### Air Nav. Pt 7

```AIR NAVIGATION
Part 7
Magnetic Fields
and The Compass
Introduction
You already know the difference between
TRUE NORTH, GRID NORTH
and MAGNETIC NORTH
But to understand aircraft compasses,
their strengths and weaknesses
we need to look into the subject of
magnetic fields a little deeper.
UK
Grid North
Magnetic North
True North
Introduction
The Earth’s magnetic field follows the same
pattern as the field round a bar magnet.
The North Pole is in fact a north-seeking pole.
Therefore, as opposites attract,
if the red end is to point to Magnetic North,
then in reality, it must be a south pole.
N
N
S
N
S
S
Introduction
The lines of force are only parallel
to the Earth’s surface at the Equator.
At the poles the lines of force are vertical !
The result is the more vertical the Earth’s field,
the weaker the directional force becomes
on the horizontal compass needle.
N
S
N
S
Introduction
At our latitude, the lines of force point down
at an angle (known as the angle of dip) of 65º,
once the angle exceeds 75º
(which occurs about 1200 miles from the Poles)
the directional force is so weak as to render
magnetic compasses virtually useless.
N
S
N
S
The magnetic variation errors
on charts in the UK are reasonably accurate,
but in the polar regions we face 2 problems.
Problem One
Variation values are unreliable and as large as
180o between true & magnetic poles
TRUE
NORTH
MAGNETIC
NORTH
The second problem is that
as the compass nears the magnetic pole,
the compass detector will try to point
directly at the pole.
This is called DIP.
A heading error of 1o can have position errors
in the order of 6 miles/hour.
eliminates this problem
and can align itself with True North
without the need for variation.
that tracks the movement of an aircraft
via signals produced by onboard accelerometers.
can achieve positional accuracies
of 1 mile/hour.
Using Ring Laser or Fibre Optical Gyros
to feed an Inertial Reference System,
which is paired with a
Global Positioning System (GPS),
can produce a position,
which is accurate to within 5 metres.
The ultimate aim is to
achieve millimetre accuracy
Aircraft Compasses
There are two main types of aircraft compass.
The simplest form is
the Direct Indicating Compass,
which looks very similar to
the car compass.
The Direct Indicating Compass (DIC),
has a magnet suspended in liquid,
which helps to dampen any movement.
Direct Indicating Compass (DIC)
The Direct Indicating Compass (DIC),
is normally used as a standby,
because of serious limitations.
Located in the cockpit,
it is affected by magnetic fields.
It only gives a correct reading in
modern aircraft may require True or Grid headings
Direct Indicating Compass (DIC)
The Direct Indicating Compass (DIC),
is normally used as a standby,
because of serious limitations.
At high magnetic latitudes
(above 70º N or S)
it becomes sluggish and unreliable
because the angle of dip is so steep
and the directional force is so weak.
The power of the Earth’s magnetic field is only
strong enough to turn a compass needle;
there is not enough torque to drive repeaters
to other crew positions in the aircraft.
Direct Indicating Compass (DIC)
The Direct Indicating Compass (DIC),
It is very simple
and therefore reliable.
It is very cheap and lightweight.
It does not require any form of power
and so will continue to work
even after a total power failure in the aircraft.
The Gyro Magnetic Compass
To overcome the limitations of the DIC,
the Gyro Magnetic Compass was invented,
that uses the long-term accuracy
of a detector unit
combined with the short-term
accuracy of a gyroscope.
The Gyro Magnetic Compass
The Gyro Magnetic Compass is made up of :
a Magnetic Detector Unit,
which electrically senses
the direction of Earth’s magnetic field
a Gyroscope
which points to a fixed point in space,
regardless of any manoeuvres
the aircraft makes.
an Error Detector,
to sense any difference between
and apply corrections to the gyro.
The Gyro Magnetic Compass
The Gyro Magnetic Compass is made up of :
a Controller or Computer
to correct the gyro for the Earth’s rotation
and the aircrafts flight path around the Earth.
a Display or Displays
various Amplifiers and Motors
to control the system.
and in some GMCs
a Roll Cut-out Switch to minimise the effect of
a turn on the Magnetic Detector Unit.
