Compass hdg / Deviation question:

[SPar.ca]

Hi folks,

Hope anyone can shed light on this (I'd have thought easy question):

You are flying a constant compass heading of 252°. Variation is 22°E, deviation is 3°W and your INS is showing a drift of 9° right. True track is ?

280° is the correct answer given. I was always taught that Variation West is best (i.e. added) and Variation East is least (substracted)... anyone able to point out my newbie mistake in thinking here? Please?

Thanks i.a.!

[Capt Pit Bull]

Difficult to say without seeing your answer.

That "add if its west" thing (which incidentally I don't care for) only works if you are converting from true.

More correctly:

Compass + Deviation = Magnetic
Magbetic + Variation = True

Where Easterly Deviation or Variation is POSITIVE
and Westerly Deviation or Variation is NEGATIVE

(these things are defined on starting with a compass reading and converting to true, but of course for pilot navigation we more often are doing the reverse... in which case subtracting a negative number requires addition)

Hence:
Compass 252
Deviation 3 West (-3)
Magnetic = 249
Variation 22 East (+22)
True = 271

Drifting 9 degrees to the right will give you a track of 280.

any use?

pb

[pitot_noob]

The way we were taught to remember was

'Cadburys Dairy Milk Very Tasty' - has always stuck in my head!

But if you're going that way, the West/best works the other way.. so you could looked at it as being "West is best... meaning the starting number is BIGGER", thus subtract if it is a west.

[Dave Clarke Fife]

'Cadburys Dairy Milk Very Tasty' - has always stuck in my head! (CDMVT)

And the other way round is True Virgins Make Dull Company, (TVMDC)

[Flightfreak]

I just use the mathematical approach like suggested by pb, simple and works like a sharm. Those mnemonics for direction calculations used to drive me knuts. :}

[Tojo]

Just remember:

Deviation East COMPASS least ie the compass direction is a lesser value than the magnetic direction.
Deviation West COMPASS BEST ie the compass direction is greater than the magnetic direction.

Variation East MAGNETIC is least ie the magnetic direction is less than the true direction

Variation West MAGNETIC is best ie the the magnetic direction is greater than the true direction.

The other thing to remember is that the figures in the question calculate a true HEADING to which the drift must be apllied to get the track.

Remembering that we always fly from heading to track on the compass the 9 degree right drift would give the track of 280. :)