Lots of people have problems with this whole subject area. These problems often stem from the fact that they have memorised the “CDTM”, East is least” and “West is best” tricks, but have never tried to understand what is actually happening. To get a better understanding of the subject we need to do a few simple demonstrations.
COMPASS DEVIATION
The term deviation refers to the situation in which local magnetic fields within the aircraft cause the compass to be deflected from Magnetic North.
When deviation is west this means that compass north is to the west of magnetic north.
When deviation is east this means that compass north is to the east of magnetic north.
To demonstrate the effects of this take a protractor and place it on a sheet of paper, with 000 degrees pointing to the top of the sheet. Draw short lines radiating out from the protractor at each 5 degree mark between 000 and 35 degrees east west.
The marks on the paper represent magnetic headings and the protractor scale represents the compass headings. With zero deviation the 000 on the compass will align with the 000 on the paper, so compass heading = magnetic heading.
Rotate the protractor 3 degrees anticlockwise (to the West). This represents 3 degrees west deviation. 003 compass heading on the protractor is now aligned with 000 on the paper. This means that with deviation west, the magnetic heading is less than the compass heading. Note that 030 compass from our original question aligns with 027 magnetic.
Rotate the protractor clockwise to align 000 on the protractor with 000 on the paper. Now rotate the protractor 3 degrees clockwise (to the East). This represents 3 degrees east deviation. 003 (compass heading) on the protractor is now aligned with 006 on the paper. This means that with deviation east, the magnetic heading is more than the compass heading.
If you repeat the above for several compass headings it will soon become very clear in your head.
MAGNETIC VARIATION
The term deviation refers to the situation in which local distortions in the magnetic field of the earth causes the direction of magnetic north to be deflected away from True.
When variation is west this means that magnetic north is to the west of true north.
When variation is east this means that magnetic north is to the east of true north.
The effects of this can be demonstrated by using the protractor again. But this time the protractor represents magnetic directions and the marks on the paper represent true directions.
Rotate the protractor 8 degrees anticlockwise (to the West). This represents 8 degrees west variation. 008 (magnetic heading) on the protractor is now aligned with 000 true on the paper. This means that with variation west, the true heading is less than the magnetic heading.
Rotate the protractor clockwise to align 000 on the protractor with 000 on the paper. Now rotate the protractor 8 degrees clockwise (to the East). This represents 8 degrees east variation. 008 (magnetic heading) on the protractor is now aligned with 016 true on the paper. This means that with deviation east, the magnetic heading is more than the compass heading. Note that 027 magnetic from our original question aligns with 035 true.
If you repeat the above processes a few times the effects of deviation and variation should become clear in your mind.
The results of all of this can be summarised as follows
When going from compass to true.
Compass + deviation East = Magnetic + variation East = True
Compass – Deviation West = Magnetic - Variation = True
When going from true to compass.
True - Variation = Magnetic - deviation East = Compass
True + Variation West = Magnetic + Deviation West = Compass
Applying this to our original question gives the following
30 Compass – 3 West Deviation = 27 Magnetic + Variation 8 E = 35 True