Small Nav Q

[Cron]

I keep getting (A). The answer is (B) apparantly. Could someone put me out of my misery. Many thanks in advance.

Given:
True track 180°
Drift 8°R
Compass heading 195°
Deviation -2°
Calculate the variation.
A 21°W
B 25°W
C 5°W
D 9°W

[Pack2]

I think the answer is 5w.

C D M V T
195
-8
187 -2 185 5w 180

Does this ring any bells ?

[Cron]

True head = 172
Compass = 195
Dev = 2W
Magnetic Head = 193
Therefore Var = 21

Where am I going wrong?

[oxford blue]

I'm not a small nav (I can think of many who are) but I'll answer your question anyway.

You are right. The answer is (A). The person who tells you that it is (B) has not applied deviation correctly.

When deviation is given as plus or minus, it is what you apply to the compass to get magnetic heading, not the other way round. So plus is East deviation and minus is West deviation. The magnetic heading is 193.

[Pack2]

well done Cron. You were right.
I can see that I should have taken the drift from the true Hdg rather than from compass Hdg.

Thanks for your input oxfordblue.

[Notso Fantastic]

I came up with (a) as well. The mnemonic for that is Cadburys Dairy Milk Very Tasty.
Compass Hdg 195
Drift 8R
Compass Track 203
Deviation -2, so
Compass Track 201 Magnetic

If this is equal to a True Track of 180 True
The variation= -21

Remember to apply the Cadburys Dairy Milk Very Tasty from left to right as you read it- C>D>M>V>T and memorise:

Variation West, Magnetic Best
Variation East, Magnetic Least
(applies for Deviation too)

(does anyone know what I'm talking about?). That was quite an exercise for my brain cells after all these years! Have I made a boob above?

[FlightDetent]

The book I learned from: Can Dead Man Vote Twice? Wicked question, so to speak.. ;) For me, the math ends up the very same as stated above. ....

FD.

[EddieHeli]

The answer is B

If your true track is 180 with 8r drift you are actually heading 172 (As you are being pushed 8 degrees to the right)
If your compass heading is 195 with a -2 degree deviation your actual magnetic heading is 197 (As your compass is reading 2 degrees less than the actual magnetic value)
197 minus 172 is 25 degrees so the answer is B.

Regards
EddieHeli

[Pack2]

My money is still with Oxford Blue answer (A) 21w.

Deviation west compass best.
Deviation east compass least

Minus = west
plus = east

Now I am going to dig out those old Nav books...

[Alex Whittingham]

Sorry EddieHeli, you've applied deviation incorrectly as well. Oxford Blue is right, it's answer (A).

The problem with using + and - for deviation and variation is that it only works mathematically correctly when working in one direction. For instance:

Compass heading 182º, Deviation -2º, Magnetic heading 180º

is correct and makes sense mathematically because 182 - 2 = 180 whereas:

Magnetic heading 180º, Deviation -2º, Compass heading 182º

is also correct but makes no mathematical sense. I would suggest a variation of Notso's idea. Convert the minus sign to 'west' then say:

Deviation West Compass Best
Deviation East Compass Least

Where 'best' means 'a bigger number'. Variation is similar, + is east, - is west as before. The rhyme is as Notso says:

Variation West Magnetic Best
Variation East Magnetic Least

[Notso Fantastic]

Alex, brilliant- I knew there was more to the poem! Who says pilots can't be poets as well! Maybe the poetry is a bit dry, but it's functional! Bit more handy than that:
"I wondered lonely as a cloud
That floats on high o'er vales and hills......"
Really, what use is that rubbish to a mechanic or engineer?

I tried to make it clear:

Remember to apply the Cadburys Dairy Milk Very Tasty from left to right as you read it- C>D>M>V>T

[G-ANDY]

Good question Cron, got any more questions to practice on??

G-ANDY

[Cron]

G-Andy, yes, I have a few more in this vain, if they are useful to folks practising. The problem is they are gleaned, filched, copied from all sorts of places and may be copyright. Does this apply to questions? If not I'll post some up. Alex?

[unowho]

Can Dead Men Vote Twice At Elections

Compass Deviation Magnetic Variation True Add East

I prefer CDMVT but the other way round is,

True Virgins Make Dull Company Add Wiskey

True Variation Magnetic Deviation Compass Add West.

Hope that helps.

Unowho

[EddieHeli]

I disagree,

I think it depends how you read the question.

The deviation is given as -2 degrees. I read this to mean that your compass is underreading by 2 degrees, therefore a reading of 195 is actually 197.

