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JSACKS

Armed VNAV and LNAV from 90 miles away !

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On a recent flight in the 744, I decided to do a diversion on a flight from JFK to IAD, so I did a HDG SEL change and got out of LNAV and then climbed from my VNAV PTH alt of 210 to FL350 and headed 180 deg in the opposite direction. I did a semi-circle around my routing to IAD and when I was about 90nm from the LNAV track into IAD, I descended to FL210 and clicked LNAV as I was at about a 45 degree intercept from the magenta line but heading toward it. To my amazement, the LNAV button lit up and stayed lit. I then clicked VNAV and it also lit up and stayed lit. Was I just lucky (or crazy?!) or is this supposed to happen?Jonathan


Jonathan Sacks

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I think it would be expected of such flight software (or any software for that matter) to behave "reasonable". If you are on a good intercept course, well positioned then why shouldn't LNAV/VNAV work regardless how you got there? No special 'trick' in software is needed to make it happen.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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Guest neeraj.pendse

You also have to note that when you program IAD as a waypoint, the FMC will use the best method of navigation (VOR/GPS/INS) to get there no matter where it is. Note that if VOR signal is not available, then the FMC will use GPS to plot a course from present position to the known lat/lon of IAD VOR. So yes, the extra work in the Boeing FMC (and PMDG FMC) software is needed to behave correctly. Note that even if you planned a flight from KSFO to KIAD DIRECT, the FMC will go into VNAV/LNAV mode as soon as you are positioned/oriented appropriately to intercept the course line plotted. Now that's over 2,000 miles :-)- Neeraj

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Guest neeraj.pendse

Oh and by the way, I would not be surprized if an airliner picked up a VOR signal at high altitude (FL210 is high) from 100 miles away. Even General Aviation aircraft (with considerably lower power reception on their NAV equipment) pick up VORs routinely from 60 nm away.- Neeraj

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>So yes, the extra work in the Boeing FMC (and PMDG FMC)>software is needed to behave correctly. I don't think you understood what I said. The LNAV/VNAV software functionality must cover situations in which this mode was disconnecetd and then it was enabled again. This is part of the standard LNAV/VNAV software. What I was saying was that there is no special need to cover siutuation in which you fly some large distances away from the original course and then want to pick up the route at some point. In other words no matter whether you return to LNAV/VNAV flying after a long pause (detour) or short pause - from the software point of view it is the same case.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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Guest neerajpendse

>I don't think you understood what I said. The LNAV/VNAV>software functionality must cover situations in which this>mode was disconnecetd and then it was enabled again. This is>part of the standard LNAV/VNAV software. What I was saying was>that there is no special need to cover siutuation in which you>fly some large distances away from the original course and>then want to pick up the route at some point. In other words>no matter whether you return to LNAV/VNAV flying after a long>pause (detour) or short pause - from the software point of>view it is the same case.Ah I see. And agreed. I was refering more to the part when you are very far away from the NAVAID to get reception, but the FMC still will figure out the best way and nav sources to get to that point.Cheers,- Neeraj

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