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  • variation and deviation

    Hi everyone,

    nearly everything is going great with GNAV, except that im getting confused with variation and deviation, does anyone have a good tip on how to apply these two factors (i know what they mean, just not how to apply the correct calculation).

    Thanks in advance,...

  • #2
    Re: variation and deviation

    Hi Daan,

    Well I'm sure you are aware of the following formula:

    compass heading= true course + variation + deviation + drift correction

    Now the method I use is to consider the following:

    - if the variation is West, I put it in the formula above as a positive number and if it is East, I put it in the formula as a negative number
    - for the deviation I apply the exact same method as for variation; West is a positive number and East is a negative number
    - as for the drift correction, if it is to the RIGHT I put it in the formula as a positive number and if it is to the LEFT as a negative number

    It really is as simple as that HOWEVER if in a question you do not see West or East and instead "-" and "+", consider all "MINUS SIGNS" as "WEST" and "POSITIVE SIGNS" as "EAST". Also if the question doesn't give drift correction but instead gives Drift Angle, it would be the opposite to what I mentioned; DA right = negative number and DA left = positive number.

    I hope I answered your question.

    Comment


    • #3
      Re: variation and deviation

      The first thing I think of is CADET

      Compass to True, ADd East. So, when going from Compass Heading to True Heading add easterly variation or deviation.
      If I'm going the other way, converting from True Heading to Compass Heading then the opposite applies, just subtract eastely variation or deviation.

      Next thing is Cork Dry Makes Virgins Tipsy (Cork Dry is a brand of Gin, just in case you hadn't swallowed a bottle before!) to remember going Compass to True, and True Virgins Make Dull Company for the reverse.

      Comment


      • #4
        Re: variation and deviation

        Hi,

        Not being as rude as the previous posters remember:
        Cadbury's Dairy Milk is Very Tasty
        stands for
        Compass Deviation Magnetic Variation True

        By convention easterly variation and deviation is positive, and westerly variation and deviation is negative.

        It works going from left to right

        098?C -1?(W)Dev. = 097?M +8?(E)Var = 105?T

        Baz

        Comment


        • #5
          Re: variation and deviation

          thnx guys, those kind of tips were just the ones i was looking for, ,
          only thing now is to ace my gnav exam :P

          thnx a lot !

          Comment


          • #6
            Re: variation and deviation

            Something to bear in mind about this stuff:

            In practical terms the most common use for these calculations in a modern setting is to convert true to magnetic.

            e.g. you're out walking. You have a map. It is far more common to measure a true bearing off the map, correct for variation, and then march on the magnetic heading. As a result, especially in the UK where variation is westerly, people pretty much just learn that you "add the variation when its westerly".

            The you hit aviation studies, and have to take on board the idea that a Westerly variation is a negative number.

            But you already have it associated with addition.

            This is enough to make many people go 'wtf?' and label the whole topic of compass corrections as 'tricky'.

            Here's the key:

            The polarity (i.e. plus or minus) for compass corrections (variation or deviation) are defined to make sense if you are starting with a compass heading and converting it to a true heading, so in fact this is the oppositre of what we are usually doing.

            To solve all these question quickly and easily just write down the sequence as mentioned above by Baz, fill in the numbers you know, then fill in the blanks with whatever is needed to make it come out correct from left to right.


            (as an aside, if you're more of a 'visual' learner this kind of task lends itself to a methodical diagram. Once you get familiar though you'll find you won't need them)
            Bill Chivers


            [CAVEAT / DISCLAIMER.

            I am not an employee of Bristol Ground School, I work for an IT company that provides them with software but I do not work on BGS's systems. My main role is as a technical instructor on airliner CBT.

            However, as I am an ex airline captain, ex groundschool instructor and ex self improver I will sometimes answer questions here if I have the time and if I have relevant specialist knowledge. I aim to be a resource to help both staff and students however please note this is because I am an enthusiast not because it's part of my job description! If I'm busy, I may not look in for several weeks.

