AIR NAVIGATION
Part 4
COMPASSES
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 aircraft’s
track and ground speed
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
Introduction
You will have learnt about the
difference between
and YOU WILL HAVE GOT LOST using the
TRUEhand
NORTH
Silva, a simple
held compass
MAGNETIC NORTH
To understand aircraft
compasses, their strengths and
weaknesses we need to look into the
subject a little deeper
Shape of the
The
magnetic
first thing
field
youaround
need a magnet
to understand is the
shape of the magnetic
field around a magnet
The Earth’s magnetic
field, follows the same
pattern as the field
round a bar magnet but
needs a little explaining
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
At our latitude, the lines
Looking
at the
diagram
on
Aofcompass
needle
will
force point down at
the
leftfollow
the lines
of force
try
to
the
lines
of
an angle (known as the
are
only
parallel
to the
force
but
is
constrained
angle of dip) of 65º;
surface
of the Earthto
at the
by
the
construction
once the angle
Equator.
Indeed,
at the
stay
almost
horizontal
exceeds 75º (which
poles
theresult
lines of this
forceisare
The
end
occurs about 1200
vertical!
that
the
more
miles
from
thevertical
Poles)
the
Earth’s
field,
the
the directional force
weaker
thesodirectional
becomes
weak as to
force
onmagnetic
the horizontal
render
compass
needle
compasses virtually
becomes.
useless.
Aircraft Compasses
We will now look at Aircraft
Compasses
There are 2 main types
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.
N 33
30
N 33
30
The Direct Indicating Compass
TheThe
points
Direct
of the
Indicating
compass
Itare
has
the
appearance
of a
onprinted
gliders
the like
Compass
(DIC),
around
the
the
squash
ball
inside
a
compass
onball,
the
equator
hand
held
of is
the
Silva
compass,
& the
bowl.
cockpit
coming
heading
has
a goldfish
magnet
is shown
suspended
against an
index
in liquid,
mark on
which
the bowl.
helps The
to
magnet
dampen
is hidden
any movement
in the ball.
The Direct Indicating Compass
The DIC has several serious limitations,
so it is normally used as a standby
Those limitations are:
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
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.
The driving power of the horizontal
portion of the Earth’s
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
The DIC only indicates magnetic
heading, modern aircraft may
require True or Grid headings
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.
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.
To overcome the limitations of the DIC, the
Gyro Magnetic Compass was invented
Gyro Magnetic Compass
It’s made up of the following
components:
A Magnetic Detector Unit,
which electrically senses the
direction of Earth’s magnetic
field and is normally situated
in the wing tip
A Gyroscope,
Z AXIS which continues to point
FRAME
to a fixed point in space, regardless
ofROTOR
any manoeuvres the aircraft may
make
Y AXIS
A gyroscope
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
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
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
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.
and
What
is more
this means
accurate
is
than
that the
thegyro,
DIC because
which is
being
connected
situated
toin
the
the
compass,
wing it is less
is constantly
affected
by the
corrected
deviating
by the
forces
from
magnetic
other extraneous
detector,
which
magnetic
is correct
fields during
in the
straight aircraft
and level flight
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
During
a turn,rate
the is
gyro
error
correction
low
(which
is unaffected
by
enough
to only
make a small
turns)
is is
more
accurate
effect
which
removed
when
the wings are levelled
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
Gyro Errors
To gyroscope
overcome this
gmc has
As the
is a the
manufactured
developed
a system
where the gyro
item, itascannot
be perfect
heading can be relied on for short
Over aperiod
period( about
of time10it minutes
will become
)
inaccurate ( this is called gyro
It can then bewander
reset by).reference to the
magnetic detector
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
Problem 1
Variation values are unreliable
and as large as 180 degrees
between true & magnets poles
TRUE
NORTH
MAGNETIC
NORTH
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.
Internal Navigation
A modern aircraft with a
The
Inertial
Navigation
heading
error
of one
System
(INS)easily
eliminates
degree can
have
this
problem
and
position
errors
in can
the
align of
itself
with True
order
6 miles/hour,
North
the is
need
which without
nowadays
not
for
variation
acceptable.
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.
Using Ring Laser Gyros or Fibre Optical
Gyros to feed an Inertial Reference
The which
ultimate
aim is to
System,
is paired
with a
achieve
millimetre
Global Positioning System (GPS), can
accuracy,
we
are
not
produce a position, which is accurate to
there
yet,
but it will
within
5 metres
happen.