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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. 

gl
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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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