# Should I fly the true bearing or the magnetic bearing?

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)?
• This question is arguably highly opinion-based, and shows no evidence of either prior research or careful consideration of the problem at hand. Mar 27 '20 at 1:41

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?

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.

• 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. Mar 27 '20 at 1:48
• @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. Mar 27 '20 at 4:18
• 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... Mar 27 '20 at 4:22
• @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. Mar 27 '20 at 4:40
• 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. Mar 27 '20 at 5:27

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.