nav question - applying variation and deviation
 

I have a question on how to apply the correction for variation and deviation.
I understand when given the variation, how to work out the Magnetic course, and also when given the Deviation in terms of a degree figure on a given heading, but give me a question where they only provide the compass deviation card, I can’t see how you work out what figure to use for deviation. For example. (Turneth to page 449 of the Thom Nav book if you’ve got it):

For Steer HDG(T) Varn HDG(M) HDG(C)
N(360) 003 012T 8W 020M ?

Now I work this out to be,
True 12 + 8W Varn = 20M as shown.
Then it loows like the deviation is 3E, East least => 20 – 3 = 17C

The answer is 22 in the book. I can’t see how 22 is the answer from any permutation of these figures.
Can anyone explain this?
Help appreciated as it’s very frustrating. The Thom book is usually pretty clear.
Thanks.

I tried to have a look at this for you - but got completely confused with what you are trying to ask....(maybe need more beer again).

I have Thom Nav book - it is third edition, dated 1997. It ends at page 362 - I have no 449. Assuming a misprint I checked pages 44 and 49. Nothing there either.

Back to your message and tried again.

One thing that would help - especially when trying to decipher from a distance - would be the use of standard phraseology.

Don't get into the habit of using two digit numbers to represent headings (if that is, as I suspect, what you have done).

I don't quite follow what the 360° bracket is there for - you have not mentioned it again....have another read and see if you have missed anything (or typed it incorrectly).

The formatting got a little messed up when I posted, so it's a bit unclear all right. I'll try to clarify.
I'm using the 2000 Reprint of the Nav book so that may explain the difference.

Basically you're given the following information,
For N(360) Steer 003
HDG 012T
Variation 8W
HDG(M) 020M
HDG(C) =?

So as I understand it, the question says, given the above information, determine the HDG(C).

Thanks for the help, I can only recommend more beer.

Mine has got a page 449 :)

ok, we've got HDG(T) - the true heading. HDG(M) is the magnetic heading, which is the true heading corrected for magnetic variation. Now, HDG(C), which is what you're trying to work out, is the corrected heading to steer - the heading that compensates for the compass error when the electrical systems in the aeroplane are on. Your aeroplane will have a card in it that will have the errors at the four or eight main points on the compass (have a look at page 38).

Now, my take on the question is that heading north you have +3 degrees of compass error (for N(360) steer 003), and heading east you have -3 degrees (for E(090) steer 087). You don't know exactly what the error is for a magnetic heading of 020, so you assume the error is linear and interpolate between the two points you do have (360 and 090) to get an error of +2 degrees, hence the answer.

(edit: However, there are errors in the questions in the Thom Nav book - and the others for that matter. It's been too long since I've used it to remember exactly what they are, but if a question really doesn't make sense Thom may have got it wrong).

Oh, and if you fly a Robin 2160, look at the compass when you turn the panel lights on. That's what your electrical systems do to your compass :eek: :ooh:

The things one needs to know to pass the exams... I doubt my £35,000 autopilot, fed off a £12,000 slaved HSI, can fly a specified numeric heading to an ACCURACY of 2 degrees. When will these people be dragged kicking and screaming out of their ivory towers?

Good, innit? Of course, you've got to work out this all out to one-degree accuracy on a whizz-wheel... :)

sound of penny dropping
 

ah it was the interpolation bit i couldn't quite work out.
i fly an r22 so i'll just be following landmarks anyways i presume. but i like to know these things.
thanks for your help.

Deviation east, magnetic least.

Therefore magnetic is LESS that corrected. 22'.

Bodie