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View Full Version : Practical usage of True Heading vs Mag Heading


foxkilo
4th Aug 2011, 03:39
I'm slightly confused and have some gap in knowledge about how True HDG and Mag HDG are used on a typical commercial flight.

Now, the gyro system in the IRU figures out the True HDG, while Magnetic HDG is calculated by accounting for the MagVar.

Aeronautical charts and flight plans are referenced to True Headings and True North (90 degrees, North Pole), is this correct?

Where does Magnetic Heading come into the picture, and why is it necessary? Which systems MUST use Magnetic Heading? Isn't it enough to just accomplish everything using True Headings? Why, for example, does the ND have the option to display Magnetic Heading?

The standby compass displays Magnetic Heading. In the event where it has to be relied on for navigation (really bad day...), isn't there a concern that there would be some deviation or error (since charts and maps are based on True Heading)?

I'm not a pilot, so I hope somebody can shed some light on these questions. Thanks.

morphmorph
4th Aug 2011, 07:48
The compass measures direction relative to the magnetic North pole, but the magnetic North pole is not located at the true North pole (0 degrees Longitude, +90 degrees Latitude). The angular difference between the true and magnetic North poles is called the 'magnetic variation' or 'magnetic declination', so to convert a bearing from a compass to a 'true' bearing you just add the magnetic variation.

The magnetic variation depends on your current position. For example, in Maine it's 20 degrees West, in Florida it's 0 degrees, and in the UK it's about 1.5 degrees West. The magnetic variation is at any point on the earth is publicly available and is also printed on aviation maps.

For small aircraft with no avionics where a pilot flies manually with reference to a compass (eg. general aviation aircraft) the pilot would plan the route using true headings then convert them to magnetic headings using the magnetic variation. For larger aircraft, the avionics would automatically convert heading data from the compass to true headings by adding the magnetic variation.

Wikipedia has a good description of it - see its 'magnetic declination' page.

BOAC
4th Aug 2011, 09:22
Aeronautical charts and flight plans are referenced to True Headings and True North (90 degrees, North Pole), is this correct? - not at 'normal' latitudes - all magnetic, which answers your next questions.

ND is normally Mag but there may be customer-selectable options for TR for polar - outwith my experience.

rudderrudderrat
4th Aug 2011, 10:43
Hi foxkilo,

Modern aircraft fitted with IRS systems can find True North reference easily.
But there are still lots of aircraft flying without IRS (or INS) systems fitted, and they rely on Magnetic North sensed systems (Flux gates etc.). In order that we can all fly in the same airspace together, the lowest common denominator (Mag North) is chosen as the reference. Those systems using IRS, simply add magnetic variation (obtained from a data base) automatically, to display Magnetic North.

Where Magnetic North is unreliable, then we all use True North as the reference.

Long range navigation over areas where the magnetic variation changes, is more easily done using Tue North as the reference.

foxkilo
4th Aug 2011, 10:48
Thanks very much for the explanations!

FlightPathOBN
5th Aug 2011, 00:10
Mag North is a left over from the Viking days of navigation.

Considering modern GPS navigation, magvar is really a pain in the ass...

even modern INS systems use GPS updates..

its a real pain in the northern airports where the magnorth is impossible to ref (as it is south) and we go to grid north, or in a Country such as Canada, being close to the mag center, have to keep renaming the runways.
not to mention designing a procedure with multiple legs and transitions, and having to convert each leg to mag...

with GPS nav, I am hoping that we are able to fix the runways at a certain designation and ref true north, rather than a meandering mag ref point...

the pole is due to flip soon, what the hell would we do then, another reason to change. :D

aterpster
5th Aug 2011, 00:45
BOAC:

- not at 'normal' latitudes - all magnetic, which answers your next questions.

True of domestic airways, but the fixed tracks in the Pacific are charted with both mag and true. It was our policy to switch our INSes to true while within the FIR.

I don't know whether there are any fixed tracks in the Atlantic. Certainly not in the NAT area.

mutt
5th Aug 2011, 00:56
aterpster, do you use True or Magnetic for the NAT area?

Mutt

172_driver
5th Aug 2011, 01:03
I don't know whether there are any fixed tracks in the Atlantic. Certainly not in the NAT area.

There are a few, to allow for non-MNPS aircraft to transit.

MNPS Operations Manual (http://www2.tech.purdue.edu/at/courses/at300/Documents/mnpsa-v8.pdf) page 51

FlightPathOBN
5th Aug 2011, 01:21
The pole is currently moving at 50 km/year...and this is the basis for navigation?

(a procedure designed in 2000 for Juneau, would not be valid today)
http://operationsbasednavigation.com/wordpress/wp-content/uploads/2011/04/Capture11.jpg

This has a much more profound effect on the Southern hemisphere.

aterpster
5th Aug 2011, 02:23
Mutt:

aterpster, do you use True or Magnetic for the NAT area?

