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Difference NDB bearing // Wrong VOR CDI

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Difference NDB bearing // Wrong VOR CDI

Postby OE-LML » Tue Mar 19, 2019 5:00 pm

Hallo!

I've spotted the following 2 things:

1.) If you follow the NDB bearing pointer on ADF gauge/ND you will miss the NDB station. There seems to be a difference of about 2° between NDB bearing on the "Map" and the bearing pointer.

2.) There is also a similar issue with the VOR course pointer/CDI.
If you fly directly towards a VOR station (and there is no crosswind) the VOR CRS should be the same as the currently flown HDG. The course pointer should be pointing to the VOR and no deviation should be visible.
But here if you fly for example HDG 360 towards a VOR and set CRS to 360, the course pointer points to the VOR but the deviation bar is not centered. If you change CRS by 2°, deviation bar is centered but course pointer won't show to the VOR station.
VOR bearing pointer doesn't seem to be affected.

All that was tested with default C172, B777 and IDG Airbuses on default Hawaiian and on downloaded scenery. Winds have been completly disabled.

Looking forward to your replies.
Last edited by OE-LML on Fri Aug 02, 2019 7:48 pm, edited 2 times in total.
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Re: Difference NDB bearing // Wrong VOR CDI

Postby WoodSTokk » Wed Mar 20, 2019 3:47 am

Can you check it also with the Citation II/SP (C550/C551), please.
I have also experienced a deviation sometimes, so I have updated the Citation II one year ago.
Would be nice, if you check that also.

PS: if there is no deviation on the Citation II, than my update is good ;)
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Re: Difference NDB bearing // Wrong VOR CDI

Postby dilbert » Wed Mar 20, 2019 2:59 pm

In pause, with map course line centered on MO, my ADF needle pointed to 315 degrees; while GPS track indicated 317, so it appears to be off about 2 degrees.
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Re: Difference NDB bearing // Wrong VOR CDI

Postby Thorsten » Wed Mar 20, 2019 3:57 pm

True vs. magnetic heading perhaps?
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Re: Difference NDB bearing // Wrong VOR CDI

Postby dilbert » Wed Mar 20, 2019 6:48 pm

Local variation at KBFM, where tested, is -.08 degrees.
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Re: Difference NDB bearing // Wrong VOR CDI

Postby Soitanen » Wed Mar 20, 2019 6:52 pm

I can confirm 2 degrees offset on ADF.

But VOR problem often lies in other space: we use fresh magnetic variation, but VOR station magnetic variation in navigation database can be outdated a lot.
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Re: Difference NDB bearing // Wrong VOR CDI

Postby dilbert » Wed Mar 20, 2019 7:37 pm

The question is what magnetic heading the ADF instrument should read when the nose, represented by instrument top, is pointed at the station. If my GPS track of 217 is True, my ADF card should have read 209 magnetic, based on the local 08 degree West variation. Appears to me the instrument has a 6 degree East deviation. However, most of this is compass deviation, as the ADF needle depends on the ADF antenna, and is within 1 or 2 degrees of rhumb line. when course centered on station.
Last edited by dilbert on Thu Mar 21, 2019 2:32 am, edited 1 time in total.
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Re: Difference NDB bearing // Wrong VOR CDI

Postby WoodSTokk » Thu Mar 21, 2019 12:53 am

I confirm the wrong ADF bearing.
Good test is on CYYD with a local magnetig declination of 18 deg. There is a ADF (M9 / 240 kHz) on field.
The bearing should be 1 or 2 degrees left or right, depending from the rwy where you stay.
The property /instrumentation/adf/indicated-bearing-deg show me more than 343 deg (thats 17 deg to the left).
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Re: Difference NDB bearing // Wrong VOR CDI

Postby OE-LML » Thu Jul 25, 2019 9:51 pm

Hallo!

Really, really, really late reply.

About the 2° ADF offset:
polly was so nice to advise me via PM that in "defaults.xml" located in "FlightGear ?.?.?\data" under
Code: Select all
<adf n="0">
/
Code: Select all
<adf n="1">
there is a line
Code: Select all
<error-deg type="double">2</error-deg>
. Just set that to 0 and the problem is solved.
Thanks again, polly, for that great help.

