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ILS Capture from Above False GS

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ILS Capture from Above False GS

Old 15th Jan 2009, 01:03
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ILS Capture from Above False GS

I just flew to Antalya Turkey, and there they always keep us high on approach, we were at 8000 ft when the GS started coming, we later on descended and caught the GS from above at 3000 ft.

I started thinking during the approach about the possibility of capturing a "false loab", I'm not too sure about the spelling on that one but anyway, I remember from my ATPL theory that the GS transmitter is not only sending the "standard 3 degree" glide slope but also some false loabs above and below the 3 degrees...

Has anyone ever managed to catch one of these false loabs? my thoughts are because almost everytime when we're flying in Turkey we're capturing the GS from above...
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Old 15th Jan 2009, 01:21
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As part of your SOPs, are you required to check your altitude when passing the Final Approach Fix inbound?

Look at this ILS. You should verify your altitude on the GS at EISEN as 1500'. Then you're guaranteed to be on the proper GS.



Second, with a groundspeed of approx. 140KIAS your sink rate should be about 700-800fpm.

Have a look at the following note. The first false glideslope is at 6 degrees and *reversed*. That's a big warning right there. The second false glideslope is at 9 degrees and properly sensed. That's three times steeper. If you need a sink rate of 2100-2400fpm just to maintain the glideslope then something is definitely wrong!

ILS (Instrument Landing System)

False signals may be generated along the glide slope in multiples of the glide path angle, the first being approximately 6 degrees above horizontal. This false signal will be a reciprocal signal (i.e. the fly up and fly down commands will be reversed). The false signal at 9 will be oriented in the same manner as the true glide slope. There are no false signals below the actual slope. An aircraft flying according to the published approach procedure on a front course ILS should not encounter these false signals.
Finally, the proper spelling is "lobe".

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Old 15th Jan 2009, 01:39
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I've seen the false glideslopes but never actually captured one. As you are no doubt aware, they can exist in multiples of the glideslope angle, depending on how many false lobes are 'powerful' enough to be detected by your ILS receiver. So, with a 3 deg G/S you may see a false G/S at 6 deg, 9 deg etc, although I've only ever experienced the 6 deg variety.

If, as in your Turkish scenario, you will be approaching the G/S from above the only thing you can do is to ensure it's the correct path by doing the 3 times table. If you're on the G/S at 20nm and 12000' you know you're on the false G/S. I add this only by way of additional information, rather than trying to 'teach' you something you undoubtedly already know.

False G/S and an early capture of the altitude when in FLCH (Open Des or whatever it's called ) and then having to quickly reduce the MCP altitude so you can keep the descent going to capture the G/S from above certainly make for 'interesting' approaches. Sometimes these things can't be helped, especially with hills around. Now add in a 'foreign' accent or two and up goes the workload.

You would think those chaps and chapesses, with big brains, who design these things could put in an aircraft filter of some kind to stop the receiver 'seeing' the false G/S. They will, of course, always exist at the transmitter but I'm quite sure a simple 3 times table machine could be added to the aircraft. Yes, I know it would have to adjust to allow for other G/S angles but certainly not beyond the wit of man, especially as the G/S angle is written on the Legs page (I assume Jambes page on the 'Bus) .

Edited to add: DOH! Note to self: don't post at the same time as other people are adding proper stuff
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Old 15th Jan 2009, 11:06
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You would think those chaps and chapesses, with big brains, who design these things could put in an aircraft filter of some kind to stop the receiver 'seeing' the false G/S. They will, of course, always exist at the transmitter but I'm quite sure a simple 3 times table machine could be added to the aircraft.
The lobes don't 'exist at the transmitter'. Rather, they are the effect of Superposition, or in layman's terms, a mixing of the original signal with another 'echo' signal. And no, there is no simple solution, as the receiver simply receives the frequency to which it is tuned, and it is not a simple task to accept some of the signal as 'good' and to reject some of it as 'bad'.

It is rather similar to the acoustic effect of mixed delayed sounds arriving from a PA system from multiple speaker sources, say at a fairground or an air display. When close to one speaker, there is no ambiguity as the strong signal from the speaker dominates. However, move away to a place where sounds from multiple speakers can be heard with similar intensity but with diffferent delay paths, and the muddled sound can be so confusing as to be completely unintelligible at times. Given that the muliple signals are indeed present, you should be completely clear that there is virtually nothing that you can do, as a remote observer or 'receiver of the signal' to separate the signals to regain the intended intelligibility.

