Talk:Instrument landing system

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[edit] ILS related aircraft accidents

ILS was instrumental in the crash of an Avianca 747 in Madrid in 1983. Can someone explain why? Antonio ILS dummy Martin

Read Here: [1]
  • The Avianca 747 did not follow the published procedure and attempted to intercept the ILS on an incorrect track. At the same time the aircraft descended below the published minimum safe altitude. A GPWS warning was received but ignored. (Crewroom mythology has it that the Captain responded to the call "Pull Up!" with "Shut Up Gringo!"). The aircraft hit a ridge to the north-west of the airfield and was destroyed. The radar controller did not advise the aircraft that it was out of position and the approach may not have been monitored on radar. The aircraft did not intercept the ILS so the ILS was not directly involved. Treesmill 08:17, 6 October 2005 (UTC)
  • I don't believe that an ILS has ever been faulted for an aircraft accident. However, that's a pretty broad statement so it may be wrong. Seems like the only accident that got close to being the ILS's fault was an aircraft landing on a runway that was under construction, but the associated ILS was still turned on. Had it been off, the aircraft would not have landed and collided with construction equipment. I don't remember the particulars, but maybe a search of NTSB accident reports would yield an answer. --Dual Freq 23:44, 1 January 2006 (UTC)


[edit] Landing from both directions

Some major airports swithces the landing directions in the same runway frequently - that is from opposite directions. How is this achieved? There might be two sets of ILS and other related instruments fixed thre? —The preceding unsigned comment was added by 213.42.2.26 (talk • contribs) SPJ June 20, 2006.

Dubai apparently only has 1 ILS, but Abu Dhabi International has ILS equipment on both runway ends, runway 13 and runway 31 are the same runway.[2] This requires two separate systems, so 2 localizers and 2 glideslopes and according to the approach plates, at Abu Dhabi two Middle markers. Both procedures appear to use the same VORDME though. For safety the Glideslopes and localizers are normally interlocked so that they are not on at the same time. Sometimes they even operate on the same frequency making interlock even more important. The ones at Abu Dhabi are different frequencies, but only one would be on at a time to prevent misleading signals resulting in aircraft landing at both ends of the same runway. --Dual Freq 11:19, 20 June 2006 (UTC)

Navtek - In the US Localizers are almost always interlocked. The primary reason is there is usually a severe interfearence when the aircraft passes over the opposing Localizer antenna even if it is on a different frequency. This is usually checked by FAA Flight Check during commissioning and if there is no interfearence the interlock is not required, however usually still used. The advantage is maintenance may turn on the unused opposing system for testing without removing the used system from service. The same problem can exist with Localizers on other runways on the airport depending on the geometry. In this case additional interlocks may be required. Glideslopes must be interlocked only if they are on the same frequency. Ref FAA 6750.49 chg 5. The same DME can be used for both systems however the identification is usually switched to be the same as the Localizer with the correct sequence. —Preceding unsigned comment added by 216.64.78.165 (talk) 17:45, 10 February 2008 (UTC)

[edit] Technical

This article is getting way too technical for the average wikipedia reader to understand. I've made some adjustments, but the tech portion might need to go to the end and a 'lighter and fluffier' section should probably lead off the article. None of the article is sourced, and the tech stuff is likely to only be found in a Wilcox or Thales tech manual. I'm not sure how any of it's going to be available to a non-pilot / ILS tech. --Dual Freq 23:44, 1 January 2006 (UTC)

