equator and magnetic poles, we say that the lines of magnetic flux
have both a horizontal and vertical component, meaning they are
aligned with the poles (horizontal component), but also bend
toward the ground (vertical component).
The vertical components of the Earth’s lines of magnetic flux
are responsible for a phenomenon we call magnetic “dip,” and the
corresponding “dip errors” that occur in flight. When a magnetic
compass is oriented level with the surface of the Earth, the magnetized needle aligns itself with the horizontal component. However,
if we tilt the compass from the horizontal, such as when we bank
the aircraft to turn, the compass needle will tend to also align itself
with the vertical component of the magnetic lines of flux. Here in
the Northern Hemisphere, that means the north-seeking end of the
compass needle will tend to point toward the ground. That causes
it to deviate from its proper alignment with the horizontal component of the flux, which is what we’re really interested in.
Now that we grasp the basic problem of magnetic dip, let’s look at
what happens when we initiate a turn from a northerly heading.
If we make a right turn (toward the east), the north-seeking end
of the needle tends to point down as it aligns with the vertical
component of magnetic flux. This causes the compass to indicate
a turn toward the north. The net result is that the compass indication lags the actual turn. Eventually it will catch up, but especially
in the early part of the turn, it appears to be “stuck.”
A handy way to remember what error will occur
when flying by the magnetic compass is “Lags
ANDS Leads.” In the north, the compass lags.
In a turn to or from the south, it leads.
Turning to or from a southern heading has the opposite effect.
If we make a left turn from a southern heading, the north-seeking
end of the compass needle tends to point upward, which causes
the needle to indicate a turn faster than the actual turn. In other
words, the compass leads the turn.
Dip errors happen not only when we bank the aircraft but also
when we accelerate the aircraft on an east or west heading. This
is because the needle rests on a precision pivot. In unaccelerated
flight, it will float in a level condition. When we accelerate, the
needle (or compass card) will tend to tip back, just as our head
might, in response to the acceleration. During a deceleration, the
compass card tips forward, again as our head might. Here comes
the dip. On an east or west heading, the north-seeking end of the
magnetized needle is pointing either left or right. If we accelerate
in this orientation, the compass card tips back, allowing the needle to align itself with the vertical component of magnetic flux.
The same goes when we decelerate.
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