EAA CPL 010 Aircraft Technical

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COMMERCIAL PILOT LICENCE

AND

INSTRUMENT RATING EXAMINATION

AIRCRAFT, TECHNICAL & GENERAL

BOOK 2

Version 1.01

PREFACE

These Study Notes have been written for pilots preparing for the South African Commercial Pilot Licence and Instrument Rating technical examination. They cover the full, published

syllabus as at April 1999.

This book has been re-written and re-drawn in accordance with the latest CAA Syllabus by Cedric Mew, Gr II Instructor. Eltanin Aerospace Academy acknowledges his invaluable

assistance.

The syllabus has been split into chapters for ease of use. Each chapter usually covers one section of Aircraft Technical. Students should complete these exercises and questions before moving on to the next chapter. In addition a book of “Typical Examination Questions” is provided in your set of Home Study Notes.

PLEASE NOTE THAT ALL THE DIAGRAMS ARE FOR STUDY PURPOSES ONLY. DISCLAIMER

While every reasonable care has been taken to ensure that these Notes are free from error, no responsibility whatever is accepted for any actions or claims resulting from the use of these Notes. By purchasing these notes you accept these conditions and agree to be bound by them.

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AIRCRAFT, TECHNICAL & GENERAL -

BOOK TWO

CONTENTS

AIM AND OBJECTIVE Page 4

AMENDMENT LIST RECORDS Page 5

Chapter 1 Airframes Page 9

Chapter 2 Aircraft electrical systems Page 15 Chapter 3 Fuel systems & fuel types Page 47 Chapter 4 Undercarriages Page 53 Chapter 5 Oxygen systems Page 59 Chapter 6 Pressurisation and Air conditioning Page 63 Chapter 7 Anti-ice and de-icing systems Page 83

Chapter 8 Hydraulics Page 93

Chapter 9 Pneumatics Page 103

Chapter 10 Power plants Page 107 Chapter 11 Engine parts and power ratings Page 109 Chapter 12 Piston engines Page 119 Chapter 13 Detonation and pre-ignition Page 131 Chapter 14 Lubrication and cooling systems Page 135 Chapter 15 Carburetion and fuel injection systems Page 141 Chapter 16 Supercharging and turbo-supercharging Page 151 Chapter 17 The gas turbine engine Page 157 Chapter 18 Aircraft Elements –Bearings, Valves, Pumps

Filters Page 175

Chapter 19 High Speed Flight Page 181 Chapter 20 Hydroplaning Page 201 Chapter 21 Fire Systems Page 207

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AIRCRAFT, TECHNICAL & GENERAL -

BOOK TWO

AIM

The aim of this course is to give the student Commercial Pilot a sound technical knowledge of the Principles of Flight and the Operation of Aircraft.

OBJECTIVE

After completing the ATG module the student will have a good understanding of the following:

! basic principles of aerodynamics

! a comprehensive knowledge of the aircraft airframe including wing, cockpit and cabin windows and landing gear etc.

! piston, turbo-propeller and jet aircraft engines ! the different aircraft systems, some of which are:

" Electrical system " Air-conditioning system " Fuel system

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AMENDMENT LIST RECORDS

Version No.

Date Details

Nov 2006

Complete re-design of these study notes. 1.01 Jan

2008

Aviation Courseware replaced all drawings with graphics drawings. Mr I. Forsyth was responsible for managing the update of the Aeronav manuals. This included the drawings and improving the text.

1.0 Feb 2008

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CHAPTER 1– AIRFRAMES & SYSTEMS OBJECTIVE

To achieve a thorough understanding of the basic principles of aerodynamics.

CONTENTS OF THIS CHAPTER:

! The airframe and systems: ! The fuselage

" Types of construction

" Structural components and materials ! Cockpit and cabin windows

" Construction (laminated glass) " Structural limitations

" Window heating ! Wing structure

" General construction " Types of wings

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AIRFRAMES – FUSELAGE AND WING CONSTRUCTION

INTRODUCTION

In flight aircraft undergo stresses in normal and abnormal operations. The whole airframe structure is designed to handle these stresses. The basic stresses are tension, compression, bending, torsion and shear loads.

Tension – is the stress acting against another force

that is trying to pull something apart. For example, engine power and propeller are pulling the airplane forward while the fuselage, wings, and tail section resist that movement because of the airflow around them. The result is a stretching effect on the airframe. Bracing wires in an aircraft are usually in tension.

Compression – is a squeezing or crushing force that tries to

make parts smaller. Aircraft wings are subjected to compression stresses. The ability of a material to meet compression requirements is measured in pounds per square inch (psi).

