Magnetic Compass Errors: Variation & Deviation

today we will talk about the magnetic compass errors specifically in this video about the magnetic variation and deviation the thing is that as we mentioned in the previous video the magnetic compass is basically a magnet that is free to rotate about a pivot point and therefore since it has such a simple design it has certain inherent errors that pilots should be aware of these errors are magnetic variation compass deviation and magnetic dip which in turn is divided into acceleration error and turning error so in this particular video we will deal with the variation and deviation errors but before going into detail with this let's remember how does a compass work real quick the magnetic compass is an instrument used to determine the orientation or heading in relation to the earth's magnetic north and it manages to do so by aligning itself with the earth's magnetic field lines within this definition we already have the basis for understanding the first type of error and it is that the compass points to magnetic north not geographic north now the earth's magnetic north is the point through which the magnetic field lines enter the planet and therefore it is the point to which all compasses point the thing is that although they are relatively close magnetic north does not coincide with true north in other words the geographic north pole and the magnetic north pole are located in different places so we would have something like this here the green point which represents the earth's geographic north is the one which is aligned with the earth's axis of rotation and it is also the point where all meridians converge while the red point represents the earth's magnetic north the point towards which all compasses point now another characteristic of magnetic north is that it does not remain static in the same position but moves over the years in these images we can see how the magnetic north pole has been moving since the 1900s from the northern part of canada towards siberia now this difference between the position of geographic and magnetic north results in some problems for navigation for example geographic north is used as a reference in navigation charts since it is the point where all meridians converge and these in turn are used to determine the course to be flown the problem is that when flying we will use a compass to follow the planned course which will point to magnetic north not geographic north this means that we will have to correct the planned course according to the difference in the position of the magnetic and geographic poles this difference between both poles is known as the magnetic variation abbreviated as mag var and sometimes referred to as magnetic declination it is defined as the angle between true or geographic north and magnetic north this variation can be to the east or to the west depending on the relative position of the poles which at the same time depends on our position on earth this may sound a bit confusing so far so let's see an example let us suppose that we are at the point marked with the white cross in this case if we draw a straight line to the true north and another one to the magnetic north the angle between these two lines would be the magnetic variation now in this particular case magnetic north is to the left of true north or in other words it is to the west of true north then we say that we have a west variation now as we previously said this variation will vary depending on our position on earth let's see why through this example here we are looking at the earth from a top point of view the red point represents the geographic north pole and the yellow one represents the magnetic north pole with this in mind suppose we are at the point marked with the white cross if from this position we look at the poles we can see that even though they are not in the same place they are in the same direction then we say that in this case the magnetic variation is zero since there is no angular difference between both poles now if we move into this new position and look at the poles we can see that now there is an angular difference between the lines in this case the magnetic north is to the left of true north or in other words the magnetic north is to the west of true north then we say that we have a west variation the same happens if we move into this other position from this point the magnetic north is also west of true north and therefore we still have a west variation and the same happens in this other position however in this other position we can see that once again there is no angular difference between both lines which means that we have zero variation and for example in this other position we have now the magnetic north to the right of true north or in other words to the east which means that in this case we have an east variation so according to what we have just seen if we have an east variation this means that the magnetic north is to the east of true north as we can see in this examples on the other hand if we have a west variation it means that the magnetic north is to the west of true north as we can see here now in case we have a zero variation this means that the magnetic north and true north are in the same direction and therefore there is no angular difference as we can see in this examples so far we have seen the general concept of magnetic variation but how do we know what is the actual value of the magnetic variation in a certain place well to do so a complete model of the earth has been created with which it is possible to determine the magnetic variation at any point on the planet here each of these lines represent a specific magnetic variation angle and these are known as isogonic lines since they connect points of equal magnetic variation we will find these lines on the navigation charts represented by dashed lines with their corresponding magnetic variation value for example in the image on the left we can see that the isogonic line represents a magnetic variation of 15 degrees west in other words if from that area we observe both poles we will see that the magnetic north is 15 degrees to the west of true north on the other hand in the image on the right the isogonic line represents a magnetic variation of 14 degrees east which means that from that area the magnetic north is 14 degrees to the east of true north now in this video we will not see in detail the exact procedure by which the planned course is corrected according to the magnetic variation however here is a short overview the magnetic courses obtain dating or subtracting the magnetic variation to the planned true course if we have a west variation we have to add and if we have an east variation we have to subtract we can easily remember this with the phrase east is least and west is best but again we will see this procedure in detail in the videos about general navigation with this being said let's move on to the next compass error the compass deviation this error is caused because any nearby magnetic interference