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    Re: Hand Bearing Compass Deviation
    From: Gary LaPook
    Date: 2015 Jun 30, 23:36 -0700

    We have s problem with magnetic compasses in airplanes. Compasses turn to 
    align the needle with the local earth's magnetic field. Standing on the 
    ground the compass needle is balanced to stay horizontal so it is constrained 
    to sense only the horizontal component of the earth's field and is not 
    affected by the dip. Some high end hand held compasses have a weight on the 
    southern end of the needle to balance the needle and keep the north end from 
    tilting downward in response to the dip. (In the northern hemisphere.) In 
    some this is in the form of a piece of wire that can be moved in and out from 
    the pivot when the compass is taken to different locations where the dip is 
    different. Compasses for use in the southern hemisphere have the weight on 
    the other end of the needle. But, in flight, when the plane is turning it is 
    banked and the local level in the plane is also banked so the compass needle 
    is freed to align with the dip as well as the horizontal component which 
    causes erroneous readings. If you start on a heading of magnetic north and 
    then start a turn the compass indicates a turn in the opposite direction, it 
    "lags," approximately the amount of the magnetic latitude. In Chicago a turn 
    from north to the east initially indicates a turn of approximately 40 degrees 
    in the opposite direction, the compass showing 320°. This effect then slowly 
    is reduced as the turn progresses, the lag gets smaller and disappears as the 
    plane turns through east. On southerly heading the compass leads the turn and 
    so the compass appears to speed up and gets all the way to 220° by the time 
    the plane has reached only south.  This also occurs when the plane 
    accelerates or decelerates on east and west headings since the compass card 
    is also tilted due to the acceleration and can respond to the dip of the 
    magnetic field. Pilots are trained to deal with these errors and usually only 
    read the compass when the plane is in
     straight and level, un-accelerated flight.
    Back to your electronic compass. Since there is now way to ensure that the 
    sensor is held horizontally it might be responding to the local dip. 
    On Tue, 6/30/15, David Fleming  wrote:
     Subject: [NavList] Re: Hand Bearing Compass Deviation
     To: garylapook@pacbell.net
     Date: Tuesday, June 30, 2015, 9:26 PM
     Magnetic Fields are
     horizontal virtually nowhere on the earths surface. They
     have dip, ie point also in a vertical direction as well as
     horizontally.When you speak of near power lines do
     you mean seeing interferrence from AC or DC power lines.
     It is likely that needle compasses do not have high
     frequency response so they would not be sensitive to 60
     cycle- this is speculation needs to be verified.Flux
     gate compasses use 10s of KHz signals to sense the magnetic
     field. Obviously their designers are aware of this so I
     would expect they filter and diesign with 60 cycle
     interference in mind, so who knows.Dave F

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