Bending – is a combination of two

forces, compression and tension. During bending stress, the material on the inside of the bend is compressed and the outside material is stretched in tension.

Torsion – is a twisting force. Because

aluminium is used almost exclusively for the outside, and, to a large extent, inside fabrication of parts and covering, its tensile strength (capability of being stretched) under torsion is very important. While in flight, the engine power and propeller twist the forward fuselage and the airframe is therefore subjected to variable torsional stresses during turns and other manoeuvres.

Shear – stress tends to slide one piece of

material over another. The aluminium skin panels of the fuselage are riveted to one another. Under flight loads shear forces try to make the rivets fails so selection of rivets with adequate shear resistance is critical.

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FIGURE 1-3

FUSELAGE – DESIGN AND CONSTRUCTION

The fuselage structures of aircraft today can usually be divided into ! The truss structure – the longerons and

subsidiary members carry the load and transmit the various stresses incurred. Construction is of wood, steel tube, aluminium tube, or other cross sectional shapes which may be bolted, welded, bonded, pinned, or riveted into a rigid assembly (Fig 1-1).

! Monocoque – The covering or skin is an integral structural or load carrying

member. Monocoque (single shell) structure is a thin walled tube or shell which may have rings, bulkheads or formers installed within. It can carry loads effectively, particularly when the tubes are of small diameter. The stresses in the monocoque fuselage are transmitted primarily by the strength of the skin (Fig 1-2).

! Semi-monocoque – The most popular type of structure used in aircraft today. Stressed skin construction has come to be the standard for most aircraft builders. Here, internal braces as well as the skin itself carry the stress. The internal braces include longitudinal (lengthwise) members called

stringers and vertical bulkhead. The metal skin exterior is riveted, or bolted and riveted, to the finished fuselage frame, with the skin carrying some of the overall loading. The skin is quite strong in both

tension and shear and, if stiffened by other members, may be made to carry some compressive load. Since the skin of the structure must carry much of the fuselage's strength, it will be thicker in some places than at other places. In other words, it will be thicker at those points where the stress on it is the greatest (Fig 1-3).

Stringer

Frame Curved skin covers airframe structure

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WING STRUCTURE INTRODUCTION

The main purpose of a wing is to give lift. The structure of a wing must be strong enough, yet still be light enough, for an aircraft to be able to generate lift.

Over the years wings have developed from an internal structure where most of the loading and a skin formed the aerofoil to the present day where every part of the structure has a structural purpose.

The materials used to cover wings have also changed over the years – from wood and fabric to metals such as aluminium and more recently – composites.

PROPERTIES OF A WING

! Light in weight – a wing structure has to be designed to cope with torsional and bending loads and at the same time be sufficiently light in weight and yet generate enough lift so the aircraft can take off.

! Longitudinal stiffness – able to cope with bending loads. A reaction force is created by the joint of the wing to the fuselage. At this point the wing is fixed and therefore the upwards force along the length of the wing tries to bend the wing upwards. Reaction forces at the joint, compression and tension resist this bending and these forces are transferred throughout the length of the wing to stop the whole structure from bending. ! Torsional stiffness - able to cope with loads which would cause twisting such as the

forces applied when using flaps and ailerons. Consider the wing to be an ‘I’ beam (Fig 1-4). If a force is applied off-centre the wing would twist. With so many variables in the types of

loading to be experienced by a wing, the wing itself could never be designed so the force always acted at the torsional centre. Therefore the wing structure must be designed so that it can cope with torsional loading.

GENERAL CONSTRUCTION

With a few exceptions most wings are constructed of a spar or spars of different shapes and construction, ribs to provide contour or shape and stringers to support the skin.

! The spar is used to attach the wing to the fuselage. A conventional structure consists of front and rear spars with the metal skin attached to the spar flanges forming a torsion box.

! The ribs maintain the aerodynamic shape of the wing, support the spars, stringers and skin against buckling and pass loads created by the engines, undercarriage and control surfaces into the skin and spars.

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! The stringers provided that they are continuous in the spanwise direction, react to bending moment in the same way as the skin. They stiffen the skin by increasing the stress at which the skin buckles.

! The skin usually of a light but strong aluminium metal is riveted to the stringers giving the framework extra strength and

rigidity.

TYPES OF WINGS

Braced or External Strut wings –

bracing struts run from the fuselage to the point halfway along the wing. They anchor the spars in torsion and relieve them of a lot of their vertical load. This type of wing is mainly used for small high wing aircraft.

Cantilever wings – Mainly used on

high performance aircraft. No external bracing struts are utilized. The wings are usually tapered from a thin tip to a thicker root where the stresses are greatest.

