ILS Sensitivity

[dcoded]

Dear All.

one thing just struck me the other day.

When in Low-Vis we establish holding-points not to interfere with the signals.

What about flying an ILS behind an A380 for example. Would he partly obscure the signals and give me false readings?
Or the signal manages to bend around the airframe and continue more or less un-obstructed to me?

[bubbers44]

Landing at MIA one day an airliner crossed my landing runway taking off at about 200 ft when I was about 50 ft. I got full left and right localizer deflections as he crossed my runway. Has anybody else had this happen?

[7478ti]

Read the AIM Section 1-1-9 para k. This issue has been well known since at least 1939,if not before. It is why we have critical and sensitive areas in ICAO Annex 10. It is also one of the many reasons why GLS (and RNP) will soon replace ILS, not the least, that ILS is far too expensive to install and maintain, and knocks itself out even in simple snowstorms.

[dcoded]

Thanks for your answers.

But, I know all about the critical areas and why they are established.

That's why I asked the question, Does another aircraft flying in front on me on the SAME approach affect the signal that I am receiving?

[bubbers44]

I am interested too because I only had this experience once that I noticed and an aircraft crossing my localizer signal shouldn't be as big a disturbance as an aircraft on the localizer in front of me. I was in a B757 with no one in front of me but the crossing airliner about 200 ft high crossing near the far end of my runway 9. My airline always wanted us to do autolands in visual conditions for periodic equipment checks and I always refused because without a sterile approach environment ILS signals were not protected.

[oceancrosser]

I´ve had similar disturbances in B757 at my home base where landing on a particular runway, the exit is a 90° turn off the runway in front of the LOC antenna, saw the preceeding (also 757) one day in CAVU wx and as he turned 90° on the end my LOC signal went full left, full right and back to middle. Happened too quickly for the A/P to do much except wiggle the wings, but in IMC approaching minima, definitely not what you want.

[LimaFoxTango]

@bubbers44

Wouldn't you advise ATC telling them your doing an autoland? Maybe they could then enforce the protected areas.

[flarepilot]

let's not write the ILS off yet...imagine having to fix things on an ILS...drive your jeep out there to the localizer shack and fix it up.

or drive your space shuttle up to the global nav sat and get your space suit on and fix her up.

yes, a snowstorm knocked out the reno ILS two years running, and then the FAA decided to put the snow proof one in...Reno usually doesn't get that much snow and they had put the cheaper model in...it worked for forty years plus.

OH, the first landing by a commercial airliner doing an ILS was during a snowstorm in Pittsburgh , PA 75 years ago.

IF you are doing an autoland you are to advise ATC.

I've been number five in line on an ILS and no reflected signal from the planes in front of me.

[bubbers44]

LFT, I was doing an ILS into DFW and the FO flying on autopilot in an MD80 when I noticed the full down deflection of glide slope and looking at DME knew things were not right so told FO to level out. We were about 1300 ft with full down glide slope about 8 miles out. I told the tower we had faulty glideslope indications because about that time it went full up and we were in the clouds. They said we are not required to protect the ILS until we are under 800 and 2 miles. A 747 was taxiing past the GS antenna and that caused the bad signal. I was a little PO'd so said should we just ask for a localizer approach from now on? Some aircraft on the ground said it is difficult to couple an approach with no working GS.

[rudderrudderrat]

Hi dcoded,
Or the signal manages to bend around the airframe and continue more or less un-obstructed to me?
Afraid not. The wavelength of ILS is about 3 metres, and any aluminium bigger than the wavelength will disturb and reflect the signal significantly.
The VHF signals are basically line of sight, and if you followed someone on the approach, you both should be able to "see" the ILS LOC signal cleanly (because it's at the far end of the runway).
However, I think the the Glide Slope transmitter could become obscured if you were both exactly in line. (But how many times have you lost the view of the PAPIs due aircraft ahead obscuring your view?) There's also a chance the reflected signal from the aircraft ahead could cause a phase difference with the direct signal and give you false Glide Slope indications.

[ampclamp]

Had a 380 do just that to an aircraft the other day. A/P and ILS checked out just fine, no defects.
I have seen this type of thing a number of times in several types. Difficult or impossible to prove but when there is no defect on board or with the nav aids a beam error is likely.

[HEATHROW DIRECTOR]

In true LVPs approach spacing is extended to protect ILS signals.

[BOAC]

We were about 1300 ft with full down glide slope about 8 miles out. - you were what?!:eek: How's your SA?

[bubbers44]

DME remember is on the localizer so less than 6 miles when we should have been at 4 miles from approach end of runway at that altitude. Lighten up a bit.

[BOAC]

?? Good luck.

[eglnyt]

On most installations the DME is situated around the midway point and zero'd at the touch down point.

In general although the aircraft in front will reflect the ILS signals its position means that those reflections will bounce off away from you and you won't see the effect. The same does not apply to reflections from aircraft on the ground passing close to the aerials.

