Magnetic Compass and Electronic Flight Display Systems

Magnetic Compass Fundamentals

The magnetic compass is the oldest and most fundamental navigation instrument on any aircraft. It requires no electrical power and serves as a legal backup heading reference on certified aircraft. The compass contains a magnetized card or needle suspended in a liquid-filled bowl that aligns itself with Earth's magnetic field lines. Because the instrument responds to any magnetic field — not just Earth's — nearby ferrous metal, electrical wiring, and electronic equipment all introduce small but measurable errors called deviation.

Deviation is different from variation (also called declination). Variation is the angular difference between true north and magnetic north at a given location on Earth's surface; it is a geographic constant printed on aeronautical charts. Deviation is caused by the aircraft itself and changes with the aircraft's heading. Deviation can be minimized but rarely eliminated entirely, so every installed compass must have a compass correction card posted nearby so the pilot can compensate.

The Compass Swing Procedure

A compass swing is the process of measuring deviation at multiple headings and recording the results. The procedure is performed on a calibrated compass rose — a precisely marked circle painted on a ramp area away from structures and vehicles — or on an indoor compass swing table that simulates heading changes with a non-magnetic rotating platform.

Before swinging, remove all unnecessary ferrous tools and equipment from the aircraft. Use only non-magnetic tools (typically brass or non-ferrous alloy) during the swing. Safety glasses are required whenever working around aircraft, and the area must be clear of unauthorized personnel and running engines or vehicles that could introduce stray fields.

The standard swing records deviation at eight headings spaced 45 degrees apart: N (000°), NE (045°), E (090°), SE (135°), S (180°), SW (225°), W (270°), and NW (315°). At each heading:

1. Align the aircraft precisely using the calibrated rose or an independent directional reference.

2. Allow the compass to settle (liquid damping takes a few seconds).

3. Read and record the compass indication.

4. Compute deviation: Deviation = Compass Reading − Magnetic Heading. A positive value means the compass reads high (east deviation); a negative value means it reads low (west deviation).

5. Compensating magnets inside the compass housing can be adjusted with a non-magnetic screwdriver to reduce deviation on cardinal headings before the final card is written.

The finished compass correction card lists the magnetic heading the pilot wants to fly and the compass heading to steer in order to achieve it. The card must be legible, permanently mounted near the compass, and formatted to match the aircraft manufacturer's or compass manufacturer's requirements. FAA regulations require the card to be present on certificated aircraft.

Glass-Cockpit Primary Flight Display (PFD)

Modern integrated avionics suites replace the traditional "six-pack" of individual round instruments with large-format color displays. The Primary Flight Display (PFD) combines attitude, airspeed, altitude, vertical speed, heading, and navigation information on a single screen. The Multifunction Display (MFD) typically shows moving-map, engine data, weather, and traffic information.

The PFD draws its data from several sensors:

• ADAHRS (Air Data, Attitude, and Heading Reference System): Combines solid-state accelerometers and rate gyros (for attitude), pitot-static inputs (for airspeed and altitude), and often an internal magnetometer (for magnetic heading). Some installations use a separate remote magnetometer mounted away from engine and avionics interference.
• GPS receiver: Provides ground track, ground speed, and position data. GPS track supplements but does not replace the magnetometer-derived magnetic heading.
• Magnetometer: A remote sensor, when used, is mounted in a wingtip or tail area to minimize aircraft magnetic interference. It feeds magnetic field strength data to the ADAHRS, which computes a heading.

Signal flow: sensors → ADAHRS unit → data bus (typically ARINC 429 or a proprietary serial bus) → display processor → screen. The avionics manufacturer's installation manual and the system wiring diagram show each connector, wire gauge, and bus address. Tracing this path is an essential maintenance skill because a heading error on the PFD must be traced back through the chain to find the faulty component.

Six-Pack vs. Glass Cockpit: Maintenance Considerations

Traditional pitot-static and gyroscopic instruments are individually replaced or overhauled on separate schedules. A glass-cockpit system integrates many functions into fewer line-replaceable units (LRUs), which simplifies some tasks but requires technicians to consult both the airframe maintenance manual and the avionics manufacturer's installation and maintenance manuals. AC 43.13-1B (Acceptable Methods, Techniques, and Practices) and applicable avionics-specific Advisory Circulars provide accepted maintenance practices. Any alteration to a certificated aircraft's avionics installation generally requires an appropriate approval — check current FARs and applicable ACs before beginning work.

Safety Summary

• Use only non-magnetic tools during a compass swing; ferrous tools change the local field and invalidate results.
• Keep the compass swing area free of vehicles, ground power units, and running equipment.
• Wear safety glasses at all times on the flight line and in the avionics shop.
• Follow lockout/tagout procedures before opening avionics bays or disconnecting wiring to prevent inadvertent power-up.
• Static-sensitive components (display units, ADAHRS) require ESD precautions — use grounded wrist straps and ESD-safe packaging.