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What are the Real World Limits on Glideslope Capture fr...

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How far above the glideslope (either in terms of dots of deviation on the GS scale or linear dist.) will the real aircraft engage the GS pitch mode?I was experimenting with GS capture on my favorite virtual 744 and discovered that while GS capture from below the glideslope was reasonable and uneventful, GS capture from above can produce a wild ride. I set up the aircraft in landing config (flaps 30, gear DOWN, Vref30+5) and maintained 5,000ft (LOC is the engaged roll mode, LOC button illuminated on the MCP, ALT is the pitch mode) until about 5 miles from the threshold (way, way above the glideslope). At this point, I pushed the APP button and GS engaged as the pitch mode (green GS on FMA) right away. The A/C proceeded to pitch down about 10-12 deg. with no regard for airspeed. As the IAS increased, FLAPS RELIEF kicked in, and closer to the ground, the ground prox. warnings went off. The pic below shows the situation at approx 500ft. AGL.Of course, nobody would fly this approach in real life, but I'm pretty sure the real A/C would also act differently (most probably by not engaging the GS mode that far off from the glideslope). So what are the rules of AFDS GS engagement in a situation like this?Thanks!Rushad

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Guest The_Glideslope

Well that's more a stunt than a landing.I'm not sure why you chose to do this because unless you turn the AP off you WILL overshoot anything above the indicated altitudes for OM, MM and IM. If a pilot passes above these above the indicated speeds on the chart, it is his decision to abort. You won't see a real 744 doing what you did. That's an expensive machine. Going down so fast would damage the aircraft and the pax wouldn't be that happy.So answearing your questions, check an ILS chart for any airport and you will see the markers and correspondent altitudes for interception. Usually you'll see Baro/Radar altitudes for each marker (like OM GS 1655'(1308')/MM GS 552(205'). Before that the chart indicates the Altitude to intercept the Glidepath and the DME to do that. For Lisbon (LPPT) Rwy 21 the alt is set to 4000 at 10 DMEs. The picture you have I can guess you were 5000 at less than 5 DME... The DH (Decision Height) is set close to Minimums so how do you expect to reach DH if you are so high above?

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also, i think it's odd that you engage the APP function 5 miles before touch down.


Tom James

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I am not sure what the real world aircraft would do in that situation as we never allow ourselves to get into that position in the first place. The whole idea behind flying a jet is stability. In normal circumstances the aircraft captures the GS from below. There are odd occassions where it must be captured from above (LAX being a frequent case where on occassions it is not uncommon to be positioned onto the GS at 4000ft where we should ideally be at 3000ft). The degree of just how much above comes down to operating experience and company operating procedures. The further out you are the more chance you have to reconfigure the aircraft (flaps out gear down etc) and descend at a higher rate to get on the the GS. As a general rule of thumb 3 times the distance and add 2 zeros will give you a reasonable height to aim for. For example 9nm out you should be at a height of 2700ft. You need to add ldg elevation to this to get the altitude reading on the altimeter. So for a 400ft elev airport you would need to see on the altimeter 2700 + 400 = 3100ft. This would put you on a nominal 3 degree slope. Most ILS GS's are from 2.5 to 3 degrees. Also on the ILS approach chart you will see specific points on the approach (called markers) or fixes that will have a crossing altitude associated with them that enables you to check the GS integrity. It is also probable that the ILS has an associated DME that can be used as a distance to touchdown to enable you to work out an approximate altitude to be at a given position. The danger of being grossly high (as you were) is that there are false GS signals radiated above the actual GS (the first at approx 6 degrees) that could cause issues for the AP. I am not sure but I vaguely recall that these indications are reversed as well so that it would be giving a fly up indication instead of fly down adding to the confusion.In the position that you were in being 5000ft at 5nm is grossly high. You should have been at around 1500ft plus ldg elev. It is not good to be in a jet descending at a high rate close to the ground. For a start the engines would have spooled right down and when it comes time to level out the aircraft will want to continue, with momentum, toward the ground and also the thrust will be slow to come on due to having to accelerate up from idle. As in your case you will get "SINK RATE SINK RATE" aural warnings and I see in your case you even got the dreaded "WHOOP WHOOP PULL UP" which is usually the last thing heard on the CVR prior to a crash. Most airlines have a policy where you have to be established on slope and on speed at a particular point in the approach (say 1000ft IMC [in cloud} and 500ft VMC [visual]) or a go around is mandatory.Sorry this appears to have become a long sermon, however this is a fairly topical point with airlines at the moment in their bid to reduce CFIT (controlled flight into terrain).CheersSteve


Cheers

Steve Hall

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Steve:<3 times the distance and add 2 zeros > That's great! I've been flying (privately) for over 30 yrs and that's the first time I ran into that rule of thumb. There are many, but just to check I grabbed my calculator and found 300~=6000sin(3); where 6000 almost equals 1 nm. I've heard a similar thing for descent from altitude, for example in a C414 at 21000 starting down 63 mi out (21x3) works if the GPS VNAV is on the blink. Thanks!!OBTW: The GA autopilots normally will not capture GS from above. Risky business, I'd say, unless you are way out there far away from the FAF.


