GTN650 Jumpy VOR

[BobTurner]

When you were at the testing location, did you also check the To-From? e.g., did the to-from flip when the OBS was rotated 90 deg from the correct bearing, or did it flip at 90 to the ‘off by 30’ bearing?

[cdeggz]

Good question. I did not check but will next time I’m in the air.

[birddog486]

I may have missed it but what antenna are you using? There's a few of us having the same issue using different Navigators. I've done a ton of troubleshooting with no luck. I think were mostly using the Archer antenna though.

I'm using the Dynon HDX with an Avidyne IFD440 hooked to the archer antenna from Aircraft Spruce.

[Ed_Wischmeyer]

Hmm. I'm not familiar with the GTN to ARINC to EFIS in the Dynon systems, but maybe the problem isn't the GTN.

Reserving the right to be wrong... (again)...

Ed

[cdeggz]

Originally Posted by BobTurner

1. Drawing a radial to an easily identified landmark, flying over it, checking your vor against the radial you drew on the map, is perfectly acceptable. Look up ‘airborne vor checkpoints’ in the AIM.
.

Hey Bob,
I think the limitation to this is it must be on a Victor airway, correct?

[cdeggz]

Originally Posted by BobTurner

There actually is some rational thought behind the ban on using GPS to cross-check a VOR. Take a look at some IFR enroute charts, find a straight line airway between 2 fairly close VORs. On some of them the heading out of one and into the other are not reciprocals. I found one out of Battle Mt (NV) where east was 064 and west into the next one was 241, a difference of 177 deg (not the 180 it should be). What's going on? The answer is the earth's magnetic North Pole moves around, and the FAA doesn't bother to re-orient the VORs to actually point at magnetic north very often (same is true of runway numbers - Tracy used to have a runway 25, now it's 26, but it took a while to be re-numbered). It all works because the charts give a "heading" that is the VOR OBS setting that's needed toward or from that VOR, even if it's not really the magnetic heading! It's just bad enough ( a few degrees) that comparing GPS (which is kept very up to date) to a VOR radial might appear out of tolerance when it actually is not.

After doing some digging it sounds like most of the Garmin GPS units account for this when the radial comes from a VOR. I've yet to confirm that in the air.

I was down at HHR and tried to use the VOT but could not even pick it up, maybe it was broken.

I flew back to Salinas and tried the VOR ground checkpoint there again and was only off by 6 degrees, strange!

I then did a few airborne checks while flying victor airways (per the GPS) and was off buy 2 degrees on LIN and 0 degrees off on HNW. I'll use that last one for my VOR check for this month and keep an eye on things. Maybe my aircraft just does not like being on the ground.

-Craig

[BobTurner]

Originally Posted by cdeggz

After doing some digging it sounds like most of the Garmin GPS units account for this when the radial comes from a VOR. I've yet to confirm that in the air.
-Craig

No, they don’t. But since the faa doesn’t let vor’s get too far off proper alignment it’s a bit tricky to confirm. Here’s an example: Find two VOR’s fairly close to each other, with an airway in between. E .g. Airway from vorX at 90 deg, goes to vorY on the 272 deg. radial. You look up the vor data in what used to be called the facilities directory (I forget the new name) and find vorX has zero deviation, vorY is mis-aligned by 2 deg. You fly over X, put ‘direct Y’ in the gps and follow it.As you near Y, you change to ‘OBS’ mode, and put 092 into the OBS setting. The needle will not center. It will center if you put 090 on the obs, because that is the actual ground track with an up to date magnetic north database.
Yes, you are correct about ground check points. They’re supposed to be on an airway because the faa does airborne surveys of airways to make sure the nav data is accurate. In the real world I doubt it makes much difference, as long as your location is getting a good signal.

[cdeggz]

Here’s the text from the 480 user manual:

“When using OBS mode with a VOR station, the GNS 480 will use the station declination of the VOR. For other waypoints, the GNS 480 will use magnetic variation based on your present position.”

I’m told the 430 does the same and I would expect the 650 to as well

[Avanza]

I would follow Walts recommendation.
To eliminate some kind of RF signal feed problem I would suggest having avionics shop plug a tester directly into the antenna port, basically a ‘bench’ check in the aircraft. First thing I would do if you were here.

Radio reception is coming from the ANTENNA. If the quality of the signal is
distorted by the installation, bad signal comes to the NAV-receiver.
An Archer dipole antenna works best in composite aircraft, not in metal
aircraft. The link is for a non sexy antenna, but it works.

https://www.aircraftspruce.com/catal...clickkey=12915

Good luck

[Carl Froehlich]

Originally Posted by Avanza

SNIP…
Radio reception is coming from the ANTENNA. If the quality of the signal is
distorted by the installation, bad signal comes to the NAV-receiver.
An Archer dipole antenna works best in composite aircraft, not in metal
aircraft.

A couple of thoughts:
- If the aircraft has something going on to “distort” reception, then the antenna makes little difference. The differences between VOR antenna selection boils down to reception gain pattern for the install, antenna to antenna feed line matching and signal attenuation in the feed line. Bottom line, when listening to a VOR signal is it full quieting or not?
- I will offer that something on the plane (e.g. propeller) could create some error with the VOR receiver measuring the phase difference between the VOR receiver reference signal and variable signal (think multi path reception). Here comparison in VOR display while changing headings would be good data.
- No, Archer style wingtip antennas will not work on a composite aircraft without a “counterpoise” to replicate the now missing metal wing. This can be done with tuned pieces of wire (radials), adding aluminum to the composite skin or such. But, there are superior VOR antenna options for non-carbon composite aircraft.

Carl