EAA Sport Aviation - October 2011

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.

TURNING ERRORS

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.

ACCELERATION ERRORS

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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