Question on Flightplan indication and VOR Radials

[skelsey 765]

 

 

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.

 

I disagree, Jim.

 

I think you'd have a point if the ND was in "Track Up" in which case I don't really know and my brain can't process how the needles would behave at this time in the morning, but in Emi's screenshot the ND is in "Heading Up". This most definitely makes the ND a big RMI and the indications make no sense.

 

The white track line, and the magenta line, indicates that the aircraft is tracking to the VOR on QDM 237. The needle should confirm this, but instead it is indicating QDM 234. Meanwhile the FMC is showing radial (i.e. QDR) 052/QDM 232.

 

You are quite correct to say that the needle will not necessarily be pointing straight up when you are in heading-up mode: but an RMI always indicates QDM, and if you are established on a steady track inbound to the station (which Emi clearly is) then the white track line should be sitting over the RMI needle.

 

 

 

you cannot navigate VOR to VOR using the ND in map mode

 

You most certainly can, if you are in heading up and use the RMI needles.

[JRBarrett 3,070]

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

I didn't see the QDM discrepancy in the FMS until taking a second look, and that may indeed be a discrepancy between the VOR's stored mag var in the sim database vs. the magnetic variation model of the entire sim world. And indeed a VOR is not "heading" aware if flying raw data using a physical or electronic HSI.

 

But, an RMI only shows relative bearing to the station off of the nose, and in that respect, navigating to a VOR using an RMI is no different than tracking to an NDB. If you were to try to fly to the VOR by placing the bearing pointer on the index, (with a crosswind), you would end up flying a curved path over the ground. Tracking with a crosswind requires taking a cut into the wind, which is going to place the RMI needle off to the opposite side by the amount of the wind correction angle.

 

I'll try to duplicate the situation depicted in the OP's screen shot in PSX tonight - I'm sure we can both agree that the database, mag var model and overall system fidelity are as accurate as it is possible to be in a simulation.

[vadriver 247]

did someone mention the difference between lnav tracks (great circle) & navaid line of sight tracks ??

[Copper. 78]

did someone mention the difference between lnav tracks (great circle) & navaid line of sight tracks ??

 

I did consider that. I don't think that's the problem here.

 

There is a problem with the VOR systems on this airplane, or, scenery is interfering with the QotS2.

 

I tried to tune and fly to 5 different VORTAC's and all but 1 presented problems, in addition, I flew to VXV, and it is not until within 1nm that you get correlation of the two data sets. Beyond 1nm, the split begins to happen.

 

Also, the pointers are not lost overhead a navaid, and, there is slight incorrect behaviour near the cone.

[AirbusCG 52]

Thank you for your research Brian, so you also get to the conclusion that there has to be a bug here, right?

 

I will raise a supportticket with PMDG then hoping that they can fix it with the upcoming updates for the 747 and 777.

I'll try to duplicate the situation depicted in the OP's screen shot in PSX tonight - I'm sure we can both agree that the database, mag var model and overall system fidelity are as accurate as it is possible to be in a simulation.

 

I would really apprechiate if you could give this a go in PSX. I unfortunately do not have PSX, else I could try it as well.

[Copper. 78]

Hello Emanuel

 

Yes, there is a bug here for sure. I don't know if it is absolutely with PMDG. Yep, you should put in a ticket. Good work! You've picked up on something we all dismissed as being an FSX-ism.

 

Tell them to read this thread.

[AirbusCG 52]

Sure, I'll do Brian! Good work from your side, too! Thanks!

[JRBarrett 3,070]

Thank you for your research Brian, so you also get to the conclusion that there has to be a bug here, right?

 

I will raise a supportticket with PMDG then hoping that they can fix it with the upcoming updates for the 747 and 777.

 

 

I would really apprechiate if you could give this a go in PSX. I unfortunately do not have PSX, else I could try it as well.

What was your flight plan and cruise altitude? I can use the same winds you had.

[SteveAull 22]

I'll have to double check but I believe the VOR needle on the RMI actually shows what radial you are on due to the fact that the input to the RMI is still a a VOR signal. VOR signals don't derive bearing info it just sends signals to tell what radial you are on. Also TO/FROM indications which in the RMI case is which end of the needle is pointing up.

