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Guest reyramon

LNAV and the Queen

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Rey,Is this occuring on every flight? If so, I'm betting on a corrupted magvar file in your FS install or something to that effect.


Ryan Maziarz
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"Could it be the IRS is aligned to the wrong coordinate, hence everything is offset?"Armen...It's difficult to describe, but...The IRS's and radio updates (including GPS) create the background map on the ND. The (real) aircraft is designed to believe that map is real and will fly to the magenta line on that map, even if the background map is created from false data (such as wrong IRS entry).If the aircraft suddenly gets correct data, the background map and magenta track will shift quickly to one side.... and the aircraft (which is represented by the fixed airplane symbol), will try to "chase" the moving magenta track (by flying to the track in the real (outside) world).Hope this makes sense.Q>

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>Rey,>>Is this occuring on every flight? If so, I'm betting on a>corrupted magvar file in your FS install or something to that>effect.Ryan,Yes this is happening with every flight! Interesting thought of you, where can I find the original one? (which file),To the other poster: yes I always do full alignment with the correct coordinates.

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Only way I know of to fix that would be to uninstall and reinstall FS, it's embedded in the BGLs I believe...


Ryan Maziarz
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Guest reyramon

well i'm going to reinstall the queen tomorrow and see if it helps any. if not then i'll reinstall fs9 to see if that file gets copies right. i'll keep you updated.

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Guest D17S

Not to get too far OT, but this might be a good place to ask. The EFIS MCP's buttonology almost seems to suggest that the airplane and the ND are working off different sources of present position data. Weird idea? Well . . Stay with me for a moment. I agree that the ND's 'map' is the present position reference for navigation, there's just something I don't understand about how a function or two works. There's a button on the EFIS MCP called "Pos"(tion). The button overlays the ND's aircraft position triangle with a horizontal bar (dog bone looking thing) for the updated GPS position. When you disable GPS updating, the symbol becomes a little three dotted airplane (looking thing).The question then becomes: So, the map is considered 'real' (the actual present position reference)and the triangle at the base of the ND simply represents the position of airplane relative to the map (This is similar to the 'painted onto the glass' function those little, bolted on horizontal coat-hanger-wire airplane symbols provided in those ancient, mechanical ADIs. It's essentially an airplane symbol bolted to the airplane's instrument panel that something moves around behind.) In other words, the "airplane" (the "you are here" triangle on the ND) doesn't have a position . . . other than to be 'painted on' to the back of the glass at the base of the ND. It's the ND's map that is displaying the present position data and that 'painted on' airplane just flys there. The airplane's position is 'fixed'. The airplane's position IS the triangle. It is literally that piece of glass.If this general understanding is accurate, then what's the purpose of the "Position" annunciation provided by the EFIS MCP?Here's a test flight I did before the patch. I flew a polar route just to get out of range of radio updating. (Just FYI, there

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Sam..... The laws of physics change at the MS poles ;) Ignore them.In real life...The triangle represents the FMC computed position. If the GPS is active, the FMC uses this position when navigating in high latitudes. If GPS is not available, then each ND switches to the onside IRU. i.e Captain's ND uses the Left IRU and the F/O's ND uses the Right IRU. The switchover from triple IRU to single IRU is fairly smooth. If the magenta line is racing away from you, then it will probably be a result of the global distortion in FS9.The aircraft should try to chase the magenta line (but this may depend on which magenta line we are talking about... the one on the Captain's ND or the one on the F/O's ND, which may show a difference in polar regions.On the real 744, if you manually tune bogus VOR's it means nothing. If there are two valid/local DME's in range, then the FMC will ignore the manually tuned VOR's and use DME stations (DME-DME updating). DME-DME updating has preference anyway. All you are doing by tuning bogus VOR's is preventing VOR-DME updating and stopping the "VOR" displays from working on your ND's. Pressing the POS button is showing you "raw data". It shows you where each (drifted) IRU thinks you are... i.e. by painting asterisks on the display... and where the GPS thinks you are...i.e. by painting >-O-< 's on the screen... and where the other nav radios think you are, by painting green lines from navaids (with bearing/distance info). If GPS is operating, then more likely than not, the GPS symbol will be will be sitting on top of the airplane triangle symbol. Priority is given to the GPS's by the FMC, so if LNAV is engaged, then the aircraft will chase the GPS symbols. Viewing raw data (by selecting POS) will tell you if something is out of whack. For example, you will see straight away if a single IRU is drifting away from the others.Hope this helps.Cheers/Q>http://forums.avsim.net/user_files/141341.jpg

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Guest D17S

Hey Ray, Hope you don

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Sam.. I'm still trying to stick to my theory that if GPS is available, the FMC/map position will use GPS position (and for all intents and purposes, the IRS is ignored*). The minor displacement of the GPS symbol and the map may simply be graphic resolution.Note that there are two GPS systems on the 744. When the two GPS signals are identical or close to each other, the graphics generator draws a single >-o-:) Perhaps you can think up an experiment for me to carry out on the real aircraft to disprove what I am saying?Cheers.Ian R>

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Guest D17S

I agree. Gotta think about this too, but I am also leaning hard toward the map being positioned by GPS . . . regardless of what we are seeing in the picture. But if this is the case, what's the point of even having GPS "POS" paints in the ND? Great stuff. I need my own airplane . . . but it's not coming for another year. Nuts! Right now, I'm leaning back and looking up. Ouch. Got that old crick in my neck again. We need that CB panel modeled.Seems a bit fuzzy now, but I was involved in setting up the GPS interface on a couple of old 747-1/200 freighters several years ago. Seems there was a GPS initialization procedure where I could manually drive GPS position around a bit. That would be the trick. Pull the breakers on one GPS, then manually drive the other GPS off position a bit and see it the map follows. How could you tell if the map moved along with the slewed GPS PP? Align one IRU (manually) to the same off-set lat/long as the offset used for the GPS. Then use the POS function to see if they line up at the triangle. Now drill down to a tight range setting and get a navaid (or anything) on the ND so you can use something for reference. Now shut down the IRU. Is the map still alive? Did it stay put? Now, are you still hooked up to slew GPS position? Slew it. Do you have control of the map?But somehow, the GPS PP has to be moved manually to see it the map follows. Can't wait to get my own toy.

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The map is drawn based on FMC position. There are 4 positions to consider- FMC, GPS, IRS (tripple mix) and radio position. IRS position will generally be least accurate, since the IRU's drift. Next will be the Radio position. Whwn a radio is tuned, the FMC knows it's EXACT position (from the database). It does some fairly simple trig calculations to calculate the plane position based on the navaid (RAD position). Next up is the GPS position, that it gets from the GPS receivers.All 3 of those are sent to the FMC. Initially, FMC position is set to IRS position. However, the IRS will drift, so after takeoff, the FMC uses the RAD and GPS position to update the FMC position. The updating calculations are stongly biassed towards the GPS position, so 99.999% of the time the FMC position will agree with the GPS position. Remember though, that the GPS only UPDATES FMC position, GPS doesn't CHANGE FMC position. Hence, I don't think the minor difference in that pic is cause for concern.Paul

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Hi, there are some add on Sceneries like EPGD for FS2002 which cause the offset errors in FS9. Jan-Paul

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Guest D17S

Paul, (et. all) Thanks for the feedback. We're gonna have some real experts around here before long. Now if we could just get Ray's Lnav to engage.

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