will cause the compass to deviate and give an erroneous indication so as we know a compass is basically a magnet that uses the earth's magnetic field to align itself properly and point to magnetic north this implies that any external magnetic interference will cause it to deviate thus pointing to a kind of new north this new north to which the compass points is known as compass north and we will abbreviate it as nc now obviously this compass deviation constitutes an error in the heading indication so in order to correct it we have to see the concept of deviation angle this is the angle between magnetic north and compass north in other words the angle between the magnetic north and where the compass is actually pointing it is important to mention that this angle is not always the same since it will depend on each aircraft the magnitude of the magnetic interference present and the heading in general terms the major magnetic interference in an aircraft is caused by the engine and the avionics equipment therefore the compass deviation will depend on the position of the engine and avionics in relation to the magnetic north fortunately the interference produced by the different components of the aircraft can be measured and corrected for each heading however there are other sources of magnetic interference that may be present such as cell phones and other portable electronic devices this is one of the reasons for restricting the use of these devices during the flight now the question is how do we know how much is the compass deviation for a particular aircraft and heading well for that there is the compass deviation card this is a card which includes compass deviation information for a particular aircraft and for different headings in this example in the upper row we find the desired magnetic heading while in the lower row we find the corrected heading to be used taking into account the compass deviation therefore the difference between these two values will be the deviation angle for that particular heading this card is normally located in the cockpit so that it is visible to the pilot for example just below the compass but let's look a little bit more in detail this card as we just said before in the upper row we find the desired magnetic heading while in the lower row we find the corrected heading to be used in the compass from another perspective we could say that in the upper part we find the heading in relation to the actual magnetic north while in the lower part we find the heading in relation to the compass north for example in this card if we want to fly with a north heading which is zero degrees we have to fly with our compass indicating heading zero zero five this means that in this case the compass deviation is plus five degrees if we want to fly heading zero nine zero we have to fly with a compass indication of zero eight eight so in this case the deviation is minus 2 degrees for heading 1 8 0 the deviation is 0 degrees and for heading 2 7 0 the deviation is plus 2 degrees now sometimes the compass deviation may be expressed in terms of east or west instead of plus or minus here we apply the same rule as with the magnetic variation east is least and west is best so in this example these would be the compass deviations expressed in terms of east and west now by this point we might be wondering how are these deviation values determined and published in this card well at some airports there are specific positions where maintenance personnel can calibrate the compass and register the information into the compass deviation card here the aircraft is aligned with different headings in a compass rose marked on the ground with the engine running then the maintenance personnel registers the different compass readings and then determine the compass deviation for each particular heading these procedure must be accomplished at regular intervals or when new equipment is installed on the aircraft in summary then the magnetic variation is the angle between true north and magnetic north and it can be either east or west depending on our position on earth on the other hand the compass deviation is the angle between magnetic north and compass north and it depends on each aircraft and its current heading usually these errors are manually corrected by the pilot during flight planning and although we are not going to go into detail with this procedures since they will be explained in the general navigation videos let's see a simple example suppose we want to fly from santa juana to servio tulio in this case to measure the course we can use this meridian and as we already know meridians are aligned with the geographic or true north so if we measure for example an angle of 120 degrees this will be the course in relation to the true north and it is called true course the problem here is that we cannot fly with one two zero indicated on the compass since the compass is not aligned with the true north but with the magnetic north therefore we have to apply the magnetic variation correction to do so we can see that there is an isogonic line close to the root which represents a magnetic variation of 7 degrees west this means in other words that in this area the magnetic north is seven degrees west of true north so as we have a seven degrees west variation we have to add that value to the true course of one two zero thus obtaining a magnetic course of one two seven however we still cannot use one two seven as reference in the compass since we haven't yet applied the compass deviation correction to do so let's suppose this is the compass deviation card for this aircraft so for a desired magnetic heading of one two seven the closest published value is zero nine zero and for that heading the compass deviation is plus three degrees which can also be expressed as three degrees west what this means is that the compass north is three degrees west of the magnetic north as we can see in the example with this in mind and having applied all the necessary corrections we obtain a compass heading of one three zero and therefore this will be the heading to be adjusted on the compass while flying this leg this was just a quick example of how the variation and deviation are used in the practice however these corrections are normally applied using a vfr navigation log while planning the flight since there are other variables to be taken into account such as the wind or the true airspeed we will deal with that in detail in the general navigation videos later on i hope the information presented in this video was useful if so don't forget to share like subscribe and leave a comment down below thanks for watching [Music]

Channel: Aviation Theory

Views: 47,091

Keywords: Aviation Theory, Aviation, Airplanes, Instruments, Navigation, Aerodynamics, ATPL, CPL, PPL, Tutorial, Explanation, Pilot, Aircrafts, Knowledge, Compass, Magnetic, Magnetic Compass, Errors, Acceleration, Turning, Variation, Deviation, Card, Correction, Heading, Course

Id: fxveRt_1ThM

Length: 15min 35sec (935 seconds)

Published: Wed May 26 2021