air navigation part 4 compasses. learning outcomes –understand the types of compass systems used...
TRANSCRIPT
LEARNING OUTCOMES
– Understand the types of compass systems used for air navigation, how they work and their limitations
On completion of this lesson, you should:
You will have learnt about the difference between
TRUE NORTH
MAGNETIC NORTH
and YOU WILL HAVE GOT LOST using the Silva, a simple hand held compass
&
To understand aircraftcompasses, their strengths and
weaknesses we need to look into thesubject a little deeper
The first thing you need to understand is theshape of the magnetic field around a magnet
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
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!
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.
A compass needle will try to follow the lines of force but is constrained by it’s construction to stay almost horizontal
The end result of this is that the more vertical the Earth’s field, the weaker the directional force on the horizontal compass needle becomes.
In an aircraft, the simplest form of compass is the Direct Indicating Compass
This looks very similar to the car compass, which can be bought from accessory shops.
Direct Indicating Compass
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
It has the appearance of a squash ball inside a goldfish bowl.
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.
On gliders the compass is on the cockpit coming
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 Direct Indicating Compass
To partially correct for these influences, when a DIC is installed in an aircraft a compass swing iscarried out.
This requires the aircraft to be placed on a compassswing bay which has the magnetic headings marked on it.
The aircraft is then turned onto the compass headingsmarked on the bay and those headings comparedwith the DIC heading.
A correction chart is then made out and mountedin the cockpit which allows the pilot to make corrections to the DIC heading while flying.
The Direct Indicating Compass
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 Direct Indicating Compass
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.
A Gyroscope
FRAME
ROTOR
Y AXIS
Z AXIS
This unit continues to point to a fixed point in space, regardless of any manoeuvres the aircraft may make
It’s made up of the following components:Gyro Magnetic Compass
Directional Gyro
Electrically senses the direction of Earth’s magnetic field and is normally situated in the wing tip
A Magnetic Detector Unit
MagneticDetector
Directional Gyro
Gyro Magnetic Compass
A controller or computer
Applies corrections to the gyro to correct for therotation of the Earth and the aircrafts flight path around the Earth
MagneticDetector
Directional Gyro
CompassComputer
Senses any difference between the gyro and magnetic headings and applies a correction to the gyro at a pre-set rate, normally done by the computer.
An Error Detector
MagneticDetector
Directional Gyro
CompassComputer
Shows the aircraft heading at required positions in the aircraft.
A Display or Displays
MagneticDetector
Directional Gyro
CompassComputer
Main Display
Secondary Display
Control the systems.
Various Amplifiers and Motors
Minimises the effect of a turn on the Magnetic Detector Unit
Roll Error Cut Out Switch
Above a designated angle of bank the MagneticDetector is disconnected from the computer and sofalse magnetic signals do not make the compassdrift.
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.
Gyro Magnetic Compass
What this means is that the gyro, which is the compass, is constantly corrected by themagnetic detector, which is correct during straight and level flight
It 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
During a turn, the gyro (which is unaffected by turns) is more accurate
When a roll cut out switch is used the magnetic detector signal to the computer is not used in turns. This normally operates at 15º angle of bank and prevents false magnetic signals causingGyro drift.
Gyro Magnetic Compass
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.
When 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.
Gyro Errors
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 ).
To overcome this the gmc has developed a system where the gyro heading can be relied on for short periods ( about 10 minutes )
It can then be reset by reference to the magnetic detector
To navigate by gmc only, this wander rate must be less than 2º/hr
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
MAGNETICNORTH
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.
Problem 2
Internal NavigationA modern aircraft with a heading error of one degree can easily have position errors in the order of 6 miles/hour, which nowadays is notacceptable.
The Inertial Navigation System (INS) eliminates this problem and can align itself with True North without the need for variation
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 navigationtechnology.