  • Too technical and too obscure. I have an excellent background in radio telecommunications and an even better one in aviation and the section headed 'Technical Description' is indecipherable. As just one example it refers to the phase of the 90 and 150 Hz signals as being adjusted to be the same, which is impossible. I tried to edit it into something meaningful but that is just not possible. The diagram with it needs some labels, but even then it is faulty. My diagram has been reduced to a size which makes it unreadable and the caption edited to make it less accurate. treesmill 18:42, 2 January 2006 (UTC)
Sorry for the reduction in size of your image. The size was so that the images wouldn't dominate the page and people could click the image to see the full size. I captioned it with the part that was beneath it and said above is etc.... However, based on my experience with ILS maintenance, I'm not sure I can agree with the way the 90/150 overlap in the image. For the purpose of a simplified overview maybe the image is OK. However, the second image, (which I didn't create), is unlabeled, but shows the simplified/ideal pattern of a dual frequency localizer. The long main lobe that has a dotted line is from the reference transmitter Carrier and Sideband (aka CSB) pattern. This signal has a maximum strength on centerline and each antenna is fed in phase with each other. The sideband only (aka SBO) pattern is shown inside of the dotted lines and properly illustrates a signal fed to two antennas 180° out of phase. Ideally the null or minimum of the sideband pattern would fall on the centerline of the runway. Most ILS theory books would probably stop with this narrow part of the pattern because it is taught using a standard, single frequency set up. The second, shorter and wider pattern, only complicates the picture. It is meant to display the signal from a dual frequency system's clearance transmitter. As for the clearance signal theory, I'm not certain I can explain it in detail from memory, but basically a second frequency is used to reduce reflections from hangers and other stuff that is outside of 10°. It also removes the 'low clearance' points in the pattern so that the user gets full scale deflection on their ILS display while outside of the localizer course. The second frequency is so close that it falls into the bandwidth of the aircraft receiver. When the aircraft enters the area where the clearance signal is stronger than the reference signal, the receiver 'captures' the second frequency and uses that signal, which is full scale fly right or fly left.--Dual Freq 00:01, 4 January 2006 (UTC)
First it is important to remember that the lobes shown in my diagram are not the boundaries of the signals. They are contours which illustrate the locus of points at which the power of that signal is at a particular level. It would be possible to choose a power level at which the lobes did not overlap, but this would give a misleading impression because the signals in space must overlap, otherwise the DDM would change suddenly across the centreline instead of being proportional to deviation as required. I am aware of the existence and purpose of the clearance signal, but in my view it is not a necessary part of a 'principle of operation' explanation. To include it a much more complicated explanation would be needed to account for the need for such a signal and how it achieves the required result and this would add nothing to the understanding of the principle of operation. treesmill 11:33, 4 January 2006 (UTC)
As for the phase of the 90 and 150, I didn't add that either, but I think it refers to what is called 'modulation equality'. Ideally you want the reference transmitter to transmit mod equality, meaning the 90 and 150 are equal in amplitude yielding 0 difference in depth of modulation (DDM) or 0 micro amps of deflection on an ILS instrument. About the only thing you can do is adjust the starting phase of the 90 and 150 signals. If your array or location are not perfect (and none are) you may have to increase the amplitude of the 90 or 150 signal to move the course signal to centerline. --Dual Freq 00:01, 4 January 2006 (UTC)
Adjusting the relative phase of the 90 and 150 signals will not affect the position of the 0 DDM locus, because the relative phase of those signals is changing continuously and rapidly (about 30 rad/s) anyway. As you say, adjustment of relative amplitude is all that is available. treesmill 11:33, 4 January 2006 (UTC)

I'm not sure any of the technical section could be easily distilled into a readable article for a layman. There is about a one foot stack of books that you get at an ILS school for theory, so I'm not sure this article is the place for that stuff. It should probably stick to the basics, ILS helps planes land using localizer, glideslope, markers and DME systems. Maybe a brief description of both would be good. --Dual Freq 23:59, 3 January 2006 (UTC)

This page, seems to have a pretty good description and overview. I'm not suggesting to copy it, but rather trying to back up my above points with at least one web reference. --Dual Freq 00:37, 4 January 2006 (UTC)

[edit] Principle of Operation

I added some comments in the first paragraph of the principle of operation section attempting to explain some of the problems with that section. It seems to be a bit redundant with the components section, I'm not sure what to do with it. I'm sure the frequency part is right, but the antenna theory part is a bit inaccurate. I don't want to offend anyone here so I just added some comments to try to make my point. --Dual Freq 00:33, 4 January 2006 (UTC)

