Direct Reading Magnetic Compass Quiz & Flashcards

Magnetic Principles

The primary function of a magnetic compass is to display the direction in which the aircraft is heading with respect to the Earth's magnetic flux.
The Earth's magnetic field is surrounded by a weak magnetic field, which terminates in two internal magnetic poles situated near the north and south geographic poles.
The total magnetic force is resolved into a horizontal component and a vertical component, with only the horizontal component being useful in determining magnetic heading.

Terrestrial Magnetism

The Earth's magnetic field is similar to a huge permanent magnet with a magnetic north and a magnetic south pole.
A freely suspended permanent magnet on the surface of the Earth will align itself with the lines of flux linking the two magnetic poles.
The magnetic pole in the northern hemisphere is a magnetic south pole, and vice versa.
The geographic and magnetic poles are not located together, with the magnetic south pole located at about 74°N 101°W.

Magnetic Variation or Declination

Magnetic variation is the difference between true north and magnetic north at a specific geographical location.
Magnetic variation is measured in degrees of error east or west of true north.
Variation changes with geographic location but is not affected by aircraft heading.
A compass cannot be compensated for variation, and the pilot must take variation into account and correct for it when navigating.

Magnetic Inclination or Dip

The lines of force of the Earth's magnetic field emerge vertically at the north magnetic pole and descend vertically into the south magnetic pole.
The angle the lines of force make with the Earth's surface at any given place is called magnetic inclination or dip.
The angle varies from zero degrees at the magnetic equator to 90 degrees at the magnetic poles.

Direct-Reading Compass

A direct-reading compass is a device that incorporates both the magnetic sensing element and display element in one component.
The primary function of a direct-reading magnetic compass is to show the direction in which an aircraft is heading with respect to the Earth's magnetic meridian.
There are two common types of direct-reading compasses: vertical panel mounted compass and suspended compass.

Compass Construction

A compass consists of a non-magnetic metal or plastic case, a magnet system, and a compass card.
The magnet system consists of a single annular cobalt-steel magnet attached to a compass card.
The compass card is graduated in 5- or 10-degree increments and is referenced against a lubber line fixed to the interior of the bowl.

Magnet System Assembly

The compass magnet system is mounted pendulously to counter the effect of dip.
The magnet system is counterbalanced for the region in which the compass is operated.
A compass from overseas must be calibrated for operation in southern latitudes before use on Australian aircraft.

Compass Fluid

The primary reason for filling compasses with a liquid is to make them aperiodic.
The liquid is used to dampen the motion of the magnet system and reduce the effects of friction and wear.
The liquid is also used to steady the magnet system and give it buoyancy.

Liquid Volume Compensation

Compass liquids are subject to expansion and contraction with changes in temperature.
The resulting changes in their volume need to be accommodated by a flexible element such as a bellows or a corrugated diaphragm.

Compass Illumination

The direct-reading compass is illuminated by a small bulb for night-flying or flying in low light situations.
Very low power lighting is used to avoid affecting the accuracy of the compass.

Deviation

Deviation is an error in the indication of an installed magnetic compass caused by local magnetic fields in the aircraft.
Deviation error changes as aircraft heading changes.
Deviation error is measured during a compass calibration and can be minimised by adjusting the compass.

Compass Compensation

Deviation is caused by magnetisable components fitted to an aircraft.
The compensator is calibrated during a compass swinging process to minimise the effects of the local magnetic disturbances.
The compensator on direct-reading compasses is a mechanical device that produces a magnetic field to correct for deviation.

In-Flight Errors

In-flight errors are caused by the acceleration of the aircraft during turns and changes in speed.
Two types of errors are introduced: turning error and acceleration error.
Turning error occurs when the pendulous card is displaced from the vertical due to the force of centrifugal acceleration produced by the turn.
Acceleration error occurs when the card's inertia lags behind the change in acceleration, causing the card to tilt and the magnet assembly to rotate.### Acceleration Error
When an aircraft in the southern hemisphere increases its speed on an easterly heading, the acceleration force displaces the magnet system, rotating it in an anticlockwise direction, indicating an apparent turn towards the south.
When the aircraft decelerates, the reverse action takes place, and the effect is for the magnet system to rotate in a clockwise direction, giving an apparent turn to the north.

Remote-Reading Compass

A remote-reading compass system consists of a magnetic field detector and a heading indicator.
The system suffers from inherent problems, including instability during turns and acceleration changes.
To improve the system, a directional gyro is incorporated, providing short-term stability and reliability of the display of the aircraft's magnetic heading.

Flux Valve

A flux valve is a magnetic field detector that detects the Earth's magnetic field as an electromagnetically induced voltage.
The sense element is pendulous and suspended from a central point of the case by a universal joint, allowing it to tilt up to 25°.
The sensing element is enclosed in a sealed bowl and immersed in a damping oil to minimize pendulous jarring caused by rapid attitude changes.

Flux Valve Construction

The flux valve consists of a central point or hub, to which three spokes are attached, set 120° apart.
Each spoke has a top and bottom leg, insulated from each other, with a collector horn that acts as a flux collector.
The spokes and horns are manufactured from laminated Permalloy, a soft iron magnet material.

Flux Valve Operation

When the flux valve is exposed to the Earth's magnetic field, the magnetic field in each spoke induces a voltage into each of the secondary windings.
The change in voltage output from the secondary coils is modified by the influence of the Earth's magnetic field, producing an output indicative of the aircraft's heading.

Slaved Gyro

A slaved gyro is an electrically driven directional gyro that provides short-term stability of magnetic heading presentation.
The gyro is remotely mounted and maintains the inner gimbal perpendicular to the outer gimbal.
The gyro erection or levelling system uses a position switch or gravity sensing mercury switch to maintain the inner gimbal in the correct position.

Compass Indicating Elements

Magnetic heading information is presented in a vertical card format on a variety of instruments.
The compass card is driven by a servomechanism that rotates in response to heading changes.
The aircraft's magnetic heading is read off under the lubber line placed at the top of the dial.

Aircraft Magnetism and Its Effect on Compasses

Aircraft have magnetism in varying amounts, which can be divided into hard iron and soft iron magnetism.
Hard iron magnetism is present in iron and steel parts of the aircraft structure, which become magnetised due to the Earth's magnetic field.
Soft iron magnetism is caused by metallic parts of the aircraft becoming magnetised due to induction by the Earth's magnetic field.

Deviation Coefficients

Deviation coefficients (A, B, C, D, and E) are used to quantify the effects of hard and soft iron magnetism on compass readings.
Coefficient A refers to installation error and is corrected by rotating the compass or flux valve until the compass reads the corrected heading.