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If you have a true bearing (track) drawn on your map but at the same time you have the magnetic bearing (as worked out from the true bearing and variation), which do you follow?

  • the true bearing on your map (and the ground features)
  • the magnetic bearing (as seen on your compass and to a certain extent, your DI)?
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  • $\begingroup$ This question is arguably highly opinion-based, and shows no evidence of either prior research or careful consideration of the problem at hand. $\endgroup$
    – quiet flyer
    Mar 27 '20 at 1:41
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This question is flawed because it is based on a fallacy: It presumes that true and magnetic bearings are mutually exclusive. They are not.

Every time you fly a track you are following both a true and magnetic course. They are simply different frames of reference.

So the correct answer is you fly both. You would normally fly the magnetic heading you computed when you reference your compass, (adjusted for wind) because that is what the instrument displays.

The track you drew on your map is the ground track your airplane should follow if your true to magnetic conversion and wind correction is right. You will cross check this against landmarks as you go. So, is this track depicted on your chart true (as you imply) or magnetic? The answer is that it doesn’t really matter, it can be either or both. It just depends on how you look at it.

It is like the temperature being at freezing, and asking whether it is 0 deg C, or 32 deg F. Which one do you want to reference?

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If you are using dead reckoning, you will need to follow your magnetic bearing including the magnetic deviation. In so doing, you will also be following your true bearing. In the air, you are following your magnetic compass and compass card unless all of your landmarks are within sight of one another. This is incumbent on not having to adjust your heading for wind.

There is no way to authenticate your true bearing except by matching the direct course between those landmarks with your map. This is not so helpful when your waypoints are more than 10 miles apart.

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  • $\begingroup$ I'm confused-- unless you have travelled so far that you no longer know the magnetic declination, even approximately, the comment about "no way to authenticate" true bearing/ heading would seem to equally apply, or not apply, to magnetic bearing/ heading. $\endgroup$
    – quiet flyer
    Mar 27 '20 at 1:48
  • $\begingroup$ @quietflyer - Sorry for being unclear. My point was that any indications of direction in the aircraft are gained by either viewing a magnetic compass or using equipment (like a DG) or calculations that were originally set or derived using a mag comp. An IRS/INS may be an exception. But, I am not too familiar with them. You would have to use your magnetic bearing in the air. Or, do the math to convert your true heading to magnetic. Otherwise, any long distance dead reckoning could be off. If you are just using pilotage, that’s a different story. Just keep your waypoints close to each other. $\endgroup$
    – Dean F.
    Mar 27 '20 at 4:18
  • $\begingroup$ Why the "more than 10 miles apart" limit? Unless you fly where it's really hazy or cloudy, of course. But in the western US, I can often navigate by landmarks that are 100 miles or more distant. E.g. once you're past the East SF Bay and can gain some altitude, the Sierra Nevada is pretty obvious. Mt. Diablo is almost as good going the other way... $\endgroup$
    – jamesqf
    Mar 27 '20 at 4:22
  • $\begingroup$ @jamesqf - You are quite right in areas of major landmarks. In the South Central US like Texas and Oklahoma, flat terrain would make it more difficult. You would have to rely on bodies of water, major roadways, antennae and turbine farms, oily tank fields, and cities which might be harder to spot 20 miles or more away. To insure you hit your waypoint, you have to use dead reckoning along with pilotage. A little bit more than just pointing the aircraft nose in the general direction of what looks like where you want to go 100 miles away. $\endgroup$
    – Dean F.
    Mar 27 '20 at 4:40
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    $\begingroup$ My point being, as Michael Hall has stated in his answer, True bearing and magnetic bearing are the same thing with different numbers. They are the same track on your map. The mag bearing just changes with variation. Unless you are using GPS waypoints, in the air, your devices for finding exact direction are going to be based on magnetic compass readings. Even using a directional gyro, it must be reset periodically during the flight. Since your main source of direction is magnetic, so should your frame of reference. $\endgroup$
    – Dean F.
    Mar 27 '20 at 5:27
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You fly the magnetic heading if you want to fly any track using a compass, or a DG set to match a compass, because that's your pointing device. It's providing a magnetic indication, so magnetic headings it is.

If you are flying in the Canadian Arctic or Alaska, different story; the compass becomes useless and all you have is a directional gyro that you set to true heading while on the ground using some ground reference, and try to make adjustments to the DG to compensate for precession as best you can (you will have established that your gyro precesses say 3 deg CCW in 15 min, so you make a 3 deg CW tweak every 15 min and you will still be in the ballpark at the end of your trip). In northern Canada, all headings and tracks are true above a certain latitude.

Farther south however, you could do the same in theory, but you have the compass available to you so you might as well use it and avoid the hassle of having to make best guess DG corrections.

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In theory it is up to you whether you want to work primarily in terms of magnetic or true headings/ bearings/ courses. Obviously if your choice is mismatched to the readout of the instrument you are looking at at a given instant, then you will have to make a mental correction, which can be either merely inconvenient, or disastrous, depending both on the particular situation, and on your competence at such things. Since there is no way to make a standard aviation magnetic compass give a readout of a true heading, the normal aviation practice, and arguably the best aviation practice, is to work entirely in terms of magnetic headings and bearings. Runway headings and AWOS/ ATIS wind direction annunciations and airway bearings on sectional charts and headings for instrument approaches are all labelled accordingly. Perhaps at some point in our ever-more-computerized future, this practice will be seen as quaint and antiquated. Setting your directional gyro (if present) to true rather than magnetic heading is always an option if you aren't afraid of the math in comparing to the wet magnetic compass, and if for some reason that's what floats your boat, and if you aren't shooting instrument approaches and landings or talking to ATC.

If you are flying in daylight over a man-made landscape resembling a north-south east-west checkerboard as can be found over much of the central US, and you aren't going to be flying in or above clouds for extended periods or giving headings to ATC or recieving headings from ATC or shooting instrument approaches, there is a significant case to be made for setting your directional gyro to the true rather than magnetic reference frame. But in such a case you could do almost as well to just "eyeball" your course without ever looking at either the magnetic compass or directuonal gyro. When it really starts to matter, in most cases you'll be best off to follow the standard aviation practice and work exclusively in the magnetic reference frame, unless you are flying with computerized displays that can do all the relevant conversions for you at the press of a button, and your contact with ATC is minimal.

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  • $\begingroup$ future edit-- fix typo in "directional" gyro near end. $\endgroup$
    – quiet flyer
    Mar 27 '20 at 12:58

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