It seems to me that the edits to the Principle of Operation section do not belong there. The position of the localiser aerial, the number and type of aerials employed, and the nature and examples of the identification signals are not part of the principle of operation, but are technical details that belong elsewhere. Similarly the mention of back beam approaches belongs in a section of its own. The description of the localiser aerial prior to the edits is undoubtedly simplified, as one would expect. Perhaps you could be more specific about where you believe it to be inaccurate. treesmill 19:36, 10 January 2006 (UTC)
Where would you prefer the localizer array and identification / backcourse information be put? I thought that a description of the array fit in well with the principle of operation. It doesn't really warrent a new section just to describe the appearance and location of the antenna array does it? Seems like the addition of a couple extra descriptive words doesn't take away from the operation section. Additionally, it might serve to help identify various componants to a layman when they visit an airport. Someone might see a group of antennas at the airport, but not know what it is. With the description, 1000 ft from departure end of runway maybe they can identify the antennas. Maybe the wording is not correct, but it seems appropriate describe elements of the system to me. --Dual Freq 02:00, 11 January 2006 (UTC)
A description of the principle of operation of any system should not include technical details which only serve to confuse such a description. If you feel that a description of the array is appropriate then since it is not part of the principle of operation, it clearly belongs elsewhere, in a new section if none of the existing is appropriate. The information on the location of the localizer aerial is misleading, the position is determined by obstacle clearance considerations (and in some cases local siting issues). A look at some UK aerodrome plates finds a variation from around 500 feet to over 2000 feet from the runway.treesmill 23:28, 21 January 2006 (UTC)
As for identification, I would think a primary concern to a pilot / user would be: am I tuned to the proper facility and is the facility operation normally. That question is answered by the morse code identification. It's also worth noting that there is no identification given by the glideslope and the signal being heard by the pilot is only the localizer. If this isn't an operational concern I don't know what else is. The backcourse is also an operational part of the localizer. If not here where else should it be put? The ID is a very important part of the signal and operation. --Dual Freq 02:00, 11 January 2006 (UTC)
Actually Dual Freq, you are quite correct! the requirement is for a pilot to identify the Identity of the ILS using the 1020 Morse code ident for that particular installation/airport runway BEFORE the pilot is legally allowed to use the facility!! the identity code is transmitted by LLZ on CSB and it is vital to the safe operation of the ILS.--Read-write-services 23:55, 5 June 2007 (UTC)
Identification is indeed a primary concern to a pilot, but this is an encyclopedia which needs to be easily accessible to a layman, and therefore needs to be structured so as to be easy to understand. Detailed information about the identification of aids is a distraction in a 'Principle of Operation' section. Similarly the back course information clearly belongs in a section of its own. It should be mentioned that the increasing use of CAT 1 and better systems is reducing the likelihood of an installation including a backcourse and for this reason there are no longer any such systems in the UK or Australia, the only places for which I have so far had time to do the research. Again, if ident and back course information are to be included it would be appropriate to create a section rather than put it somewhere it doesn't belong.treesmill 23:28, 21 January 2006 (UTC)
I disagree with the idea 90 Hz comes only from one side and 150 Hz only from the other side. I thought the whole point of having a highly complex phased array was that the CSB signal is transmitted with each pair in phase giving a maximum on centerline and the SBO signal transmitted with each pair 180 degrees out of phase yeilding a minimum on centerline. And the composite signal removing the flag and moving the needle in the appropriate direction. I don't think that it can be distilled down to 90 Hz out of one antenna and 150 Hz out of another. Maybe it just needs to say there's an array and 90 Hz tones dominate one side and 150 Hz dominates the other. The reality is that there is 90 and 150 on both sides, but 90 Hz dominates the left and 150 dominates that other. There is more on one side, but both are present. The 1020 Hz ident goes out with the CSB as well, not that that matters. --Dual Freq 02:00, 11 January 2006 (UTC)
The text doesn't say that the signals only come from one side or the other, and if the text is not clear enough the diagram makes it obvious that the signals overlap. Bear in mind that the CSB and SBO signals are an enhancement to the basic principle to improve the quality of the localizer signal and avoid false localiser indications. Again they are not part of the principle of operation. Different manufacturers use different techniques to achieve the required radiation patterns, no doubt for patent reasons, but all of them involve feeding different signals to some of the aerials effectively creating multiple aerials. I am working on a technical section describing the use and purpose of these signals but I do not want to introduce CSB and SBO without providing information on the reason for the use of these signals and how they achieve what is required. This is what was missing from the garbled technical description which you correctly criticised. However this will take some time.treesmill 23:28, 21 January 2006 (UTC)

