Magnetism

Understanding Magnetism in Aircraft Instruments

Magnetism underpins several core aircraft systems, most notably the direct reading magnetic compass and remote indicating compass systems used across PPL, CPL, and ATPL training. The Earth’s magnetic field varies with location, and the inclination (magnetic dip) increases with magnetic latitude, strongly influencing compass behavior. As dip grows toward the poles, turn and acceleration errors become more pronounced. Because the local magnetic environment also changes, a direct reading compass should be swung (checked and compensated) whenever there is a large, permanent change in magnetic latitude, and in accordance with manufacturer instructions and aviation regulations after maintenance or major modifications.

Two operational error families dominate compass use in the Northern Hemisphere. First, acceleration error: on easterly or westerly headings, acceleration causes an apparent turn toward the North and deceleration toward the South (ANDS). That’s why an aircraft accelerating on 045° can read below the runway heading, and why a take-off or landing on a westerly heading shows an apparent turn north on the take-off roll and south during deceleration after landing; on a northerly heading, no apparent turn is seen during deceleration. Second, turning error: due to dip, the compass lags/leads during turns, increasing with magnetic latitude and bank angle. The classic Northern Hemisphere rule is UNOS—Undershoot North, Overshoot South—meaning you must stop the turn early when rolling out on northerly headings and slightly late on southerly headings. For example, to roll out onto 180° with 20° of bank near 20°N, you’d anticipate the lead/lag and stop the turn around 200°.

Compass accuracy is further affected by aircraft magnetism (deviation). Hard iron magnetism is permanent; soft iron magnetism is induced and varies with the local field. Compensator magnets reduce residual deviation, and the remaining values are recorded on the compass deviation card, which you use to derive compass heading from magnetic heading (and vice versa) during procedures. To minimize interference from airframe and electrical systems, remote indicating compasses place the detector (flux valve) in a low-disturbance area such as a wingtip; the flux valve electrically drives indicators (e.g., RMI/HSI) for improved accuracy and stability.

Magnetism also appears in other aircraft systems. A magnetic circuit-breaker provides rapid (quick trip) protection by using magnetic force to open the circuit under overcurrent. Engine RPM indicating systems can be magnetic: an electronic tachometer sensor often uses a notched wheel passing an electromagnet (variable reluctance pickup) to produce pulses proportional to speed, while some installations use a three-phase tachometer system comprising a three-phase generator, a synchronous motor, and a magnetic tachometer. Understanding these devices helps you connect theory to cockpit procedures and aligns with ATPL systems knowledge and operational best practices.

Topics covered in this question bank

• Magnetic dip, magnetic latitude, and their effect on compass errors
• Compass swinging: when and why to swing; regulatory and procedural considerations
• Acceleration and turning errors in the Northern Hemisphere (ANDS and UNOS), including rollout techniques and the influence of bank angle
• Hard vs. soft iron magnetism, compensation, and use of the compass deviation card
• Remote indicating compass systems: flux valve placement (e.g., wingtip) and integration with aircraft instruments
• Magnetic circuit-breakers and magnetic/electronic tachometer principles
• Procedural examples for take-off, landing, and turning using a direct reading compass