AIR NAVIGATION

Part 4

COMPASSES

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:

Introduction

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.

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

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.

To overcome the limitations of the DIC, the Gyro Magnetic Compass was invented

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.

VC10 Instruments

Main Compasses

VOR Repeaters

TACAN Repeaters

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.

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.