can a VOR give a wrong reading?

[Yorks.ppl]

Is it possible that a VOR/OBI can give wrong reading?
I was tuned to Ottringham VOR, morse ID confirmed everything as normal. I was overhead kirkbymoorside (I was confident of position I know the area well and had just overflown my brothers house) but the bearing I had set to be correct at kirkbymoorside showed a large deflection indicating I should fly right.
I centered the needle and noted the bearing, then out of interest I flew as the needle said. It was still not centered as I aproached sutton bank so I turned round and came home. I re checked the morse ident several times and re checked the bearings back on the ground. I asked the instructors at the club, no one could suggest what had happened. Any body got any ideas?

[Flap40]

The VOR transmitter has inbuilt systems to check its accuracy. If it drifts by more tha a degree or two (can't remember the exact figure) it shuts down.

What bearing did you expect to be using at Kirkbymoorside? What was the actual bearing on the OBI that centred the needle?

<<edit: Ground equipment accurate to ± 1°. Airborne equipment meant to be within ± 3° to be certifed as acceptable. Max error, therefore, ±4°. Do this with two receivers (to do a position fix) and you could be in the region of 8° out. If 60 miles from the beacon, that puts you 8nm away from your 'fix' and, if travelling at 120 knots groundspeed, and taking two minutes to do the fixing and plotting, you will have flown 4nm from where you started. VOR fix gives you a rough indication of where you used to be - not where you are.

Not much to do with the original question - I just got carried away after saying ±1°.>>

[Yorks.ppl]

Flap 40,

I dont have the bearings with me but from memory there was around 15 degrees difference at kirkbymoorside to what I was expecting.

[Genghis the Engineer]

If you were reasonably low, that could be terrain?

Or it could be that your receiver is out of calibration of-course, which for that magnitude of error is probably more likely.

G

[Northern Highflyer]

I recall that you fly from Sherburn ?

Some of the Warriors there have RNAV kit fitted. A common mistake when using this is that you have ENR (en route) selected rather than VOR. If so, you will ident the VOR without any problems but it will be transposing the VOR to some other point programmed in by a previous user. Therefore the readings would be different to what you expected.

I am surprised none of the instructors there remembered this, if that is where you flew from.

[Flap40]

Okay

I centered the needle and noted the bearing, then out of interest I flew as the needle said. It was still not centered as I aproached sutton bank

If I understand this correctly, you centred the needle and took a reading, then returned the OBS to your original expected bearing (which said fly right) and proceeded towards Sutton Bank.

Sutton Bank is left of the OTR-Kirkbymoorside radial not right so it is little wonder that the needle did not centre.

[Yorks.ppl]

Sutton Bank is left of the OTR-Kirkbymoorside radial not right so it is little wonder that the needle did not centre.

Not if you are flying south its not, I had turned and was heading south, I entered the radial as a position fix at kirkbymoorside and it said fly right towards sutton bank.

Northern highflyer,
I have never been shown how to use the RNAV so dont normally switch it on but I wonder if this could have been the cause, I tuned the VOR into NAV/comm 1 but perhaps the OBI was still swiched to RNAV is that possible? it would explain everything. I think I will book a couple of hours and see if I can get the hang of the RNAV.

[Flap40]

I agree that Sutton Bank is right of the Kirkbymoorside-OTR radial but it is left of the OTR-Kirkbymoorside radial as I wrote.

BTW. If your compass says Sutton Bank is South of Kirkbymoorside I think that a dodgy VOR is the last of your worries :)

[Brooklands]

yorks.ppl

Another possibility is that you'd got the reverse radial set (ie from instead of to or vise-versa). If this was the case then following a fly right indication would only make things worse.
I have never been shown how to use the RNAV so dont normally switch it on but I wonder if this could have been the cause, I tuned the VOR into NAV/comm 1 but perhaps the OBI was still swiched to RNAV is that possible
The only RNAV setups that I've ever used are the KNS80 and KNS81, which are both Nav only receivers. I've never heard of a OBI display having the RNAV function on it - its normally in the receiver. I'd recommend that you get an instructor to show how to check that the RNAV isn't turned on on the receiver, as the consequences of using it without realising the RNAV has been enabled could be embarrassing to say the least.