The aircraft in front could in some circumstance block the ILS signal but the placing of the signal sources means that it won't normally be in the path between you and the signal source. Again aircraft on the ground are more likely to be between you and the sources.

[bubbers44]

DME on the localizer has always been on the localizer antenna so a bit of math was required to figure touchdown DME. Maybe that has changed in the last 10 years. It would simplify cross checking distance to altitude a lot. DME should be co located with the GS antenna to be more meaningful. I haven't flown in 10 years because of the old age 60 rule so maybe things have changed.

[DaveReidUK]

At UK airports, at least, approach plates typically show the DME zero-ranged to the threshold, regardless of the physical location of the antenna.

[HEATHROW DIRECTOR]

Hmmm.. I always thought the DME was zero ranged to touchdown.

[Chris Scott]

Re possible LLZ (localiser) interference from preceding a/c, eginyt is right. If you are on the GS, you should always have line of sight on the LLZ antenna, unless either another a/c is departing, or you are at a platform height on long final.

HEATHROW DIRECTOR,
Agreed for the UK.

bubbers44 and eginyt,
Think you are both partly right on the DME location, because bubbers seems to be thinking American and eginyt British.

Can't quote the relevant books, and I'm not currently flying, but in the 1980s, most large US airfields seemed to have the DME TRx antennae at the LLZ antenna. No bias is applied to the readout, so the reading at the touchdown point had to be quoted on the chart (say, 2.4).

In the UK, the DME is generally somewhere at the side of the Rwy, but not normally abeam the touchdown point. A bias is then built into the reading transmitted to the a/c, so that a/c on the LLZ will receive a distance to touchdown. If there is an ILS/DME approach in both directions on the same Rwy, it would make sense for the DME to be placed equidistant from both thresholds, but that used not to be the case at Gatwick, where the bias had to be changed when they switched from 26L to 08R. I think that may have changed now.

The bias sysyem can create confusion when an a/c is using the DME readings for another purpose, such as a SID. And the DME reads negative when you are between the threshold and the ground antenna.

I notice at least one airfield in the US with multiple runways, LAX, has decided to reduce the number of DME installations. They seem still to have the DME with the LLZ, but only at one end of each Rwy. So, to give a very rough example (in the absence of a chart), if you are touching down in one direction the DME reading might be 2.4, reducing. Landing the other way, it might be 0.5, increasing. (The latter case means that the DME would read zero 0.5 nm before touchdown.)

Confused? Just do what it says on the chart!

[DaveReidUK]

Hmmm.. I always thought the DME was zero ranged to touchdown. That may well have been the case once, but it certainly isn't nowadays.

http://www.avgen.co.uk/DME.jpg

[Chris Scott]

Hi DaveReidUK,

I see you are right: the LHR 27L ILS DME currently (well, 2010) reads zero at the threshold, not the touchdown point. I wonder why that should be so, and whether it was ever as HEATHROW DIRECTOR and I think we recall.

[eglnyt]

With a display resolution of 0.1 Nm and an accuracy of 0.1 Nm you'd be hard pushed to tell the difference.

[bubbers44]

Things have changed in the last 10 years for the better. We always had to subract about 2 miles from Localizer DME to calculate touch down DME. This is a great improvement I wasn't aware of.

[flyboyike]

Why 2 miles? What if the runway is less than 2 miles?

[bubbers44]

Then you subtract runway length, what ever that may be.

[flyboyike]

Good thing too, because if you subtract 2 miles from, say, Rwy1 at DCA, you'll be swimming.

[bubbers44]

true, but most pilots are inteligent enough to figure it out.

[bubbers44]

add an l to intelligent.

[flyboyike]

Hey, if we were good at math, would we do this for this kind of money? It's just that until today I never heard the 2-mile figure before. Besides, how would you account for displaced thresholds and such?

[bubbers44]

LAX, MIA and DFW usually have two miles of runway and when the LOC DME was at the far end you subtracted distance to figure touchdown point.

[dcoded]

Thank you Chris Scott for a most informative answer.

My question has been answered and MODs can now lock it if they so desire!:ok:

[bubbers44]

Using LOC DME must be different now but even when you had to do the math it gave you a heads up if the glide slope didn't match the 300 ft per mile profile. Nobody uses this as their primary descent profile, it is a backup to verify the ILS or what ever descent profile they are using.

[Ozlander1]

add an l to intelligent.

And I get "intellligent". :ugh:

[bubbers44]

Age does screw up your vision doesn't it. I could have sworn I wrote intelligent. I'm only 69.

[bubbers44]

Looking back I did leave out the L so I fixed it by adding another. I hate it when my dead english teacher keeps waking me up in my dreams.

[Ozlander1]

Looking back I did leave out the L so I fixed it by adding another. I hate it when my dead english teacher keeps waking me up in my dreams.
I still remember mine. And that was a loooong time ago. ;)