GTN650 Jumpy VOR

[cdeggz]

Hello Brains, I have a GTN650 tied to my AFS Panel and see some strange behavior.

-When tuned to a VOR, I can receive a signal, GTN650 can decode the identifier, but the radial is ~30degrees off and the CDI needle wanders back and forth, almost a full deflection. This happens in the air and at VOR ground checkpoints. Video: https://www.icloud.com/sharedalbum/#B0SGqQrCLJI489P
-Tracking an ILS works without issue
-Tracking a GPS track is no issue.


Curious where to start troubleshooting. I don't have a spare 650 laying around to troubleshoot with. Since the other data feeds to the AFS panel are clean, I suspect the issue lies in the GTN itself or some interference perhaps.

-Craig N299PW

[BobTurner]

I think you can receive SJC from the ground at Reid-Hillview? Do you still get the wig-wag with the engine off? I suspect one hour of time at a decent avionics shop will tell you if the 650 VOR is out of wack. Then you have to decide on paying Garmin's fixed price repair policy, or just never using the VOR.
The rules for the instrument test have changed, so now it is possible to take the flight test without a VOR.

[cdeggz]

Thanks Bob, yes I think I can get SJC and Woodside from KRHV. I went down to KSNS for the cheap fuel today :-)

My next step is to open circuits one at a time to see if it affects the reading, and as you mentioned try with the engine off too.

Good to hear about the VOR not being needed for the instrument test! I hate having anything INOP on the plane though.

Do you have a recommended avionics shop?

[cdeggz]

While we're on the subject: What's up with 30-day VOR check method not allowing cross-reference with GPS? Having a single-VOR plane with no VOT or VOR checkpoint at my home field makes for a real pain to comply. Is this just FAA behind the times?

-Craig

[BobTurner]

Originally Posted by cdeggz

While we're on the subject: What's up with 30-day VOR check method not allowing cross-reference with GPS? Having a single-VOR plane with no VOT or VOR checkpoint at my home field makes for a real pain to comply. Is this just FAA behind the times?

-Craig

I honestly don't have a recommendation on an avionics shop. I used the shop at Visalia and it was okay but that was so long ago I doubt it's still the same guy!
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.
As to your predicament: you're allowed to make your own checkpoints when nothing else is available. Draw a line on a sectional to something prominent, like "brushy peak", note the VOR bearing to it, fly over it, check. You're allowed to be off by 6 deg, so it's generous. And no one says you cannot use GPS to help you find Brushy Peak! Or....I have an SL30, which uses an all-digital means to decode OBS settings. Its manual actually says, "Well, the FARs do require you to check the OBS every 30 days for IFR, but believe us, it will either work or totally fail, it will never ever be off." (!) Anyway I'd draw a line on a sectional from SJC to the runup area at RHV, call it your checkpoint, check the VOR once a month while in the runup area.

[Ruready]

Google Vor BENDS

[cdeggz]

Ahh that makes a ton of sense. I will lower my frustration level.

Regarding drawing a radial, my understanding is that is not an acceptable method per the FAA regs, correct? Seems very reasonable though!

That said, my understanding is as long as you don't plan to use VORs for IFR navigation, the 30-day VOR check is not even required. If you unexpectedly lose GPS coverage, then reverting to an un-checked VOR would be legal.

[BobTurner]

Originally Posted by cdeggz

Ahh that makes a ton of sense. I will lower my frustration level.

Regarding drawing a radial, my understanding is that is not an acceptable method per the FAA regs, correct? Seems very reasonable though!

That said, my understanding is as long as you don't plan to use VORs for IFR navigation, the 30-day VOR check is not even required. If you unexpectedly lose GPS coverage, then reverting to an un-checked VOR would be legal.

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.
2. True, the 30 day vor check is only required for ifr operations where the vor is used. And in an emergency you may break any rule needed to meet the emergency. But don’t be surprised if the faa comes after you for ‘careless and reckless’ operation, for poor preflight planning, if you get into trouble because the vor accuracy is way off.

[bjdecker]

You didn't mention this, but I thought I should ask.

During the power on of the 650 (and Xi), you will get a prompt for the Database page, followed by the Instrument Self Test Page. On this page, you will see that the 650 is driving its electrical outputs to the following:

CDI - Half Left deviation, TO indication, no Flags
GS - Half Up deviation, no Flags
Annunciators - ON
Selected Course/OBS - 150 (or 149.5)
DTK - 150 (149.5)

In this condition, what is displayed on the AFS5600?

[Avanza]

If VOR signal passes through prop, arc modulation can be changed.
CDI needle may fluctuate as much as plus or minus 6°.
Try to fly abeam a VOR station at 10 nm distance.


Good luck