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ahinterl

NAV1/NAV2

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Sorry, must be blind or what, but I didn't find anything about this in the manuals:Under what circumstances does the FMS switch between NAV1 and NAV2 as source for the lateral nav mode? As far as I understand NAV1 is driven by MCDU1 and NAV2 by MCDU2...And how can I force the FMS to use a specific NAV?

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I don't think it does, not in the way that you mean. The MD-11 does not navigate using NAV1 and NAV2 they way you might in a Cessna. It navigates from one point in space to another using GPS and INS to keep track of where it is. But each of which has an error margin associated with it, so, just like a human pilot, the navigation computer needs external landmarks to confirm it is where it thinks it is, or to at least quantify the error. Where it uses VOR/DME is to confirm it is where it thinks it is. It does this by using logic such as "In the point of space where I think I am, VOR station X should be a distance of a on radial b, and VOR station Y should be a distance of c on radial d. If they are not, then I need to adjust where I think I am." Wether it uses NAV1 or NAV2 to tune the next station depends on which channel it is no longer using.

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I see that my post is misleading somehow (to a certain extent, I've expected this to happen). So, let me clarify what I mean:I don't speak of VOR reception or OBS here.What I mean is the NAV mode of the FMS. The FMA announces either NAV1 or NAV2 in the center column when NAV is pressed. For some reason, I sometimes get NAV2 instead of NAV1. As far as I know, these two annunciations base on two different computers, which get their data from MCDU1 or MCDU2 respectively. Last time I got this was after I played with the electrics bus tie switches. I got this corrected after putting MCDU1 in standby navigation and back to normal mode.Since I havent't found something in the manuals so far, I now ask why it happens that NAV2 is announced, what must happen to make the FMS logic switch to NAV1 (and back), and if/how I can manually force the usage of NAV1 or NAV2.And because I didn't pay too much attention to the respective chapter in the manuals: Next question would be how separated the three MCDUs in the sim plane are (guess there's an option in the menus or so to share data between them)...

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I see that my post is misleading somehow (to a certain extent, I've expected this to happen). So, let me clarify what I mean:I don't speak of VOR reception or OBS here.What I mean is the NAV mode of the FMS. The FMA announces either NAV1 or NAV2 in the center column when NAV is pressed. For some reason, I sometimes get NAV2 instead of NAV1. As far as I know, these two annunciations base on two different computers, which get their data from MCDU1 or MCDU2 respectively. Last time I got this was after I played with the electrics bus tie switches. I got this corrected after putting MCDU1 in standby navigation and back to normal mode.Since I havent't found something in the manuals so far, I now ask why it happens that NAV2 is announced, what must happen to make the FMS logic switch to NAV1 (and back), and if/how I can manually force the usage of NAV1 or NAV2.And because I didn't pay too much attention to the respective chapter in the manuals: Next question would be how separated the three MCDUs in the sim plane are (guess there's an option in the menus or so to share data between them)...
NAV1/NAV2 indications in the FMA stand for the respective FCC (flight control computers) lateral navigation modes. There are, of course, two FCC on board. One for FMC1, one for FMC2.Those computers, as you said, combine the data coming from the two CADC (central air data computers) with the MCDU computers, the FMCs...Now, in normal operations, if you press AUTOFLIGHT button one time, the system takes the data from the FMC1 and the FCC1 to control autopilot and you will see NAV1 mode in FMA central column.If you depress again the AUTOFLIGHT button on the FCP, you will engage data coming from FCC2 and FMC2 and so you will see NAV2 mode active in the FMA.So, to answer your question, you can switch between NAV1 and NAV2 modes using the AUTOFLIGHT button on FCP, and i think also that the automatic switch between the NAV1/NAV2 that you experience is happening because you arm the autothrottle before the flight pressing the AUTOFLIGHT switch and, when you airborne, you press again the AUTOFLIGHT button to engage the lateral and vertical modes of AP...so, simply press the AUTOFLIGHT button a third time and you will return to initial NAV1 mode...BTW, the FMC in MD11 are two, one linked to one MCDU...the MCDU on the aft pedestal is used for maintenance only and is linked to CFDS (Central Failure Detection System) and is called CFDIU...(for that explanation about CFDS i had to take out my printed manuals, because my memory is not so good...tongue.gif)Hope this helps,Cheers,

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I see that my post is misleading somehow (to a certain extent, I've expected this to happen). Since I havent't found something in the manuals so far, I now ask why it happens that NAV2 is announced, what must happen to make the FMS logic switch to NAV1 (and back), and if/how I can manually force the usage of NAV1 or NAV2.
Hi Andreas,NAV1 is the lateral guidance data of AP1 which is needed by the AFS to follow the Captains instructionsNAV2 is the lateral guidance data of AP2 which is needed by the AFS to follow the F/O instructionsIf NAV1 is "active" NAV2 is in "armed" mode, vice versa.If you want to change between NAV1 and NAV2 you have to push the "Autoflight" button once in the case of at least one AP is already active.That means in normal operation: after 3 times pushing the autoflight button you have to see NAV2/AP2 in PFD.First time pushing: arming "autothrust" on ground (before 80knots), Second time: make NAV1/AP1 guidance activeThird time: change between NAV1/AP1 and NAV/AP2 hope it helps youedit: next time i have to be faster in completing my post;-)

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Hi Andreas,NAV1 is the lateral guidance data of AP1 which is needed by the AFS to follow the Captains instructionsNAV2 is the lateral guidance data of AP2 which is needed by the AFS to follow the F/O instructionsIf NAV1 is "active" NAV2 is in "armed" mode, vice versa.If you want to change between NAV1 and NAV2 you have to push the "Autoflight" button once in the case of at least one AP is already active.That means in normal operation: after 3 times pushing the autoflight button you have to see NAV2/AP2 in PFD.First time pushing: arming "autothrust" on ground (before 80knots), Second time: make NAV1/AP1 guidance activeThird time: change between NAV1/AP1 and NAV/AP2 hope it helps youedit: next time i have to be faster in completing my post;-)
I love to answer those technical questions...i must admit that i'm crazy, but i like to study how an aircraft really works reading manuals...(and PMDGs ones are amazing...)So, it's a pleasure to open my manual everytime to review what i don't remember or what i don't be sure about...Big%20Grin.gifAnd, most important, i love MD11...of courseCheers,

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Indeed, I remember that I've read in the manuals that every time AUTOFLIGHT is pressed, it switches between the systems! My head is so full of ballast that I don't see the wood because of all the trees...But: Better to ask an "unnecessary" question than to not ask at all...

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