1

What will the magnetic compass which has an Easterly deviation of 10 read when the true heading is North

Will it read 350W or 10E?

I understand that the 10E East variation means that the underlying magnet is swung 10 deg clockwise instead of pointing to true North. So it is now pointing to 10E. But I guess since the readings are done from opposite side of the compass for the end user, the reading shown will be 350W.

Help!

edit : The question is different from what is answered at How do I convert true heading to magnetic heading or the other way around? -- My question is mainly regarding how to visualize the same. - John's answer below is perfect.

user007
  • 155
  • 5
  • I understand the math and what is explained on it - but more confused on how compass mechanism works - esp given compass is read from behind – user007 Mar 27 '21 at 05:51

1 Answers1

3

I assume your question is about Declination/Variation, not Deviation that is addressed with the compass correction card. It's confusing because you are reading the compass card from behind so to speak and you are rotating around the magnet, which is always aligned with Magnetic North, when you turn.

So forget about the magnet alignment and just consider the compass card indication that is telling your where the nose of the airplane is pointing, and think it through this way:

If you are flying straight toward the Magnetic Pole on the 10 Deg E Variation line, your compass heading will be 360/0 Magnetic, and your True Heading will be 10 Deg, because the nose is pointed 10 Deg to the right of the True North Pole with your compass showing 360/0 Magnetic.

If you are flying straight toward the True North Pole, your true heading is 360/0 True, and your Magnetic heading will reflect the fact that you are pointing 10 degrees to the left of the Magnetic Pole in order to point straight at the North Pole. This will show as 350 on your compass.

Hence the "East is Least" rule for Easterly Deviation (West of the Agonic Line). To fly a specific True Heading, point so that the Compass indicates 10 Deg LESS (least=less=subtract from True for a Magnetic heading to fly) than your target True Heading; to fly 360 True, Point at 350 Magnetic.

So, per your question, if you are flying 360/0 True, that is, pointing straight at the North Pole, your compass will show 350 Magnetic if you are on the line of 10 Deg Easterly Declination/Variation.

enter image description here

John K
  • 130,987
  • 11
  • 286
  • 467
  • The OP asked about Deviation, and also introduced "Variation" into the question. These have different meanings in aviators parlance. Your illustration appears to be laying out "Variation" a.k.a. "Declination" and you have labeled it "Deviation", which is usually used to describe magnetic disturbances peculiar the the aircraft. – 2NinerRomeo Mar 26 '21 at 22:43
  • Good point. I meant declination and just wrote deviation without thinking about the terminology. However, his question clearly is about Declination/Variation or he/she wouldn't be asking about true north vs mag north. If it was about Deviation, it the question would just be about use of the correction card. Also note it says "variation" farther down. Thanks for bringing it up. I fixed my post. – John K Mar 27 '21 at 00:22
  • Thanks for the explanation John. This is very helpful! – user007 Mar 27 '21 at 05:46