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Professional Pilot Training (includes ground studies) A forum for those on the steep path to that coveted professional licence. Whether studying for the written exams, training for the flight tests or building experience here's where you can hang out.

9th Mar 2011, 16:08

Join Date: Jun 2007
Location: London
Posts: 12

First off, hopefully i've posted this in the right forum. If not - apologies.

Secondly - i'm currently going over some notes and am trying to brush up on my knowledge. Im currently stuck on what is a seemingly easy question

Given compass heading 030, deviation 3W and variation 8E, what is the true heading?

a. 030°
b. 025°
c. 019°
d. 035°

I dont want anyone to just give me the answer - i want to understand it, and hopefully it'll stick in my head this time! I seem to remember the whole "West is best (i.e add the figure on) - East is least (subtract)" part of the groundschool, but would be grateful if someone could explain it better here - or point me in the way of some theory so i can read up.

Many thanks.

9th Mar 2011, 16:38

Join Date: May 2007
Location: Brighton
Age: 34
Posts: 120
Ok, i'll do my best to come up with the answer. Do you remember "CDMVT"?

This is always the method i used and got tought: -

ALWAYS DRAW IT OUT SO U CAN SEE WHATS HAPPENING: -

←Add if East →Subtract if East
←Subtract if West → Add if West

030 3W 033 8E 025

So from what i can see it's answer B 025 degress's. Is this the right answer?

Hope this helps
9th Mar 2011, 16:40

Join Date: Nov 2006
Location: uk
Posts: 180
Use CDMVT, college d**ks make virgins tremble!!
9th Mar 2011, 17:18

Join Date: Feb 2002
Location: Sunny Solihull
Age: 62
Posts: 204
The West best East least is correct but only when working from TRUE to COMPASS as taught at PPL level.

The question has been asked in reverse, therefore: -

C D M V T
030 3W 027 8E 035

Check the answer in the PPL method - 035(T) - 8E = 027(M) + 3W = 030(C)
QED unless I am having a bad day!

There are loads of this in the JAA GN & RN exams often tied up with CRP5 work and seems to be a weak area for students.

All this should be explained in any good navigation book from your course provider.
9th Mar 2011, 18:53

Join Date: Sep 2006
Location: World citizen
Posts: 315
-> -
<- +

Cv Dm Cm Dv Cc
025 -8 033 +3 030

Th (Cv) = 025

The French way !!!
9th Mar 2011, 19:05

Join Date: Feb 2001
Location: The Winchester
Posts: 5,428

or

True Virgins Make Dull Company.

Simples....
9th Mar 2011, 21:16

Join Date: Jan 2011
Location: England
Posts: 642
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
9th Mar 2011, 21:38

Join Date: Nov 2007
Location: Wherever I go, there I am
Age: 39
Posts: 675
Or...you just click the magic button on any good GPS unit.
10th Mar 2011, 23:37

Join Date: Jun 2008
Location: Dark side of the Moon
Posts: 37
Keith.... Your post had just made my night as I was sitting in bed doing Crp5 revision and was a bit stuck.

Thanks so much
11th Mar 2011, 17:22

Join Date: Jan 2002
Location: where the north starts
Posts: 95
Perhaps those that got it wrong should now delete their posts?

I'm with KW and RichardH
14th Mar 2011, 16:42

Join Date: Jun 2007
Location: London
Posts: 12
Big thanks to Keith for his very informative post - that explains a lot.

Good to know that i wasn't the only one having problems understanding it too - judging by the amount of incorrect posts!

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