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tcas_climb

IRS NAV ONLY

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Just a quick question hereIm doing a flight from EHAM to CYYZ right now, and with GPS INHIBITed, i had received IRS ONLY NAVIGATION with IRS NAV on the ND, fine.....however going south of Iceland, the Radios auto-tuned IMG VOR which was over 246nm away and although i wasnt receiving neither radial nor dme, it switched from IRS NAV to R/I NAV......is that supposed to happen?EDIT sorry FS9 MD11

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yes i was on the NAT. I had received the IRS ONLY NAVIGATION message in the CDU some minutes after receiving the last VOR over Scotland. However passing south of Iceland, over 260nm away, it tried to tune another vor at least it was displayed on the ND but i wasnt receiving any radials or dme, and it changed from IRS NAV to R/I NAV, moving away from Iceland, some minutes i once again received the IRS ONLY NAVIGATION message and it changed back to R/I NAV.

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IRS NAV only basically means the FMC isn't getting posn updates with VORs. It could either be that you turned off automatic tuning on both NAV radios and you haven't updated a new VOR within 15min (I think) or there are no VORs in your area. It happens in the oceans and deserts all the time. If you were to lose RNAV capability at this point or the primary flight displays, you would have to resort to dead reckoning for navigation.For the VOR that the FMC tuned over Iceland, not all VORs have DME.

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IRS NAV only basically means the FMC isn't getting posn updates with VORs. It could either be that you turned off automatic tuning on both NAV radios and you haven't updated a new VOR within 15min (I think) or there are no VORs in your area. It happens in the oceans and deserts all the time. If you were to lose RNAV capability at this point or the primary flight displays, you would have to resort to dead reckoning for navigation.For the VOR that the FMC tuned over Iceland, not all VORs have DME.
Yep i know, but as i said, i wasnt receiving a radial at all, and at 246nm, thats out of range of FS's VOR signals

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TCAS,From what I understand, it should be tuning VOR's along the way, but since you were out of range on the NATS, you'd be out of range, and shouldn't get any signal. You should thus have stayed in IRS NAV.

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yes i was on the NAT. I had received the IRS ONLY NAVIGATION message in the CDU some minutes after receiving the last VOR over Scotland. However passing south of Iceland, over 260nm away, it tried to tune another vor at least it was displayed on the ND but i wasnt receiving any radials or dme, and it changed from IRS NAV to R/I NAV, moving away from Iceland, some minutes i once again received the IRS ONLY NAVIGATION message and it changed back to R/I NAV.
Bryan,Looks as normal system operation to me, when GNS is inhibited.INGO (ING) is a VOR/DME.For radio updating, the FMC makes at regular intervals, a list of stations it can use for this, by selecting stations around the aircraft position (300nm also have read 370nm) from the nav database.On this list several criteria are applied (figure of merit, stations deselected etc.)From the remaining list the FMC will tune navaids to calculate aircraft position.From the ranges you mention (246 and 260nm) it's normal that the FMC tuned ING to try a radio update, and informed you by showing R/I NAV.(after the message it changed back to IRS NAV you mean.)Regards,Harry

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Harry thanks!So what you are saying is that even if the aircraft tries to tune the VOR and doesn't receive any signal at all it switches to R/I NAV in anticipation of receiving the VOR or DME signal?

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So it seems R/I indicates that the FMC is tuning a navaid, not necessarily receiving the navaid. Gotcha!

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Harry thanks!So what you are saying is that even if the aircraft tries to tune the VOR and doesn't receive any signal at all it switches to R/I NAV in anticipation of receiving the VOR or DME signal?
Bryan,Yes, looks like they programmed this autotune feature like the real one.Which doesn't surprise me at all.Harry

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