I therefore stand by my answer.

why do you say Convert the minus sign to 'west'

surely East is Least, West is Best

therefore East is minus West is plus


Regards
EddieHeli

[Alex Whittingham]

EddieHeli, the problem with the rhyme 'east is least, west is best' is that it doesn't tell you what is best. The conversion of + to E, - to W is just a convention, its just so everybody understands it the same way.

[212man]

Does "Meridien east, Greenwich least" help things at all? !!!;)

[unowho]

E H. Alex is RIGHT you are WRONG

Quotes

"The angle between Magnetic North and the direction indicated by a compass needle is called the angle of DEVIATION"

"Deviation is the angle between Magnetic North and Compass North measured in degrees East or West FROM Magnetic North"

"The signs for plus and minus are sometimes used as a prefix instead of the suffixes East or West, in which case you will see that the rule to apply the deviation (as stated) to the compass heading to obtain the Magnetic headin"

Compass 195º Deviation -2º = Magnetic 193º

Hope that clears things up.

EH is it different in helios????????????????

Unowho

[EddieHeli]

unowho

I disagree and stand by my answer.

To use your quotes

"The angle between Magnetic North and the direction indicated by a compass needle is called the angle of DEVIATION"

If you have a deviation of -2 degrees then you must add 2 degrees to your compass heading to get magnetic north.

To visualise this draw a line straight up and call this magnetic north.
now draw a line 2 degrees to the left of it (i.e. to the West as everyone is saying minus is to the West).

using your own quote
"Deviation is the angle between Magnetic North and Compass North measured in degrees East or West FROM Magnetic North"

Although my compass heading is now saying north It is actually pointing 2 degrees to the west of north.
Therefore I must add these 2 degrees on to my compass heading
to get true magnetic.

so 195 compass heading must have 2 degrees added to give true magnetic therefore 197 is the answer.

The answer is in your own quote "FROM Magnetic North"

Yours and everyone elses answers are going FROM COMPASS NORTH.

You are all saying that magnetic north is 2 degrees to the west of your compass North.

If you take the two degrees off you are then out by 4 degrees.

[Right Way Up]

<<Although my compass heading is now saying north It is actually pointing 2 degrees to the west of north.
Therefore I must add these 2 degrees on to my compass heading
to get true magnetic.>>

No, if your compass reads North and it is pointing to the west of mag N. then you must takeaway 2 degrees to get mag hdg!

If CompNorth datum is to the left of MagNorth datum by 2 degrees, you use up 2 degrees of your Compheading before you reach the zero of Mag North.

[EddieHeli]

You can work this out with basic maths in both directions given what we know using two well known sayings. Substitute Virgins or Chocolate as desired for your own versions.


if we use the saying
"True virtue makes dull company. Add whiskey."
true+variation=magnetic+deviation=compass
The plus signs are used for westerly changes ("add whiskey" = add westerly) If the variation or the deviation are EASTERLY, then use a minus sign.


We know the following
true = 172 (Nobody disputes this 180 minus 8 degrees drift)
variation = x (to be calculated)
magnetic = y ( to be calculated)
deviation = -2
compass = 195
we know variation and deviation are westerly so we add.

172 + x = y (magnetic)
y + (-2) = 195 (compass)
y must equal 197 as we know that 197 - 2 = 195
therefore 172 + x must equal 197
therefore 197 - 172 = x
therefore x (variation) = 25

therefore Answer = B

If we now do the reverse calculation using the saying
"Can dead men vote twice at elections?"

compass+deviation=magnetic+variation=true
The plus sign is used for easterly changes ("at elections." = add easterly) If the variation or the deviation are WESTERLY, then use a minus sign.

Again we know the following
true = 172
variation = x (to be calculated)
magnetic = y ( to be calculated)
deviation = -2
compass = 195
we know variation and deviation are westerly so we subtract.

195 - (-2) = y (minus minus 2 = plus 2)
therefore y (magnetic) = 197
197 - x (variation) = 172 (true)
therefore x (variation) = 25 (as 197 - 25 = 172)

Variation = 25

so I still think the answer is B

Regards
EddieHeli

[avoman]

Sorry Eddie I think you are wrong too!
I have swung lots of aircraft compasses and calculate the deviation thus:
deviation = magnetic - compass
that is
D = M - C
(swap it around to get C + D = M , that is Cadburys Dairy Milk!)
All this Easterly and Westerly business is unnecessary for deviation on aircraft, it scares me, I have nothing to do with it. Unfortunately we are lumbered with it for variation for historical reasons.
If I am wrong there are many aircraft flying with wrong magnetic and gyro compasses!
The original question is simply solved with
CDMVT to get answer A.

