Aircraft Flux Valve Operation and Heading Indication

Direct-Reading Compass

• 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 of American design and Suspended type of British design.

Construction of Direct-Reading Compass

• The compass consists of a non-magnetic metal or plastic case which houses the magnet system.
• The magnet system consists solely of a single annular cobalt-steel magnet, to which is attached a compass card.
• The suspension consists of an iridium-tipped pivot secured to the centre of the magnet system and resting in a sapphire cup supported in a holder or stem.

Magnetic Inclination

• The compass magnet system is mounted pendulously, with the magnet below the point of suspension to counter the effect of dip.
• Gravity acting on the magnet assembly pulls it into a smaller angle, reducing the apparent dip angle in aircraft compasses to approximately 3 degrees between the latitudes 60° north and south.

Compass Fluid

• The primary reason for filling compasses with a liquid is to make them aperiodic.
• Liquids used include mineral or alcohol, and some special silicone liquids.
• Desirable properties of a liquid include having a low freezing point, low viscosity, no corrosive properties, and remaining free from discolouration.

Compass Illumination

• Direct-reading compasses are illuminated by a small bulb for nighttime or low-light flying.
• Power to supply this is usually fed through either screened cable or a twisted pair to minimize the magnetic field produced by current flow in the wire.

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, and is measured during compass calibration.
• The adjustment phase of the calibration procedure is called compass compensation (compass swinging).

Compass Compensation

• Any components fitted to an aircraft that are magnetisable, i.e., contain iron, have the potential to distort the Earth's magnetic field.
• Every compass is fitted with a compensator which is calibrated during a compass swinging process to minimise the effects of the local magnetic disturbances.

Slaved Gyro

• Remote compass systems commonly use an electrically driven directional gyro, often remotely mounted, to provide short-term stability of magnetic heading presentation.
• The gyro slaving system detects any mismatch between the flux valve and the gyro heading output caused by real and apparent gyro drift.

Compass Indicating Elements

• Magnetic heading information is presented in a vertical card format on a variety of instruments depending on the system.
• The compass card is driven by a servomechanism which 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.### Magnetic Poles
• The magnetic pole in the northern hemisphere is actually a magnetic south pole, and vice versa.
• This explains why the north pole of a suspended bar magnet (or a compass needle magnet) points towards the north.
• The geographic and magnetic poles are not located together.
• The Earth's magnetic south pole is located at approximately 74°N 101°W, which is 2000 km from the geographic North Pole.

Magnetic Variation or Declination

• The grid of an aeronautical navigation chart is aligned to the geographic north pole (True North).
• A magnetic compass aligns to the Earth's magnetic flux, which may be oriented either side of true north (Magnetic North).
• The difference between true north and magnetic north at a geographic location is termed magnetic variation, variation, or declination.
• Variation is measured in degrees of error east or west of true north.
• Variation changes with geographic location, but it is not affected by aircraft heading.
• An installed compass and a compass that has been removed from the aircraft are both equally affected by variation.
• Magnetic variation maps show lines of equal variation, called isogonic lines (or isogonals).
• Isogonic lines enable pilots to determine the magnetic variation at a specific location.

East and West Variation

• There is no practical way to compensate (adjust) a direct reading standby compass for variation because variation changes as the aircraft travels.
• Variation is the difference between true north and magnetic north at a specific geographical location (aircraft position).
• Variation does not change as aircraft heading changes.
• A compass cannot be compensated for variation.
• The pilot must take variation into account and correct for it when navigating.

Magnetic Inclination or Dip

• In a magnet, the lines of force emerge vertically at the north magnetic pole and descend vertically into the south magnetic pole.
• The Earth's lines of force behave similarly, emerging at the South Pole and returning at the North Pole.
• The lines of force form great arcs around the surface of the Earth, being horizontal only at the magnetic equator.
• The angle the lines of force make with the Earth's surface at any given place is called magnetic inclination or magnetic 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.
• Acceleration error occurs when an aircraft in the southern hemisphere on an easterly heading increases its speed, displacing the magnet system and allowing it to rotate 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

• An instrument panel usually has a concentration of magnetic fields around it due to the location of electrical wiring looms in the vicinity of the cockpit.
• To eliminate the problem of interfering magnetic fields, the magnetic compass sensing element is mounted as far away as possible from the influence of soft and hard iron magnetism and electrical interference.
• A remote-indicating compass system consists of a magnetic field detector and a heading indicator.
• The system suffers from inherent problems, but the incorporation of a directional gyro provides short- and long-term reliability of the display of the aircraft's magnetic heading.

Flux Valve

• The flux valve 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 (Hooke's joint).
• The flux valve assembly 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, and a collector horn that acts as one of three individual flux collectors.
• The spokes and horns are manufactured from laminated Permalloy, a soft iron magnet.

Flux Valve Errors

• When the flux valve is exposed to the Earth's magnetic field, the collector horns help concentrate the magnetic field, which takes the path of least resistance through the legs.
• The direction and intensity of the magnetic field through each leg depend on the angular relationship of the flux detector to the Earth's magnetic field.
• The flux valve is subject to turning error and acceleration error, which are classified into two distinct errors.

In-Flight Errors

• In-flight errors are caused by the pendulous card being displaced from the vertical due to aircraft acceleration.
• The magnet system is influenced by the vertical component of the Earth's magnetic field and attempts to rotate to align with it, even though the aircraft's heading might not have changed.
• The effects of acceleration on the pendulous card can be classified into two distinct errors: turning error and acceleration error.
• Turning error occurs when an aircraft is in a coordinated turn, and the pendulous card remains vertical to the plane of the aircraft.
• Acceleration error occurs when an aircraft is accelerating or decelerating, and the card's inertia lags behind the change in acceleration.