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False ILS Glideslope

Started by Britjet, Mon, 10 Sep 2018 17:31

Britjet

I wonder if we have any experts here who can shed light on what inhibits the extra lobes that are sometimes mistaken for a false glideslope. I don't think I have ever knowingly encountered a false glideslope for real, although of course it is a potential hazard and is trained as such. This is despite being on lots of "high" approaches!
In PSX it seems to happen frequently - in fact at any time that I am high on the approach initially, (which happens a lot to me - doh!).
Does it occur these days? How do airports and/or aircraft get around it? I thought that false glideslopes were a thing of the past unless NOTAMed as such , but I actually don't know.
Any thoughts?

Peter

Avi

Some six years ago I made a post in the Alpha test forum which was Hardy's inspiration for the G/S malfunction:

Quote
Something similar to the Eastok Avia incident (false G/S) happed in LLBG few years ago.
On an early morning hour when there was a fog and low visibility, aircrafts were instructed to do an ILS approach to runway 26.
3 aircrafts did it (or tried to) but during the descent, on GS, the (very aware) crews realized the aircraft's altitude didn't fit with the DME and canceled the approach.
I don't remember the numbers but when approaching runway 26 you first fly above hills east to the airport so they were relative low.

When the airport authority went to check what was the problem with the GS antenna they found out that wild bush grew up so much that it twisted the antenna signal. Once they clear the area the problem was solved. There were no problems with the equipment itself.

Of course this kind of problem can't be detected by the aircraft systems so no flag can be displayed. It is all about the crews to save the day.

Cheers,
Avi Adin
LLBG

emerydc8

QuoteI don't think I have ever knowingly encountered a false glideslope for real, although of course it is a potential hazard and is trained as such.

Add me to the list.

Jon

double-alpha

I encountered this situation a long time ago (I can't remember the details!!!) during a visual approach with an ATR.

We were too high, and we used the ILS to help us with the GS... we got the wrong GS before being established on the LOC (bad idea).

In my Airbus FCOM, there is a procedure called « glide interception from above », which should only be applied when established on the localizer ( but no other limitation, or extra lobe caution)... very useful in KLAX, by the way

Will

I've never seen one in real life, probably because in real life I never intercepted a glide slope from above.

I'm glad Hardy added the training opportunity to PSX though.
Will /Chicago /USA

tango4

Those secondary lobes are definitely not a thing of the past.
But the reason you will not encounter them frequently is because you have to be VERY high on the approach to encounter them.
For a standard 3 degrees ILS if I recall correctly you have the two primary lobes between 0 and 3 degrees, and between 3 and 6 degrees. The secondary lobe will be received when you are between 6 and 9 degrees.
What this means is that up to 6 degrees, the glide signal is valid. Above 6 degrees, you should have an inverted signal.


We had a very serious incident a few years ago involving an A340.
Without going into too much details that I will not put on a public forum, basically the aircraft had been vectored way too high, and the pilots failed to do their DME cross-check because they were "behind their aircraft". Problem is the aircraft tried to follow the inverted glideslope and came very close to a stall.


Now I cannot guarantee that all ILS work in precisely the same way as I am by no means a radio navigation specialist, but that's the way I understood it when we were briefed following that incident.


Charles

Hardy Heinlin

Avi, was that LLBG case related to the "normal" 6 and 9 degree effects, or to random interference which may also occur due to vehicles etc.?

Random interference can be simulated by the features on the "Malfunctions" page. The interference effect is static when the aircraft isn't moving. The instrument indications move randomly only because of the aircraft's motion through the "comb".

As for the 6 and 9 degree effects: I think this is an important training feature in PSX. I don't want to miss it.

Charles, as far as I understand it, the next higher, false beam is not at 7.5 degree between 6 and 9. I think it's at 6. And then another one at 9.


Cheers,

|-|ardy

tango4

You might be right on that one Hardy (I would even say that you most probably are !).
I'll try to ask someone working on our ILSs  in the coming days to get a confirmation on that one and report back if I can get a more precise information.

Avi

I think it was related to a random interference (I think the 6 and 9 cases came much latter).
Avi Adin
LLBG

Hardy Heinlin

From memory, I think the EFIS G/S scale maximum is 0.5 degrees (2.5 dots).

So the 6 degree beam will be alive on the EFIS when you are between 5.5 and 6.5 degrees.

If I recall correctly, the diamond in PSX will jump from full up to full down when the signal rises through 4.5 degrees. Perhaps that's the point what Peter refers to? He's not above 5.5 seeing the wrong G/S alive, but he just sees the jump?


|-|

tango4

Here is a picture similar to what I remember in our briefings.


https://www.dropbox.com/s/9yh54asol5ad9oa/v5ap5%20%281%29.gif?dl=0


Not sure I interpret it correctly but what I get with this one is that below 6 degrees you receive the 90Hz "go down" signal.
Then if you find yourself above that you receive the 150Hz signal which is the opposite glide. And if you go above 9 degrees then it once again tells you to go down (this secondary reversal rings a bell in my mind).
Once again, I'll try to check that with ILS guys to see if they can get me a more definitive answer.


