# Aircraft Magnetic Field Effects: Deviation, Compensation, and Compass Swing

> Learn how aircraft-induced magnetism creates compass deviation, and how swing, compensation, and deviation cards are used to maintain heading accuracy.

- **Subject:** 022 — Instrumentation
- **Section:** 022-03 — Magnetism – Direct Reading Compass and Flux Valve
- **Topic code:** 022-03-02
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/instrumentation/magnetism-direct-reading-compass-and-flux-valve/aircraft-magnetic-field/

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The aircraft itself generates a magnetic field due to its metal structure, electrical circuits, and engines. This disruptive field affects the compass accuracy.

## Compass Deviation
**Deviation** is the angular difference between **Magnetic North** and the **Compass Heading**. It is the error caused by the aircraft's own magnetism.
*   **Deviation West (-):** Compass points West of Magnetic North ("Compass Best").
*   **Deviation East (+):** Compass points East of Magnetic North ("Compass Least").

## Types of Magnetism
1.  **Hard Iron Magnetism:** Permanent magnetism acquired by the aircraft's steel components during manufacture or due to lightning strikes/maintenance. It is relatively constant.
2.  **Soft Iron Magnetism:** Temporary magnetism induced in the aircraft's structure by the Earth's field. It changes as the aircraft changes heading or latitude.

## Compass Swing
A **Compass Swing** is a calibration procedure performed to measure and correct deviation.
*   **Process:** The aircraft is positioned on a known heading (using a calibrated "Compass Rose" on the ground). Engines and electrical systems should be **running/on** to simulate flight conditions.
*   **Correction:** Adjustable magnets in the compass housing are used to cancel out most of the deviation (Hard Iron).
*   **Residual Deviation:** The error that cannot be removed is recorded on a **Deviation Card** placed near the compass.

**When is a Compass Swing required?**
*   After long-term storage (aircraft stationary > 1 year).
*   After a lightning strike.
*   After significant changes to electrical systems/avionics.
*   After maintenance involving magnetic materials.
*   When errors are suspected.

## Conversion Formula (Cadet)
To convert between headings, use the mnemonic **C-D-M-V-T**:
**C**ompass $\pm$ **D**eviation = **M**agnetic $\pm$ **V**ariation = **T**rue.
*(East is Plus, West is Minus when going Left $\rightarrow$ Right).*

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.