Echo deconvolution or removal is a huge subject, and is an inexact science. Seismology is one field where much work has been done, and the Cepstrum, with its multiple Fourier Transforms, is a potentially very good way to mathematically deconvolve a mixed signal to reveal the intended source signal. However I don't believe it will be arriving in aircraft ILS receivers any time soon. For now, the altitude check at aproximately 4NM, as cited by zerozero, is our best defence against false glideslope lobes.
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Old 15th Jan 2009, 11:17
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Excellent Air New Zealand video here

YouTube - NZ60 Erronious ILS Incident, Apia

PS. The video is in three parts.

Last edited by forget; 15th Jan 2009 at 11:34.
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Old 15th Jan 2009, 22:21
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Yes, I have been PNF on a jet that captured the 6 degree lobe (Airbus 320). I called go-around; to be honest it is bloody obvious (unless you are completely behind the aircraft and very unstable as happened with the captain flying it). But I learnt a lesson - don't expect someone who is very behind an aircraft to fly a go-around correctly!
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Old 15th Jan 2009, 23:23
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Very interesting reading...

But regarding the ROD, if you're capturing from above the ROD is probably gonna be around 1500 FPM or maybe more already so it might take some time before you actually realize that its the wrong lobe that has been captured...

I'm just speculating here, but I would say that capturing from above and from high altitude could be a great source of danger... and it's bad procedure from the ATC to keep aircraft high like that...

well well, thanx for the great input, it's been interesting =)
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Old 15th Jan 2009, 23:46
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Well yes, you might descend at a high rate to intercept, but if the aircraft has achieved glide slope capture and is still going down at 1400 feet/min something is very wrong - plus the attitude is wrong and it looks wrong if visual.

Regarding ATC, if you are kept high, it isn't great work from them; but you can always ask for delaying vectors etc. to give you more time. The pilots are to blame if you push on regardless while hot and high.

The airline I am currently flying for is a big advocate of the discontinued approach. On a tangent, most airlines teach us in the sims go arounds from decision altitude, not from further out on the approach. Infact it is far easier at a higher altitude just to level off, start going up and reconfigure and have another go at an approach, than sticking in TOGA thrust.
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Old 16th Jan 2009, 00:20
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I just flew to Antalya Turkey, and there they always keep us high on approach, we were at 8000 ft when the GS started coming, we later on descended and caught the GS from above at 3000 ft.
Be aware that the normal limit of the GS is 10nm. However, even in my meager experience I have seen several undulating captures within 10nm. As PF I now don't engage APP until Flap 5 (some drag to reduce unwanted acceleration) and at a reasonable altitude. I'm usually managing the descent in V/S shortly before capture within the distance limits.
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Old 16th Jan 2009, 09:33
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Thanks PilotMike. I didn't realise it was all so complicated and that probably explains why the big-brained people haven't done it. There's me thinking my 3 times table machine would solve world hunger and cure cancer

Echo deconvolution
I'm going to try and remember that phrase and get it into dinner party conversations
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Old 16th Jan 2009, 09:44
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I had one recently, when tail wind kept us high with ATC speed reductions. Instaed of chasing GS, we levelled off at 4000'. Then ATC requsted us to descend to 3000', but at same time Acft detected false GS ABOVE us and started climbing frantically!!!

Good lesson... disarm GS if you arent interested in capture... before the surprises.
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Old 16th Jan 2009, 11:02
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ILS GS capture

Attempting ILS glideslope capture from above was an automatic instrument rating test fail item when I was a boy in the mlitary. It was considered dangerous for good and sensible safety reasons.
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Old 16th Jan 2009, 17:29
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Here's an interesting bit of trivia--at least for the American pilots.

The GS signal is "flight checked" by the FAA only from the FAF inbound down to DA.

In other words, if you couple the autopilot to the GS outside of the FAF (in the USA) you do so at your own peril.

So, keeping in mind what 'mike rondot' says about joining the GS from above, I would submit, if you chose to join the GS from above, *outside* of the FAF, it should only be used for *advisory* purposes and NOT be used to control your descent.

I'd recommend making that descent on V/S to altitude pre-select, until the aircraft is fully established within the service volume of the ILS.
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Old 16th Jan 2009, 18:11
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Oh yes...

You have lobes on the localiser too, you know! I remember once watching the system lock onto one of those and try to capture it. Didn't work very well; it just turned in, lost the signal and then went into basic "heading and altitude" mode pointed completely in the wrong direction. Our mistake was to have "approach" armed as we flew in "heading and altitude" mode, forgetting about possibly catching a localiser side-lobe. The "LOC" annunciation went from white to green and the airplane turned off the intercept heading while we (somewhat new to EFIS) did that "What is it doing now?" number.

When you are established high on the localiser then you might want to use "navigation" mode, when you track the localiser but do not leave the pre-set altitude, until you are close to the final approach fix at the correct altitude. You won't just have the machine trying to fly an impossible angle but it will not get you to the fix at the correct altitude.