            Additionally, my responses are intended to provide broad education; I do not maintain currency on the latest peculiarities of the sylabus or the latest 'oddball' questions.

            Accordingly, BGS staff instructors must be viewed as the definitive reference in all matters regarding examinations, and should be your first port of call if you have any difficulties with your coursework
            .]

            Comment


            • #7
              Re: variation and deviation

              This is exactly where I am... In the "wtf?!" situation. And I can get out of it.

              When I read all the courses I have, it says that variation is the angle between True track (the reference) and Magnetic. Easterly (positive) variation, being when the Magnetic North is at the right of the True north.
              http://en.wikipedia.org/wiki/Magnetic_declination

              So, If I have a Magnetic heading of 110? with a variation +10 East, I should have a True track of 100?.

              In all the question of the INS PT03, it's always the opposite...

              I have the same problem for the deviation.

              ------------------
              Concerning the drift:

              Let's say I have a track of 360? with a 20?left (port) drift. I thus imagine the plane following relatively to the earth a True north and the nose of the plane shifted to the left by 20?. So the heading should be 340?...

              By doing this, my results are always opposite to the answers.


              I really can not just apply one of your mnemonic without understand. So please help me!
              Jean-Michel Maguin

              Comment


              • #8
                Re: variation and deviation

                Primax,

                I perhaps understand where your confusion is coming from.

                The signage '+' or '-' relates to a clock-face. As the fingers on the clock move clockwise, it's '+'; as the fingers on the clock move anti-clockwise, it's '-'. In this case, the clock is now a compass face.

                Therefore, W = '-' and E = '+'

                The convention of 'plus' (E) and 'negative' (W) are only applicable when mathematically converting from a Compass Heading through to a True Heading (CDMVT); not True to Comp.

                However, if you need to convert a True Heading to a Compass Heading (TVMDT), then you must change the signage in order to mathematically calculate the answer. For example, if your True Heading was 090?T and the Variation is 10?W (-10? by convention), the Magnetic Heading is actually 100?M... you've 'added' 10? to find the correct answer of 100?M.

                The logic of 'plus' or 'minus' is with reference to a clock-face of where the magnetic pole lies with respect to the true north pole (East or West; East being 'plus' and West being 'minus'). Conveniently, the calculation (when using the conventional signage) is correct when converting a Compass Heading to True Heading... but NOT in the case of making the mathematical calculation of True Heading to Compass Heading.

                My explanation is perhaps all a bit long-winded; but I hope that it helps you with your understanding of the very confusing convention of + or -.

                The 'drift' issue is perhaps a little easier to understand. If you have ten degrees of left drift (-10), then the wind must be from the right, so you must turn right to counteract the drift; add ten degrees. Remember, it's only a clock-face; anticlockwise it's minus; clockwise it's plus.

                Regards

                John

                Comment


                • #9
                  Re: variation and deviation

                  Thank-you for your answer.

                  I realise is it just a matter of convention. For example, I understood by "drift" a correction already applied to the aircraft, not a "force" to counteract. I should have the same kind of trouble with the True/magnetic conversion...

                  I saw your answer and here is my problem:
                  "The convention of 'plus' (E) and 'negative' (W) are only applicable when mathematically converting from a Compass Heading through to a True Heading (CDMVT); not True to Comp."

                  In the wiki page I linked in my previous post, there is a clock-like picture showing a Easterly/Positive variation. Looking it, it's clear to me that we:
                  - start form the True North (Ng)
                  - apply a 'positive' transformation to the East
                  - Find the Magnetic North (Nm)

                  Isn't that a True to Magnetic heading conversion ??

                  I know I am wrong but I can not figure out why. I probably have to admit that conventions on this topic are the opposite of my way of thinking...

