Magnetic compass and heading errors

Name the heading reference before comparing numbers

True heading is the direction the nose points relative to geographic north. Magnetic heading uses magnetic north. Compass heading is the indicated reading of the aircraft's magnetic compass after its local magnetic influences. A comparison is useful only when you know which reference each display uses.

Track is the direction of movement over the ground. A crosswind can make track differ from heading even with serviceable instruments. Likewise, a display configured for true heading may differ from a magnetic compass because it uses a different north reference. Learn the installed display's labels and settings before treating a numerical difference as a failure.

A conventional liquid compass is a direct magnetic reference with no requirement for the main electrical bus to spin a gyro. It is useful redundancy, but it must be serviceable and free enough from magnetic interference to read reliably. “Independent of the electrical bus” does not mean independent of nearby magnetic fields or aircraft motion.

IFH 5-10–5-15: magnetic compass and remote heading systems · PHAK Chapter 8: magnetic compass construction and errors

Variation belongs to the location; deviation belongs to the aircraft

Variation is the angle between true and magnetic north at a location. Use current chart or approved navigation information. It changes geographically and over time, so an old textbook example is not the current variation for that city. Variation is not repaired by adjusting the aircraft compass.

Deviation is compass error caused by the aircraft's own magnetic influences, including magnetized structure and electrical equipment. It can vary with heading and equipment configuration. Compensation reduces it; the remaining corrections are recorded on the aircraft's compass correction card. Follow the card's wording, such as “for magnetic heading … steer compass …,” rather than assuming an unstated sign convention.

For a fictional conversion with no wind correction: desired true heading 010°, variation 10° west, gives magnetic heading 020°. If this aircraft's example correction card says “for 020° magnetic, steer 018° compass,” its compass target is 018°. The location and card are invented teaching values, not operational data. Converting true to magnetic adds west variation and subtracts east variation; reverse the operation when converting back.

Course planning also requires wind correction to obtain a heading. This example is a heading-reference conversion only. Adding variation to a plotted course does not by itself supply the heading to fly in wind.

IFH 5-12–5-13: variation, deviation and correction card

Why motion disturbs a direct-reading compass

The Earth's magnetic field is inclined as well as horizontally directed. Magnetic dip is that inclination. The vertical component becomes more significant toward the magnetic poles; close to a magnetic pole, the horizontal component may be too weak to provide a useful heading reference.

A conventional float compass is arranged to read in a near-horizontal plane. Banking or accelerating changes the forces on the suspended assembly, allowing dip and inertia to produce a misleading heading indication. This lesson's dynamic-error examples assume a conventional liquid compass, moderate latitude in the Northern Hemisphere, and ordinary airplane maneuvers. They are not universal rules for electronic AHRS heading or every vertical-card compass design.

Near the magnetic equator, dip-related errors are reduced; their size and behavior depend on latitude, maneuver and instrument design. In the Southern Hemisphere the north/south sense of the conventional acceleration and turning corrections reverses. Use the appropriate training and aircraft guidance for the operating region.

IFH 5-10–5-14: magnetic field, dip and compass errors · PHAK Chapter 8: magnetic dip and compass limitations

A speed change can look like a turn

In the stated Northern Hemisphere conditions, acceleration on an easterly or westerly heading can make a conventional compass swing toward north; deceleration can make it swing toward south. ANDS means acceleration-north, deceleration-south. It describes the error, not the airplane's required response. The acceleration error is greatest on east/west headings and absent on exact north/south headings in the simplified conventional case.

For example, during a straight eastbound acceleration, the compass may momentarily show a northerly change while a reliable gyro and turn instrument support the intended straight path. Banking south to “fix” the compass could create a real heading error. Maintain the intended flight condition using the cross-check, let the speed change end, and read the compass after it settles.

ANDS reverses in the Southern Hemisphere. Do not apply it as a generic explanation for a persistent heading discrepancy, or to an AHRS display merely because that display shows magnetic heading.

IFH 5-13–5-14: acceleration error, Figure 5-21

Turning changes the indication before it settles

Banking a conventional compass allows magnetic dip to create a turning error. In the stated Northern Hemisphere conditions, a turn beginning from a northerly heading can initially indicate the opposite direction and then lag; from a southerly heading the indication leads the actual turn. A stable compass heading after the bank and turn end is a better comparison than a moving indication.

When using the compass itself to terminate a turn toward north, the traditional correction is to begin rollout before the compass indicates the northerly target; toward south, let the indication pass the southerly target before rollout. UNOS means undershoot north, overshoot south. It describes compensation using the indicated compass target, not permission to finish on the wrong actual heading.

The necessary allowance depends on latitude, bank, rollout and instrument behavior. This lesson supplies no universal number of degrees. An instructor must teach the technique for the aircraft and conditions, then verify the stabilized heading after rollout. The north/south sense reverses in the Southern Hemisphere; do not transfer the mnemonic unchanged.

Common error: reading a turning compass as though it were a stable heading gyro, or resetting a gyro to that moving reading. The result can create a new error in an otherwise serviceable heading indication.

IFH 5-13–5-14: northerly and southerly turning errors · PHAK Chapter 8: compass turning errors

Use a stabilized comparison and investigate interference

For a normal airborne heading check, establish straight, level, unaccelerated flight and allow the compass to settle. Compare like references, account for deviation with the correction card, and follow the heading-system procedure. Small residual oscillations may require reading an average; turbulence can make a useful comparison difficult. Do not keep adjusting a gyro to each swing.

Keep magnets and magnetic accessories away from the compass. A phone mount, headset component, tool, or new electrical device placed nearby may change deviation. A compass that sticks, leaks, has a large unexplained discrepancy, or lacks required correction information needs evaluation under the aircraft's maintenance and airworthiness procedures.

A slaved heading system uses a remote magnetic sensor and processing, not the same freely swinging float shown here. It can have magnetic-interference or source faults, and ground surface materials can affect some installations. Such an error may not self-correct; compare heading against an available reference and correct it as the manufacturer directs before departure. Know its source flags and comparison method. A matching second electronic heading display may use the same magnetometer and AHRS.

During preparation, explain what you would use to compare each installed heading source, when that comparison is valid, and how wind or a reference setting could account for a difference. This ground reasoning supports instructor-led instrument flying; it does not demonstrate compass-turn proficiency.

IFH 5-14–5-16: oscillation, vertical-card and remote heading systems · AIM 1-1-15(4): heading errors from magnetic field disturbances

FLIGHT SCENARIO

What would change your plan?