Air Navigation_Part 4

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AIR NAVIGATION
Part 4
COMPASSES
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LEARNING OUTCOMES

• On completion of this unit, you should
• Be able to carry out calculations to determine
  aircraft distance, speed and time
• Understand the principles of vectors and the
  triangle of velocities to establish an aircrafts
  track and ground speed

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LEARNING OUTCOMES

• Understand the principles of the 1 in 60 rule
• Understand the types of compass systems used for
  air navigation, how they work and their
  limitations
• Know the hazards that weather presents to
  aviation

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Introduction
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You will have learnt about the difference between
and YOU WILL HAVE GOT LOST using the Silva, a
simple hand held compass
TRUE NORTH
MAGNETIC NORTH
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To understand aircraft compasses, their strengths
and weaknesses we need to look into the subject a
little deeper
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The first thing you need to understand is
the shape of the magnetic field around a magnet
Shape of the magnetic field around a magnet
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The Earths magnetic field, follows the same
pattern as the field round a bar magnet but needs
a little explaining
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The red end of a magnet (known as the North Pole)
is in fact a north-seeking pole
Therefore, as opposites attract it can be seen
that if the red end of our compass needle is to
point to the North Magnetic Pole
then in reality the North Magnetic Pole must, in
magnetic terms be a south pole
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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
becomes so weak as to render magnetic compasses
virtually useless.
Looking at the diagram on the left the lines of
force are only parallel to the surface of the
Earth at the Equator. Indeed, at the poles the
lines of force are vertical!
A compass needle will try to follow the lines of
force but is constrained by the construction to
stay almost horizontal The end result of this is
that the more vertical the Earths field, the
weaker the directional force on the horizontal
compass needle becomes.
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Aircraft Compasses
We will now look at Aircraft Compasses
There are 2 main types
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In an aircraft, the simplest form of compass is
the Direct Indicating Compass (shown right),
which looks very similar to the car compass,
which can be bought from accessory shops.
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The Direct Indicating Compass
The Direct Indicating Compass (DIC), like the
hand held Silva compass, has a magnet suspended
in liquid, which helps to dampen any movement
The points of the compass are printed around the
equator of the ball, the heading is shown
against an index mark on the bowl. The magnet is
hidden in the ball.
It has the appearance of a squash ball inside a
goldfish bowl.
on gliders the compass is on the cockpit coming
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The Direct Indicating Compass
The DIC has several serious limitations, so it
is normally used as a standby
Those limitations are
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The Suspended Magnet Will Only Give A Correct
Reading In Steady Straight Level Flight. During
Turns Acceleration The Magnet Is Swung To One
Side And Gives False Readings
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The DIC is located in the cockpit, and there it
is affected by the magnetic fields emanating from
both the metal the aircraft is made from and from
the various electrical circuits in the aircraft.
These other magnetic fields badly affect the
accuracy of the DIC.
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The driving power of the horizontal portion of
the Earths magnetic field is only strong enough
to turn a compass needle it does not have
sufficient torque to drive repeaters at other
crew positions in the aircraft
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The DIC only indicates magnetic heading, modern
aircraft may require True or Grid headings
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At high magnetic latitudes (above 70º North or
South) the DIC becomes sluggish and unreliable
because the angle of dip is so steep and the
directional force is so weak.
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Advantages of the 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.
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To overcome the limitations of the DIC, the Gyro
Magnetic Compass was invented
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Gyro Magnetic Compass
Its made up of the following components
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A Magnetic Detector Unit, which electrically
senses the direction of Earths magnetic field
and is normally situated in the wing tip
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A Gyroscope, which continues to point to a fixed
point in space, regardless of any manoeuvres the
aircraft may make
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Z AXIS
FRAME
ROTOR
A gyroscope
Y AXIS
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An Error Detector that senses any difference
between the gyro and magnetic headings and
applies a correction to the gyro at a pre-set rate
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A controller or computer that applies corrections
to the gyro to correct for the rotation of the
Earth and the aircrafts flight path around the
Earth
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A display or displays to show the aircraft
heading at required positions in the aircraft.
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
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The basic principle of the GMC is that it uses
the long-term accuracy of the detector unit
combined with the short-term accuracy of the gyro.
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What this means is that the gyro, which is
connected to the compass, is constantly corrected
by the magnetic detector, which is correct during
straight and level flight
and is more accurate than the DIC because being
situated in the wing it is less affected by the
deviating forces from other extraneous magnetic
fields in the aircraft
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If a roll cut out switch is used no error is fed
from the magnetic detector to the gyro in the
turn, if a roll cut out is not present, the error
correction rate is low enough to only make a
small effect which is removed when the wings are
levelled
During a turn, the gyro (which is unaffected by
turns) is more accurate
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A gyro magnetic system has considerably more
torque than a DIC and can therefore provide
outputs to repeater units in other positions in
an aircraft and/or computers in the aircraft. The
output to these repeaters can be easily modified
so that they can display either true or magnetic
heading
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Gyro Errors
To overcome this the gmc has developed as a
system where the gyro heading can be relied on
for short period ( about 10 minutes )
As the gyroscope is a manufactured item, it
cannot be perfect
Over a period of time it will become inaccurate (
this is called gyro wander ).
It can then be reset by reference to the magnetic
detector
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Inertial Navigation, GPS and Beyond
Throughout the UK the variation errors on maps
charts are reasonably accurate, but if we go into
polar regions we face 2 problems
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Problem 1
Variation values are unreliable and as large as
180 degrees between true magnets poles
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Problem 2
The second problem is that as the compass nears
the magnetic pole the compass detector will try
to point at it. this is called dip.
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Internal Navigation
A modern aircraft with a heading error of one
degree can easily have position errors in the
order of 6 miles/hour, which nowadays is
not acceptable.
The Inertial Navigation System (INS) eliminates
this problem and can align itself with True North
without the need for variation
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A typical inertial navigation system can achieve
positional accuracies of one miles/hour. Whilst
this accuracy may appear good, it is still a long
way short of the latest development in
navigation technology.
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Using Ring Laser Gyros 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, we are not there yet, but it will
happen.