 

Maybe it is different in a Boeing aircraft.

 

 

Steve Aull

NAVAIDS Maintenance Tech

[AirbusCG 52]

What was your flight plan and cruise altitude? I can use the same winds you had.

 

Here you go:

https://www.dropbox.com/s/ktgjlm7rh9wc7h5/EDDFMMMX_PDF_1486591652.pdf?dl=0

 

That's the original OFP I used on that flight.

 

You may want to carry a little extra fuel though, I found the PMDG to use not quite the amount of fuel it should use according to simbrief, so I set the performance to -7%!

Not sure if this is a simbrief of a PMDG thing.

 

I'll have to double check but I believe the VOR needle on the RMI actually shows what radial you are on due to the fact that the input to the RMI is still a a VOR signal. VOR signals don't derive bearing info it just sends signals to tell what radial you are on. Also TO/FROM indications which in the RMI case is which end of the needle is pointing up.

 

Maybe it is different in a Boeing aircraft.

 

 

Steve Aull

NAVAIDS Maintenance Tech

 

Correct, the RMI should show your radial.

It should be the same on all aircraft.

 

The confusing thing here is that the RMI needle, the FMC predicted VOR position from the green POS indicator and the actual position of the VOR shown on the flightplan are different.

That makes no sense, at least one of the three position sources is indicating a wrong position.

[downscc 1,327]

I'll have to double check but I believe the VOR needle on the RMI actually shows what radial you are on due to the fact that the input to the RMI is still a a VOR signal. VOR signals don't derive bearing info it just sends signals to tell what radial you are on. Also TO/FROM indications which in the RMI case is which end of the needle is pointing up.

 

Maybe it is different in a Boeing aircraft.

 

 

Steve Aull

NAVAIDS Maintenance Tech

Good point Steve.  The Radio Magnetic Indicator is pointing to the station, the compass card is slaved to the aircraft heading and has no relationship with VOR radials.  If you dial in a VOR course and use the VOR indicator then it is centered when you are on that radial.  And as already pointed out, radials of a VOR do not necessarily correspond to magnetic bearings because the VOR deviation is set and doesn't change annually or even every three years.  For example, Seattle VOR has a declination of 19E dated 2000 whereas the charts show the airport variation is 16.1 E changing 0.1 deg/year W.

[AirbusCG 52]

You have a point Steve, if only the POS indication, which is an FMC computed radial would not point to the same direction as the RMI radial.

I just had a look into the navdata used by PMDG, there is no magnetic variation set for a VOR.

 

So either PMDG has another database of VORs variations somewhere that they use to correct this indication, or there has to be another mistake in the system, or at least in my mental model of what is happening here.

 

I do however believe we might be thinking to complex here. Does anyone even know for sure whether VORs even have any kind of variation correction implemented in FSX/P3D?

Heck, do they even have a variation correction?
In all theory books they always teach you to use the VORs variation when calculating radials... on the other hands side, it's 22:45 and I'm up since 18 hours...

 

Suggestions welcome!

[downscc 1,327]

database of VORs variations

 

FSX/P3D VORs do have a variation setting.  It is independent of magdev.bgl.  It is possible to update the VOR data, but it has to be done VOR by VOR using a tool such as EasyNavs.  There is no reason an aircraft needs this information.

 

The FMC does not compute VOR radials.  Radial information comes from the NAV receiver, this information is explicitly a part of the received RF signal.

[SteveAull 22]

Also if you put the ND in VOR mode and center the needle then the course should correspond to the RMI VOR pointer. If your on the 25 degree radial in VOR mode then the RMI needle should also be on the 25 degree mark. These two should be the same because they are reading the same VOR signals which have the same declination. 

 

I can see where a magenta line drawn that represents a known airway than the track should match the published airway crs as shown on paper charts. The reason I think you don't I think has to do with old declination values in FSX.

 

Steve Aull

[downscc 1,327]

 

 

If your on the 25 degree radial in VOR mode then the RMI needle should also be on the 25 degree mark.

 

Careful, the RMI is only pointing to the VOR and the compass card is slaved to the aircraft not to the VOR.  They will line up if your local variation and the VOR declination are the same, but they seldom are.