OK, I moved the backcourse stuff to the a different section, I think they are fairly common in the US especially at rural airports that only have one ILS. A quick search of airnav.com via google yields 185 results to the string "bc rwy". I figure there has to be at least that many. The LOC 1000 ft thing is a rule of thumb, and I agree there is a wide variation of distances so I changed it to beyond the departure end. I'm still not certain where the LOC Ident part should go, maybe a new part of the components section called localizer. I think it is worth mentioning especially since the ident comes from the localizer only, no ident from glideslope. As for the CSB and SBO signals, I'm not talking about clearance transmitters / dual frequency equipment, I'm just talking about a standard-issue, single transmitter, null reference equipment. I don't know any ILS equipment that does not use CSB and SBO to create the radiated signal. However, I don't know too much about end-fire or sideband ref glideslopes, but I assume they have CSB and SBO as well. Most of the stuff I have seen is old Wilcox and new Thales equipment, but I think the main difference in manufacturers is how the signals are made and how the antennas are fed with respect to phasing, I think they all have to have CSB and SBO signals. --Dual Freq 00:45, 22 January 2006 (UTC)

I'm not suggesting that the backcourse stuff be removed, simply that the absence of backcourse in higher quality ILS systems should be mentioned. As far as I have been able to establish, as well as the US installations, there are a number in Canada and one in Scandinavia. There seem to be none in the rest of Europe, none in S. America, Africa, the middle-east, Asia or New Zealand. We know there are none in Australia and the UK. This is not comprehensive, it covers a little over 2300 ILS installations which I can search electronically. I notice that the list which your link produces includes a lot of duplicates.treesmill 22:10, 22 January 2006 (UTC)
Sorry about the duplicates, I only looked at the first page or so and I figured it was a ballpark number anyway. This Instrument Flight Procedures (IFP) Inventory Summary I found via google, says there are 98 B/C approaches (I assume that's only the US). I'm a bit surprised that there are only 87 Cat III procedures. I thought there would at least be more Cat III than B/C procedures. In any event, I moved the B/C stuff to the variations section.
Usage of V-ring antennas allows a runway to have a non-precision approach called a localizer backcourse. This lets aircraft land using the signal transmitted from the back of the localizer array. This signal is reverse sensing so a pilot would have to fly opposite the needle indication. Log periodic antennas are highly directional and do not provide a sufficient signal to support a backcourse. In the United States, backcourse approaches are commonly associated with Category I systems at smaller airports, that do not have an ILS on both ends of the primary runway.
I think that should be sufficient, feel free to translate it to layman's terms if it's too complicated. --Dual Freq 23:30, 22 January 2006 (UTC)
I'm certain that all modern ILS systems create the beam structure by transmitting CSB and SBO signals, though I wouldn't bet that there aren't some ancient systems remaining in the less civilised parts of the aviation world. When the radiation patterns of the various CSB and SBO signals are summed, the result is similar to the two beam structure described. I believe that trying to describe this process accurately would make the description of the principle of operation far less accessible to the layman, many of whom don't even understand the word 'antenna'. It still wouldn't address the reasons for using this technique. I'm also doubtful about the reference to '8 or 14 directional antennas' since current systems may employ 8, 12, 14, 15 or 24 aerials and there are probably others I haven't researched with other numbers. References to log-periodic or v-ring aerials are equally mysterious without at least some explanation of what they are and why they are used and preferably also pictures. treesmill 22:10, 22 January 2006 (UTC)