                  Regards,
                  Jean-Michel Maguin

                  Comment


                  • #10
                    Re: variation and deviation

                    The compass that you're looking at within the 'Wiki' page is in fact a 'True Reading' compass face. The needle (roughly pointing in the direction of 030?T) is actually reading 000?M (Magnetic North). Therefore, try looking at the diagram as a Magnetic Compass, where the compass needle will be pointing at Magnetic North (000?); True North would then become 330?M (on the compass face). The variation is 30? East of True North, therefore, if we subtract 30? from our magnetic heading of 000?, the direction of True North becomes 330?M (000?T).

                    The 'Wiki' diagram is somewhat confusing, because it's a 'True Reading Compass' and therefore not representative of what you're trying to achieve. It's simply trying to show you the signage convention; which is positive when the magnetic north is east of true north.

                    Confusing huh?

                    Cheers

                    John

                    Comment


                    • #11
                      Re: variation and deviation

                      I first thought I understood because I totally agree with you. In your example, we have a 30?East of true north variation, thus, we substract 30? from our compass heading (000?) to obtain the True heading (330?).

                      Finally, for a Magnetic heading to True heading correction, with a +30?E variation, we made a subtraction.

                      Sorry to boring you but I?am still confuse when everybody agree with the opposite:


                      Originally posted by Positive_Rate View Post
                      The first thing I think of is CADET

                      Compass to True, ADd East. So, when going from Compass Heading to True Heading add easterly variation or deviation.
                      If I'm going the other way, converting from True Heading to Compass Heading then the opposite applies, just subtract eastely variation or deviation.
                      Jean-Michel Maguin

                      Comment


                      • #12
                        Re: variation and deviation

                        I found this in an other thread:

                        Originally posted by John H View Post
                        Hi malekien

                        I think you are getting confused the 'S' is for starboard not south ? With 11? W variation remember ''variation west magnetic best '' ie bigger number than true, so add 11? to 352? = 003? . With - 5? deviation this means westerly so is added to the magnetic heading to give 008? compass. The question tells you that you have drift 10?R. so you must turn 10? L to compensate. So you would fly a compas heading of 358? .

                        John H
                        According to john, a Westerly Variation (negative by convention) means that the Magnetic Heading is greater then True Heading... Just the contrary of this

                        I am deep inside the wtf
                        Jean-Michel Maguin

                        Comment


                        • #13
                          Re: variation and deviation

                          What I found in the progress test:

                          "Variation is the angle between True North and Magnetic North. Westerly variation means that Magnetic North lies to the West of True North - or True North lies to the East of Magnetic North."

                          It's seems that the "West Magnetic Best" mnemonic is wrong, isn't it?
                          Jean-Michel Maguin

                          Comment


                          • #14
                            Re: variation and deviation

                            Indeed, variation is the angle between True North and Magnetic North.

                            Westerly variation means that Magnetic North lies to the West of True North.

                            Therefore, if I have a Magnetic Heading of 090?(M) and the variation is 10?W; what is the True Heading? Ans = 080?(T). Variation West, Magnetic Best.. you've subtracted 10? (Var) from the magnetic heading to obtain the True Heading which is less than the Magnetic Heading.

                            If the True Heading is 090?(T) and the variation is 10?W; what is the Magnetic Heading? Ans = 100?(M). Variation West, Magnetic Best... you've added 10? (Var) to the True Heading to obtain the Magnetic Heading, which is greater than the True Heading.

                            Therefore, in both of the above statements, Variation West, Magnetic Best is correct. Don't be confused by the term 'best'. It doesn't mean that you always ADD the variation; it simply means that when the variation is westerly, the Magnetic Heading will always be greater than (better than) True Heading i.e. best!

                            When the variation is easterly, Variation East, Magnetic Least, means that the magnetic heading will always be less than the True Heading.

                            Regards

                            John

                            Comment


                            • #15
                              bearing

                              Given: Magnetic heading 280? VOR radial 090? What bearing should be selected on the omni-bearing selector in order to centralise the VOR deviation needle with a "TO" indication? the answer is 270 degree how do they get it?

                              Comment

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