VOR's on half mil chart

[TotalBeginner]

Are the VOR compass roses on the half mil chart aligned with magnetic north, or are they aligned with the 360 radial of that particular VOR.

I am under the impression that magnetic north changes over time, so surely there must be an error between "true magnetic north" and the magnetic north that the VOR was aligned with when it was installed?

Hope this makes sense? :ugh:

[slim_slag]

This might be of interest

http://www.naco.faa.gov/index.asp?xml=naco/faq#q2f

[Halfbaked_Boy]

They are aligned with magnetic North - partly because the only natural reference pilots have to direction is the compass, and so it would make sense to at least keep correction to a minimum.
Regarding your second question, I couldn't comment but I 'assume' that they are either altered mechanically or they automatically align themselves with M' North through the use of a compass... anyone?

[Blinkz]

VOR radials themselves are magnetic tracks and so the roses are also magnetic tracks. The VORs get calibrated for the correct variation (the difference between mag north and true north) altho the actual poles move pretty slowly so the errors are small between map issues etc.

[TotalBeginner]

Wow, +/- 6 degrees, that's quite a difference.

[DB6]

'Are the VOR compass roses on the half mil chart aligned with magnetic north, or are they aligned with the 360 radial of that particular VOR'
Totalbeginner, both mate. Have a look at your chart and you will see that there is a difference between the VOR alignment (magnetic North) and the longitude lines on the chart (true North). That's the variation. Aviation maps are reissued every year or so, so the VOR alignment and lines of variation (faint dotted ones) are updated regularly, as Blinkz says. The VORs themselves are calibrated/realigned regularly too, which is one reason they will go out of service and you need to check the idents don't say TST (test). They are not allowed to get to 6 degrees out before they are realigned though, not in the UK anyway. That would mean that an airway based on a VOR would move 6 nm laterally at 60 nm from the beacon, which would create a LOT of chaos. That probably relates to +/- 5 degree accuracy figures (class B? my nav exams were a while ago now).

[TotalBeginner]

Sorry guys, I don't think I worded my first question very well. What I really wanted to know is, when they plot the rose on the chart, do they distinguish between magnetic north, and the magnetic north that the VOR is aligned with?

For example, if we take the Sumburgh VOR/DME. The current magnetic variation there is 4.6W However the VOR is aligned with the magnetic variation of 2000, which was 5.45w. So do they plot the rose at 4.6W or 5.45w?

I know that it is negligible, as we are talking less than a degree, but it's just out of interest.

[Charley]

Careful here. What has been said here is largely correct but could be considered misleading.

Strictly speaking, the difference between the VOR alignment and true north is not variation, but is actually called declination. This is because the VOR's alignment with respect to magnetic north is only correct when it is installed and then again when it is recalibrated at intervals thereafter. Consequently, the alignment of any given VOR may not exactly match magnetic north. Variation is, of course, the difference between mag north and true north - so be wary when using the angle between VOR north and true north to describe it.

Furthermore, be wary of what constitutes a 'regular interval'. VORs are updated regularly but it isn't necessarily something that occurs every few weeks. In fact, it can be years before VORs are updated. Thankfully, in the UK at least, the annual change of variation is so small that this isn't so much a problem.

To put it into practical terms, here's a quote for the Aberdeen VOR from the UK AIP ENR chapter 4 (link here (http://www.ais.org.uk/aes/pubs/aip/pdf/enr/20401.PDF), AIS login required). The ENR section contains similar info on all the UK's en-route VORs.

Aberdeen ; VOR/DME ; Var 4.4°W - 2005 ; VOR 4.60°W - 2001

This suggests that the variation at the Aberdeen VOR in 2005 was 4.4°W. However, the VOR was last aligned with mag north in 2001 and at that time the variation was 4.6°. Therefore there is now a discrepancy of 0.2° so, in theory, if you flew along the 360° radial you wouldn't be flying a track of 360°M, you'd be diverging to one side.

Most people in Europe aren't affected by this to any great extent, but there are places in the world where the differences between VOR declination and variation can become significant. I've seen this first hand in the United States, where some airways between two VOR's can have the radial from one VOR be a degree or two different from the reciprocal of the radial at the other VOR.

Similarly, this is why those who use GPS to aid their IFR flying can track an outbound radial only to note that the GPS indicates a magnetic track of a degree or two difference.

I've rambled on enough for now... :zzz:

[Edited to add: TB; my understanding is that the rose remains aligned with VOR north, so that one may plot the correct radial required. This is certainly true of the VOR rose on NOAA charts in the States and I believe it is the case here in the UK as well. Hopefully someone else can confirm this...)

[Whopity]

I recall certain VORs in very Northern Latitudes are aligned to Grid North using a Polar Stereographic as the North datum! Makes for interesting navigation!