Mutt

I am long out of the actual flying and I never flew the NAT. Further, I was speaking of the L-1011 where we could select true or mag anywhere.

Perhaps with today's high-tech aircraft it is done differently. I don't know.

But, it seems to me flying a NAT track true would make more sense.

foxkilo
5th Aug 2011, 03:09
By the way, what about the flight plans and nav databases uploaded into the FMC / FMGES?

Are the lat / long coordinates referenced to Magnetic North, or True North?

PantLoad
5th Aug 2011, 03:13
At my old carrier, we did everything in TRUE...planning, plotting...the
whole enchalada. The very last step was to convert to MAG to actually fly the segments.

Dunno if that was smart, stupid, or otherwise....I just did as I was told.
:*:*:*:*:*



Fly safe,


PantLoad

galaxy flyer
5th Aug 2011, 03:16
We used True in th Carousel INS in the C-5 nd it was indeed handy in oceanic airspace--plotting was easier, re-routes were easier to recompute flight plans, made sense on plotting charts. Now, in the Global and other civie planes, we use a magnetic--OK but not as simple. I agree with FlightPathOBN, get rid of magnetic.

GF

Microburst2002
5th Aug 2011, 09:47
at high latitudes, magnetic becomes a problem, so True is more convenient. But if you keep going up the latitude, then even True is very difficult and you have to go Grid.

I am curious. Airlines flying through the NP do they use Grid? or there is other alternative?

aterpster
5th Aug 2011, 13:14
foxkilo:

Are the lat / long coordinates referenced to Magnetic North, or True North?

You're kidding, right?

What is your house referenced to? :confused:

aterpster
5th Aug 2011, 13:17
FltPathOBN:

The pole is currently moving at 50 km/year...and this is the basis for navigation?

About 10 years ago one of the Flight Standards Aces at the FAA floated a proposal to converted to a true north system in the U.S. We asked, "What about the 10's of thousands of aircraft with only magnetic compasses and a DG or only a flux gate DG/HSI?"

galaxy flyer
5th Aug 2011, 17:48
At least in my experience, there is no need for grid, the Global just uses true, but in the "keyhole" the pilot has to manually select True.

GF

FlightPathOBN
5th Aug 2011, 18:31
By the way, what about the flight plans and nav databases uploaded into the FMC / FMGES?

The nav database coding uses true north, actually just waypoint to waypoint, so there really arent any bearings. The different FMC's calculate things somehwat differently, the Smiths box has a grid model of the earth built in, with 'lookup'.

In the 424 code header for the procedure you place the magvar, so that the FMC knows, which is nice, because as the magvar changes, this is the only mod to the file you have to do.

punkalouver
5th Aug 2011, 19:09
In northern Canada, most of the carriers will operate in True(a few cowboys stay in magnetic and will still work in many areas). The aircraft will typically depart from an airport in a magnetic area of operation such as YFB or YZF and then if heading farther north, about 40-50 miles to the north, the compasses will be switched over to true. This is where the area of true operation is required. In the eastern arctic the variation is westerly, in the western arctic it is easterly variation, so you have to adjust the compasses the proper direction. It is near 40 variation out of YFB. If you forget and are accidentally still in magnetic and then use true bearings on an approach, you might get a big surprise.

Of course your aircraft has to have the capability for free gyro operation. The easy way to switch over is to set your GPS in true and then check the bearing to an NDB off the GPS and set your RMI heading accordingly using the NDB needle. Then when you get near destination use the destination NDB or a nearby NDB to re-adjust for approach. Some NDB's are higher power than others.

When lined up with the runway in areas of true ops for departure, set the compass to runway heading. If there is no NDB at the destination you can also correct for planned convergence while lined up on the runway for takeoff. Going 10 of longitude east and the runway heading is 297 set 307 when lined up. The saying east is least(subtracted) for variation doesn't work for convergence. It is the opposite.

Of course for the convergence correction which is part of "Apparent Precession" on the gyro, you are supposed to multply the longitude change by the sine of the mid-lattitude.

It is also mandatory to have an astrocompass on board in this area, so if you really need it, you can do a sun or star shot. There is a formula for quickly setting it up. GMT times 15 subtract westerly longitude. An E6-B, whizwheel etc can be used instead in a pinch but you do have to have the sun visible and you may have to change heading significantly to get the shot which will give you your present heading. At night, stars are used. Planets not used.

Cloudy skies....you are on your own.

Spooky 2
8th Aug 2011, 09:05
Just as a point of order, Boeing offers a Grid option for the 777/747 and 787. It displays on the ND but has no LNAV navigation capability.

Microburst2002
8th Aug 2011, 12:49
So grid navigation is not used very frequently, I assume.

I guess that if you use True North navigation, then near the NP you fly great circle tracks between each two waypoints, as computed by the navigation systems, while seeing how the heading changes (without any roll), right?