Earth Magnetism & the Magnetic Compass

B. an apparent turn towards north

ANDS (northern hemisphere): Accelerate - North, Decelerate - South. Dip makes the magnet's centre of gravity lag behind the pivot, turning the card.

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D. 119, 3L, 122, 2E, 120, +4, 116

Check every step: 119 with 3 L drift needs heading 122; 2E variation: 122 - 2 = 120 magnetic; +4 (east) deviation: 120 - 4 = 116 compass. The others fail a step (e.g. 123 true with 2W must give 125 magnetic, not 121).

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A. Isogonals

Lines joining places of equal magnetic variation are isogonals (isoclinals join equal dip, isogonals equal variation).

B. turning

The largest errors of a direct reading compass in flight are the turning and acceleration errors (due to dip).

D. directly with the horizontal component of the earth's magnetic field

Compass sensitivity (directive force) is proportional to the horizontal component H of the earth field - which is why a compass is best midway between the magnetic poles.

B. 0 deg variation

Agonic lines connect points where the magnetic variation is zero; isogonals join points of equal variation.

B. find the deviation, reduce it with the compensators and record the residual deviation

A compass swing measures deviation on a number of headings, reduces it with the compensators and records the residual deviation on a deviation card.

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C. 90 deg

At the magnetic poles the lines of force are vertical: dip 90 deg. At the magnetic equator they are horizontal: dip 0 deg.

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C. Dip is zero

At the magnetic equator the field is horizontal, so dip is zero. Variation and deviation are not zero there.

C. Approximately midway between the magnetic poles.

A compass is most effective where the horizontal field is strongest and dip smallest - roughly midway between the magnetic poles.

A. 358

Heading = track - starboard drift = 342 T; + 11 W = 353 M; deviation -5 (west) is added going to compass: 358 C.

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A. True north is east of mag north

West variation means magnetic north lies west of true north, so true north is east of magnetic north.

D. towards; increase

In a sustained turn the liquid is dragged round with the bowl, in the direction of the turn. Turning towards the nearer magnetic pole, the turning error already swings the magnet system in the direction of the turn, so the swirl adds to it; turning away, it partly cancels it.

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C. more than 225 deg

The correct answer is: more than 225 deg.

A. North and South geographic and magnetic poles

Isogonals converge at the magnetic poles and also at the geographic poles.

B. 091°

TH = 070 - 10 (right drift) = 060; MH = 060 + 35 (west variation) = 095; CH = 095 - 4 = 091.

B. H=T cos dip

Horizontal component H = T cos (dip); vertical component Z = T sin (dip).

A. is permanent in nature

Hard iron (steel parts) keeps its magnetism - it does not change with heading or latitude, though the DEVIATION it causes changes with heading. Soft iron magnetism is induced by the earth's field and changes as the aircraft moves.

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A. there is a large and permanent change of magnetic latitude

Deviation from soft iron changes with magnetic latitude, so after a permanent move to a very different magnetic latitude (and after lightning strike, major repair or new equipment) the compass is swung.

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D. an isoclinal

Isoclinal = equal dip; aclinic = zero dip (the magnetic equator). Isogonal = equal variation; agonic = zero variation.

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A. 180

Magnetic variation can theoretically be as large as 180 deg.

C. aclinic

A-clinic = no dip. Do not confuse it with the agonic line, which has zero VARIATION.

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C. variation

Variation (declination) = true vs magnetic north, a property of the place. Deviation = magnetic vs compass north, a property of the aircraft.

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B. in a wing tip or the tail

The detector unit (flux valve) is placed as far as possible from the aircraft's hard- and soft-iron sources and electrical fields - typically a wing tip or the fin - and feeds the gyro-stabilised indicator remotely.

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C. an apparent turn to the north

In the southern hemisphere everything reverses: Accelerate - South, Decelerate - North. So a landing on a westerly heading shows a turn towards north.

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B. the horizontal

Dip (inclination) is measured from the horizontal down to the line of force; the needle's red end dips downwards in the northern hemisphere.

B. magnetic dip together with the centripetal force in the turn acting on the offset centre of gravity of the magnet

Dip tilts the magnet system so its centre of gravity is offset from the pivot; in a turn the centripetal force acts on that offset mass and swings the card. Liquid swirl adds to it.

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A. 356 deg

TH = 350 - 10 = 340; MH = 340 + 11 = 351; CH = 351 - (-5) = 356.

D. 090 and 270

The error comes from the dip-offset magnet swinging across the heading. On east/west headings the force acts across the magnet and turns it most; on north/south it acts along it (no error).

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C. an agonic line

An agonic line joins places of zero magnetic variation; an isogonal joins equal variation and an isoclinal joins equal dip.

C. blue pole near the north pole of the earth and the direction of the magnetic force pointing straight down to the earth's surface

The earth behaves as a magnet whose blue (south-seeking) pole is near the geographic north pole, with the field pointing down there.

A. (+) easterly

Magnetic north east of true north is easterly (+) variation.