I wikilinked the log-periodic antenna, but there is no wikipedia article for v-ring antenna. The external links section on the bottom has a page with v-ring antenna pictures and the Thales link has a picture of a dipole reflector antenna array, so I thought that the links would be sufficient since no-royalty free pictures of those seem to be available. As for other antenna systems, I'm not sure what else there would be to list, there are some traveling wave antennas out there and maybe some Alford loop antennas out there somewhere too, but from a US standpoint, log-periodic 8 or 14 antenna systems 'seem' to be the dominant systems. I mentioned the V-ring antenna to open the door for the backcourse part and all the antennas I listed above, except the alford loop, are directional antennas and with the back course section, directivity of the array is what allows or excludes a backcourse. I also said 'generally' in the statement, indicating that I was painting with a broad brush, not trying to list all possible types. Maybe that belongs in a localizer specific section, but that paragraph is pretty much the only one in the article that discusses the localizer. Is that one sentence disruptive enough that it should be removed completely? --Dual Freq 23:30, 22 January 2006 (UTC)
I don't know if you have read the 'log-periodic antenna' article, if so you will have seen that it doesn't help to explain what they are and why they are used in this application. 'Generally' conveys that most systems are of this type, which may be the case in the US but is certainly not generally true. There is a risk of making this (as with many other Wikipedia entries) applicable only to the USA. Other aerial systems are Dipole Array and Slotted Cable and I believe taht both of these can support a back-course. The problem becomes that to be reasonably general and not misleading the sentence should read "...generally consists of 8, 12, 13, 14, 15, 16, 20, 21 or 24 directional antennas such as Log-periodic, V-Ring, Alford Loop, Dipole array, Travelling wave or Slotted Cable antenna." This is hopelessly unwieldy and doesn't add anything useful to a description of the principle of operation. treesmill 20:26, 23 January 2006 (UTC)
In my view the article needs some re-organisation. The 'Components' section should follow 'Principle of Operation' and should include a specific sub-section 'ILS Back-course'. This needn't mention any specific aerial type. This would be followed by the sections 'ILS categories', 'Limitations and alternatives' and 'The Future'. A new section called 'Technical Details' or 'Technical Description' should be added, before or after 'The Future' into which should go much of the stuff we have discussed. I would do it myself, but I won't be able to do the job properly for 3 or 4 weeks. treesmill 20:26, 23 January 2006 (UTC)

Navtek - The diagrams and technical discription of the Localizer just isn't quite right. First of all the Log Periodic Antenna just does not have a narrow beam. The narrow defined centerline is the result of the out of phase SBO signals sent to each pair of antennas. I didn't see in the technical part that the SBO 90 and 150 hz audio signals are 180 degrees out of phase with each other. It can be seen then if the RF phasing of the SBO is adjusted to provide maximum addition of the 150 hz signals on the pilots right side this same phasing will cause maximum addition of the 90 hz on the left side. This is a linear addition (and cancellation of the opposing audio tone) on each side of centerline to at least 10 degrees. The phase of the sideband signal reverses at the null point on centerline. I've been working on and installing these systems for Wilcox/Thales for 38 years and would be pleased to answer any questions. —Preceding unsigned comment added by 216.64.78.165 (talk) 18:14, 10 February 2008 (UTC)

[edit] Marker Beacons

Explain the use of the word promulgated under 'marker beacons' - even knowing the definition of the word, its use clarifies nothing - how is the height promulgated? —Preceding unsigned comment added by 81.158.108.46 (talk) 23:22, 28 October 2007 (UTC)

