Intercepting from Above

[keithl]

For years I have preached that the ILS GS should always be intercepted from below, to avoid the risk of false lobes. Now I see from this (http://www.srg_gad_gasil4of2003.pdf ) (page 16) that there are some that may be intercepted from above - "where published" and with a suitable DME.

The editor of the publication couldn't give me any examples, so may I request same from pPruners, so I can see for myself?

[Captain Stable]

Can you check your link please keith?

[keithl]

First link I've tried... Try this (http://www.caa.co.uk/docs/33/srg_gad_gasil4of2003.pdf) . If it doesnt work just go to the latest GASIL Leaflet at the caa website.

[Crossunder]

But is there a law against intercepting from above? You'd know soon enough if you were on a false GS because of the steep approach angle, and you'd never go below the "real" GS, as there wouldn't be any fake ones to follow anymore? And you would not be cleared for an ILS if there was no altitude reference-point (OM or DME distance)?

[keithl]

No, Crossunder, there's no law that I know of against it. But it is possible to intercept the false GP. As an examiner, in my flying days, I have seen it done. And because of the high RoD, there's no question of using an even higher one to recover to the true GP, so the approach has to be thrown away. Undesirable from many points of view, I'm sure you'll agree.

Hence my post. I'd like to see actual examples so that I can see what the wording is, why it may be desirable, what is special about those locations. Remember the wording in GASIL is "where published" so I am assuming some sort of reassurance to cautious souls like me is included in the words. If false lobes are somehow shielded, for example, well I'd like to know that.

[Flight Detent]

Hi all,
I notice in the B737-700 flight manual (vol II), that it says that GS intercepts from above or below are acceptable, provided the localizer has been captured.
I understand that this is only from an aircraft systems point of view.
Cheers:suspect:

[keithl]

Thanks, FD, that's an interesting (tho' slightly different) point. It's one to be careful of, because one has to watch the words. We recently bought some new aircraft, the manufacturer's tech manual of which had similar words. When we went back to them and asked "So can it distinguish a true from a false GP?" the answer was "Oh no, that's the responsibility of the crew. We mean that the equipment is capable of intercepting a GP signal from above".

The 737 may be different, but I'd suggest it's worth checking.

[exeng]

We strive for continuous decent approaches (particularly in the UK where at the major airports ATC monitor all approaches for compliance)

To achieve a good CDA will mean that you capture the LOC and the GS at the same time. Continuous monitoring of distance to run and ajdusting V/S is required to acheive this.

If you always aimed to capture the G/S from below you would never achieve a continuous decent approach, which would result in increased noise pollution for all.

Capturing a false G/S from above should be obvious given that distance to touchdown is available by DME or ATC 'track miles to go'. (As well as a very high V/S) With modern and slippery jet aircraft it is unlikely that a succesful landing could be achieved from a false G/S.


Regards
Exeng

[keithl]

Thanks for all replies so far, everyone's trying to be helpful but no-one's answered the question: Any actual examples of "where published" so I can read the words?

[exeng]

I can't help you with any published material.

In the world of everyday airline operations all glideslopes can be, and are frequently, intercepted from above. (We could be talking semantics here however as it is all about how much 'above' the G/S you are. 500 ft above at 10 miles is probably OK depending on headwind etc, but 500 ft at 5 miles isn't OK for a jet maybe OK for a turbo-prop)

It is a requirement that you are established on the correct glidepath by a certain altitude specified by most operators in order to achieve a 'stabilised approach'. Checking you are on the correct glidepath is achieved by checking DME etc.


Regards
Exeng

[OzExpat]

I've been following this thread from the outset keithl, as you would probably have expected. I'm concerned by the use of the phrase "but only where specifically published". If it is indeed published, it suggests that the procedure was deliberately designed that way. If so, it is very bad practice because of rate of descent considerations.

However, the phrase is a bit of a catch-all type of statement that might be employed by ATC. Or even as a local area restriction for noise abatement or due to a need to avoid Restricted airspace, or some such thing. I can't see any situation where a procedure designer would deliberately require an intercept from above, unless there was a note relating to maximum IAS to limit the rate of descent a bit.

Aside from obstacle protection, the main consideration with an ILS procedure design is to allow aircraft to be stabilised on the approach as early as possible. I'll keep monitoring this thread in the hope that someone can provide an example of a published requirement to intercept from above.

[keithl]

Ozex: Yes, I thought you'd be there, and you know my views from previous discussions. I do understand about CDAs, but they are not relevant to my current concerns. You clearly understand why I want to get the actual wording.

I'm going to ask the question again now on the ATC forum. Maybe someone at an A/D with this notation will be able to help.

[keithl]

Copied to here, along with answers, from ATC Forum

For years I have taught that the ILS G/S should be approached from below, to avoid the (first) false lobe. Now this (http://www.caa.co.uk/docs/33/srg_gad_gasil4of2003.pdf) (page 16) indicates that some A/Ds have some procedure annotation indicating that it may be intercepted from above.

