Magnetic Compasses and Heading References
Domain 5 covers the direct-reading magnetic compass and the remote-indicating systems that replaced it as the primary heading source. The exam tests the compass errors, the flux-valve/slaved-gyro arrangement, and the arithmetic of variation and deviation.
Direct-reading compass
Magnets attached to a float in a fluid-filled bowl, pivoted so the assembly aligns with the horizontal component of the earth's field. The fluid damps oscillation and supports the float.
Errors
- Variation - the angle between true and magnetic north at the aircraft's position; taken from the chart.
- Deviation - aircraft-induced field error, recorded on the deviation card.
- Dip (inclination) - the vertical component of the field pulls the north end down; worse at high latitude. It causes turning error and acceleration error.
- Turning error, northern hemisphere - turning through north the compass lags (under-reads); through south it leads. Reversed in the southern hemisphere.
- Acceleration error, northern hemisphere - accelerating on an east or west heading produces an apparent turn towards north; decelerating gives a turn towards south.
- Oscillation - the compass swings about its final heading after a turn.
Remote-indicating compass
A flux valve (flux gate) senses the horizontal component of the earth's field and sends an AC signal to a slaving amplifier that torques a directional gyro; the output feeds the HSI or ND as magnetic heading. It is more stable than the direct compass and largely free of dip errors, but the gyro can drift if slaving fails.
Worked example
True heading 045°, variation 12°W, deviation 4°E on that heading. True → magnetic: add westerly variation → 057°M. Magnetic → compass: apply deviation, subtracting easterly → 053°C. Cross-check with "variation and deviation east, magnetic and compass least".
Traps that cost marks
- Adding when you should subtract - always state which conversion you are performing.
- Confusing variation (position, from the chart) with deviation (aircraft, from the card).
- Getting the turning-error direction backwards between hemispheres.
- Assuming a flux valve senses the total field; it is designed around the horizontal component.