The use of marker beacons and their placement on the glideslope is determined by the operator of the airport as well as local geographical constraints. There is no set in concrete position data for every marker beacon location (except they need to be coincident with the localiser/gp centreline). Therefore it is difficult to give you an exact place for a beacon. There is an outer and middle and sometimes an inner marker, that's about as far as it goes for "usual situations".--Read-write-services 03:16, 29 October 2007 (UTC)

[edit] Reorganization of article

I have reorganized the article a bit, and removed some of my previous edits that were questioned above. I hope this helps things. I'm not sure I could do the tech section properly, so I've left it out for now. --Dual Freq 02:05, 24 January 2006 (UTC)

[edit] Wrong Pic and other notes

The pic of the "localizer system" is actually a pic of the approach lighting system, and needs to be changed. The ILS glidepath equipment is usually located next to the "captain's bars" while the DME may be co-located or at some other location on the airfield.

Also, in 17 years of flying, I've never heard of a Transponder Landing System. I've flown many fully-instrumented aircraft, including 737s, B-52s, T-38s, T-37s, C-130s, C-172s, and C-150s. While it appears plausible in theory, similar to a PAR, I've never heard it briefed or read it in the FAR/AIM.

Please elucidate! Dr1819 20:35, 30 June 2006 (UTC)

Sorry, but given GPS/WAAS/LAAS/JPALS, MLS is dead. Way to expensive, both for the airport as well as the carrier. Abandonded by the military. Too many far cheaper alternatives available.

I can testify that both localizer picture depict localizer antennas as well as approach lighting. The one at the lead is a system common in Europe, called a Dipole reflector array. The other shows a 14 element log-periodic system. Which one do you think is wrong? I don't know what a captain's bar is, hopefully not somewhere that the pilot gets drunk at prior to a flight, but the GS is usually located around 1000 ft from the approach threshold offset on either side of the runway. The glideslope picture shows a 3 antenna glideslope array, probably a capture effect type system, with a DME antenna just above the building, what's wrong with that picture? Read the TLS article and you'll see that it's a system designed for small airports and it just started up around 2000. There are no published procedures for them they are controlled locally. As for TLS, it's in the AIM right here. I'm pretty sure its just a CAT I system, but its better than nothing for a small airport with terrain that can't handle an ILS. I don't think that its a problem to list those things in the see also section, they are all systems that allow some form of precision approach. --Dual Freq 21:20, 30 June 2006 (UTC)
TLS (Transponder landing System) is a viable and usable system for the Australian Military we use it here for difficult terrain areas where you may even have a intentionally bent localiser centreline for landing on short and hidden runways (during military operations). The system is at least Cat II-possibly CAT III for really difficult areas. The TLS system will be installed soon around a particular place here in Australia as part of a Multi-Lateration system. WAAS and LAAS are going to be installed around again at some major airports here in Australia, they are definitely not dead. The picture is correct, it is a Normarc Log Periodic Dipole Array (LPDA)-a system used primarily by the Australian Air Force-I teach this product to them. Dual FreqJust a note to you. Your explanations to some questions are reasonably accurate-you know your information-good work--Read-write-services 22:28, 23 January 2007 (UTC)
Image:Ojw ILS gs illustration.png
GS and LOC positions at Cranfield, from Jeppersen data
Surely the glideslope transmitter needs to be adjacent to the threshold or touchdown zone (see Runway article for definitions), so that in the vertical direction, it guides you to the beginning of the runway rather than to the far end. Ojw 09:57, 5 June 2007 (UTC)
Runway markings


What is your question? As a real lift example, using data from Airnav, Chicago Ohare Runway 27L ILS equipment is located at Glideslope- 41°58′04.47″N, 087°53′15.050″W and the Localizer is at 41°58′08.418″N, 087°55′32.617″W. Using a measuring tool, it looks to me that the Glideslope is around 1000 feet from the approach threshold and a bit 0ver 400 feet on the left side of the runway. The Localizer appears to be about 1280 ft from the departure end of the runway. Click the coordinates above and use a aerial view from Google maps and Terraserver, urban images, has high resolution imagery of O'Hare as well. --Dual Freq 11:32, 5 June 2007 (UTC)

