The compass continues to operate despite failures in the electrical, vacuum or pitot static systems.
Compass turns are typically performed in simulated or actual failures of the directional gyro or other navigational instruments.
The bowl and pin assembly is enclosed in a case filled with non-acidic kerosene.
As the aircraft (and the compass housing) turns, the bowl remains somewhat stationary with respect to the Earth due to the magnetic attraction.
Thus the pilot will always ignore the magnetic compass while turning, but periodically check it in straight and level unaccelerated flight.
Several types of error will affect the heading indication provided by a magnetic compass if the aircraft is not in steady straight and level unaccelerated flight.
The compass dial will tend to align itself with the geomagnetic field and dip toward the northern magnetic pole when in the northern hemisphere, or toward the southern magnetic pole when in the southern hemisphere.
To help negate the effect of this downwards force, the center of gravity of the compass bowl hangs below the pivot.
[1] Compass navigation near the polar regions, however, is nearly impossible due to the errors caused by this effect.
When in steady straight and level flight the effect of magnetic dip is of no concern.
However, when the aircraft is accelerated or turned to a new heading the following two rules apply: First, when on an easterly or westerly heading and the aircraft accelerates, the center of gravity of the bowl lags behind the pivot, making it tilt forwards.
The error is neutralized when the aircraft has reached its velocity and the magnetic compass will then read the proper heading.
Pilots in the northern hemisphere remember this by the mnemonic ANDS: accelerate north, decelerate south.
The pilot community uses the mnemonic UNOS (undershoot North overshoot South) to memorize this rule in the Northern hemisphere.
A pilot would begin to roll the aircraft out of the bank at 308 degrees read from the compass to fly on a north heading.
For example, a left turn made from a heading of west to south east (SE).
The compass would initially show a heading that is correct as the turn gets closer to south the compass would indicate a lead heading of the greatest error, as the aircraft passes through south the error would decrease and show less of a lead.