AIR NAVIGATION

Part 7

Magnetic Fieldsand The Compass

Introduction

Magnetic North True North

UKGrid North

You already know the difference betweenTRUE 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.

S

N

IntroductionThe 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.

S

N

N

S

IntroductionThe 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.

S

N

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 rendermagnetic compasses virtually useless.

S

N

N

S

Inertial Navigation and BeyondThe magnetic variation errors

on charts in the UK are reasonably accurate, but in the polar regions we face 2 problems.

Problem OneVariation values are unreliable and as large as

180o between true & magnetic poles

TRUE NORTH

MAGNETICNORTH

Inertial Navigation and BeyondThe 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 BeyondA heading error of 1o 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 BeyondA 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 toachieve millimetre accuracy

Aircraft CompassesThere 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 inSteady 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 steepand the directional force is so weak.

The power of the Earth’s magnetic field is onlystrong enough to turn a compass needle; there is not enough torque to drive repeatersto 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 CompassTo 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 CompassThe 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 CompassThe 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 CompassThe 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

Y AXIS

Z AXIS

The Gyro Magnetic Compass

FRAME ROTOR

Y AXIS

Z AXIS

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.

The Gyro Magnetic CompassDuring 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 CompassGyro 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 CompassGyro Errors

A gyro suffers from Real and Apparent errors.

Apparent errors, are caused because we fly around a rotating Earth.

Apparent errors follow simplemathematical formulae and are easily corrected,

in many cases they are compensatedfor automatically using a corrector unit

built into the compass system.

Check UnderstandingWhich of the following statements

is true?

Lines of magnetic force are parallel to the Earth’s surface at the Equator

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 South Pole

When what angle is exceededdoes directional magnetic forcebecome so weak as to render

magnetic compasses virtually useless?

90 degrees

65 degrees

75 degrees

45 degrees

Check Understanding

Check UnderstandingAs a compass nears the magnetic north pole

the magnetic detector will try and point directly at the pole.What is this called?

Tilt

Dip

Waver

Drop

Check UnderstandingWhat principle is used in

an Inertial Navigation System to calculatethe position of the aircraft?

A gyroscope feeds position to the computer

Initial location on the ground is setand accelerations are measured

The navigator must update the system all the time

Compass headings and doppler values

Check UnderstandingAn Inertial Reference System paired with a

Global Positioning System (GPS), can produce a position

accurate to within how many metres?

8 metres

4 metres

5 metres

2 metres

Check UnderstandingWhich of the following is true about

the Direct Indicating Compass?

It will continue to work after a total power failure in the aircraft.

It is constantly corrected by the magnetic detector.

It is very reliable at high magnetic latitudes.

It gives True and Magnetic headings.

Why are RAF aircraft fitted with a Direct Indicating Compass?

It give a True North reading

It is the most accurate compass system available

It is reliable and needs no power supply

It is not affected by turns and acceleration

Test Questions

Check UnderstandingWhich of the following is an advantage

of a Gyro-Magnetic Compass over a Direct Indicating Compass?

A GMC does not work well during tight turns

A GMC can feed repeaters around the aircraft

A GMC requires no power supply

A GMC is cheaper

Which of the following is not a component of a Gyro Magnetic Compass system?

A turn/accelerator cut-out switch

A flux valve magnetic detector

A suspended magnet

A gyroscope

Test Questions

Check UnderstandingWhich of the following is true about

the Gyro Magnetic Compass?

The gyroscope takes over from the flux valve during turns.

The flux valve takes over from the gyroscope during climbs and descents.

The flux valve controls the speed of the gyroscope.

The gyro magnetic Compass is less accurate than the DIC.

AIR NAVIGATION

End of Presentation