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GTN650 Jumpy VOR

cdeggz

Well Known Member
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
 
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.
 
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?
 
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
 
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.
 
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.
 
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.
 
Self Test Ok?

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?
 
VOR deflection

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
 
If VOR signal passes through prop, arc modulation can be changed.
CDI needle may fluctuate as much as plus or minus 6°.

Good luck

Yes, this happens when the prop ‘cuts’ the signal at something close to its 30 Hz modulation frequency, sort of a strobe effect. Changing rpm usually makes it go away.
 
Yes, this happens when the prop ‘cuts’ the signal at something close to its 30 Hz modulation frequency, sort of a strobe effect. Changing rpm usually makes it go away.

Interesting theory - but.....

We have been chasing this problem with the 650 in my old RV-10. I was an early adopter of the 650 (2011 timeframe) and paid the price chasing what Garmin told me were "known software problems). But after that pain, it worked as advertised - with no VOR drift as best I can remember (I did my IFR training and check ride in this plane).

The check of the current problem included parking the plane at a VOR test site. Everything turned off (both masters off) with power applied only to the 650 and an EFIS - engine shut down. Same problem. At this point the new owner took the 650 to Garmin when at Oshkosh to check it out. They found no problems. He did get talk into trading it in (along with a pile of cash) to get the new 650 version. Same problem.

So - what do I conclude? Same plane, same install using two different 650s, same problem. The only change that is common is the updated 650 software from the original install. As other RFI type causes have been explored and ruled out, this is the only remaining unknown.

Carl
 
The only change that is common is the updated 650 software from the original install. As other RFI type causes have been explored and ruled out, this is the only remaining unknown.

What EFIS? What interfaces for NAV(CDI/HSI) are employed - RS-232, ARINC, or Analog?

Painful exercise, but you could wire in another display head to the 650, in place of the EFIS, and see what it does (Dynon D10A, Garmin G5, GI-275).

Asked previously, what does the 650 power up self test show on the EFIS -- a stable CDI/To/From or does it wobble?
 
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GTN650 component?

Isn't this an almost identical problem as thread https://vansairforce.net/community/showthread.php?t=213175 ?

OP there seems to be quite astute - tried all the common things, down to snake oil guesses like floating an antenna feed-through fitting.

Beat freq interference and multi-path VOR needle waving is not usually described as "jumpy".

Was in the biz forever, looks like a box problem to me. And maybe a future Garmin service letter.

Ron
 
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.
 
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?

Yes the instrument self test page all checks out. Ironically that test is sending out magenta (GPS) info which I'm not having trouble with. I'm told by AFS that the GPS and VOR/LOC come over separate data streams so that check does not really help check the VOR output!
 
Unfortunately I agree. But try inexpensive fixes first!

Today I found I can't receive a VOR on the ground at my home airport (KRHV) so I flew down to the VOR checkpoint at KSNS for testing.

I was surprised today to see there was no wagging of the CDI, however the VOR error was around 30 degrees.

I powered on everything I could find in the plane and there was no change.

I then 1 by 1 powered everything off, lastly cutting the engine, and the 30-degree shift was still present.

The OBS reading on the EFIS matches the OBS on the GTN, but I ran through that recalibration step anyhow. Still no help.

Looks like I need to find someone to radio swap with me or get this thing on a test bench. Garmin wants $1350 to fix it of course. Probably not worth it to me. Which brings me to another question:

Are the INOP labeling and deactivation requirements applicable to experimentals? If it were applicable, how would one "disable" the VOR radio only? Looks like in the settings I can disable VOR/LOC but I'd like to keep my ILS functionality.
 
What EFIS? What interfaces for NAV(CDI/HSI) are employed - RS-232, ARINC, or Analog?

Painful exercise, but you could wire in another display head to the 650, in place of the EFIS, and see what it does (Dynon D10A, Garmin G5, GI-275).

Asked previously, what does the 650 power up self test show on the EFIS -- a stable CDI/To/From or does it wobble?

BJdecker this is an interesting idea. Data is ARINC. I do have a G5 in the plane so I could try seeing what it will display. I don't have high hopes though. Magenta self-check is stable and accurate.
 
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?
 
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.
 
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
 
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?
 
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
 
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.
 
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
 
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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/catalog/avpages/av532.php?clickkey=12915

Good luck
 
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
 
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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

Thanks for this reference, I learned something. No wonder the database updates are so expensive - they must include mag declination data for all the vor’s, too. Maybe I should have guessed this - I’m pretty sure that de-commissioned VORs disappear from the gps database.

Have you actually pulled the wingtip and looked at the antenna? I’ve seen everything wrong that you can imagine: ground leg a foot away from the end rib, radiating leg against the rib with the ground leg further out(!). I’ve even seen one builder who thought he could tie two Archers (one from each wingtip) together (bad idea).
 
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