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Nixon

Using VOR's to check ND integrity

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Hi everyoneIn the cruise I tune in the VOR's on my flight plan and the inbound course to that VOR. If selected on the options page, a green dotted course line appears. I have until now assumed that if this course line lies on the magenta flight plan line, then the map integrity has been cross checked by the VOR. Thinking more about this I am now wondering if the green line is generated as part of the ND display and therefore is not an actual indication of the VOR radial. Therefore I should select the VOR display and if the beam bar is aligned then that proves the map integrity.So am I right in my assumption that the only way to check the map integrity with VOR's is by checking it on the VOR display?

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Hi everyoneIn the cruise I tune in the VOR's on my flight plan and the inbound course to that VOR. If selected on the options page, a green dotted course line appears. I have until now assumed that if this course line lies on the magenta flight plan line, then the map integrity has been cross checked by the VOR. Thinking more about this I am now wondering if the green line is generated as part of the ND display and therefore is not an actual indication of the VOR radial. Therefore I should select the VOR display and if the beam bar is aligned then that proves the map integrity.So am I right in my assumption that the only way to check the map integrity with VOR's is by checking it on the VOR display?
Your basically looking at the same thing.The Green lines are infact vor radials from the tuned vors. If you use the ND in VOR mode or map mode, you will get the same vor accuracy, just in a different graphical format. It may be easier to see your aircrafts deviation from the radial with the ND in VOR mode when compared to the map mode(only because of the way it displays), but in VOR Mode the needle will indicate deviation relative to range of the tuned station. (Closer you are to the station the larger the needle will displace for a given offset range from the radial)Edited as I confused myself writing that.. ;)^^^^ What a bunch of jiberish... LOLEasier to understand version> The VOR mode will give you angle of deviation, where the Map mode will give you an actual graphical representation of the distance off the radial centerline... JB

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no you can enter the VOR ident into the fix page and cross check the bearing and distancealthough thinking about it that might be slightly off with regards to the distance, as the fix page gives horizontal distance and the vor antenna give slant distance

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I found this in my Big Boeing FMC Users guide,that covers the 747-400,777,767 and the 757. I was suprised to see the amount of drift that was acceptable.I have not found this criteria for the NG. My question is, how can you have map shift in the terminal arrival area,if the GPS is auto updating?

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I found this in my Big Boeing FMC Users guide,that covers the 747-400,777,767 and the 757. I was suprised to see the amount of drift that was acceptable.I have not found this criteria for the NG. My question is, how can you have map shift in the terminal arrival area,if the GPS is auto updating?
I don't think the GPS is part of the IRS system.Could your chart be referencing just the IRS (without GPS updated) as it states? Edit... I was gonna look it up in the FCOM, but can't tear my self away from work. I'm sure Andrea knows the answer. But it's my understanding that, the IRS alone, updates aircraft position through calculations based on aircraft spatial acceleration.JB

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I found this in my Big Boeing FMC Users guide,that covers the 747-400,777,767 and the 757. I was suprised to see the amount of drift that was acceptable.I have not found this criteria for the NG. My question is, how can you have map shift in the terminal arrival area,if the GPS is auto updating?
Not all FMC (specially the old units) have any GPS navigation. I think when in Boeing aircraft FMC had GPS navigation, the variant was called "Pegasus" - at least in 757/767. I bet there are still 757/767 flying around without the Pegasus FMC.

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HiThanks for your replies. I have looked at FCOMv2 and on p 10.40.5 it says under option VOR course lines displayed, Manually tuned VOR radials "When a navaid is manually tuned, the selected course and reciprocal are displayed". I have always assumed that information source for the course lines was derived from the station. Am now wondering if the information is instead derived from the GPS,IRS mix. If that's the case then the VOR display would have to be used to check the ND integrity.With a fix such as a waypoint, I presume the position must again be derived from GPS IRS mix. I have always presumed that the position of a VOR fix is derived by using the same mix. It's only tonight that I started to question exactly how VOR positions are arrived at when displayed on the ND and I have not found in the FCOM any definitive answer to that.So in summary it seems that the view is that my original assumption stated in my first post is correct.