B. Q

Coefficient C comes from athwartships hard iron Q (and soft iron fZ).

B. keeping the magnet mass close to the pivot, using several short magnets, and damping liquid

Aperiodic = settles without oscillating. Mass near the pivot (low moment of inertia), short magnets and damping filaments in liquid do it.

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B. Clockwise giving an apparent turn toward the north.

Accelerating on an easterly heading in the northern hemisphere the card turns clockwise, an apparent turn towards the north (ANDS).

C. Is strongest at the magnetic poles

The total magnetic force is strongest at the magnetic poles.

B. A value representing the deviation registered on headings East and West

Coefficient B is the deviation that is greatest on easterly and westerly headings (it comes from fore-and-aft hard iron P).

B. Coefficient C = +4

Coefficient C = (deviation on 000 - deviation on 180) / 2 = (+3 - (-5)) / 2 = +4.

B. less than 060°

Turning right through north in the northern hemisphere the compass lags - UNOS (undershoot north) - so after 90 deg of turn it reads less than 060.

D. 033°

TH = 348 + 17 = 005; MH = 005 + 32 = 037; CH = 037 - 4 (east deviation) = 033.

A. magnetic pole movement causing numerical values at all locations to increase or decrease

Variation charted at a place changes slowly with time because the magnetic poles move - the numerical value at different places may increase or decrease.

A. more than 235

Rate 1 = 3 deg/s, so 30 s = 90 deg: the real heading is 235. Turning through SOUTH in the northern hemisphere the compass LEADS (reads ahead of the turn) - UNOS: Undershoot North, Overshoot South - so it over-reads, showing more than 235. Hence roll out early on northerly headings and late on southerly ones.

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B. Underread and increased

Turning through north in the northern hemisphere the compass under-reads (it lags the turn). The liquid in the bowl is dragged round in the direction of the turn and carries the magnet with it, so the error is increased.

C. Is Approximately the same at magnetic latitude 50°N and 50°S

H is about equal at equal magnetic latitudes N and S, and falls toward the poles.

A. +P

Permanent (hard iron) magnetism along the fore-and-aft axis is +P.

B. Compass deviation

Deviation is the angle between magnetic north and compass north.

A. P

Hard iron P (fore and aft) produces coefficient B, Q (athwartships) coefficient C and R (vertical) the vertical-force terms.

A. Variation

Variation is the angle between true north and magnetic north; deviation is between magnetic north and compass north.

B. varies with magnetic latitude but not heading

Soft iron is magnetised by induction from the earth's field. Vertical soft iron sees the vertical component Z, which grows with magnetic latitude but is the same on every heading.

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C. no apparent turn

Acceleration and deceleration errors come from the dip (vertical) component acting through the offset centre of gravity of the magnet system. On north and south headings the force lies along the magnet, so there is no turning moment - no error, either hemisphere.

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D. the north magnetic pole, downwards, dip

The red (north-seeking) pole points to the north magnetic pole and, away from the magnetic equator, dips below the horizontal along the lines of force. The angle below horizontal is the dip.

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A. Is used to establish aircraft magnetic heading during a compass swing

A landing compass is a hand-held reference compass used to establish magnetic heading during a compass swing.

A. Deviation card

Residual deviation after the swing cannot be removed; it is recorded on the deviation card and applied by the pilot.

A. north

Accelerating on an east or west heading in the northern hemisphere the compass shows an apparent turn towards the north (ANDS: Accelerate North).

B. the horizontal direction of the earth's magnetic field at that place

Magnetic north is the direction a freely suspended, horizontally constrained needle settles - the horizontal component of the field. It does not in general point exactly at the magnetic pole.

A. variation at a place changes slowly year by year

The magnetic poles wander, so the variation at any place drifts (typically a few minutes of arc a year). Charts state the variation for a year and the annual change.

B. senses the direction of the horizontal component of the earth's field electrically

The flux valve has no moving magnet: its windings sense the horizontal field electrically and the signal slaves the directional gyro, so it senses rather than seeks north.

B. before the compass reads 360

Turning towards north the compass lags, so it still reads less than the turn made: stop the turn early - Undershoot North.

B. strongest near the magnetic equator

H = T cos dip: greatest where dip is zero (magnetic equator), zero at the magnetic poles - which is why the compass is useless near the poles and best roughly midway.

B. lag behind the turn (or even show a turn the other way)

Turning error: through north in the northern hemisphere the compass lags (sluggish); through south it leads. Hence UNOS - Undershoot North, Overshoot South when rolling out.

C. neither, because the variation changes along it

A rhumb line keeps a constant TRUE direction; a great circle is the shortest. A constant magnetic track only matches a rhumb line where the variation stays the same.

B. H decreases, Z increases

At the magnetic equator the field is horizontal (H maximum, Z zero); at the magnetic pole it is vertical (H zero, Z maximum). H = T cos dip, Z = T sin dip.

B. about zero

Turning error is largest through north and south and close to zero through east and west, where the dip force lies along the turn.