Compass Errors on Take-off and Turn at 62°S
An aircraft is operating from an airport at 62°S. The active runway is 27 (magnetic), and the crew is cleared for take-off. During the take-off roll, with acceleration about 0.35 g, the standby magnetic compass begins to indicate a steady left turn, even though the aircraft is tracking the centreline. After lift-off, the crew turns right to intercept a northerly track. As the turn passes through north, the compass reads about 020° while the standby horizon and the slaved compass system show 350°. The first officer suggests rolling out on the magnetic compass. The aircraft has a serviceable AHRS with a flux valve, and the captain's PFD heading agrees with the standby horizon. Magnetic variation is 18°E. The airport has a 2,400 m runway and no nearby alternates.
1. Explain the compass indication during the take-off roll and state what the crew should use as the heading reference.
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On the take-off roll the aircraft is accelerating on a westerly heading. The magnetic compass card is affected by the vertical component of the Earth's field, which is large at 62°S. The acceleration produces an apparent precession of the card, and in the Southern Hemisphere the indicated error is a turn toward south. From heading 270, south is to the left, so the compass shows a steady left turn. The magnitude is greatest on east-west headings and zero on north-south headings, so the effect is strong here. It is not a real turn. The crew should ignore the compass during the roll, keep the aircraft on the centreline, and use the AHRS heading or the runway direction. After lift-off, the compass should settle in straight and level flight; if it does not, suspect a compass fault or a nearby magnetic disturbance.
2. Explain the compass reading during the turn through north and whether rolling out on the magnetic compass would be correct.
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The turn is passing through north, where the Southern Hemisphere turning error is largest and the compass leads the actual heading. The compass reads 020° while the true heading is 350°, an over-read of about 30°. If the first officer rolled out when the compass showed 360°, the aircraft would actually be on about 330°, so the aircraft would under-turn and end up west of the intended track. The correct action is to use the AHRS heading as the primary reference, roll out on 360° by the AHRS, then allow the compass to settle in straight and level flight and compare it with the AHRS. Any residual difference can be noted as a compass deviation. The magnetic compass is only reliable in steady, unaccelerated flight, away from magnetic disturbances, and should not be used as the primary turn instrument.
3. Why does the slaved compass system not show the same errors, and what are its limitations?
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The slaved compass system uses a flux valve, usually in a wingtip or fin, to sense the horizontal component of the Earth's magnetic field. The flux valve is not tilted with the aircraft's manoeuvres in the same way as a wet compass card, so it does not suffer the same dip, turning, and acceleration errors. Its signal is used to slave a directional gyro, which provides a stable heading. However, the system has limitations: local magnetic anomalies, electrical loads, or a flux valve failure can cause errors. If the flux valve fails, the system may revert to a free-gyro mode, and the heading will drift and must be updated manually. The AHRS in this aircraft provides heading from a gyro with magnetic slaving; if the magnetic reference is lost, it may use a free gyro or IRS heading. The captain's PFD agreeing with the standby horizon supports the AHRS, but the crew should still cross-check the standby compass in steady flight.