I know about CDAs and am not interested in them (helo ops). I really want to read the words so I can see exactly what is intended. I have asked on Tech Log, and I have contacted the publication's editor, but no-one can provide examples.

Can anyone here point me in the right direction? (That being an air trafficker's job, after all!) Otherwise this would appear to be an unsupported assertion.

[brimstone]

Hi keith - I've been following your thread on Tech Log with interest.

I am of the opinion that the author of the article in GASIL 4/2003 may be guilty of loose wording and is in fact referring to airports where CDAs are standard procedure. ie the London airports and where the ideal profile is to intercept the glide-path at the appropriate height for the distance without recourse to level flight.

I think the phrase "specifically published" probably refers to the UK AIP entries for these airports where detailed procedures regarding the requirement to attempt to achieve CDAs at all times are laid out.

As far as I am aware at other UK airports CDAs are considered to be a "best practice" to be aspired to when possible, but normal practice would be to intercept the glide-path from below.

[Jerricho]

For Heathrow, CDA's are considered not only best practice, but are monitored and evaluated as well, being one of our Key Performance Indicators (KPI's) between the hours of 2300 and 0600 local (or 0700 for Easterly operations). If for any reason during a night shift, when the noise thing is the most "sensitive" to the locals, a CDA is not complied with, the details (operator, time etc) are recorded and past on to the powers that be, who I believe contact the operator if it resulted from the operation of the aircraft.

[bluethunder]

Shoot me if I am wrong.
But doesn't a false glidepath on an ILS appear on the earliest on 9 degrees on a 3 degreeglidepath? Being far off the localizer this could fool you to seem OK. But if you are established on the localizer this could be easily checked with among others OM crossing altitude or a decent rate of half of your ground speed. And if you are above, wouldn't it be safe to put in the localizer minima until you are established on the ILS glidepath?

[keithl]

I wouldn't say you're wrong, bt, only that my understanding was that the first false GP can be as low as 6 (maybe 6.5 deg). I'm not among my books at present, so can't follow that up further (but will).

On your other point, it is the same one that others have made on this topic. My answer is "yes, you can x-check against various fixes. And yes, you can immediately tell that you are on a false one, but by then it's too late to recover to the true one, so you have to go round, so it's better to be absolutely certain before you start that you are intercepting the correct one".

Everyone quotes these cross checks will be a certain defence against the false GP, and so they are if everythings going well. But do late/rushed descents no longer happen? My experience in the sim suggests otherwise...

[keithl]

Thanks, Brimstone, I am inclined to agree that it is a case of loose wording. I am taking it up "through channels", but before I make official waves, I need to see if anyone out there can find an example of what he appears to mean.

Jerricho, I'm still trying to see the connection betwen your reply and my question! Could you elaborate, please?
But as you're at Heathrow, perhaps you are familiar with the words the GASIL Ed. is talking about, in which case I'd be very grateful for Chapter and Verse.

[OzExpat]

I couldn't agree with you more kiethl. There have been plenty of well documented cases (none of which are currently at my finger tips, apart from NZ60's now famous erroneous GP incident - but that's another matter entirely, of course). The fact is that it IS possible for an aircraft to be hot and high, not properly configured.

At such times, pilots still naturally want to try to land on time, to save money for the company and minimise complaints from the pax. So they setup a higher rate of descent than normal, with a higher groundspeed than normal, then try to play catchup with the checklists and aircraft configuration. This can work out okay but the odds are not good because Murphy's Law intervenes so that the aircraft intercepts a false GP and the crew is too busy to notice it due to a higher workload than normal.

So yes, it happens. Unfortunately.

[Jerricho]

Sorry Keith, my reply was more towards Brim's comment about CDA's being best practice to be aspired to whenever possible.

Of course, saftey is always a factor. And we do try to issue descent instructions to prevent you guys being above the glide path. But there are one or two operators that during our "CDA critical" night movements point the nose at the ground and level for ages and ages.

Gotta be honest...........that GASIL stuff is white man speaking with forked tongue.

[Captain Stable]

Like keithl, I still can't find anything documented.

Lest anyone be tempted still, as OzEx points out, be tempted to follow it even once they have identified it as a false GP, they need to consider that the aircraft's energy will be significantly higher than when on a "normal" ILS approach.

Kinetic energy is significantly higher, speed often ditto, and all that neergy has to be lost on the round-out. This will involve landing longer or faster (or both) than normal, and your landing distance performance tables are then useless. If you are heavy, chances of going off the far end (at worst) or heavy brake use (at best) will be the inevitable result.

Not to be recommended, as you could (a) bend it or (b) find yourself in the FM's office with hat and without tea and biccies.

