharing your thoughts- Keep em Coming!!:ok:

BEagle
1st Nov 2003, 16:27
Most German approaches specify a mandatory continuous descent technique and low drag approach; their radar vectoring usually facilitates it.

In a rather elderly 4-jet, at a typical aerodrome we used to fly through the overhead (airpace restriction) at 4000 ft aal and 210 KIAS, then descend to 2500. At 7 DME outbound, select flap and slat to the 'take-off' setting and decelerate to 170 KIAS, then at 9 DME, select gear down and start a descending base leg turn to level at 2000 ft and not below Vat + 30. Intercept the localiser, then select approach flap just prior to GP intercept, maintaining not below Vat + 20. At 1500, select land flap and maintain not below Vat + 10 until 200 ft aal, then decel to Vat.

This is quite noisy, so the low drag equivalent with a continuous descent technique becomes:

Through the overhead at 4000 ft aal and 210 KIAS. Maintain this to 9 DME outbound, start the base turn then select flap and slat to the 'take-off' setting, decelerate to 170 KIAS and select gear down on a descending base turn to intercept the GP (but not outside localiser coverage). Keep descending on the GP as the localiser is captured, maintaining 170 KIAS to 4 nm from touchdown, then select flap to approach and subsequently to land, increasing thrust as required to stabilise at not below Vat + 10 until 200 ft aal.

Makes far less noise but is slightly more difficult to fly at the ILS intercept point as things happen rather quickly at that point. Prompt and accurate trimming and thrust management are essential to fly a safe approach from 4 miles - so practice was needed! However, when Wx was close to the limit it was necessary to adopt the 'normal' high drag technique as the autopilot system needed time to stabilise; hence land flap and not below Vat + 10 was established as early as possible on the glideslope after a level segment at 2000 ft aal, giving the autopilot the maximum possible chance to achieve accurate localiser and glideslope tracking with the autothrottles holding the stabilised Vat + 10.

This was in a 35 year old 4-jet with a fairly primitive autopilot system, extremely noisy engines and a 'busy' flight deck. Modern ac such as the Airbus series should be able to cope with low drag continuous descent approaches routinely using AFS/ATS.

Dan Winterland
1st Nov 2003, 17:28
Continuous Descent Aprroaches (CDAs) also required at LHR. A league table of those who comply is published by BAA.

Spitoon
1st Nov 2003, 17:49
I think you'll find that the 2.5 mile level segment is specified for procedure design purposes (i.e. not vectored but flown with reference to ground based aids). The ICAO doc that covers this stuff is Doc 8168 PANS-OPS.

Hudson
1st Nov 2003, 19:10
Flyingwithoutwings. You are right. The FCTM states that flaps are not normally used for increasing the descent rate...and that normal descents are made in the clean configuration to pattern or instrument approach altitude...if greater descent rates are required, extend the speed brakes. All this adds up to the aircraft should normally be in level flight when reducing speed prior to extending flaps. While the FMC may gradually reduce the speed in the descent before initial flap selection to flaps 1 and 5 - the aircraft is not in level flight - it is still going down and the flaps are therefore being used as speed brakes - in my view.

OzExpat
1st Nov 2003, 21:25
ICAO Doc 8168 (Pans Ops), Chapter 21 is the reference, but it is only for procedure design purposes. As a procedure designer, I can tell you that I don't care whether you use it that way or not. It is strictly a procedure design principle that the Intermediate Approach Segment includes a 2NM level segment, as a minimum.

If there's a track change between the Initial and Intermediate segments, greater than 30 degrees, we have to provide additional length in the level segment. I don't have my Pans Ops book in front of me (it's well after hours here now), but I think the minimum distance increases to 2.5 NM for turns of between 30 and 40 degrees between the Initial and Intermediate segments.

From the standpoint of procedure design, this part of the approach is referred to as the Intermediate Approach segment. The sole purpose of this segment is to establish the aircraft on the LLZ prior to GP intercept. That way, if pilots fly the way the procedure is designed, GP intercept occurs from below it.

I recognise that this is not a fuel-efficient way of intercepting the GP.

If I was you, I wouldn't get too hung up on what is built into the design of these approaches. The chances are that your company SOPs and your FMS have already figured out how to deal with it. The only thing that you really need to worry about is the possibility of a "false glidepath" and how to identify it - and I'm sure that your company will have SOPs to address that as well.

I don't know if TERPs provides any similar design constraint, so I don't know if American pilots will know about it or not.

reynoldsno1
3rd Nov 2003, 05:19
TERPS does not specify a level segment for the intermediate approach, but does specify a 5NM portion with a descent gradient not exceeding 5% - all of this forprocedure design puposes only.

FlyinWithoutWings
3rd Nov 2003, 11:08
HUDSON-
"While the FMC may gradually reduce the speed in the descent before initial flap selection to flaps 1 and 5 - the aircraft is not in level flight - it is still going down and the flaps are therefore being used as speed brakes - in my view."

I shouldn't think that just because the aircraft is descending with the Flaps out, the Flaps are being used as speedbrakes- as long as you are maintaining the flap maneuvering speed. Even Boeing does NOT have a prblem with this as this is standard procedure say during a "Delayed Flap Approach", where you would be descending with Gear Down Flap15, then extending to landing Flap by 1000 ft agl.

Beagle, Hudson, Spitoon, Ozexpat, reynoldsno1 your replies appreciated.
:ok:

Actually- the origin of this thread was a General Operating Circular issued by our Flight Operations stating that pilots are intercepting the Glidepath on a continous descent profile, ie., without a level flight segment, and the ATC has objected to this. Hence, they have said that the pilots should be in level flight at the Glideslope intercept altitude, atleast 2.5nm prior to Glidepath capture, and that this was required as per Doc 8168 Vol I/II.

