VOR error question

chucky

chucky

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Chucky
Is there anything that could cause to independent VORs on the same plane (I assume same antenna) to have the same error - in the specific case I'm thinking about 10 degrees? That is to say, they both read about 10 degrees off from what the GPS reads out. This came up recently, and I'm rather puzzled.
 
Well 10° is a bit large but a big part of it could be the difference between the VOR's declination and current magnetic variation.

Which VOR are you talking about?

Joe
 
I thought about that (and argued about that with my CFII) but that can't be it. It was pretty much every VOR up the east coast. We'd be on the airway according to the GPS, and 10 degrees off (or so) the radial that the IFR charts told us we should be. We'd get a similar disagreement on direct-to-VOR navigation.
 
chucky said:
Is there anything that could cause to independent VORs on the same plane (I assume same antenna) to have the same error - in the specific case I'm thinking about 10 degrees? That is to say, they both read about 10 degrees off from what the GPS reads out. This came up recently, and I'm rather puzzled.
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What VOR? Did you happen to be where local magnetic variation is ten degrees?
 
Well, pretty much every VOR on V1 south of New York. So, yes, the local variation was 10 degrees or more, but the radials listed on the IFR charts should be the radials you dial in, regardless of how the magnetic variation has changed since the VOR was commissioned, right? I'm gonna go haul out my Instrument Flying Handbook now to check....
 
chucky said:
Well, pretty much every VOR on V1 south of New York. So, yes, the local variation was 10 degrees or more, but the radials listed on the IFR charts should be the radials you dial in, regardless of how the magnetic variation has changed since the VOR was commissioned, right?
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Right. Was your GPS providing true or magnetic?
 
Both VOR/CDI's on every VOR? Something ain't right. You need to be checking that you didn't kick the GPS into "true" mode (variation along V1 is about 10 degrees, more or less), or that someone didn't misadjust both VOR's. Also, check the mag var mode of the GPS to make sure you didn't bump it out of "auto" and have zero mag var loaded manually.
 
Well, it isn't a plane I'll have access to for a little while, but that aside, whether the GPS was showing true or magnetic, it'll show me on the airway or off if I have the route entered correctly, and if I'm on the airway, the VOR needle should center when I dial in the radial listed, right?

EDIT - Ron, yes, it was both VORs, and they were off by a consistent amount on each VOR.
 
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chucky said:
Well, it isn't a plane I'll have access to for a little while, but that aside, whether the GPS was showing true or magnetic, it'll show me on the airway or off if I have the route entered correctly, and if I'm on the airway, the VOR needle should center when I dial in the radial listed, right?
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Right. I thought you were saying the bearing readings in the GPS were 10 off the bearing readings on the VOR's, not that the VOR's were showing you 10 off while on the centerline of the airway per the GPS flight plan.

EDIT - Ron, yes, it was both VORs, and they were off by a consistent amount on each VOR.
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Sounding more and more like CDI adjustment. How do they check out on a VOT or a VOR ground check point?
 
That check hasn't been done yet, and since both I and the owner of the plane will be gone for a while, it may not get done right away. We had done the dual VOR check, and they checked out. Obviously they need to be checked with a VOT or ground check point - I was just wondering if there was one problem that could affect two receivers.

The other thing was that I didn't notice the problem on the first part of the trip - I thought everything was working fine. Since I was using GPS as the primary source, it's entirely possible that I just centered the needles, and saw what I wanted to see instead of noticing that the CDI was on the wrong tick mark. I noticed it when I decided to fly one transition with the VORs instead of GPS, just for practice. Then I started checking them on every subsequent VOR. Yet another lesson to check and double check everything - assume something is wrong rather than assume that everything is right.

Also, SBY was our fuel stop.
 
chucky said:
I was just wondering if there was one problem that could affect two receivers.
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Not that I can think of. The only part they have in common is the antenna, and I can't see how that could cause a deviation error.

Also, SBY was our fuel stop.
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You shoulda let me know -- I know where the good lunch places are, as there's no restaurant on the field.
 
I had forgotten there was a PoA regular based at Salisbury, or I would have asked. We ate at a diner called something like Dayton's, maybe? Perfectly acceptable.
 
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