Dan Downs KCRP

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Thank you all very much for your responses.Somehow, I think the intent of my post has been lost in translation, so let me try to clarify. It was flying into LAX (as Steve mentions) above the glideslope that I first noticed that the AFDS engaged the GS mode while I was at least a couple of dots above (but still far enough out to get to the glideslope by the OM). While I have no RW knowledge, I did recall from a few years of experience flying virtual 744s with PS1, that the GS mode did not engage (it remained armed) until you had descended a lot closer to the glideslope (using perhaps another pitch mode). Since the PMDG 744 behaved differently, I decided to experiment. The intent of the experiments was to determine just how far above the glideslope the AFDS as modeled in the PMDG aircraft, would engage GS as the pitch mode.After flying progressively higher approaches (to the point of the ridiculous one in my earlier post) and finding that each time the GS mode immediately engaged and potentially put the A/C in harms way, I was curious as the what the real A/C was programmed to do in situations of intercepting the glideslope from above.So, I was hoping that one of our many knowledgeable posters with real world experience in 744's would be able to provide the criteria for engagement of the GS mode when the glideslope is intercepted from above.Rushad

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Guest Matthias1975

Hi there,apart from all the technical stuff being duscussed here, I am just curious why all you people do always want to make an automated landing. We all know this system is very well modeled on the PMDG 747 and behaves like the real one. Nevertheless I do all of my landings manualy...besides a few with very low visibilty this winter. Nothing

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>>Somehow, I think the intent of my post has been lost in>translation, so let me try to clarify. Rushad,I know exactly what you are after and I clearly recall the limits of A/P when applied to ILS were discussed here at length. I don't know if this was a real 747 driver or someone was quoting Boeing manuals but this topic had indeed been discussed before. I still remember for example some limits as to LOC capture. Try to search this and the old forum and you may find it.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg


Michael J.

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>It is also probable that the ILS has an associated DME that can be used as a distance to touchdown to enable you to work out an approximate altitude to be at a given position. Hi all,Speaking of ILSs that are equipped with a DME, there's something I want to know about the distance given on the PFD. Is the distance that's shown on the PFD just below the ILS identifier the distance from the runway threshold or touchdown point? This is what I've always thought but every time I make an ILS landing equipped with a DME, the distance from the touch down point or runway threshold is actually about 2 miles less than what's shown on the PFD. When I come over the thresthold, the distance indicated on the PFD is 2 miles, and I'm flying right over the threshold. I'm on the correct frequency for the landing runway I'm approaching. So the distance indicated on the PFD is the distance from the other end of the runway. Is this a problem in FS or is this how it's indicated in the real world? Is the purpose for the DME is to give the pilot the distance from the landing runway threshold or touch down point and not the other end of the runway? Ken.

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I did recall from a few years of experience flying virtual 744s with PS1, that the GS mode did not engage (it remained armed) until you had descended a lot closer to the glideslope (using perhaps another pitch mode). Since the PMDG 744 behaved differently, I decided to experiment. People take PS1 too serious at times. FYI it has it's faults still too so best not to judge reality by it unless of course one knows for a fact that a certain system modeling is correct beyond a doubt. There are many different things between the two, some even correctly modelled in the PMDG and not in PS1, this of course comes back the other way too. It's harder even with the F series because of the inherent differences in the aircrafts systems................Randy J. Smith................A PROUD MEMBER OF THE PMDG BETA TEAM[h4]Evolution is a process that results in heritable changes in a population spread over many generations[/h4]


Randy J Smith

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The DME distance in white on the PFD, when an ILS is tuned, is to the touchdown point as they are co sited with the GS transmitter antenna (unless otherwise stated). The DME on the ND is to a separate DME that may or may not be co-sited with the VOR. The ILS approach plate will specify which DME to use. A non ILS DME could be placed anywhere so for example if it was located on one end of the rwy you would get near enough to a zero reading when at one end of the reading and a couple of miles approx when at the other end. This needs to taken into account if flying a non precision approach such as a VOR or NDB.CheersSteve


Cheers

Steve Hall

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>The DME distance in white on the PFD, when an ILS is tuned,>is to the touchdown point as they are co sited with the GS>transmitter antenna (unless otherwise stated). The DME on the>ND is to a separate DME that may or may not be co-sited with>the VOR. The ILS approach plate will specify which DME to use.>A non ILS DME could be placed anywhere so for example if it>was located on one end of the rwy you would get near enough to>a zero reading when at one end of the reading and a couple of>miles approx when at the other end. This needs to taken into>account if flying a non precision approach such as a VOR or>NDB.>Cheers>SteveThat's what I thought. But why does my PFD indicate 2 miles from the runway when I'm flying over the threshold? Ken.

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Hi Matthias,>just curious why all you people do always want to make an>automated landing. We all know this system is very well>modeled on the PMDG 747 and behaves like the real one. Just note that at todays real life operations, with al the complex transitations, STARS or SID's a lot of SOPS's don't allow to much handflying. Complex airliners as the B744 are flown by the numbers. Don't forget that most t/o's of B744's are standard done with LNAN /VNAV armed from the ground.

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Guest andyman

Because the ILS signal thingy-mabobber is at the opposite end of the runway, which is why the AUTOPILOT can track the centerline during an autoland. So this measurement is how many miles to the opposite end of the runway.I'm going to guess the runway was anywhere between 10,000-and 12,000 feet.

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>Because the ILS signal thingy-mabobber is at the opposite end>of the runway, which is why the AUTOPILOT can track the>centerline during an autoland. So this measurement is how many>miles to the opposite end of the runway.>>I'm going to guess the runway was anywhere between 10,000-and>12,000 feet.Hi Andy,Yes, you're right. The runway is about 10,000 feet. I've thought about what you said but it seems strange why this signal locator would be placed at the opposite end of the runway since it's measuring distance to it. If I remember correctly, there are separate vertical and horizontal signals being emitted from two separate antennas, the horizontal being for the localizer and the vertical being the glide slope. It seems like from looking at diagrams I've seen in the past, the horizontal antenna is located at the opposite end of the runway and the vertical antenna is located about 1,000 feet from the landing runway threshold. It's been a while since I've seen these diagrams. I supposed if the vertical signal antenna is located at the opposited end of the runway, the pilots has to take that into consideration, but this does explain the 2 mile difference. Ken.

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