Aircraft fields and compass deviation
The Earth supplies the reference field, but the compass sits inside another magnetic field made by the aircraft. The angle between magnetic north and the direction taken up by the compass magnet is deviation.
What produces aircraft magnetism
Iron and steel structure, engines, control cables, electrical equipment and current-carrying wiring can distort the Earth's field at the compass. Some material retains magnetism after the magnetising influence is removed. This is hard-iron magnetism. Other material becomes magnetised mainly while it is within the Earth's field. This is soft-iron magnetism.
Hammering, vibration and construction while the airframe is aligned with a magnetic field can leave permanent magnetism. Electrical loads add fields whose strength may change when equipment is selected. The total aircraft field at the compass is therefore the vector sum of several sources.
Hard iron magnetism
Hard iron supplies a permanent aircraft-fixed field. Its force is resolved into the P, Q and R components below. The source force does not depend on heading, although the compass deviation that it produces does because the aircraft axes rotate relative to magnetic north.
Soft iron magnetism
Soft iron is magnetised by the surrounding field. Its induced field therefore changes with aircraft attitude, heading and magnetic latitude. Vertical soft iron is magnetised by Z, so its effect disappears at the magnetic equator and increases with dip.
| Source | Behaviour | Effect at the compass |
|---|---|---|
| Hard iron | Permanent field fixed in the aircraft | Its force is nearly constant, but its angular effect increases as Earth's horizontal force H becomes weaker |
| Horizontal soft iron | Induced mainly by the horizontal Earth field | Produces semicircular and quadrantal patterns according to location and orientation |
| Vertical soft iron | Induced by the vertical component Z | Zero at the magnetic equator and stronger as dip increases |
| Electrical field | Depends on current and equipment state | Can change deviation when a circuit is switched |
The aircraft axes P, Q and R
The permanent hard-iron force is resolved into three aircraft-fixed components. P acts fore and aft along the longitudinal axis, Q acts laterally across the wings, and R acts vertically. P and Q directly disturb the horizontal compass indication. R becomes important when the compass system tilts or the aircraft manoeuvres.
| Component | Positive direction used in the model | Main coefficient association |
|---|---|---|
| P | Forward along the fuselage | Semicircular coefficient B |
| Q | To the right wing | Semicircular coefficient C |
| R | Vertically down | Tilt-related effects rather than a simple level-flight cardinal correction |
Naming and applying deviation
Deviation is easterly, or positive, when the north-seeking end of the compass magnet lies east of magnetic north. It is westerly, or negative, when it lies west. To pass from compass heading to magnetic heading, apply deviation with its sign. A compass heading of 090 degrees with 5 degrees east deviation gives 095 degrees magnetic. A compass heading of 095 degrees with 5 degrees west deviation gives 090 degrees magnetic.
Deviation coefficients A to E
A compass swing turns a set of observed errors into a compact mathematical model. The first three coefficients explain the dominant constant and semicircular errors; D and E describe quadrantal error.
The coefficient patterns
| Coefficient | Pattern with compass heading θ | Typical origin | Where prominent |
|---|---|---|---|
| A | Constant | Lubber-line or detector alignment | Same signed offset on every heading |
| B | B sin θ | Longitudinal hard iron P plus related soft iron | Maximum on east and west, zero on north and south |
| C | C cos θ | Lateral hard iron Q plus related soft iron | Maximum on north and south, zero on east and west |
| D | D sin 2θ | Symmetrical horizontal soft iron | Quadrantal, with two cycles per revolution |
| E | E cos 2θ | Asymmetrical quadrantal or alignment effect | Quadrantal, shifted by 45 degrees from D |
Reading the B and C curves
A positive B curve is zero on north, maximum east on 090 degrees, zero on south, and maximum west on 270 degrees. A positive C curve is maximum east on north, zero on east, maximum west on south, and zero on west. Reversing the associated magnetic pole reverses the sign of the curve.
