Transcript 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.
Magnetic North UK Grid 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 S N 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
Inertial Navigation and Beyond
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 180 o between true & magnetic poles TRUE NORTH MAGNETIC NORTH
Inertial Navigation and Beyond
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.
Inertial Navigation and Beyond
A heading error of 1 o can have position errors in the order of 6 miles/hour.
The Inertial Navigation System (INS) eliminates this problem and can align itself with True North without the need for variation.
Inertial Navigation and Beyond
A computer-based navigation system that tracks the movement of an aircraft via signals produced by onboard accelerometers.
Inertial navigation systems can achieve positional accuracies of 1 mile/hour.
Inertial Navigation and Beyond
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 Steady Straight & Level Flight. It only indicates magnetic heading, 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), does have three advantages.
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 the gyro and magnetic headings 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 to show the heading.
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 ( about 10 minutes ) 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.
Apparent errors follow simple 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?
Compass headings and doppler values 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.