Sorry I could not post the picture directly I'm on my mobile phone and could not successfuly do it.




Hardy Heinlin

That picture helps and will make Peter happy. Indeed, there's no beam at all at 6°. The 6° section isn't a copy, it's an elimination. The next positive signal is only at 9°. A lot of references in the media and Internet are wrong according to this.

Britjet

The PSX indications that I have been getting are constant, not a quick flicker. They look believable apart from the obvious rate of descent and height checks.
In PSX I have been typically 1000 to maybe 1500ft high at, say, 8 miles in these situations, not good, but not particularly extreme..
Peter

tango4

Here is a recent example involving a B747 where the aircraft intercepted what I guess is the 9 degrees false glide (not the inverted signal coming before that) that led to a crash.
https://aviation-safety.net/database/record.php?id=20170116-0



mark744

Quote from: Britjet on Mon, 10 Sep 2018 23:05
The PSX indications that I have been getting are constant, not a quick flicker. They look believable apart from the obvious rate of descent and height checks.
In PSX I have been typically 1000 to maybe 1500ft high at, say, 8 miles in these situations, not good, but not particularly extreme..
Peter

I recently had very similar
Mark

Hardy Heinlin

In the next update I'll remove the 6° beam, and add another one at 15°.
In other words, 6 and 9 will become 9 and 15.

I guess the signal gaps at 6° and 12° are too small to cause the indication to disappear for more than a fraction of a second, if at all. So I just let the indication invert when passing 6 or 12.
In previous versions this happened at 4.5 and 7.5.


|-|ardy

tango4

I could just talk with our ILS guys and they confirmed me the version I mentioned above. The inversion comes between 6 and 9 degrees. The false glide is at 9 degrees.
I just also found another document we had following the mentioned incident that confirms this version. It just adds that the ILS signal is deemed RELIABLE between 1,35 and 5,25 degrees.
I guess beyond those areas you start getting a weaker signal. And perhaps the switch when approaching 6 degrees is not instantaneous because both signals would be mixing (the real glide and the false one) but this is sheer assumption on my part.

richjb

Quote from: Britjet on Mon, 10 Sep 2018 17:31
I wonder if we have any experts here who can shed light on what inhibits the extra lobes that are sometimes mistaken for a false glideslope. I don't think I have ever knowingly encountered a false glideslope for real, although of course it is a potential hazard and is trained as such. This is despite being on lots of "high" approaches!
In PSX it seems to happen frequently - in fact at any time that I am high on the approach initially, (which happens a lot to me - doh!).
Does it occur these days? How do airports and/or aircraft get around it? I thought that false glideslopes were a thing of the past unless NOTAMed as such , but I actually don't know.
Any thoughts?

Peter

Peter,

I had one instance where I captured a false GS in a modern, EFIS airplane and that was Learjet 31A.  The story begins with that famous line "There I was, fat, dumb, and happy...".  We were being vectored into our home airport (KICT) and for some reason that I can't recall, we were kept high.  We were given an intercept heading and cleared for the approach.  We remained at 4000' MSL, 1000' above the GS intercept altitude and intercepted from below what we thought as the real glideslope.  We intercepted a false glideslope and popped out of the clouds overhead the airport at about 1,000' AGL.  There was no DME on this approach, so all we had was radial off the ICT VOR to check our altitude at the FAF.  The fact that we were expecting a visual breakout didn't help.


More recently, I have had more problems capturing false localizer courses.  I have captured them in the DA2000 and CL300 (Rockwell Collins PL4 and PL21) and even in the Lear 45 (Honeywell PRIMUS).  Usually, this is the result of premature arming of the LOC or APP mode while too far to one side of the LOC centerline and/or being too far out on the approach.  Some approach controls in the US have a nasty habit of clearing the aircraft to intercept the localizer beyond 18 NM from the runway, the maximum distance if hasn't been flight checked for ESV.  It doesn't take much to intercept a false LOC that far out.  Just like a GS, you'll intercept and track the false course toward the runway; however, there could be intervening obstacles or other aircraft in the way.  My hapbit now is to use the FMS until I'm within the normal service volume of the ILS LOC.


Best regards,


Rich Boll
Wichita KS

Jeroen Hoppenbrouwers

I flew a few ILS approaches in a 172 a long time ago, and the instructor purposefully (he knows me and hey, he is on this Forum, gosh) drove us through a few of these lobes. The indicator needles swayed all over the scales, I remember. The aircraft swayed all over the runway, too, but that was due to a different reason   :-D


Hoppie

cagarini

Just for curiosity sake, I asked at one of the few X-Plane 11 add-ons support forum ( an Airbus A320 ) because I had experienced, under some occasions, what I thought might have been the case for "False Glideslopes", and among other posts on that thread, there's this one, which of course most of you know from memory, but I didn't and found pretty interesting:

https://forums.x-plane.org/index.php?/forums/topic/157716-false-ils-glideslopes/&do=findComment&comment=1485319