About the wrong VOR CDI:
Ok, I'm convinced, it's due to old database. I can't imagine that anyone will ever update thousands of VOR stations.
I don't know anything about programming, but for the future wouldn't it be easier to set all headings to TRUE in the navigation database and implement some kind of virtual magnetic variation map that changes the headings to MAGNETIC depending on the location?

Looking forward to your replies.
CU.
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Re: Difference NDB bearing // Wrong VOR CDI

Postby Octal450 » Fri Jul 26, 2019 3:55 am

OE-LML wrote in Thu Jul 25, 2019 9:51 pm:that in "defaults.xml" located in "FlightGear ?.?.?\data" under
Code: Select all
<adf n="0">
/
Code: Select all
<adf n="1">
there is a line
Code: Select all
<error-deg type="double">2</error-deg>
.


That's stupid - why on Earth would that be there - I am killing it aircraft side in next libraries update.

Kind Regards,
Josh
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Re: Difference NDB bearing // Wrong VOR CDI

Postby Johan G » Tue Aug 13, 2019 5:52 pm

OE-LML wrote in Thu Jul 25, 2019 9:51 pm:About the wrong VOR CDI:
Ok, I'm convinced, it's due to old database. I can't imagine that anyone will ever update thousands of VOR stations.
I don't know anything about programming, but for the future wouldn't it be easier to set all headings to TRUE in the navigation database and implement some kind of virtual magnetic variation map that changes the headings to MAGNETIC depending on the location?

The headings in the apt.dat is very likely magnetic because both charts and publications like the AIP AD2 (typically a list of aerodromes with all relevant information, or a reference to where that information is found) as well as most of the navigation using those are all magnetic. For some odd reason winds are typically reported in true headings.

Unfortunately as you have indirectly noticed, there is a discrepancy in that the apt.dat file now is very old, but the magnetic model FlightGear uses is up to date.[1]

[1] World Magnetic Model 2015, WMM2015, according to simgear/magvar/coremag.cxx (perm). See also the wikipedia article World Magnetic Model (perm).
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Re: Difference NDB bearing // Wrong VOR CDI

Postby legoboyvdlp » Tue Aug 13, 2019 7:34 pm

"The ICAO minimum accuracy for NDBs is ±5°"

Wikipedia says this. However a random error surely is better than a default constant two degrees?
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Re: Difference NDB bearing // Wrong VOR CDI

Postby OE-LML » Thu Aug 22, 2019 6:17 pm

Hallo!

Unfortunately as you have indirectly noticed, there is a discrepancy in that the apt.dat file now is very old, but the magnetic model FlightGear uses is up to date.

Yeah, runway headings are quite up to date.
But wouldn't it be possible to use this magnetic model for VORs too, so that the 0° radial will be the actual 0° MAG radial?

Btw: Does ATC (and ATIS) also report winddirection in TRUE e.g. during take off and landing clearances?

However a random error surely is better than a default constant two degrees?

@ Development-staff: No, please, no! No more simulation of errors. :D

CU.
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Re: Difference NDB bearing // Wrong VOR CDI

Postby Isaak » Thu Aug 22, 2019 7:37 pm

OE-LML wrote in Thu Aug 22, 2019 6:17 pm:Hallo!

Btw: Does ATC (and ATIS) also report winddirection in TRUE e.g. during take off and landing clearances?

CU.


As far as I know (but I don't own any RL license) these are reported in magnetic degrees, otherwise it would be impossible for GA traffic with only a magnetic compass on board to quickly determine the crosswind factor. I wouldn't want to look up the airport magnetic variation and recalculate the corrected heading while on short final :-)

Edit: the runway names and headings are also determined by the magnetic north, so I see no reason to report the wind in true hdg.

Edit2: a quick search on the internet doesn't help me, seems the base rule is that written (E.g. METAR) is true and spoken (E.g. ATIS) is magnetic, but many internet users report airports with +10° magnetic variation broadcasting an ATIS identical to the METAR. I'm confused now.
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Re: Difference NDB bearing // Wrong VOR CDI

Postby Johan G » Sat Aug 24, 2019 6:40 pm

Isaak wrote in Thu Aug 22, 2019 7:37 pm:[...] these are reported in magnetic degrees, otherwise it would be impossible for GA traffic with only a magnetic compass on board [...]

Also, a backup magnetic compass would still work even when absolutely all systems are gone in an aircraft. There are reason why multimillion dollar aircraft with double and triple redundant systems still have a magnetic compass for when everything else fails. :wink:
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