Magnetic vs True
 

A small question for you...

When a controller asks you to turn left heading 250 (say) will the aircraft fly magnetic or true heading ?

Thanx in advance for your replies.

Headings are always magnetic.

The reason is that headings are flown for relatively short preiods of time only, the flight path being interfeered with more by the wind than the change of variation from one end of London FIR to the other, and finally it has to be computed by the computers that have only recently (the last 15 years) been installed.

"...and finally it has to be computed by the computers that have only recently (the last 15 years) been installed."

I think you could double that value.. and still be relatively safe. How old is the INS-equipped 747? (I've been working on them for over 25 years). ;-)

Rgds.
Q.

The reason (partly) that I asked is that in the UK Air Pilot, the wind direction for landing and taking off is given in mag degrees, whereas the wind direction en-route is given in degrees true. Reasons?

Also, when downloading via systems such as ACARS, is the heading info given in mag or true?

Rgrds,

RS

will the aircraft fly magnetic or true heading ?

What the a/c will do will depend whether your heading ref is set to Mag or True (as available on some modern a/c eg 777).
However ATC in non polar latitudes always expect a MAG heading to be flown.

Headings in Mag since all types have at least a wet compass (which reads in Mag). Many only have a wet compass for directional information.

Runways in Mag since that matches the a/c compass information.

Winds in True since the variation changes from place to place, the wind is forecast for relatively large areas. How is the forecaster supposed to know exactly what variation is appropriate for where you intend flying?

...but winds for landing and take-off are in magnetic since you use them in relation to the runway direction...

As invalid says, normally all headings, wind info etc. are magnetic UNLESS you are flying in polar regions where magnetic info is not a lot of use, so modern jets have a switch that transfers the FMC's to true headings.


[This message has been edited by HugMonster (edited 19 October 2000).]

Wrong, not all winds are in mag. As Tinstaafl says enroute winds/wind charts give winds in true as they span vast areas with changing variation.
Only runway winds are in mag as they are used in relation to the runway and are only local, with negligable change in variation.

So, to recap so far...!

The heading flown is always MAG unless flying polar routes. Does it change from Mag to True at a pre-defined point then... e.g above 60 North all headings are flown (and given) as True?

Winds are only mag near the runway (and then only round non-polar airports, I guess). Otherwise they are always given as true. I must confess, I thought most winds were measured from airports (ie. that's where the reporting stations for VOLMET etc were) - does this mean that surface wind is given in Mag degrees, but wind at altitude is given in degrees true?

If you're flying en-route (to repeat an earlier question), what heading is it that appears in the ops room via your ACARS system? Mag, true, or whatever is currently selected via your FMS?

Sorry to keep on like this, but I confess I'm still a touch confused! It sounds as though whilst en-route two aircraft could be given the same heading, and manage to fly true tracks that were several degrees apart.

Oh yes! I wasn't even thinking of Rwy / terminal winds. Just en-route & grid sector.

How dopey of me to leave them out! http://www.pprune.org/ubb/NonCGI/redface.gif

The heading flown is always MAG unless flying polar routes. Does it change from Mag to True at a pre-defined point then... e.g above 60 North all headings are flown (and given) as True?

Absolutely correct. And yes, it changes at a specific latitude, and if my memory serves me correctly it is at 63 north but I'm not sure and could be wrong.

Winds are only mag near the runway (and then only round non-polar airports, I guess). Otherwise they are always given as true. I must confess, I thought most winds were measured from airports (ie. that's where the reporting stations for VOLMET etc were) - does this mean that surface wind is given in Mag degrees, but wind at altitude is given in degrees true?

Basically an airport wouldn't tell any aircraft the wind at altitude. They publish it to metoffices who compile the wind charts. And yes, they publish it to them in TRUE.
However, when landing or on a wind check, they will give the pilots the mag wind, since the pilot is thinking in mag and wants to know cross/headwind factors on the active runway (which is in mag as well). They measure this wind by a simple windmeter outside, which is aligned with magnorth.
I've never actually looked into this, but I think even the polar regions would still use mag headings for the runways and thus the surface winds, since even there on the approach the variation shouldn't change that much. However, I've not checked this and could be wrong.

If you're flying en-route (to repeat an earlier question), what heading is it that appears in the ops room via your ACARS system? Mag, true, or whatever is currently selected via your FMS?

We don't use ACARS, but I would say it reports whatever the FMS gives at that time, which would be mag unless in polar regions (above the specific latitude).

Many thanx, Phoenix and others. Much appreciated.

Rgds

RS.

Just to correct myself: I've looked it up and the FMS automatically switches from mag to true above 73 north and below 60 south...

I'd still like to know for sure about polar airfields using mag, though. I'm trying to find out.

Px- Generalising a bit, but N polar directions - rwy, sfc winds, en-route tracks and winds are all true. Certainly in Canadian Northern Domestic Airspace due proximity of Mag pole. Russians use T up that way as well I think. Alaskans stay in M because its not that much of a factor.

Polar Tracks are all T even when they start at low latitudes. The transition points, M to T, will vary with routes being flown.

In the lower lats, rwys, sfc winds given by ATC, ATIS, enroute tracks, headings are mag. Volmet, forecast sfc winds, metars and winds aloft are T. This last item is a favourite check question for those new to transatlatic ops "What`s the X-wind component on rwy X @ Y ?".

Down South, do what you like. US Mil will almost universaly use Grid for everything. Antipodean Mil as well. Brits will use Mag cos they generally don`t get near the Magnetic S Pole, but winds aloft ? - pick a datum. Around these parts 3 identical a/c could easily fly the same ground track with 3 completely different headings (as they could anywhere else if you ignored the conventions ).

Sorry RevStar if this is more information than you needed.

TAF winds are given as true winds.
ATIS winds are magnetic,
Tower gives you magnetic winds as well.
Enroute winds are true.
Basically, all winds are true, unless you need the wind for take off or landing and there you need magnetic, because the runway heading is magnetic.
Magnetic heading: mag compass doesn't work at high latitudes, INS/IRS systems don't align above certain latitudes north and south. It's published where to switch modes.

true :)
I do not have a high level can, 3/4 in front of me, but I think you guys are correct, above 60, although I think the lateral tracks A,B,C,etc true, but extend below the ADIZ, say ending at Uranium city, or Ft Smith, cannot remember exactly, ( actually is`nt Ft Smith on 60?) anyway, and I am sure ATC guys can correct me, but I always believed that when we are put on a heading, as per maintain current heading when you are already INS to a point, it allows for drift, I think this because I see it quite a bit over say Rhine, and the previaling wind will then take us away from same level traffic on TCAS whereas the INS track will correct, correct?

RevStar,

Winds are not mag near the runway. All written weather reports, including Surface Winds (METARS), are in degrees TRUE. The only time you will hear magnetic wind direction is when it is spoken (as opposed to written) i.e. ATIS, Tower, etc.

Hope above helps,

Stamatis

As I was told in groundschool, if it comes out of someones' Mouth (i.e Tower, ATIS), it is Magnetic.