This document discusses magnetism and compasses. It describes the basic magnetic properties of permanent bar magnets and explains how compasses work by aligning with the Earth's magnetic field. It discusses different types of iron and their magnetic properties. It also describes various aspects of the Earth's magnetic field, including magnetic poles, magnetic declination, dip, and anomalies. Sources of deviation in aircraft compasses are explained, including effects of hard and soft iron. The process of swinging a compass to determine and correct for deviations is also summarized.

1. MAGNETISM AND
COMPASSES

2. DRC or standby compass
 Primary function: show magnetic heading
 Nowadays: heading reference instrument,
relegated to standby role
 Its carriage in all types of aircraft is still a
mandatory requirement of JAR

3. MAGNETIC PROPERTIES
 3 principle properties of a simple
permanent bar magnet:
– It attracts other pieces of iron and steel
– Its power of attraction is concentrated at each
end of the bar
– When freely suspended it always comes to rest
in an approximately north-south direction (If
displaced from that direction, it will return to
same alignment always)

4. Two poles: RED : North seeking pole (south pole)
BLUE: North pole
– Like poles will repel and unlike will attract
– A magnetic field of influence surrounds the
magnet (reflected in lines of force that may never
be broken and will never cross each other)
– A magnetic field is strongest where the lines of
force are closest together (close to poles)

5. Hard iron and soft iron
 HARD IRON: magnetic materials difficult to
magnetise, but once in a magnetised state, retain
magnetism for long periods of time
– Coercive force: resistance to magnetisation and
demagnetisation
 SOFT IRON: metals easily magnetised. They
lose magnetised state once the magnetising
force is removed.

6. Magnetosphere: shields the surface of the Earth from the charged
particles of the solar wind. Generated by electric currents located in
many different parts of the Earth. Compressed on the day (Sun) side
due to the force of the arriving particles, and extended on the night
side. (Image not to scale.)

7. Terrestrial magnetism
Early measurements of magnetism showed the
existence of the Earth’s magnetic field (weak) and
the existence of strong:
– Magnetic Pole under surface on Northern H
– Magnetic Pole on SH
The positions of this poles are not permanent, they
are slowly moving. ( from 2007 it is proceeding
quicker and to Siberia: 55km/year)

8. Terrestrial Magnetism
 The position of the Magnetic North Pole is
84.742ºN 129.077ºW, North of Canada
Source: NOAA (2010)
 The Magnetic South Pole is 64.049ºS
137.227ºE , south of Australia
Source: NOAA (2010)

9. The source of the terrestrial magnetism, based on
observations in the space surrounding the Earth,
seems to be a fairly short magnet, located close
to the centre of the Earth
A plane passing through the magnet and the centre
of the Earth would trace and imaginary line on
the Earth’s surface called a magnetic meridian

10. Terrestrial Magnetism
 The magnetic poles are not antipodes
 The lines S to N represent the direction of the
Earth's magnetic field
 The lines of force will be seen that they do not
always coincide with the Earth's horizontal.

11. Difference Earth’s MF – bar magnet
 Its points of maximum intensity are not at the
magnetic poles, but at 4 other positions
(magnetic foci)
 Two of them are close to the poles

12. VARIATION
 Variation is the angle between TN and MN and is
measured in degrees East or West from the TN
(0º-180º)
 Isogonals are pecked lines on a map or chart
joining places of equal magnetic variation
 Agonic Line is the name given to isogonals
joining places of zero variation

14.  Magnetic field is not regular.
 There are a large number of local irregularities
(magnetic anomalies) (ferrous nature of rocks
disturbing Earth’s magnetic field)
– Large changes in VAR over very short distances)
 Nowadays the most important anomaly being
studied is the South Atlantic one. (area approx
5,000,000 km2) near Brazil coast
– Region at which the magnetic field is being weakening.

16. ANGLE OF DIP
 The Earth’s lines of magnetic flux are not
horizontal, don’t lie parallel to the Earth’s
surface at all points.
 The Angle of Dip is the angle in the vertical
plane between the horizontal and the Earth's
magnetic field at a point

18.  The Magnetic North Pole is the position on
the surface of the Earth where the dip (or
inclination) is plus 90 degrees
 The Magnetic South Pole is the position on
the surface of the Earth where the dip (or
inclination) is minus 90 degrees

19. Angle of dip
 Isoclinals are lines on a map or chart joining
places of equal magnetic dip
 Aclinic Lines is the name given to isoclinals
joining places of zero dip.
 Dip may be described as magnetic latitude:
– Zero at the magnetic equator
– 90º at the magnetic poles

21. Earth’s total magnetic force
 The direction of the Earth's magnetic field at
any point may be split into its Vertical and
Horizontal components
 If the value of dip angle is ϴ:
H = F cos ϴ
Z = F sin ϴ

23. When moving from the Magnetic Equator
towards one of the magnetic poles:
- F will increase
- H will decrease because of dip
- Z will increase

25. Aircraft magnetism
 Challenge to designers:
– DRC must be located where pilot can
readily see it
– DRC in the cockpit is surrounded by
magnetic material and electrical circuits
– Such magnetic influence provides a
deviation force to the Earth’s magnetic field:
compass needle will not point to the local
meridian

26. Aircraft magnetism
Such magnetic influences may originate from:
- components of the aeroplane's structure,
- items of the traffic load, cargo and
passengers baggage
- items placed near to the compass
Deviation caused by a/c magnetism can be
analysed and errors can be minimised

28. Aircraft magnetism
 Deviation is the angle measured at a point
between the direction indicated by a
compass needle and the direction of
Magnetic North
 It is termed East or West according to
whether the Compass North lies to the
East or West of Magnetic North.