[bluethunder]

Keithl,
I tried to find documentation on where the false glidepaths appear, but haven't succeeded so far. So I would like to see what you can find.
And, sorry, I haven't either found an approach where such an approach would be the normal procedure. Would like to hear if there is such, as well.
And then a correction to my own posting after having read it a number of times; Setting the localizer minima when flying an ILS approach could create a big hazard (B-737-700). If you at any point decide to do a missed approach, the Flight Director/Auto Pilot will aim for the altitude set in the MCP panel.
It will be a judgement call if you are intercepting the glidepath from above, but latest, as my SOP says, the approach should be stabilized from 1000' RA meaning:
Aircraft in landing configuration
Aircraft on the correct fight path
Airspeed within target speed +10/-5 kts
Sink rate maximum 1200 fpm
Engines at stabilizid forward thrust
Landing check list completed.
I am sorry for the lengthy post, but I didn't want anyone to get new ideas based on my previous posting.
I haven't come up with any answers to your question but I will follow this thread, as I am curious myself.

[keithl]

BT and all interested parties: I have now found three relevant references.

1. My old CFS notes (circa1980) say that the first false GP is "not below 10deg".

2. "Ground Studies for Pilots", Taylor & Parmar, 1990 quotes "above 6deg".

3. "Air Pilot's Manual", Thom, 1990 says "above 12.5deg".

So, I don't think I've proved anything, but it would be wise to assume the lowest figure until we've proved otherwise. Is there a "Calibrator" out there?

[safetypee]

Some of the older ILS that I flew on, gave false glide slopes at the harmonics of the GS angle i.e. 6 deg, 9 deg, and 12 deg, for a 3 deg slope. The signals were reversed at alternating angles; thus although a false path existed at 6 deg the fly up / down indications indicated in the opposite sense. Some 9 deg GS were use for experimental work on steep approaches. Some military ILS with GS as low as 2.5 deg had correspondingly lower false slopes at 5, 7.5 and 9 deg, but again with alternating reversals of the fly up / down indications.

There is nothing to say that you must not join a GS from above, but it is not good practice. Good situation awareness is always required, but with joins from above or when flying a CDA a mental plot of altitude vs range is essential. Remember 3 deg slope is approx 300 ft/nm thus you should be on the slope by 1500 ft (aal) at 5nm.

Also, remember that there are similar side lobe effects on the LOC, thus automatic / FD systems should not be armed for capture until close to the centreline, which in turn must be verified by other means. ADF locator, map, crosscut DME, etc. For me this is a far greater hazard than a false GS; early LOC capture and turn in can quickly erode safe altitude margins.

[keithl]

That's most interesting, safetypee. So a consistent argument that would draw all the evidence together would be that:
For a standard 3deg GP:
a. The first false GP is at 6 deg, but
b. the first false one that gives logical demands is at 9deg
c. Falsies do appear at 12deg and upwards but may be discounted due to the extreme error needed to find them.

Anyone unhappy with that?

For the record I am therefore happier about intercepting from above (within reason), but still have a problem with the unclear wording in GASIL.

[safetypee]

keithl, don’t be too quick with the logical conclusion. My experiences were with older ILS (many still in service), but I suspect that even the latest equipment has many pitfalls for the unwary aviator.

I also note that at least one of the major manufacturers, whilst accepting joins from above, cautions against arming the automatic capture systems until the aircraft is safely clear of false GS or LOC signals. Although the human can deduce that the indications are reversed, automation just blindly follows the received signal.

The UK NATS give advice on the use of ILS in AIC Pink 141 (http://www.ais.org.uk/aes/pubs/aip/pdf/aic/4P141.PDF) (AIC 34/1997) the link may have restricted access. If registration is required start with: www.ais.org.uk

The following extracts are relevant:

“The GS coverage is 10 nm +- 8 deg from LOC centerline. Vertical coverage is between 0.45 x and 1.75 x GS angle (i.e. 1.35 deg to 5.25 deg above the horizon for 3 deg slopes)”. I assume this is the calibrated (guaranteed) signal, thus does not exclude false GS beams outside of these limits.

“Where special approach procedures have been devised in which the GS may be joined from above, pilots must be aware that false GS may exist at an angle of about an angle of 2 x GS angle (i.e. 6 deg for 3 deg slope). This false GS must not be used for descent guidance. Establishment on the promulgated GS should be confirmed by the relationship between aircraft height and the distance to runway threshold.” i.e. approx 300 ft/ nm for 3 deg GS.

For LOC the useable coverage is 25 nm for +- 10 deg and 17 nm for +-35 deg, both from the front course. (Does this imply no back course ILS in UK, I think so?) These values are less for steep approach ILS installations. There is a caution about the risk of early LOC capture even within these limits.

[keithl]

Thanks again, sp. Your point about the crew's responsibility for being clear of false signals is one I referred to earlier in this thread (23 Dec 10:53) so I agree there. I do have the AIC you refer to, but it is not quite detailed enough. Interestingly, the reference to "where special approach procedures have been devised in which the Glide Path may be joined from above..." takes us right back to my original question, i.e. "Where might that be then?"