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3 Answers

how do I explain magnetic dip

Asked by: 13358 views Instrument Rating

turning errors

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3 Answers

  1. Jon on Jul 31, 2012

    I think the section on wikipedia does a great job:


    especially when you pull out the chart from

    http://en.wikipedia.org/wiki/Magnetic_dip –>

    The summary as best as I can explain it is the north end compass points downwards in the north pole because the magnetic field lines point into the ground. To balance the compass needle manufacturers add a weight on the south end of the needle.

    Imagine flying on an easterly heading and your compass is pointing north. Now you accelerate and since the north end of your compass needle has less mass than the south end of your compass needle, the north end accelerates faster than the south end (Force=mass*acceleration rearranged is a=F/m… more mass means less acceleration).

    Looking at the compass top down orientated towards the north, it looks like the north end of the needle rotates clockwise and looking at the compass from inside the airplane it looks like we are turning towards the north as we accelerate. Hence “Accelerate -> North” and similarly, “Decelerate -> South” yielding the acronym ANDS.

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  2. Wes Beard on Aug 01, 2012

    There are two important things to remember to explain how magnetic dip affects the magnetic compass.

    1. The magnetic flux lines going from one poll to the next are not always parallel to the surface of the earth. These lines are almost purpendicular at the poles and parallel at the equator.

    2. The magnet inside the compass is always trying to align itself with the magnetic lines of the earth (and other metallic options). This means at the equator the magnet will try to remain parallel to the surface of the earth (which is what we want resulting in no error) and will try to position itself purpendicular at the poles. This is not what we want. Anytime we turn the magnet will swing and align itself better showing magnetic dip errors.

    It is this reason why we have the saying UNOS (or OSUN) meaning undershoot north and overshoot south. As we are turning towards north the compass will lag our heading by the same amount as our latitude above or below the equator. The compass will move faster as we approach a southerly heading by our latitude. For example if we are in the middle of Texas around 30° north latitude. As we turn north from a westerly heading (right hand turn), the compass will read 330 when we are pointing due north.

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  3. Brian on Aug 01, 2012

    This depends largely on your audience. For a private pilot, I stick with the standard acronyms: Overshoot north – Undershoot south (UNOS) || and || Accelerate North – Decelerate South (ANDS).

    More or less, my goal here is not to teach them how to handle these errors. Rather, to show they exist in easy to remember acronyms that can be expanded on later. As private pilots I feel it is enough, at this level they can stop turning and wait a few seconds to read their heading on the off 1 in a million chance they ever use the darn thing.

    For the IFR pilot I feel it is imperative to not only know of their existence, but to understand how to cope with them should they ever find themselves partial panel IMC.

    I do this by drawing a circle with the following heading marks: 360-30-60-90-120-150-180-210-240-270-300-330. Next to each heading mark I note the degrees to lead or lag a turn, 0 for east/west, 10 for 30 north and south of east/west (300/240/60/120 all get 10s), 20 for 60 north and south of east/west (330/210/30/150 all get 20s), and a turn to north or south (360/180) get 30s.

    This may seem complex, but take a moment to get it drawn on paper and it’s pretty easy. Now we talk about it and come up with a half dozen or so examples that we will fly during the flight to prove this diagrams usefulness. For instance, if we are making a left turn from 180 to 330 we know that is a northerly heading and requires us to undershoot. Our diagram says we undershoot by 20 degrees, so we should turn to 3500 and level off. Try this example out, come up with some more of your own examples, and see what happens.

    Point is, for this level of pilot I look for application and a deep understanding of the knowledge. Since, of course, this level of pilot may someday need to apply such knowledge single pilot IMC with a vacuum failure.

    On a side note, I try to avoid the theory behind the why that has been largely touched on in replies thus far. Not because it’s not good knowledge, but I can’t find a practical use for it. To me it’s merely good trivia for the pilots lounge. Where as what is presented above can help reduce stress, reduce workload, and give some precision to a not so precise real world emergency (partial panel IMC). My opinion and two sense of course. 🙂

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