[unowho]

We all have been wrong for about 100 years and thank you EH for pointing it out. Now let the rest of the world continue to be wrong and except the fact that we are all wrong and you are RIGHT.

I know which way I am going do these calculations from now on.

Last post on this.

RGDS unowho

[EddieHeli]

If we ignore the minus sign and apply the sayings as above then the answer is indeed 21.
But which way is correct and why ?

I don't mind eating humble pie but someone please explain the reasoning, not just we told you so.

Incidentaly I looked up this minus being west convention but teh only navigation text book I could find says the opposite, that east is minus. It was a nautical book but its the same.


if we use the saying
"True virtue makes dull company. Add whiskey."
true+variation=magnetic+deviation=compass
The plus signs are used for westerly changes ("add whiskey" = add westerly) If the variation or the deviation are EASTERLY, then use a minus sign.


We know the following
true = 172 (Nobody disputes this 180 minus 8 degrees drift)
variation = x (to be calculated)
magnetic = y ( to be calculated)
deviation = 2
compass = 195
we know variation and deviation are westerly so we add.

172 + x = y (magnetic)
y + 2 = 195 (compass)
y must equal 193 as we know that 193 + 2 = 195
therefore 172 + x must equal 193
therefore 193 - 172 = x
therefore x (variation) = 21

therefore Answer = A

If we now do the reverse calculation using the saying
"Can dead men vote twice at elections?"

compass+deviation=magnetic+variation=true
The plus sign is used for easterly changes ("at elections." = add easterly) If the variation or the deviation are WESTERLY, then use a minus sign.

Again we know the following
true = 172
variation = x (to be calculated)
magnetic = y ( to be calculated)
deviation = 2
compass = 195
we know variation and deviation are westerly so we subtract.

195 - 2 = y
therefore y (magnetic) = 193
193 - x (variation) = 172 (true)
therefore x (variation) = 21 (as 193 - 21 = 172)

Variation = 21

the answer is A

Regards
EddieHeli

by the way unowho - chill out

have you never been wrong ?

I thought the idea of these discussions was to thrash out ideas till we found the answer.

I have supplied my reasoning with all of my answers. If my thinking is wrong well fair enough, but explain it better so that I can see why - otherwise why should my thinking change?

Someone said that convention says that -(minus) is west but I read this article which says the opposite.

Here\'s a clip from a navigation article by Jim Sexton on sailnet found at
http://www.sailnet.com/collections/articles/index.cfm?articleid=sexton050

Secondary magnetic fields in the vicinity of the compass can also affect the compass readings. These secondary magnetic fields are caused by the presence of ferromagnetic objects, electronics, and electrical wires in the boat. This error can be reduced by changing the position of the small compensating magnets in the compass case. However, it is not possible to remove all of these errors on all headings. The end result will be a compass card showing the number of degrees of error in the compass when you are on various compass headings. This error is called deviation, and like variation must also be considered when determining heading relationships.

The correction for variation and deviation is usually expressed as east or west and is computed as a correction to true heading. In order to make this computation easier we usually convert the east or west values to a plus or minus value and add them algebraically. If variation or deviation is east, the sign of the correction is minus and if west, the sign is plus. A good mnemonic for remembering this is "east is least and west is best."


regards
EddieHeli

[EddieHeli]

Avoman,
I trust that all of your compass swings are in order.
Thats not the point.
If you swung the compass in my aircraft I would not expect you to simply leave a note saying deviation -2 degrees as it is open to mis-interpretation as this thread has shown (Even if only by me!!).

All of the deviation cards I have seen say something like for 180 steer 178 etc.
This is unambiguous and easy to follow.

As I said earlier, the answer you get to the question posed depends how you read the question.

Before everyone shoots me down in flames as the lone voice against the world let me explain.

Crons original post said the answer was given as B : 25W.
He also said he made the answer A : 21W. Everyone else agreed that 21W was the answer, however I pointed out that depending how you read the question you can indeed get the answer 25W.

The difference is based on how you interpret the -2 degree deviation. Everyone seems to think that it means 2 degrees west. However the question doesn't say 2 degrees west is says simply -2 degrees.
(If it did say 2 degrees West then I think everyone agrees that the answer would be 21W).

Before you all groan please read on.