Hi, MLS is dead...hum maybe, but I know it has been under test in 2005 on the CDG aircraft Carrier. For civil aviation it still doesn't meet the precision for Cat III landing.bloublou 03:16, 8 October 2007 (UTC)

Hi all,

the GP (glidepath antenna system) is usually situated adjacent to the runway, and there is usually some setback from the landing threshold, The beam pattern is adjusted to appear that it is leaving the ground at the threshold (it is actually reflected from a ground-plane-earth mat (beam forming area)) at an angle of 3degrees. The associated LLZ antenna system is located at the far end of the runway furthest away from the approaching (landing) aircraft so that the two major lobes of SBO (sideband only) signals are aligned with the physical centreline of the runway. Does this help? --Read-write-services 23:43, 5 June 2007 (UTC)

[edit] Correct information

Hi User:Dual_Freq, your latest correction is right. GP/GSs are nominally 3degrees above the horizontal-not horizon! --Read-write-services 00:20, 29 March 2007 (UTC)

Hello again, someone edited the article back to horizon, changed it back to correct angle. Also, there seems to be some misinformation regarding ALS approach lighting system-the ALS and ILS glideslopes are different! the visual assistance provided by ALS is directed to a pilot via the cockpit windscreen-this is different angular wise to the ILS GP which is detected by an antenna mounted near the front wheels of an aircraft, therefore the angles are for different purposes one visual (ALS), the other radio reception of the GP (ILS). The localiser antenna for an aircraft is usually mounted in the leading edge of the aircraft's rudder as this is the 'centreline' of the aircraft. Cheers--Read-write-services 23:27, 18 September 2007 (UTC)
The above information relating to ALS angles could be misleading. Most of the ALS is not set to radiate in a narrow range of angles because it is essential that it should be clearly visible to a pilot who is not in the optimum position for the approach. PAPI, VASI or other approach slope indicators are usually set at the same angle as the ILS glideslope if one is installed on that runway. The difference between the glideslope aerial position and the eye position is managed by moving the longitudinal position of the indicator, not by changing the angle which would obviously be most undesirable because the indication would change relative to the ILS glideslope during the approach. Because the eye-to-glideslope aerial height varies between large and small aircraft, airports capable of accepting the largest types and equipped with PAPI often have two PAPI indicators, the lower one (nearest the aiming point) being for the use of aircraft with a small eye height and the upper one being used by 747s and other large aircraft. In similar circumstances VASIs use an arrangement using three indicators, the lower pair being used for low eye heights, the upper pair for large eye heights.
Strictly speaking the localiser aerial is not in the rudder, which is the moving vertical surface, but in the fin.
I see that my change to 'some' on connection with marker beacons has been reverted to most. This is incorrect. Marker beacons are rare in much of the world because of the superiority of DME. Even in older installitons it is not uncommon for markers to have been replaced by DME. There is only one installation in the UK with marker beacons, I don't think there is one at any of the major airports in Europe, I can't speak for the smaller ones because I haven't operated to all of them and I don't have time to research them all. It may be that in the US things are different, but again I don't have time to research this and WP should not be US-centric. One surprising factor in this is the arrival of the mobile phone which has driven up the cost of aerial sites by a factor of 10. treesmill 20:44, 31 October 2007 (UTC)
  • Hi treesmill, Thank you for the correction to the Fin rather than Rudder.