If you have "approach" pre-selected then the machine won't know the difference between a false LOC or GS signal and the real one so that it will just lock on and try to fly it once that course guidance signal comes close to centre: one more example of how people are smarter than computers! Of course Situational Awareness comes into this; it is no good just sitting there expecting the machine to do all the work for you.
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Old 17th Jan 2009, 02:46
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Airbus has a simple procedure for capturing GS from above. You set VS of 1500 to 2000 fpm depending on how far off slope you are and this will bring you down to the GS when it captures (GS*) and you set GA altitude.

However, many lazy unthinking pilots forget to set an altitude ABOVE current altitude when starting the procedure resulting in an altitude capture at just the wrong time before GS intercept.

Never forget your 3 times tables and you won't go far wrong in any descent. Unfortunately many of today's young uns can't seem to do this without a calculator which is slightly inconvenient on the approach!

Published GS height/altitude over the OM or equivalent position (typically 4 to 5 DME on newer ILS installations should always be checked and should be in your company SOPs.

To the guy with the Captain who was "surprised" I'd say, "think of every approach as an approach to a GA" (and every takeoff as a takeoff to a reject) and you'll rarely be surprised. When you're going think of stopping and when you're stopping think of going!
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Old 17th Jan 2009, 19:24
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I've been vectored above the GS a few times, most recently at Bournemouth the other day. Presumably I would have captured the 6deg one had I carried on. I was visual so disengaged the AP and proceeded manually.
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Old 17th Jan 2009, 21:52
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Well, there is no FAF for a precision approach, the final approach starts at the FAP, which is wherever you happen to be when you start down the glideslope. You are the, by def, on final approach.

USA does have it's own non ICAO method of establishing instrument approach procedures (TERPS vs PANS OPS), but that does not mean that you are "at your own peril" with the glideslope coupled at any time, as long as you are not below the relevant altitude (MSA, intial approach altitude or whatever depending on how far out you are).

Coupled or not, the autopilot is NEVER responisble for terrain separation, you, the pilot, are. GS, VS or whatever descend mode you may be using.
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Old 18th Jan 2009, 05:37
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The last sentence is absolutely true.

But anytime you're using a signal beyond the "operational service volume" (do you prefer that terminology?) you do so at your own.....risk.

Perhaps "peril" is a little too dramatic for you.
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Old 18th Jan 2009, 11:20
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Many years ago I was flying a 737-200 to Manila. Due noise abatement we were kept high at 4000 feet until localiser intercept which was around 8 miles where the glide slope should have been on slope at 2400 ft. We expected full scale fly down at LLZ intercept but instead the glide slope indicator on both captain and F/O ILS showed only slightly high by less thah half a dot. Cloud tops were 3000 ft and base 1200ft. Despite full flap and idle thrust we stayed on the apparent glide slope which gave a well over 1000 fpm rate of descent. Ground speed showed no wind. We broke visual around 1500 ft and the runway perspective showed we were definately high with glide slope indication just slightly high. The T-VASIS was useless with red and white lights in unlikely combinations.

Landed Ok and sent report to Manila ATC. A year or so earlier, an Air Manila (?) Boeing 707 from Guam had crashed short of the runway in perfect weather. A couple of fatalities I think, and the captain swore he was on the ILS glide slope right down to impact. In other words he blindly followed his instruments (even though the sink rate was out of this world).

The ILs was tested shortly after we had complained, and the calibration aircraft reported a perfectly flyable glide slope around seven degrees as well as three degrees. This was the result of poor maintenance and it was just by luck we picked it up because of the initial high vectoring. If other aircraft had had similar experience they did not report it. It also explained that the 707 pilot was probably telling the truth a year earlier when he said he followed a steady glide slope to impact.

Asked by the investigators why he didn't go around from the obviously unstable approach, he replied he thought the noise of all engines spooling up from idle thrust would only frighten the passengers! At the time of impact the 707 had full flap, speed brakes extended and idle thrust with a sink rate of around 2000 ft per min. Obviously the captain was a proponent of the saying "Real Men don't go around."

The T-VASIS? The all over the place T-VASIS was caused by severe earth tremors a few days earlier which moved the light boxes. When asked why the T-VASIS was not turned off when it was obvious that damage to the boxes had occurred, ATC advised they were abiding by a standing directive that the T-VASIS must be switched on for all jet arrivals. The Directive omitted to say the VASIS must also be serviceable.. .
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Old 18th Jan 2009, 11:42
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False LOC

In the early 1970s, a DC-8 freighter captured a false LOC beam into Cold Harbor(?), Alaskan Aleutian chain, and hit a mountain.

GB
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