Deriving A, B and C from cardinal readings
Suppose the observed deviations are north +7 degrees, south +1 degree, east 0 degrees and west +4 degrees. Then A = (7 + 1 + 0 + 4) ÷ 4 = +3 degrees. B = (0 minus 4) ÷ 2 = minus 2 degrees. C = (7 minus 1) ÷ 2 = +3 degrees. The first-order model is therefore deviation = 3 minus 2 sin θ + 3 cos θ.
Check the result on 090 degrees. Sine 90 degrees is 1 and cosine 90 degrees is 0, so deviation = 3 minus 2 = +1 degree. If the recorded east value differs, the remainder represents observational error or coefficients D and E that the three-term model does not contain.
With A zero, B minus 2 and C +3, the stationary headings satisfy tan θ = minus 2 ÷ 3, giving about 326 degrees and the reciprocal 146 degrees. Zero-deviation headings satisfy tan θ = +1.5, giving about 56 degrees and 236 degrees. Substitution in the equation confirms which pair gives zero.
Compass swing, correction and records
A compass swing compares the aircraft compass with a reliable magnetic datum on a series of headings, corrects the dominant errors and records the residual deviation that remains.
The three aims
- Observe the difference between magnetic heading and compass heading on selected headings.
- Reduce the correctable coefficients as far as practicable.
- Record the remaining deviation on a card or graph for use in flight.
The aircraft is positioned on an approved compass-swinging site. A landing or datum compass supplies magnetic heading. The aircraft must be in its normal flight configuration, level, with engines and relevant electrical and radio services operating. Tools, watches and other ferromagnetic items are kept away. An unusual magnetic payload may require a swing with that load aboard.
Correction of coefficients
| Coefficient | Correction | Heading used |
|---|---|---|
| A | Align the lubber line, compass body or remote detector mechanically | Constant error is visible on all headings |
| B | Use the fore-and-aft corrector or the appropriate micro-adjuster | Correct on an east or west heading |
| C | Use the lateral corrector or the appropriate micro-adjuster | Correct on a north or south heading |
| D and E | Reduce only with the specified soft-iron correctors or system adjustment | Assess on intercardinal headings |
After adjustment, carry out a check swing on eight or twelve headings. The remaining deviation is the residual deviation. Place the deviation card where the pilot can read it from the operating position. The card converts between compass and magnetic heading for the installed condition.
Occasions for swinging the compass
- When a compass component is installed or replaced, or compass accuracy is in doubt.
- At the maintenance interval specified for the aircraft.
- After a significant modification, repair or replacement involving magnetic material.
- After major shock, a lightning strike, or important radio and electrical modification.
- Before carrying an unusual ferromagnetic load when its effect may be significant.
- After a move to an operating region with a large change of magnetic latitude.
- After long storage on one heading, especially if the aircraft was not moved.
Change of magnetic latitude
At the magnetic equator H is strongest and Z is zero. Hard-iron angular deviation is then least, and vertical-soft-iron deviation is zero. Poleward, H decreases while Z and dip increase. Both angular effects become larger. Hard-iron deviation keeps the same sign in both hemispheres, while vertical-soft-iron behaviour follows the changed direction of Z.
For a simple comparison, if a fixed hard-iron transverse force is 2 units and H falls from 20 to 10 units, the small-angle ratio doubles from 2 ÷ 20 = 0.10 to 2 ÷ 10 = 0.20. The hard-iron source did not strengthen; the Earth's directive field weakened.
Accuracy limits
The Oxford training source quotes 10 degrees as its maximum residual direct-compass deviation after compensation. Treat that as a source-book examination figure. For an Indian aircraft, the approved maintenance data and the applicable DGCA requirement control serviceability and release to service.
Direct indicating magnetic compasses
The direct indicating magnetic compass lets the pilot read heading directly from a pivoted magnet assembly. Its usefulness depends on three requirements: horizontality, sensitivity and aperiodicity.
Compass requirements
The magnet system must lie nearly horizontal, respond to the weak directive force H with little friction, and settle promptly after displacement. These requirements are linked, so each construction feature often helps more than one of them.