If my compass card says for 180 steer 178 I hope you would agree that my compass is underreading by 2 degrees.

i.e. my compass deviation is -2 degrees as deviation always refers to the compass as it is has deviated from magnetic by the given amount. (You cannot say that magnetic has deviated from your compass).

In the question it states deviation -2 degrees. I read this as above i.e. the compass has deviated by 2 degrees less than magnetic i.e. it is underreading by 2 degrees.

Therefore a compass reading of 195 is actually 197 magnetic, hence the answer calculates to 25W.
(equivalent to above i.e. for 197 steer 195).

To get 21W as the answer you must interpret the -2 degrees deviation to mean something different to my explanation.

It depends on whether you read the question as given as I did, or apply your interpretation of the -2 degrees to mean plus 2 degrees West

Regards
EddieHeli

[Alex Whittingham]

Have you noticed how everyone has shut up?

[Dockjock]

Can someone explain to me how right drift causes you to drift to a smaller true track? In my mind if I was supposed to be tracking 180 and was drifting 8 right, I would be tracking 188.

I therefore get 5W as the answer.

[butpau]

Hi Eddieheli – Great post and excellent thought process.

You succeeded in undermining my entire belief system on this subject and I have had to scurry back to the scared scriptures for reassurance. All I can add is that if you follow the ‘conventional’ rules as stated earlier by all those who favour Answer A, you will also answer correctly the only two question I can find in the JAA databank. Question 3080 and 3081 are similar but ask you to caculate the Compass Hdg, but the same rules apply and you need to accept the a minus deviation figure is West and should be added to the compass reading to give magnetic – 195 plus –2 equals 193.

Q3080 – Given True course 300, drift 8R, var 10W and Dev –4. Calc compass heading.
Ans – (a)306 (b)322 (c)278 (d)294

Q3081 – Given True Track 352, Var 11W, Drift 10R and Dev –5. Calc compass heading.
Ans – (a)358 (b)346 (c)025 (d)018

Note: - true course and true track are the terms used in the question.

What do you think the correct answers are or should I say, the answer that JAA give as being correct.

Could you do me a favour and promise not to turn attention on greater issues such as the existence or otherwise of God, the tooth fairy or santa. I really don’t think I could handle the truth.

Cheers.

[Pack2]

Dockjock
In the original post the Question gave the "True Track" as 180.
Think of this as the "track made good" along the ground.

If there was no wind then the "true track" would be the same as the "true Hdg" but we have 8R drift giving us the 180 true track or track along the ground measured in degrees true.

so lets remove the 8R from the true track and we are left with a true Hdg of 172. We know that the magnetic Hdg is 193 so the difference is the variation. 21w...

Hope that helps

[avoman]

EddieHeli, rest assured the info is used to produce a for....steer....card working in the correct sense.
How fine it would be to adjust out all the deviation errors. Never happens though!

[Mike Allen]

It's quite correct to use CDMVT, but lets go back to the basic definitions:
Variation is the angular difference between True North and Magnetic North. The sense is defined as West IF magnetic North is west of True, and vice-versa.
Deviation is the angular difference between Compass North and Magnetic North. The sense is defined as West IF Compass North is West of Magnetic North.
To put this in context:
Draw a vertical line on a sheet of paper. This will be True North. Now draw an aircraft pointing left to right. The aircraft will be heading 090T.
Now draw a line 10 degrees to the left of true North. This will be magnetic North. Because the line is to the left of True this will give 10 deg West Variation. The Magnetic heading will now be 090 + 10 = 100M.
Now draw another line to the left of Mag North, again 10 degrees. This will be Compass North. The Compass North is West of Magnetic so the Deviation will be 10 West.
The aircraft has not moved so the compass heading will be 090T + 10W Var + 10 W Dev = 110 Compass.
The way I teach this is to use CDMVT but link it to the Cardinials -
Draw a cross, vertical, and annote the 4 ends with N,S,E and W (I trust you can cope with this one!).
Start in the centre of the cross. If you go right you are going East - so add easterly variation. If going left you are going West, so add Westerly variation.
C D M V T
Mag to True you are going right thus add east Var.
For deviation, which is usually shown as + or -, we normally write from left to right thus if going from left to right you apply the sign as given. If going "the wrong way" right to left you reverse the sign.
C D M V T
If dev is -10 then: Comp - 10 = Mag
Sounds awkward, but in practice its very easy.
By the way the correct answer is 21W!

Mike
Chief Ground Instructor
OAT