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Emi

Question on Flightplan indication and VOR Radials

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I noticed this on both, the 747 and the 777 already, maybe one of you can help me.

 

I am on a direct routing to a VOR via the FMCs LNAV function.
The VOR is also tuned in both NAV radios.

Here is the corresponding ND indication I see:

 

ZuBlVEW.jpg

 

Obviously the VOR radial indicated does not match up with the flightplan indication on the ND.

One possible reason could be magnetic variation, which is 5W at KTRI airpot (closest to my position) and 3W at VXV VOR.

However the difference between the two would be just 2° while the difference on my ND indication is 5°.

Furthermore the closer I get to get VOR, the smaller the difference should become until they finally match up shortly before entering the cone of silence. But this did not happen.

 

The POS function matches the indication of the VOR reciever, so obviously the aircraft still has the correct FMC position.
This is reflected in the ANP which at the time of observing this was 0.06.

 

Here are two more screenshots taken shortly before overflying the station:

As you can see the difference still persists.

 

9tkXaaP.jpg

 

f1ABv4N.jpg

 

Does anyone have an explanation what is going on here?

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A VOR doesn't have it's magnetic declination updated very often.  It is common to see a variation of several degrees between the compass and the VOR and therefore very common to be flying the 236 radial inbound on a 238 track.  I think the FAA gets around to updating the VOR on a periodic flight check schedule, because when the adjustment is made the navaid needs a flight check.  Flight checks are expensive.

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Thank you for the insight, I never noticed that flying here in Germany. But then again Germany is a totally different country flying wise.

If the issue is with the VOR station being miscalibrated, why does the green POS line also show the "wrong" radial?

If my understanding of the POS function is correct it should generate a "fictional" radial from the FMC position to the databaseposition of the VOR which you can use to compare to the raw data indication to check the accuracy of your FMC position.
Would this line not point to the VOR on the active leg of my flightplan then?

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If the issue is with the VOR station being miscalibrated, why does the green POS line also show the "wrong" radial?

 

I never said miscalibrated.  They are never miscalibrated, there is a monitoring system that shuts down the navaid if something is out of tolerance.  The declination is set correctly but the Earths magnetic poles move around.... no one wants to waste money resetting the declination every year or two.

 

The angle you see is because your ND is in heading mode, switch to track and it will line up.  This just the wind correction angle.

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The angle you see is because your ND is in heading mode, switch to track and it will line up.  This just the wind correction angle.

 

The white track line is lined up with the flight plan route and there is still an angle between the track line and the VOR indication. So they don't seem to match yet.

Switching to TRK up does not make the difference either, there is still an angle of a few degrees between the lines.

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Remember that the mag var data hasn't been updated since the release of FSX over ten years ago. Thats why you will see the white dashed runway extended centerline and the computed magenta line on final to be off slightly.

 

This may help......

 

https://www.aero.sors.fr/navaids.html

 

 

Steve Aull

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Thanks Steve, I had this set installed already and it was active for the flight where I took these screenshots.

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Emi,

 

My only thought is that you are very close to the station (like, practically overhead). With that in mind, I'm not sure I'd be paying too much attention to the needles at that range -- they become hyper-sensitive and the RMI starts doing weird things.

 

What's it like when you're a bit further, say 10-20nm, away from the VOR?

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Em, you may not find my answer useful

 

But I think it is a problem with either:

-FSX's placement/co-ordinates of the NAVAID, and or,

-if 3rd party software is installed, they may have the NAVAID incorrectly placed at the wrong co-ordinates, and or,

-the Nav Database (ndb) provider has set the NAVAID to the wrong co-ordinates.

 

Considering that you have the magvar updated, all things being equal, the magenta and VOR arrow should align, like you say. If you are depending on these NAVAIDs for navigation, you should check the FSX and ndb data files, if that's possible, and cross-check their latlon positions with real world data, found on charts, otherwise, google.

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What's it like when you're a bit further, say 10-20nm, away from the VOR?

 

Ignore me. Just re-read your post and saw the top screenshot.

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Em, you may not find my answer useful

 

But I think it is a problem with either:

-FSX's placement/co-ordinates of the NAVAID, and or,

-if 3rd party software is installed, they may have the NAVAID incorrectly placed at the wrong co-ordinates, and or,

-the Nav Database (ndb) provider has set the NAVAID to the wrong co-ordinates.

 

Considering that you have the magvar updated, all things being equal, the magenta and VOR arrow should align, like you say. If you are depending on these NAVAIDs for navigation, you should check the FSX and ndb data files, if that's possible, and cross-check their latlon positions with real world data, found on charts, otherwise, google.

 

Hi Brian,

 

no worries, I posted in the public forums and not directly with PMDG support not because I wouldn't find the communities answers not useful :)

I fact I want the communities opinion because I can't really imagine this to be a bug in PMDGs systems. But more and more clues seem to suggest it is.