Can ATC object to something that is NOT wrong in the first place- as many have said that the 2.5nm level segment is a DESIGN requirement. I appreciate the ATC could/should do anything that improves flight safety- but waht are they thinking here??????:confused:

OzExpat
3rd Nov 2003, 20:51
Can ATC object to something that is NOT wrong in the first place
On the face of it, this seems to suggest that ATC has either misunderstood, or misinterpreted, Pans Ops. However, there may be other issues that are relevant to ATC in your area of ops, FlyinWithoutWings.

I think that, rather than conjecture the reason, it might be a good idea for your company to ask them about it. Perhaps you can suggest that your Fleet Captain put in a polite letter that seeks clarification from them.

Or maybe there's an ATCO or two floating around here who can shed some light on it for us?

keithl
3rd Nov 2003, 21:47
I thought that some level segment was always necessary (though maybe not over a specified distance) in order to ensure the GP was intercepted from below. Thus avoiding false lobes. Some of the methods described above don't seem to have that protection. Any thoughts?

DFC
3rd Nov 2003, 22:38
In most cases where I have come across the continuous descent approach, a DME is provided.

Thus while not capturing the glideslope from below does leave the posibility of establishing on a false glideslope, SOPs which require crosschecking of height versus distance on a regular basis and crosschecking rate of descent required against groundspeed can ensure that the correct glideslope is being followed.....regardless of how it is captured.

Regards,

DFC

FlyinWithoutWings
4th Nov 2003, 18:30
OzExpat- Our flight dept./fleet capt. seems to agree with the ATC interpretation of the PANS OPS., or if not agree, have anyhow issued instructions to pilots to follow the ATC interpretation. I wanted to basically find out and get views on the ATC inerpretation.

keithl-
"I thought that some level segment was always necessary (though maybe not over a specified distance) in order to ensure the GP was intercepted from below. Thus avoiding false lobes. "

The Boeing737 FCTM says that the Glideslope may be captured from above or below. Besides there are numerous ways to detect if you are on a false glideslope (also mentioned in the Boeing FCTM):
1. Abnormal altitude range distance relationship (Normal would be aprox. 300ft HAT per NM of distance from the runway for a 3 degree glideslope)
2. Abnormal pitch and ROD would be apparent on a flase glidepath.

Also don't have my GSP handy at the moment, but remember reading that the first false lobe of the glideslope would be occur at a sufficient height, that it would be very obvious that your are getting a false glideslope. Infact, I remember getting a false glideslope at an altitude of about 8000ft at a distance of some 17nm to touchdown on the localizer ( please don't ask how we got there
:ugh: )

DFC- totally agree with you.

Cheers....:ok:

BOAC
4th Nov 2003, 18:50
Hudson - I think someone in your company has got a little bit confused! Extending flap in descent is NOT, as others have said, NECESSARILY using it as a speedbrake. Do your SOPs require a level off if ATC ask for a manoeuvre speed during your descent that requires flap extension? In the UK that would get ATC quite excited!

Most (UK) companies try to conform to the BAA/CAA requirements for CDAs and the false GP capture is guarded against with a check of altitude (height) against position (DME or NDB) as discussed here and at length on another thread.

FlyinWithoutWings
4th Nov 2003, 19:02
BOAC-

"Most (UK) companies try to conform to the BAA/CAA requirements for CDAs "

What are CDAs? Where would I be able to read about it?

Thanks--

OzExpat
5th Nov 2003, 17:13
keithl... You would probably only need to fly level if your company SOPs tell you to do so. The point raised by DFC, subsequent to your post, is valid and relevant. The design of the procedure does not account for a false GP and the procedure designer doesn't care whether a pilot opts to intercept from above or below the GP.

The design of the procedure includes the level segment for the pilot who wants to fly level to help slow down and make any necessary configuration changes prior to GP intercept. We figure that pilots will have ways to determine whether or not they have intercepted a valid GP.

FWW... perhaps I misunderstood you intially. It would now seem that what I should have suggested was that, as your Fleet Captain has agreed with the ATC ruling, you're stuck with his directive as a SOP. That being the case, it's a bit irrelevant as to whether or not ATC's ruling is correct.

keithl
5th Nov 2003, 18:44
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.

FlyinWithoutWings
6th Nov 2003, 00:57
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:

OzExpat
6th Nov 2003, 17:26
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.

Captain Airclues
6th Nov 2003, 17:49
FlyingWithoutWings

Is this (www.dft.gov.uk/stellent/groups/dft_aviation/documents/page/dft_aviation_503311-02.hcsp#P88_9713) what you are looking for?

Airclues

keithl
6th Nov 2003, 18:01
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.

BOAC
7th Nov 2003, 03:59
Captain A - direct hit! Thanks
FWW - apologies for all the TLA's:O

OzExpat
7th Nov 2003, 23:14
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.

FlyinWithoutWings
9th Nov 2003, 20:26
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:

OzExpat
12th Nov 2003, 19:12
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 (http://www.caa.govt.nz/)

and

ILS Glide Path could be a serial killer (http://www.airmanshiponline.com/fall2000/11-MXP%20July92%20english.htm)

lucille
21st Nov 2003, 02:55
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.

FlyinWithoutWings
21st Nov 2003, 13:06
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??????

OzExpat
22nd Nov 2003, 14:08
lucille...

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!
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.


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
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.


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

lucille
22nd Nov 2003, 21:52
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".

OzExpat
24th Nov 2003, 17:09
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.