In aviation, aircraft compass turns are turns made in an aircraft using only a magnetic compass for guidance.
Description
A magnetic compass aboard an aircraft displays the current magnetic heading of the aircraft, i.e., the aircraft's directional orientation relative to the Earth's geomagnetic field, which has a roughly north-south orientation. The compass can be used in turns to verify the aircraft is travelling in the desired direction at the conclusion of a turn. The nature of the instrument and the alignment of the magnetic pole of the earth cause the magnetic compass to have several significant limitations when used for navigation. A pilot aware of those limitations can use the compass effectively for navigation. The compass continues to operate despite failures in the electrical, vacuum or pitot static systems.
Compass turns (turns using the magnetic compass as the primary reference instrument) are not standard practice in modern aircraft. Compass turns are typically performed in simulated or actual failures of the directional gyro or other navigational instruments. A magnetic compass is a simple instrument when the compass is not moving and is on the earth. A magnetic compass installed in an aircraft is subject to compass turning errors during flight. Pilots must compensate for such errors when using the magnetic compass.
Most of the errors inherent in the heading indications of a magnetic compass are related to the compass' construction. An aircraft compass consists of an inverted bowl with a magnetized bar attached. The bowl is balanced on a low friction pin. The bowl and pin assembly is enclosed in a case filled with non-acidic kerosene. The magnetized bar tends to orient the assembly with the local geomagnetic field. The bar turns the visible bowl of the compass. The outside surface of the bowl includes markings to indicate a magnetic heading. As the aircraft (and the compass housing) turns, the bowl remains somewhat stationary with respect to the Earth due to the magnetic attraction. In summary, the aircraft is free to turn around the stationary bowl.
The standard practice when flying with a gyro-stabilized compass (or heading indicator) is to read the magnetic compass only while in straight and level unaccelerated flight. This reading is then used to set the gyro-stabilized compass. The gyro compass will read correctly in a turn, whereas the magnetic compass can't be read properly while turning. Thus the pilot will always ignore the magnetic compass while turning, but periodically check it in straight and level unaccelerated flight.
Compass errors
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.
Pitch limits
A limitation imposed by a compass' construction is that the balancing bowl's pin, which is connected to a pivot point, only allows, in most compasses, the bowl to tilt by approximately 18 degrees before it will touch the side of the casing. When this happens its freedom to rotate is lost and the compass becomes unreliable.
Magnetic dip
A second limitation is magnetic dip. 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. At the equator this error is negligible. As an aircraft flies closer to either pole the dipping error becomes more prevalent to the point that the compass can become unreliable because its pivot point has surpassed its 18 degrees of tilt. Magnetic dip is caused by the downward pull of the magnetic poles and is greatest near the poles themselves. To help negate the effect of this downwards force, the center of gravity of the compass bowl hangs below the pivot. 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.