VOR Bearing Innacuracy?

[kaosfere 3,115]

I've made a few early flights in the DC6 and the prop-head in me is loving it.  However, I've stumbled upon what seems to be a navigation oddity.   I like to fly vintage planes entirely via radio nav, so I've disabled the GPS.    I've noticed that when tracking a VOR the bearing indicator seems to be a few degrees off course, and the direction depends upon whether i'm inbound or outbound.

Say I have the OBS set for 280.  It seems as though if I'm flying on an outbound I actually end up right of course, on a radial closer to 283.   If I'm flying inbound I will be left of course, on a radial closer to 277.   Has anyone else seen this sort of behavior?

[BlueStar 323]

Sounds like the magvar needs to be updated.  

blaustern

[kaosfere 3,115]

Good call, I loaded another plane and saw the same issue.

 

Frustratingly, I already have magvar updates in place, no idea why it's doing this.  At any rate, I'll troubleshoot it as a general problem.   Thanks.

[serviceceiling 135]

Are you compensating for wind?

[downscc 1,327]

1 hour ago, kaosfere said:

Good call, I loaded another plane and saw the same issue.

 

Frustratingly, I already have magvar updates in place, no idea why it's doing this.  At any rate, I'll troubleshoot it as a general problem.   Thanks.

Has nothing to do with magvar....even if updated annually.  The basic VOR transmitter has a "north" calibrated into it, and is independent of any external magnetic field (or local magnetic variation).  This calibration is not done annually, because when it is done it requires a flight check and that is expensive.  There's not enough FAA flight check birds to visit every VOR every year.  If you look up any VOR with the web site Airnav, you will find the current FAA calibrated "magnetic deviation" for that station.  Compare that with a nearby airport and notice the difference to the airports magnetic variation.  I think VORs are flight checked every 3 to 4 years, not sure because it's been years since I knew any flight check people.

Also, the VOR calibration might be as old as the scenery.  Updating global magvar does not update VORs.  I use the Hers Sors utilities to update navaids on a case by case basis.

Finally, the King radio equipment is TSO'd for IFR but the tolerance when ground checking your OBS is +/- 4 degrees.  Sim pilots are used to exactness that does not exist real world.

[skelsey 765]

As Dan says, this is not to do with the sim data -- it is a real life 'problem'. Although VORs are flight tested on a reasonably regular basis, many will have not had their internal magnetic variation data updated for 30+ years; the 090 radial from the XYZ VOR will always be the same track over the ground, but it may not necessarily equate to a magnetic track of 090 depending on the local shift in variation since the VOR was last/originally calibrated.

It's much cheaper to change the numbers on the charts than it is to re-calibrate the physical VOR ;).

[Samany69 41]

as people already said , i will also that there will fluctuations and deviations as you move closer or further from an NDB and/or VOR and DME station. there is nothing wrong with the planes. ( i'm not sure about this one , but i Think The Fluctuation begins on : 45 Degrees For NDB , 60 Degrees for VOR and For DME there will be a small Error in Distance Calculation around 85-90 Degrees ) ( all degrees are relative to the Horizon => 0 parallel to Horizon Surface , 90 Degrees=> Totally Normal To Station )
 

ADF/NDB Errors

Ionosphere Error: Specifically during periods of sunset and sunrise, the ionosphere reflects NDB signals back to Earth, causing fluctuations in the ADF needle.

Electrical Interference: In areas of high electrical activity, such as a thunderstorm, the ADF needle will deflect toward the source of electrical activity, causing erroneous readings.

Terrain Errors: Mountains or steep cliffs can cause bending or reflecting of signals. The pilot should disregard erroneous readings in these areas.

Bank Error: When an aircraft is in a turn, the loop antenna position is compromised, causing the ADF instrument to be off balance.


VOR Errors

Errors are likely to exist in any VOR system due to a number of causes. These include ground station error, site effect error, error due to vertical polarization effects and airborne equipment error. The algebraic sum of all these errors is known as the aggregate error.  the pilot is concerned primarily with the aggregate error.

Ground Station Error
This is a systematic error associated with the transmitter, aerial and earth systems and power supply that is with the actual ground equipment. Error on a particular bearing is very small, and is plotted accurately on commissioning. It is similar to quadrantal error in an airborne ADF system. Ground station error, in practice, is usually less than ± 2°.

Site Effect Error
There are, superimposed on the ground station error, site effects which are due to topographical features near the ground station. These site effects modify the ground station errors. The combined effect of these errors are determined at various altitudes at the time of commissioning and the overall error must be less than ± 3°.
Terrain effects caused by the VOR radials being distorted by signals reflected from rough terrain may be experienced under certain circumstances. These effects are evidenced in the form of slow or quick oscillations of the deviation indicator. The VOR track is said to band or scallop depending on the rate of oscillation. Bends on VOR radials normally do not exceed 2° from the average alignment of the track and the scalloping amplitude must be less than ± 2°.

Vertical Polarization
If vertical polarisation effects are presented they are detected in manoeuvres which tilt the aircraft aerial, that is in a turn by an aircraft. When in a banked attitude, the receiving antenna on the aircraft, instead of sampling only the horizontally polarised VOR signals, may pick up large sloping obstructions. Under these circumstances, the deviation indicator is seen to move abnormally in either direction, thus giving rise to incorrect guidance information.

Airborne Equipment Error
As the name implies, airborne equipment error is the error attributable to the various components of the VOR equipment in the aircraft. In well designed and built equipment it is generally less than ± 2°.

an E&E Engineer may Correct me if i'm wrong 

[downscc 1,327]

14 minutes ago, Samany69 said:

Ionosphere Error: Specifically during periods of sunset and sunrise, the ionosphere reflects NDB signals back to Earth, causing fluctuations in the ADF needle.

I had to ponder this for a moment (I am both an EE and a retired USAF Ground Comm/Elect Engineer) then I realized this point is in regards to long distance ADF signals.  We don't have many of these stations anymore in the US, a few along the coasts for oceanic and up towards Alaska.  Most of the time the ADF frequency is well below the LUF (old HF term) so these effects are only during the periods when you can get a D layer.

Something to point out to pilots is that the VOR radials are only flight checked at a distance from station when they comprise a published airway, so if you are tracking a radial that is not a published airway then the abnormal effects such as scalloping can be significant.

Edited February 24, 2018 by downscc

[Samany69 41]

8 hours ago, downscc said:

I am both an EE and a retired USAF Ground Comm/Elect Engineer

Sir . My pleasure to see you here. would you allow me to send you a private message. i'm really interested to talk to you .

Sincerely.

[randomTOTEN 13]

Rob,

In both cases the aircraft is deviating north of your intended course, per your description. Is this a simple case of a wind from the south, and you not correcting for the drift?

You say that you're using VOR only, no GPS. If the needle is centered with OBS set to 280, how exactly are you determining that the bearing indicator is off? Are you talking the RMI bearing when you are centered on the CDI? If so I think I remember seeing that, if I remember the aircraft heading on the RMI was off a little bit.

Robert Toten