Small Nav Q
Join Date: Jan 2004
Location: Marlow UK
Posts: 137
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Mar 2000
Location: Edinburgh, UK
Posts: 62
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jun 2003
Location: SPAIN
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
The world has it wrong
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
I know which way I am going do these calculations from now on.
Last post on this.
RGDS unowho
Join Date: Jan 2004
Location: Marlow UK
Posts: 137
Likes: 0
Received 0 Likes
on
0 Posts
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/a...leid=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
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/a...leid=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
Join Date: Jan 2004
Location: Marlow UK
Posts: 137
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Sep 2000
Location: Canada
Posts: 268
Likes: 0
Received 0 Likes
on
0 Posts
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.
I therefore get 5W as the answer.
Join Date: Jul 2003
Location: Dublin, Ireland
Posts: 6
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Aug 2004
Location: Australia
Posts: 58
Likes: 0
Received 0 Likes
on
0 Posts
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
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
Join Date: Mar 2000
Location: Edinburgh, UK
Posts: 62
Likes: 0
Received 0 Likes
on
0 Posts
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!
How fine it would be to adjust out all the deviation errors. Never happens though!
Join Date: Sep 2002
Location: Oxford
Posts: 3
Likes: 0
Received 0 Likes
on
0 Posts
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
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