Vertical card and grid ring
| Feature | Vertical card compass | Grid ring compass |
|---|---|---|
| Other name | B type or E type | P type |
| Indication | Circular card attached to the magnet system, read at a lubber line | Rotating grid ring aligned with the north reference |
| Use | Common main compass in light aircraft and standby compass in larger aircraft | Older, accurate and stable installation |
| Limits | Simple and direct, but subject to dip and motion errors | Heavier, bulkier and costlier; read in straight and level flight after unclamping and aligning |
| Damping | Liquid and low-inertia magnet assembly | Liquid plus damping wires gives stronger damping |
Horizontality
A free magnet tries to align with the total Earth field, so it would be horizontal only at the magnetic equator. The compass instead suspends its magnets pendulously, with the centre of gravity below the pivot. Weight supplies a restoring couple that opposes the tilting couple from the vertical field Z.
In middle northern magnetic latitudes the north-seeking ends remain slightly down, about 2 degrees in the Oxford treatment. In the southern hemisphere the south-seeking ends dip. Some designs also shift the pivot towards the nearer magnetic pole. Perfect horizontality is not possible, and the remaining tilt is the source of motion errors.
Sensitivity
Sensitivity is the ability to align with H. It increases with the magnet's magnetic moment and with the local horizontal field. Designers use several short, strongly magnetised bars or a circular magnet rather than one long heavy bar. An iridium-tipped pivot in a jewelled cup reduces friction. The liquid lubricates the pivot and its buoyancy reduces the effective weight on the bearing.
Aperiodicity
Aperiodic, or dead beat, means that the system settles quickly after displacement instead of oscillating repeatedly. Several short magnets and a light alloy framework keep mass near the pivot and reduce moment of inertia. The bowl liquid damps movement, while grid-ring damping wires move through the liquid and increase the damping effect.
“Settle down quickly on a steady indication.”Oxford ATPL Book 10, chapter 29
“The system becomes aperiodic, meaning dead beat.”R.K. Bali, Air Navigation, chapter 8
Acceleration, deceleration and turning errors
Motion errors arise because the magnet system is pendulous and the field is inclined. They are zero at the magnetic equator and become more serious as magnetic dip increases.
Acceleration and deceleration
On an east or west heading in the northern hemisphere, acceleration tilts the compass and produces an apparent turn towards north. Deceleration produces an apparent turn towards south. In the southern hemisphere the indications reverse: acceleration appears towards south and deceleration towards north.
The acceleration error is greatest on east and west headings, reduces as the aircraft approaches north or south, and is zero on north and south headings. It is also zero near the magnetic equator because Z and dip are zero. A useful general statement is that acceleration indicates towards the nearer magnetic pole, while deceleration indicates towards the magnetic equator.
Turning error and UNOS
When an aircraft banks, the pendulous compass assembly does not remain in the aircraft's horizontal plane. Its magnets respond to both H and Z, so the indicated rate and amount of turn differ from the real turn. The error is greatest when turning through north or south and small when turning through east or west.
In the northern hemisphere, the compass is sluggish when turning through north and lively when turning through south. Roll out before the indicated northerly heading and after the indicated southerly heading. This is UNOS: undershoot north, overshoot south. In the southern hemisphere the pole and equator relationships reverse.
| Situation | Northern hemisphere indication | Pilot cue |
|---|---|---|
| Acceleration on east or west | Apparent turn north | Treat it as acceleration error, not a real heading change |
| Deceleration on east or west | Apparent turn south | Expect the opposite apparent swing |
| Turn through north | Compass lags or underreads | Roll out early, undershoot the indicated north |
| Turn through south | Compass leads or overreads | Roll out late, overshoot the indicated south |
Approximate rollout example
At magnetic latitude 45 degrees north with a 16 degree bank, the approximate allowance is 45 minus 8 = 37 degrees. Turning from east towards north, begin rollout near 360 minus 37 = 323 degrees. Turning from west towards north, begin near 000 + 37 = 037 degrees. It is an approximate training relationship, not a substitute for the aircraft procedure.
Liquid swirl and serviceability
The same liquid that damps oscillation can be carried around by the bowl during a prolonged turn. A useful compass therefore needs both correct magnetic behaviour and sound mechanical condition.
Liquid swirl
During a sustained turn, friction drags the bowl liquid in the direction of the turn. The moving liquid then drags the magnet assembly. When the aircraft rolls out, liquid momentum continues briefly and can carry the compass indication in the direction of the former turn. This is liquid swirl.
