VOR/DME - Updated for Magnetic Variation?

[CSCOT]

Low time newbie student here:

Studying radio navaids and VOR.

So far all sources are saying that VOR Radials are based on magnetic bearings, not true bearings.

I was wondering therefore how these are updated for the annual rate of change to magnetic variation?

[Gertrude the Wombat]

I was wondering therefore how these are updated for the annual rate of change to magnetic variation?

They aren't. That's why each one has its own compass rose drawn on the map, with its own private "north".

[FlightDetent]

Yes they need to be updated. The little arrows point to magnetic north from the station's location thus indicating the variation. As MN moves, charts need to be reprinted.

Heading compass / gyro is magnetic, the complete navigation reference system is magnetic, VORs need to be magnetic too in order to fit to the system. It would be overly complicated to apply correction for NavAid variation in practical use.

Same reason why the ATC reported wind is also magnetic - provided for practical an immediate use. Unlike METARs and TAFs, which for practical reasons need to be geographic.

cheers, FD.

[Talkdownman]

I expect a man with a screwdriver pops round every couple of years and gives them a little tweak...

[CSCOT]

MagVar

FlightDetent - thanks ok, charts reprinted with corrections each year. In marine charts we're only given a variation from a specific year and a predicted average annual rate of change to apply corrections manually on a case-by-case basis.

Thanks

[BEagle]

VOR transmitters are aligned using 'station declination' to a magnetic north value which existed on a certain date. But they are not updated with change of local magnetic variation until the difference between variation and declination becomes significant - at which time the 'man with the screwdriver' adjusts them and the new orientation is publicised.

Given the overall system accuracy of a typical GA avionic fit and the relatively short distance from the station used for navigation, it's highly unlikely that the difference is significant.

Try flying in a straight line between 2 VORs and watching the GPS CDI if you have one....

[FlightDetent]

2OP:

I guess the full story is that for different reasons aeronautical charts get reprinted somewhat frequently so the magnetic variation and it's depiction is being continiously updated with sufficient accuracy.

Using an oldish (2 years) chart and not applying the "annual rate of change" for MV is in practical terms acceptable, as in short distance (<30 NM ?) from the Navaid the angular error represent only a small physical displacement. On a written test, different story.

cheers, FD. (crossed post with BEagle)

[Jan Olieslagers]

a man with a screwdriver pops round every couple of years and gives them a little tweak

Probably. But the person* AND the screwdriver AND the tweak must be backed up by a ream of paper full of rubber stamping and signatures on dotted lines.

*of any sex!

[wanabee777]

The screwdriver can't be magnetic.

[Discorde]

With almost universal use of GPS and INS I would think that before too long aeronautical navigation will switch to true (geographic) north for reference. No need for VORs to be 'tweaked' and runway designators to be periodically repainted. Ditto QFE, QNH and Standard Pressure Datum will disappear - all altimetry will be referenced to GPS sea level (with baro referenced back-up).

[Talkdownman]

gives them a little tweak

Originally Posted by Jan Olieslagers

any sex!

Thread drift...

[Cough]

Discorde, I very much doubt that. (GPS defining ALT that is..)

Firstly, it takes a bizarrely small box to interfere with GPS signals...

Secondly, Airliners (wrong forum I know!) fly efficiently at a certain pressure altitude. Changing levels just because the temp changes isn't going to be great.

Thirdly, for such a small change, the cost to worldwide operators is massive.

[Discorde]

Firstly, it takes a bizarrely small box to interfere with GPS signals

I envisage a transition period where GPS and baro run concurrently until the bugs have been ironed out. Local QNH would be set on the baro altimeters, periodically updated. There would be no transition altitude. If GPS is accurate enough for instrument approaches it's accurate enough for vertical separation, assuming equal accuracy in all three dimensions.

Secondly, Airliners (wrong forum I know!) fly efficiently at a certain pressure altitude. Changing levels just because the temp changes isn't going to be great.

Not sure this is relevant to vertical traffic separation, which is the rationale behind the Standard Pressure Datum setting.

In the UK we frequently have problems when low pressure weather systems cross the area. Crews mis-setting or neglecting to change altimeter settings is a headache for ATC, the problem compounded by the associated turbulence distracting crews at critical times.

[Talkdownman]

Originally Posted by Discorde

Not sure this is relevant to vertical traffic separation, which is the rationale behind the QNE setting

With respect, do you mean the Standard Pressure Datum?
The QNE is not a 'setting'. The QNE is the reading in feet on an altimeter with the sub-scale set to 1013.2 hPa when the aircraft is at aerodrome or touchdown elevation.

[Discorde]

Thanks TDM. I've edited my previous posts accordingly.