I corrected the installation of markers to most because yes in the US it is most, in Australia it is most and in most poorer nations eg: south eastern Asia etc. Markers are the most used progress-on-glideslope indicators. This fact does not make the article US-centric (believe me I wouldn't do that, I have had MANY discussions about British/Australian English versus US English), It just makes it (as far as geography and numbers are concerned), correct. Most installations around the world have markers. In other parts of Europe (i.e. apart from the UK), markers are more common-but yes they are being replaced SLOWLY (a case of if it isn't broken..) Further to this I would appreciate some further discussion before I revert it to "most" or we keep it at "some" could you please enlighten me as per some statistics? maybe i'll do it too.--Read-write-services 23:27, 31 October 2007 (UTC)

I haven't corrected 'most' to 'some' because I prefer to discuss it here before doing so. I have already given here the definitive information to which I have ready access. Otherwise I can only fall back on my experience of operations which encompasses major airfields the Americas through Europe and Africa to Asia except the Far East. Markers are not installed at most of the places to which I operated. treesmill 15:03, 2 November 2007 (UTC)
Hi there again, I feel that in many cases we are dealing with semantics. The original concept for the total "Instrument Landing System" was to use 75 Mhz marker beacons. The system was designed this way. DME is replacing markers for various reasons-usually cost of land etc. to install marker sites. I think that just about all readers would agree, that whatever the "improvements" to the system are, the original ILS used markers-this is indisputable. The article whether it uses the term most, some, many etc. is irrelevant, however, from a purely encyclopedic view, (which is what Wikipedia is), all old, most updated and a greater than 60% around the Australian/South East Asian Airspace use markers, as they are are a simple (and cheap) 2 Watt AM TX versus an expensive DME that is legally only allowed to be used in landing (you would already know this anyway). I am curious though from a pilot's point of view, isn't the DME range just another thing to distract you? Isn't a flashing/beeping coded signal a better thing for you?? Rather than having to read distance versus altitude at the same time while progressing on the Glideslope?--Read-write-services 22:44, 6 November 2007 (UTC)
DMEs typically augment marker beacons, rather than replacing them. One reason is not all aircraft equipped with ILS will have an operating DME on-board (either one is not installed or it is in-op.) When DME replaces the marker beacons, then technically the approach is an ILS/DME approach and "DME Required" will be noted in the approach plate. Another reason for keeping the marker beacon is convenience... the middle marker is typically located at the CAT I decision height and the inner marker (if present) at the CAT II decision height. 76.66.1.194 (talk) 20:38, 14 January 2008 (UTC)
I have a contrary view to the comment that major airfields in Americas + Europe + Africa + Asia not having markers. The majority of CAT II and CAT III approaches will have an inner marker (at least). In the US, examples include ORD ILS Rwy 14L CatII, DEN ILS Rwy 34L Cat III, SFO ILS Rwy 28R CatII (both MM and IM), and JFK Rwy 22L Cat II (also having MM and IM.) The lack of Markers for CAT I approaches is usually made up by having intersecting VOR radials, not just by replacing them with DME. —Preceding unsigned comment added by 76.66.1.194 (talk) 20:55, 14 January 2008 (UTC)

[edit] Add Head Up Display to "Use of the ILS"

Added a minor reference to HUD and also clarified "autoland" to be coupled to a Flight Control Computer but a manually flow approach using a HUD can also be used. As an aside some companies make both Head Down and Head Up Displays and also autoland systems. In a sense this puts the HUD group (less stuff to maintain so lower cost, less safety concerns from computer failures, so lower cost) in direct competetion to the Autoland group (more precise, better passenger comfort, etc.) - Brian (ZazenCID 22:51, 31 May 2007 (UTC))

[edit] Add Reference to FAA Order 8400.10 All Weather Operations

FAA Notice 8200.97 (Airman and Aircraft Approval for Reduced Visibility Flight Operations, dated 9/20/06) and its contents are incorporated into section 7 of Order 8400.10, Chapter 2, All-Weather Terminal Area Operations via N 8000.340 (which contains the new material for 8400.10) effective 12/12/06. And contrary to what is on the first page of this Notice, all Notices are effective until specifically canceled by another notice. <sigh> - Brian (ZazenCID 16:37, 1 June 2007 (UTC))