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.
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
13.
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.
15.
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
17.
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
20.
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
22.
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
24.
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
27.
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.
29.
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
36.
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.
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
55.
56.
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
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
65.
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
69.
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
72.
73.
74.
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
78.
79.
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)