Intercepting from above
 

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 (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?

Can you check your link please keith?

First link I've tried... Try this . If it doesnt work just go to the latest GASIL Leaflet at the caa website.

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)?

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.

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:

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.

Keithl
 

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

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?

Kiethl
 

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

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.

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.

Intercepting from Above
 

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 (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.

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.

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.

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?

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...

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.

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.

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.