Here's what they look like:
KNS80
http://www.seaerospace.com/king/kinggra/kns80.gif
KNS81
http://www.seaerospace.com/king/kinggra/kns81.gif

Brooklands

[IO540]

Technically speaking it is perfectly possible for a VOR receiver to indicate anything whatsoever, while correctly receiving the morse ident, and the flags showing in sensible positions.

That's because there is no separate subsystem in the receiver which would indicate something is wrong.

[Yorks.ppl]

Flap 40, why are you trying to muddy the water, I was tuned to Ottringham VOR, morse ID confirmed everything as normal. I was overhead kirkbymoorside (I was confident of position I know the area well and had just overflown my brothers house) but the bearing I had set to be correct at kirkbymoorside showed a large deflection indicating I should fly right.

Did you read my post, the obi said fly right, which is towards sutton bank.

What makes you think that I said sutton bank is to the south of kirkbymoorside?
I was flying south, then noticed the anomally with the VOR so I tracked west a little to see what happened.

I was hoping for helpfull answers, perhaps if you read some (all of them actually) of the other posts you will see the sort of thing I had in mind.

I believe northern highflyer has the most likely answer anyway.

[B2N2]

Forgot what it's called but you can have engine induced VOR needle fluctuations within a specific RPM range.

[MikeJ]

The answer to the original question is yes, and IO540 gives it straight.
Some years ago, when FM immunity was all the 'buzz', but I had not done anything about it, I was flying SVFR, ORTAC to Guernsey on GUR radial (yes, my GPS was on as well).

All normal and calm, needle central, when it suddenly went hard left, no indications of error. Thought 'those French radio stations', and on return spent £?000 getting FM immunity for my Class 1 equip.

Immediately after, attempted an IMCR renewal test, during which we discovered it was generally indicating accurately N-S, but E-W was 15deg wrong! Test aborted.

Umpteen £££ later, was found to be a "sin-cos potentiometer" fault in the indicator. As IO540 says, you get no indication of error.

Yes, I have had twice had GPS go wrong, but never without it being blindingly obvious!

MikeJ

[IO540]

I've never tried this, and I have never heard of any demonstrated instance of an FM Immunity problem (amazing considering the cost of compliance) but I would expect the morse ID to be audibly affected if a FM station was clobbering the VOR signal.

[Daysleeper]

Is it possible that a VOR/OBI can give wrong reading?


The VOR transmitter has inbuilt systems to check its accuracy. If it drifts by more than a degree or two (can't remember the exact figure) it shuts down.


Not always. Depends how well maintained the VOR is! This sort of failure very nearly caused the loss of a BMed A320 at Addis Ababa a couple of years ago. But then in UK airspace its more likely something aeroplane related.

[High Wing Drifter]

The VOR transmitter has inbuilt systems to check its accuracy. If it drifts by more than a degree or two (can't remember the exact figure) it shuts down.
If I remember correctly, required accuracy is +/-5deg 95% of the time. So what about the other 5%? I guess that is to account for the site error and scalloping referred to earlier in the thread.

[DRJAD]

Yorks.ppl,

I, too, used to fly those Warriors quite frequently, and suspect that Northern Highflyer's theory is the correct one.

Ask at the Sherburn fllightdesk for a copy of the relevant pilots guide manual, one of the instructors readily gave me a copy when I started hiring those a/c. (Or there may be one available from the web.)

The receiver is a KNS80 as far as I recall.

[slim_slag]

Have never seen VOR accuracy expressed using a confidence interval, but I guess there has to be one in there somewhere. Have seen GPS accuracy expressed in this way.

I think the VOR transmitter is +- 1 deg, and the VOR receiver must be +- 3 deg, which works out to around +- 4 deg total acceptable 'lack' of accuracy. If you are 4deg off, by using basic trigonometry you can calculate the distance from the transmitter when you will find yourself outside an 8nm wide airway. 4 / TAN(4 deg). This works out to be about 51nm.

At this point the airway will cease to be 8nm wide and will extend outwards by 4.5 deg from the transmitter. FAR 71.75 describes, no doubt a similar reg is found elsewhere too.