30.  In order to discuss the causes of deviation
in detail, the magnetic properties of an
aircraft are divided into those that are
caused by:
– Hard iron magnetism
– Soft iron magnetism

31. Hard magnetism in aircraft
 Is permanent in nature
 Caused by steel components used in its
construction
 Such components are difficult to
magnetise but once magnetised, hold their
magnetic field for a long time.

32. Hard magnetism in aircraft
 This magnetism has its origin in components
permanently installed in the aircraft.
 So: Direction and force of hard magnetism relative
to the compass position will be the same for all
attitudes and headings
Hardened steel materials used in the engines, the
fuselage and in bolts and nuts all over the a/c

33. Hard magnetism in aircraft
 Distance between magnetic steel component
and sensitive part of the compass is important
The force of the field is reduced by the square
of the distance from the magnetic source

34. Hard magnetism in aircraft
 To study their influence on the sensitive
part of the compass, all hard iron
sources are considered simultaneously,
based on the direction and force of the
magnetic field they produce in the
position where the sensitive unit of the
compass is installed.

35. Hard magnetism in aircraft
HARD IRON DEVIATION FIELD COMPONENTS
Component Aircraft axis along Positive direction
which the component of component
acts
P Fore-aft axis Forward
Q Lateral axis Starboard
Z Vertical axis Downward

37. Vertical hard iron magnetism
 In straight and level flight:
 If compass needle is kept horizontal, the
directive force from the vertical magnetic field
will not cause deviation.
 It will cause dip or raise of one of its ends
 If not: turn/nose up or nose down
 the vertical magnetic field no more vertical
with respect to compass card
 Its horizontal component will cause deviation.

38. Component +R

39. Vertical hard iron magnetism
 Whenever the aircraft is banked or pitched,
the deviation values are likely to change
 These changes are predictable but as they
vary rapidly, it is not common to record
them for cockpit use.
 Deviation effects in a turn are more
complex due to sideways acceleration
making the compass card to leave the
horizontal

40. Soft magnetism in aircraft
 Soft iron magnetism is referred to metal
easily magnetised but that will lose its
magnetism with same facility.
 It is temporal induced magnetism due
to the Earth´s magnetic field, acting as a
focus for it and causing a localised
intensifying of that field.

41. Soft magnetism in aircraft
 It is due to the Earth´s field which gives
the components of the soft metal a
variable magnetic value which depends
on the forces H and Z

42. Soft magnetism in aircraft
 The strength will therefore vary with the
relative direction of the Earth’s field and
the strength of the Earth’s field.
 So: It will vary with heading, attitude and
position
 Its effects are analysed by considering
the equivalent effect from a series of
imaginary soft iron bars aligned
horizontally (h bars) or vertically (z bars)

43. Horizontal soft magnetism
 Magnetic intensity and polarity of the h
bars will vary directly with the strength of
H (Deviation force will increase as H
increases)
 As magnetic latitude varies, the directive
force of the compass also varies directly
with H.
 both effects tend to cancel each other
and deviation doesn’t change with
latitude

44. Soft magnetism in aircraft
 Magnetic intensity and polarity of z bars vary with Z
component of the Earth’s field.
(As Z increase, the strength of the deflecting force will
increase)
 As magnetic latitude varies, the directive force of
the compass also varies directly with H.
 Deviation due to z bars changes as Z/H
 Polarity inverts when crossing the magnetic
equator

45.  X’ = X + aX + bY + cZ + P
 Y’ = Y + dX + eY + fZ + Q
 Z’ = Z + gX + hY+ kZ +R

46. Soft magnetism in aircraft
 Effects and strength of their magnetic
field around the compass are easy to
calculate but difficult to compensate for
in isolation.
 They are most often only recorded as a
part of the total deviation registered
during a compass swing.