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have always assumed that information source for the course lines was derived from the station
In a conventional Vor system, The ground based vor transmitter is sending an omnidirectional radio signal only.. It's the combination between the Vor Reciever and OBI that translates the needle position based on frequency lag from the station. I'm sure the web has more info on how it technically works.
Am now wondering if the information is instead derived from the GPS,IRS mix. If that's the case then the VOR display would have to be used to check the ND integrity.
I'm not that technically savy with the real NG. But I'm sure that the actual VOR recievers on the plane are prmarily responsible for the VOR indications on either the VOR or MAP mode, if not purely responsible. But there may be a composite of info that includes data from the rest of the nav systems on the NG. Fail your IRS and GPS and see if both VOR and MAP modes will succsesfully indicate and track a Radial. Then do the opposite and fail just the VOR recievers.
With a fix such as a waypoint, I presume the position must again be derived from GPS IRS mix. I have always presumed that the position of a VOR fix is derived by using the same mix.It's only tonight that I started to question exactly how VOR positions are arrived at when displayed on the ND and I have not found in the FCOM any definitive answer to that.So in summary it seems that the view is that my original assumption stated in my first post is correct.
Any fix that has altitude asociated with it will be a fix in 3d space and obviously not on the ground. So yeah, I'm sure the fixes generated by the FMC that can contain altitude constraints will be a data composite of multiple sources.If you wanna back up your theory, assuming the NGX is simulating all of that correctly, then do a test and fail certain systems.Edit... After thinking about it... Considering that Conventional VOR Navigation is less accurate the further you are from the station, but at the same time it can be more accurate (the closer you get to the station) than Rnav equipment (GPS, INS, IRS), I'm sure there is a gradual handoff of primary data that the FMC prioritises to plot position.Andrea would know.. JB

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If you'd like to check for navigational errors, here's two things to try:1. Put a VOR into the FIX page2. Dial the same VOR manually3. Hit POS on the EFIS control panel4. Compare the two readings.andCheck the POS SHIFT of the Left and Right IRS.

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@ non-GPS airplanes: Those will still update the FMC position via DME/DME, VOR/DME or VOR/VOR scan, so basically you should only get a position error if you manualy tuned the recievers to an out-of-range station.

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Nixon,To answer your question in your original post, I figured I would do a quick test. (Plus I was curious)I failed both the right and left nav recievers and the ND in both MAP and VOR mode would not indicate any vor stations or radials.Then it apears that the green radials and GREEN VOR position in MAP mode are being derived solely by the NAV Recievers and COURSE Selectors just like a conventional VOR system and not a composite of data from the other systems.JB

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Thanks Jack for doing thatSo my original assumption was correct.
So am I right in my assumption that the only way to check the map integrity with VOR's is by checking it on the VOR display?
No, I don't think so. It's not the only way. You can check many ways. You can use the VOR display, the map display, and like Mat said and probably the easiest way is to check the POS page to directly compare all the nav data sources. But to strictly compare the ND magenta course lines vs the NAV radials (victor airways/Jet Airways) would probably be the most intuitive just by zooming in and looking at the map display.

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As stated before, keep in mind that the actual data from the VOR will be a little different. In our ops, the non flying pilot must all ways tune/display the nav radios progressively along the airway when nav aids are available. On the FMS map and CDI, you will indicate right on the airway. On the non flying pilot side, the nav aid CDI will be off course depending on the distance from the nav aid. I also demonstrate this to my students during training flights. I will have them load and fly a VOR/NDB FMS approach. I tune and display the nav aid on my side. During the app i have them cross check both sides to compare the information. Alot of guys are shocked by the amount of disparity between the two. I use this demo to show that it's best to fly it in blue(FMS) instead of green(nav aid mode) if available. In my ops it's legal for us to both fly it in FMS mode as long as one pilot has the nav aid tuned and displayed as a back up. In this case all is needed is to slave it to a pointer on the map or RMI.

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