An aviation compass deviation card is a cockpit reference document that records magnetic compass deviations for specific aircraft headings, helping pilots correct their compass readings. It is a small but vital tool that supports accurate navigation by accounting for magnetic errors introduced by the aircraft’s own structure and electrical systems. For pilots, understanding how to interpret and apply the card is a routine part of preflight preparation and in-flight decision-making. This guide explains the purpose of the deviation card, how deviations arise, how to read and apply the card, and how this information fits into broader compass checks and instrument crosschecks.
What Is a Compass Deviation Card
A compass deviation card is a compact reference, often mounted near the compass or included in the aircraft’s flight manual, that lists deviation values for a range of headings. Each entry on the card shows how much the magnetic compass differs from magnetic north for that specific heading, due to magnetic influences from the airframe, wiring, and installed equipment. Pilots use these values to adjust their heading readings, turning the observed compass heading plus or minus the deviation to arrive at a corrected heading. The card is tailored to a particular aircraft and should be updated whenever compass installation, avionics, or major airframe changes occur.
How Deviations Occur in Aircraft
Deviation is distinct from variation, which is a geographic difference between true north and magnetic north. Deviation arises from local magnetic fields generated by the aircraft itself, including ferrous metals, batteries, alternators, magnetos, wiring harnesses, and avionics installations. Because these influences vary with aircraft configuration—such as gear and flap settings, fuel load, and even the presence of maintenance tools—deviation can change slightly across conditions. A well-executed compass swing, performed by a qualified technician, quantifies these effects at multiple headings and produces the data used for the deviation card.
Key Sources of Magnetic Deviation
- Ferrous airframe components and control cables
- Electrical wiring and current in bus bars
- Magnetos, starters, and generators
- Battery installations and bonding practices
- Avionics racks and handheld radio emissions
How to Read and Apply a Deviation Card
Reading a deviation card typically involves locating the heading nearest to your current magnetic heading, then applying the listed deviation to obtain a corrected heading. Deviations to the east are usually positive (add) and deviations to the west are usually negative (subtract), but pilots should verify the card’s own labeled convention, because practices can differ. The table below illustrates a typical format and the kind of values you might see in a light single-engine aircraft.
| Magnetic Heading | Deviation (°) | Corrected Heading | Source Type |
|---|---|---|---|
| 000° | +3° | 003° | Compass swing |
| 090° | -2° | 088° | Compass swing |
| 180° | +4° | 184° | Compass swing |
| 270° | -1° | 269° | Compass swing |
Example Correction Workflow
- Note your observed magnetic heading from the compass.
- Find the nearest heading on the deviation card and read the deviation value.
- Add or subtract the deviation according to the card’s convention to obtain the corrected heading.
- Use the corrected heading for pilots who reference magnetic headings in navigation, while also crosschecking with GPS and other instruments.
When and How Deviation Cards Are Generated
A deviation card is usually produced after a compass swing, a ground calibration that measures deviation at standard headings, often by maneuvering the aircraft into specific headings and recording the magnetic response. Maintenance personnel then plot the results, produce a table or rose diagram, and issue a dated card to be kept in the aircraft. Because structural changes, avionics upgrades, or even rebalancing of accessories can alter deviation, periodic rechecks are recommended, particularly before long trips or when installing new equipment.
Integration With Other Compass Procedures
The deviation card is one part of a broader set of compass and navigation practices. Pilots also rely on magnetic variation charts to correct for geographic differences, and they use attitude and heading references to maintain control when magnetic indications are weak or turbulent conditions make compass use challenging. Crosschecking the magnetic compass with GPS track, radio magnetic indicators, and attitude indicators helps reduce the risk of navigation errors. Understanding deviation, variation, and the limitations of each instrument leads to more robust decision-making in all phases of flight.
Practical Tips for Pilots and Operators
- Keep the deviation card current by documenting any compass work, rechecks, or equipment changes.
- Verify the correction convention on your card (east positive, west negative, or other) before applying it in flight.
- Note that rough terrain, nearby structures, or parked equipment can temporarily alter local magnetic effects.
- Use the deviation card during preflight planning to anticipate required heading changes for your route.
- Combine card corrections with GPS and other electronic navigation data for a resilient, redundant check.
For pilots and operators, the aviation compass deviation card remains a foundational element of magnetic navigation. By clearly documenting how the aircraft’s magnetic environment affects the compass, the card enables reliable corrections that support safer, more accurate flights. Regular compass checks, careful recording of deviations, and disciplined use of the card help maintain confidence in heading information across varied operating environments.