47. Deviation coefficients
•Before compass swing (check on compass
accuracy and documentation of deviation values)
deviations caused by aircraft magnetism on
various headings must be determined
•Values of deviations are analysed into
coefficients of deviation

48. Deviation coefficients
•Coefficient A: constant on all headings. Due to misalignment
of the compass lubber line.
•Coefficient B: results from deviations caused by P+cz
(deviations vary as the sine of the heading. Max on E/W
•Coefficient C: results from deviations caused by Q+fz
(deviations vary as the sine of the heading. Max on N/S

49. • C.A = DevN + DNE+DE+DSE+DS+DSW+DW+DNW
8
• C.B = Dev East – Dev West
2
• C.C = Dev North – Dev South
2
• Total deviation = A + B sin (hdg) + C cos (hdg)

50. Compass swing
 Special calibration procedure
 To determine to which extent compass
readings are affected by aircraft hard
and soft iron magnetism.
– Deviations may be determined
– Coefficients calculated
– Deviations compensated

51. When must a compass be swung
 New a/c from manufacture or New compass fitted
 Periodically or when specified in maintenance schedule
 After major inspection
 change of magnetic material in the a/c
 a/c moved involving a large change in latitude
 Lightning strike
 Same heading for more than 4weeks
 Carrying ferrous freight
 For issue of a C of A
 Any time when compass or residual deviation in
compass card is in doubt

52. JAR LIMITS
 JAR 25 for large a/c requires a deviation card
(level flight with engines running) near the
instrument
 Placard must show each MH readings not
greater than 45º steps
 Deviations greater than 10º not allowed in any
heading after compensation
 Distance compass-mag. material: no dev > 1º
 (same for electrical equipment when ON)

53. JAR LIMITS
 Use of undercarriage or flight controls: no dev
>1º
 Coefficient B/C not >15º(DRC) or 5º(RIC)
 Greatest dev on any heading after correction:
• DRC:3º
• RIC:1º

54. Compass swing includes:
Preparations: including finding suitable location
Correction swing: establishing the present
deviation on selected headings and correcting as
much as possible.
Check swing: registration of residual deviation
on as many headings as required.
Producing a deviation curve: based on the
residual deviation values found on check swing
Filling out the deviation card: for presentation
in the cockpit

57. Deviation Card
 This Card shows the pilot what
corrections need to be made to the
actual magnetic compass reading in
order to obtain the desired magnetic
direction
 This correction usually involves no
more than a few degrees

60. Dynamic Errors of the Compass
Turning and Acceleration Errors

61. Dynamic Errors of the Compass
• Dynamic errors of the magnetic compass
will occur when the aircraft turns,
accelerate or decelerate
• This type of errors will occur because the
compass card CG is located below the
compass card suspension point

62. Dynamic Errors of the Compass
• When accelerating, decelerating or turning
the aircraft, the compass card CG will be
displaced and the compass card will be
tilted
• When tilted, the vertical force will cause the
compass card to rotate and present a false
heading

63. Dynamic Errors of the Compass
• The magnetic compass card CG is below
the point of suspension in order to keep the
compass card horizontally stable during
level flight
• The vertical force of the Earth’s magnetic
field will pull the north pole of the compass
needle down on the northern hemisphere
and up on the southern

64. Dynamic Errors of the Compass
• This effect will partly be balanced by the
low position of the centre of gravity
• The exception is at the Magnetic Equator,
were there will be no such effect

66. Dynamic Errors of the Compass
• A swirl error will occur when the liquid in
the compass is not turning at the some rate
as the aircraft
• The liquid will indicate a too large heading
change after the turn has been completed

67. Turning Errors
• Turning error is most apparent when turning
to or from a heading of north or south
• This error increases near the poles, due to
the magnetic dip and the vertical
component of the Earth’s magnetic field
• There is no turning error flying near the
equator

68. Turning Errors
• When turning from a northerly heading in
the northern hemisphere, the compass gives
an initial indication of a turn in the opposite
direction
• It then begins to show the turn in the proper
direction, but it lags behind the actual
heading

70. Turning Errors
• The amount of lag decreases as the turn
continues, then disappears as the aircraft
reaches a heading of east or west
• When turning from a southern heading,
the compass gives an indication of a turn in
the correct direction, but it leads the actual
heading

71. Turning Errors
• This error disappears at east and west
headings
• You must lead the roll-out for turns to
north
• And lag the roll-outs for turns to south

75. Turning Errors
• To compute lead or lag in the roll-outs, use
the local latitude plus or minus the normal
roll-out lead (one-half the bank angle)
• Example: At 35ºN a right turn to north
requires a roll-out point of 318º(360-35-7)
(42º for LT)( and a right turn to south
208º(180+35-7) (152º for LT)

76. Acceleration and Deceleration
Errors
• Magnetic dip causes the acceleration and
deceleration errors, which are fluctuations
in the compass during changes in speed
• In the northern hemisphere, the compass
swings toward the north during
acceleration and toward the south during
deceleration

77. Acceleration and Deceleration
Errors
• This error is most pronounced when you
are flying on headings of east or west and
decreases gradually as you fly closer to a
north or south heading
• The error doesn’t occur when flying
directly north or south headings

80. Acceleration and Deceleration
Errors
• The memory aid: ANDS (Accelerate North,
Decelerate South)
• In the southern hemisphere, the error
occurs in the opposite direction (accelerate
south, decelerate north)