 

Going through your possibilities finds me one problem:

The raw data signal and the FMC signal point to the same position as you can see when comparing the green POS line which is FMC generated with the raw data VOR needle.

This means that, unless there is a bug in the system, the position of the navaid in the flight sim and in the database match.

 

BUT if the position of the navaid in the database matches the one in the flightsim, why does position on the magenta routeline not match with the VORs position indicated by the POS indicator?

Expecially since this happened at more than one VOR and even in two aircraft (777 and 747) already.

 

Does anyone have an explanation for this?

  • Upvote 1

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And an additional question- which is indicating incorrectly?

1) The pointer and fmc synthesized radial line, together, or,

2) The VORTAC position

 

I cross-checked my Navigraph and FSX-native "VXV" lat/lon's with the real position of the VORTAC. All 3 match.

 

There is something definitely wrong, good catch! You should report it...

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Hi Brian,

 

no worries, I posted in the public forums and not directly with PMDG support not because I wouldn't find the communities answers not useful :)

I fact I want the communities opinion because I can't really imagine this to be a bug in PMDGs systems. But more and more clues seem to suggest it is.

 

Going through your possibilities finds me one problem:

The raw data signal and the FMC signal point to the same position as you can see when comparing the green POS line which is FMC generated with the raw data VOR needle.

This means that, unless there is a bug in the system, the position of the navaid in the flight sim and in the database match.

 

BUT if the position of the navaid in the database matches the one in the flightsim, why does position on the magenta routeline not match with the VORs position indicated by the POS indicator?

Expecially since this happened at more than one VOR and even in two aircraft (777 and 747) already.

 

Does anyone have an explanation for this?

What you are seeing is normal, and is not a bug. It is important to understand that the representation of waypoints and navaids on the navigation display do not necessarily show their exact position. The magenta line shows the track the aircraft is following towards a particular waypoint. What it does NOT show is the amount of wind correction angle (crab) that the aircraft is maintaining to assure that it will arrive at the waypoint in question.

 

In your example screen shot, you have a right crosswind. Therefore the nose is turned about 4 degrees into the wind. The aircraft is flying (slightly) sideways, relative to its track over the ground.

 

The green bearing indicator shows the position of the navaid RELATIVE TO THE NOSE OF THE AIRCRAFT. I put that in caps because it is an important concept to understand. The nose in your example is NOT pointing directly at the VOR - it is pointing to the right of the VOR, therefore the bearing needle is deflected to the left. The aircraft (as a whole) is tracking to the VOR, but it is doing so with the nose pointed right of the ground track.

 

If the crosswind was stronger, the aircraft would have to turn its nose even farther right to track to the VOR, and the green needle would be deflected even more to the left.

 

The ONLY time the bearing needle would point directly at the VOR symbol would be if there was either no wind, or a direct headwind or tailwind, with no crosswind component.

 

The magenta line on the Nav display shows the aircraft's ground track - it does not show the amount of "crab". To "see" the crab - look at the heading indicator in the lower half of the primary flight display, and/or turn on the PFD FPV (Flight Path Vector)

 

In your example screen shot of the ND, you have your heading bug set right at the top off the index, matching the magenta track line. If you look at the heading display in the PFD, you will see that the heading bug is offset to the left, just like the green bearing pointer in the ND.

 

Because the ND magenta line shows wind-corrected ground track, the nose of the aircraft symbol will always appear to be pointing directly at the waypoint it is navigating to - it would be disorienting if it did not. But, in reality, the waypoint being navigated to is not necessarily directly off the nose

 

For this reason, you cannot navigate VOR to VOR using the ND in map mode, because the apparent position of VORs are offset. To navigate VOR to VOR, you must put the ND in VOR mode, which gives an electronic representation of an HSI, where you set a course in the FMS NAV/RAD page, and center the needle.

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Disregarding the FMC synthesized radial line

The VOR sys 1/2 pointers are both pointing in one direction, and the FMCS plots the waypoint off to its right.

You've got 3 separate systems showing two different things... two are showing the same thing, one is showing a different thing... which do you believe?

Something there is wrong irrespective of crab angle error, or if track-up is fitted or not.

 

EDIT

Em, why are you in TRU heading mode?

 

After further thought, and I can't believe my oversight and I'm very embarrassed by this...

 

VHF.OR is typically 130nm for high alt, and your close in shot may be due to distortion due to cone-of-silence. But I don't know whats going on with the synthesized radial line.

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The magenta line shows the track the aircraft is following towards a particular waypoint. What it does NOT show is the amount of wind correction angle (crab) that the aircraft is maintaining to assure that it will arrive at the waypoint in question.

 

 

Doesn't the angular difference between the heading and track show the "correction angle"? 

 

Are you saying you can't use raw data to check your track?

 

If I was "on track" and there was no mag variation between my current position and the VOR station, I would expect my aircraft to be on the radial (or its reciprocal) which corresponds to the track line.... irrespective of my heading. Isn't that the whole point of the VOR system? (It's not confused by heading).

 

Cheers

John H Watson

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