The Gyro Magnetic Compass
The principle of the
Gyro Magnetic Compass (GMC)
is that it uses the long-term accuracy
of the detector unit
combined with the short-term accuracy
of the gyroscope.
FRAME
ROTOR
Z AXIS
Y AXIS
The Gyro Magnetic Compass
The gyro, connected to the compass,
is constantly corrected by the magnetic detector,
and is more accurate than the DIC,
because being situated in the wing
it is less affected by the deviating forces
from other magnetic fields in the aircraft.
FRAME
ROTOR
Z AXIS
Y AXIS
The Gyro Magnetic Compass
During a turn, the gyro is unaffected
and more accurate,
so it takes over from
the magnetic detector flux valve,
and if a roll cut-out switch is used
no error is fed from the magnetic detector
to the gyro during the turn.
A gyro system has more torque than a DIC
and can provide outputs to repeater units
and/or computers in the aircraft.
The Gyro Magnetic Compass
Gyro Errors
A gyro suffers from Real and Apparent errors.
Real errors are caused by inaccuracies
during the manufacturing process.
Over a period of time it will become inaccurate
( this is called gyro wander ).
To overcome this the gyro heading
can only be relied on for short period
before being reset by the Magnetic Detector
The Gyro Magnetic Compass
Gyro Errors
A gyro suffers from Real and Apparent errors.
Apparent errors, are caused because we fly
around a rotating Earth.
mathematical formulae and are easily corrected,
in many cases they are compensated
for automatically using a corrector unit
built into the compass system.
Check Understanding
Which of the following statements
is true?
Lines of magnetic force are parallel
to the Earth’s surface at the South Pole
Lines of magnetic force are vertical
to the Earth’s surface at the Equator
Lines of magnetic force are parallel
to the Earth’s surface at the North Pole
Lines of magnetic force are parallel
to the Earth’s surface at the Equator
Check Understanding
When what angle is exceeded
does directional magnetic force
become so weak as to render
magnetic compasses virtually useless?
45 degrees
65 degrees
75 degrees
90 degrees
Check Understanding
As a compass nears the magnetic north pole
the magnetic detector will try and point
directly at the pole.
What is this called?
Drop
Dip
Waver
Tilt
Check Understanding
What principle is used in
an Inertial Navigation System to calculate
the position of the aircraft?
Initial location on the ground is set
and accelerations are measured
The navigator must update
the system all the time
A gyroscope feeds position
to the computer
Check Understanding
An Inertial Reference System paired with a
Global Positioning System (GPS),
can produce a position
accurate to within how many metres?
2 metres
4 metres
5 metres
8 metres
Check Understanding
Which of the following is true about
the Direct Indicating Compass?
It gives True and Magnetic headings.
It is constantly corrected
by the magnetic detector.
It is very reliable at
high magnetic latitudes.
It will continue to work after a total
power failure in the aircraft.
Test Questions
Why are RAF aircraft fitted with
a Direct Indicating Compass?
It is not affected by turns
and acceleration
It is the most accurate
compass system available
It is reliable and
needs no power supply
It give a True North reading
Check Understanding
Which of the following is an advantage
of a Gyro-Magnetic Compass over
a Direct Indicating Compass?
A GMC is cheaper
A GMC can feed repeaters
around the aircraft
A GMC requires no power supply
A GMC does not work well
during tight turns
Test Questions
Which of the following is not a component
of a Gyro Magnetic Compass system?
A gyroscope
A flux valve magnetic detector
A suspended magnet
A turn/accelerator cut-out switch
Check Understanding
Which of the following is true about
the Gyro Magnetic Compass?
The gyro magnetic Compass is less accurate
than the DIC.
The flux valve takes over from the gyroscope
during climbs and descents.
The flux valve controls the speed
of the gyroscope.
The gyroscope takes over from
the flux valve during turns.