Swirl is a separate source of error from magnetic dip. At the magnetic equator, where dip-related turning error disappears, liquid swirl can still disturb the indication. Elsewhere it may increase or reduce the observed turning error according to hemisphere, direction and the heading through which the turn is made.
Serviceability checks
| Check | Acceptable observation | Fault indicated |
|---|---|---|
| Body, glass and lighting | No cracks or dents; lighting and luminous marks usable | Mechanical damage or unreadable indication |
| Compass liquid | Clear, with no sediment, discolouration or bubbles | Corrosion, extra friction or a leaking seal |
| Bowl suspension | Moves gently in every direction without metal contact | Restricted suspension or poor mounting |
| Grid ring | Rotates through 360 degrees and locks positively | Ring or clamp fault |
| Friction test | After deflection 10 to 15 degrees each way, returns within 2 degrees of the original reading | Excess pivot friction |
| Damping test | After a 90 degree deflection held 30 seconds, returns through 85 degrees within the manual limit; Bali notes about 6.5 to 8.5 seconds as typical | Incorrect damping or liquid condition |
Magnetic installation discipline
Bali gives training limits for material placed near the compass. A single non-electrical item should not introduce more than 1 degree, with the combined non-electrical effect not more than 2 degrees. The same 1 degree single-item and 2 degree aggregate values are stated for electrical equipment and wiring. Approved aircraft data remains controlling.
When to trust the reading
Read the direct compass in straight and level flight at constant speed whenever accuracy matters. Do not rely on a value taken during acceleration, deceleration, a bank, a sustained turn or while the liquid is still settling. Cross-check gross disagreement against other heading sources, then apply the published aircraft procedure.
Remote indicating and gyro magnetic compasses
A remote indicating compass moves the magnetic detector away from cockpit interference and combines magnetic north-seeking with a stable display. A slaved gyro system uses magnetic information to correct gyro drift slowly.
Detector or flux valve
The detector unit is mounted where aircraft magnetic interference is small, commonly near a wingtip or high in the fin. Three sensing elements are arranged about 120 degrees apart. The unit is pendulously mounted so it remains approximately horizontal and senses the horizontal component of the Earth's field.
A flux valve does not carry a freely rotating compass card. It senses the magnetic meridian and produces electrical signals whose relative values describe heading. A transmitting system carries those signals to one or more indicators. Remote installation reduces deviation and parallax and allows repeaters to feed other equipment.
Direct indicator, directional gyro and slaving
| System | Reference | Main strength | Main limitation |
|---|---|---|---|
| Direct reading compass | Magnets align directly with H | Independent, simple standby source | Deviation, dip errors, swirl and reading limits |
| Directional gyro indicator | Gyro rigidity after manual alignment | Stable in manoeuvre, without compass oscillation | Not north-seeking; drifts and needs periodic realignment |
| Remote magnetic compass | Remote detector senses the magnetic meridian | Detector can be placed away from interference; easy repeater display | Electrical and system complexity |
| Slaved gyro magnetic compass | Gyro display with slow magnetic correction | Combines north-seeking reference with gyro stability | Heavier, more expensive and dependent on electrical power |
Why the gyro is corrected slowly
A gyro has rigidity in space, so it gives a steady heading presentation during normal manoeuvres, but it does not find magnetic north by itself. The remote detector supplies a long-term reference. A low slaving rate, approximately 5 degrees per minute in Bali's outline, corrects gyro drift without copying rapid compass swings. A roll cut-out can interrupt slaving when bank exceeds about 10 degrees because the detector is no longer in its best sensing attitude.
Advantages and limitations
- Remote location reduces deviation from cockpit wiring and magnetic structure.
- The detector senses the meridian without a heavy card rotating on a central pivot.
- Gyro stabilisation greatly reduces acceleration and turning indications at the display.
- Repeaters can supply the same magnetic heading to several instruments and systems.
- The installation is heavier, costlier and more complex than a direct compass.
- Electrical failure can remove the remote indication, so an independent standby source remains important.