744 IRS

[Guest Andrew1981]

hi , I noticed that the PMDG 744 FMC Position is displayed correctly depending on if you have the GPS NAV active or not . But the autopilot will nevertheless follow use GPS to navigate. With only the IRS Navigation active, in the ND it's clear that the Autopilot doesn't follow the displayed track . In this pic it's not clear why it keeps flying slightly right off track :http://img212.imageshack.us/img212/4590/irsweirdo0ut.jpgBut here it

[cowpatz 243]

Hi there. Not sure of your name as you did not sign your post.From what you have described and shown it would appear to be that this is reasonably well modelled in the sim.You mention in your last question "Is it possible to update the IRS position from the VOR stations ?" Well the answer is you cannot update the IRS position from anything only the FMC postion. Once the IRS units are alligned during preflight then that it is it. Over time they will progressively accumulate errors, sometimes positive and sometimes negative. Prior to GPS the raw FMC position was a result of a weighted average position based on the 3 IRS units. This FMC position could only be updated on takeoff and enroute via various ground nav stations, VOR, DME, LOC. When you push the TOGA switch on TO the FMC position is updated to the landing threshold or to the position shift position that you can enter as a plus or minus figure on the TO page ie. intersection TO. Once airborne, and within range of ground nav stations the FMC position will be updated. Once outside of the range of these stations then FMC position will be updated to the raw IRS position.When GPS came along this was used to update FMC position. As we know GPS is extremely accurate and therefore takes the highest priority for FMC position updating. This GPS updating can be inhibited by using the GPS NAV INHIBIT key.In your first picture (unfortunately you only selected the position button for the ND on the second pic)the GPS NAV has been inhibited and therefore the FMC position is now the average of the IRS positions. Looking at the second picture you can see why it went right as the IRS positions are all out that side. If you left it long enough the map would progressively update as well.In the second pic you have re-enabled GPS NAV updating and therefore the FMC position is updated that of the averaged GPS position because it knows that this is most likely to be more accurate than raw IRS position. Because you did this over a short period of time the map did not have time to update and so when you went back to GPS updating the FMC postion just came straight back onto the magenta track line.VOR or DME updating automatic and is used anytime the GPS is disabled and within range. Make sure that the VORS are not left in manual so as as to get the best automatic navaid update.In practice I cannot think of when you would need to disable GPS updating apart from some approaches not referenced to the WGS-84 reference datum and god only knows where that would be.CheersSteveCheersSteve

[Guest Andrew1981]

>Hi there. Not sure of your name as you did not sign your post.There is at the signature picture ;)>In your first picture (unfortunately you only selected the>position button for the ND on the second pic)the GPS NAV has>been inhibited and therefore the FMC position is now the>average of the IRS positions. Looking at the second picture>you can see why it went right as the IRS positions are all out>that side. If you left it long enough the map would>progressively update as well.>In the second pic you have re-enabled GPS NAV updating and>therefore the FMC position is updated that of the averaged GPS>position because it knows that this is most likely to be more>accurate than raw IRS position. Because you did this over a>short period of time the map did not have time to update and>so when you went back to GPS updating the FMC postion just>came straight back onto the magenta track line.>VOR or DME updating automatic and is used anytime the GPS is>disabled and within range. Make sure that the VORS are not>left in manual so as as to get the best automatic navaid>update.Thanks for the reply.Just to correct some misunderstandings I didn't switch the GPS off to take the first picture it was off since I aligned the IRS on the ground . I just turn it on to take the pic and then deactivate it again. So the Autopilot was flying along a quite different route from what displayed in the ND display .>In practice I cannot think of when you would need to disable>GPS updating apart from some approaches not referenced to the>WGS-84 reference datum and god only knows where that would>be.Of course in FS GPS is perfect , but IRL GPS may not be always accurate enough . Appart that not all 744 have GPS installed.Best Regards Andrew Angelopoulos

[Qavion 19]

"I noticed that the PMDG 744 FMC Position is displayed correctly depending on if you have the GPS NAV active or not . But the autopilot will nevertheless follow use GPS to navigate."PMDG's behaviour doesn't sound right to me (although I haven't independently confirmed it).With the GPS position purged, the magenta track should visibly shift if the IRS or DME-DME position is different from the GPS position. That is to say, it will move fairly quickly.. you won't have to go away for 5 minutes and come back again to notice the difference. The aircraft (symbol) will appear offset if the map shift is large... and it will chase after the magenta line. As you say, it remains offset in PMDG.Inhibiting the GPS or purging Radio Navaids is quite different from flying out of range of navaids. If you fly out of range of navaids, there is NO map shift to the averaged IRS position (unless you are in polar regions). The FMC remembers the difference between the radio position and the IRS position... and it continually adds that bias to the mix when the aircraft flies out of range of the stations. Of course, the IRS will continue to accumulate errors.. and you may see a map shift when you fly back into radio range. Be careful... if you purge the radio position, say, in the middle of the ocean, the bias will be removed completely.. and you will be stuck with pure IRS position."With only the IRS Navigation active, in the ND it's clear that the Autopilot doesn't follow the displayed track . In this pic it's not clear why it keeps flying slightly right off track :"Did you look at the display at the lowest range prior to inhibiting the GPS. You may find that PMDG flies slightly offset anyway. Once in cruise, I tend to put my ND on the highest ranges, and because of the resolution, the airplane appears on the track.I believe some improvements were made in the 744F... Which version are you flying?Cheers.Q>

[cowpatz 243]

Sorry Andrew I did not see your name on the pic :-hang Qavion is correct in that without radio updating the position bias is maintained by the FMC from the IRS position and I assume the same logic should apply to the GPS. I did not realise that you had it inhibited from the beginning. I have found that there is a regular offset at times....maybe it is incorrectly modelled. Will have to muck around with it some more.Steve

[Guest Andrew1981]

>Did you look at the display at the lowest range prior to inhibiting the >GPS. You may find that PMDG flies slightly offset anyway. Once in >cruise, I tend to put my ND on the highest ranges, and because of the >resolution, the airplane appears on the track.>I believe some improvements were made in the 744F... Which>version are you flying?Yeap when I Enabled the GPS I found that there is a small and constant offset but it's much smaller. No mater what, with the GPS off it will stay way off the displayed track. This can be confusing when flying off-track to avoid traffic or WX and then rearming the LNAV cause of the difference between displayed and followed track. I use the PMDG 400F v1.01 I tried and it's the same both in the 400 and 400F recently reinstalled . However I never tried before getting the Freight .