D Spar construction – The front spar is

placed as near as possible to the point of maximum thickness of the wing. The skin of the leading edge is rigidly attached to it to form a D-shaped tube relieving the torsional stresses of the wing. The rear spar forms a mounting for the aileron and flaps and is

connected by the skin and a light rib structure to the D-spar.

Torsion Box

construction – A torsion

box runs along the length of the wing providing all the torsional and longitudinal stiffness required in the wing. The torsion box is formed by the upper and lower surfaces of the skin

rigidly attached to the front and rear spars in the form of a box. The skin between the spars is corrugated in order to increase the load carrying capacity and this is then covered with a thin metal sheet.

Fig. 1-5a External Strut Wings

Fig. 1-5b Cantilever Wing

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Stressed Wing Construction A light alloy skin is riveted to the framework and is designed

to stiffen the wing by taking some of the load. It produces a relatively strong wing without too large a weight penalty.

COCKPIT AND CABIN WINDOWS INTRODUCTION

Since the modernization of aircraft into the jet age, cabin and cockpit windows have had to be adapted to withstand high speed and pressurization which they were not required to do in the aeroplanes of yesteryear. Several design factors and materials had to be incorporated into the manufacturing of modern cockpit and cabin windows such as strength, heat

resistance, icing, durability and visibility etc. Cockpit windows are subjected to numerous stresses such as bird strikes, loading imposed by flight manoeuvres and pressurization. With advancements in

technology various types of glass and synthetic glass have been created:

! Monolithic acrylic ! Laminated acrylic ! Acrylic/glass laminates ! Glass/glass laminates ! Cold heating film for

heating windows ! Wire gribs

Fig. 1-5e Stressed Skin Wing

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CONSTRUCTION

Depending on the type of aircraft the windows would be constructed from especially hardened polycarbonate, laminated glass and/or acrylic materials.

Cabin windows are normally constructed of single layer acrylic while various types of construction processes are used for cockpit windows.

Plastic interlayers generally made of Polyvinyl Butyral (PVB) are placed between sheets of tempered glass to form a laminate. A wafer thin coating of polyurethane is vapour

deposited between the layers of acrylic and glass to raise the chemical and mechanical resistance of the materials. The acrylic layer serves to prevent the glass from shattering.

HEATING

Windshield heating is provided by supplying power to heat up a wafer thin film of metal oxide that is vapour deposited on the inside of the outer pane. The heat generated provides protection from misting and icing.

Fine heating wires such as used in cars is also incorporated into the glass to provide protection.

Cockpit windows are also vapour coated with a layer of gold or silver in order to reflect solar radiation and protect the cockpit from heating up.

One additional advantage of heating is that the vinyl, incorporated into the windscreen, becomes palpable and is therefore able to withstand the impact of bird strikes.

LIMITATIONS

Due to the extreme conditions in which aircraft operate the construction of cockpit windows and cabin windows is such that they have to be strong enough to withstand:

! temperature differences,

! surviving the impact of weather such as rain, hail, ice ! pressurization differences

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CHAPTER 2 – AIRCRAFT ELECTRICAL SYSTEMS Section 1.01 CONTENTS OF THIS CHAPTER:

! Current and voltage ! Electricity and magnetism ! Aircraft generating systems

" Power Supplies " Generator systems " Alternators

! Comparison of alternators and

generators

! Switches

! Voltage regulators

! Constant speed drive unit ! Vibrating voltage regulators ! Monitoring devices

" Reverse current relays " Sensor and warning lights " Ammeter

! Electrical distribution " Busbars

" Terminals wiring fuses and switches " External power supplies

! Batteries " Construction " Types of batteries

" Hazards and safety precautions

! Ignition systems " Magnetos – types " Magneto faults/failures " Battery ignition system " Ignition timing

" Single and dual point ignition " Impulse coupling " Induction vibrator ! Electrical miscellaneous ! Abnormal operation/emergency handling ! AC power supply ! Ammeter ! Brushless generator ! Busbar ! Cabling ! Electric motors

! Engine starter motors ! Fault protection ! Frequency controller

! Light aircraft electrical circuits ! Indictor lights

! Inverters ! Load sharing ! Monitoring

! Multiple AC generator operation ! Protection unit

! Static electricity ! St. Elmo’s fire ! thermocouples

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AIRCRAFT ELECTRICAL SYSTEMS

INTRODUCTION

Many older aircraft had no electrical systems except for the magneto, which supplied energy for the ignition. Most contemporary aircraft have a full electrical system as well as a separate self-contained magneto or ignition system, which operates independently from the main electrical system. This means that if the aircraft electrical system is switched off, the engine will still run using power from the magneto/s. Most light aircraft use a 12v or 24v DC system consisting of the following basic components:

! alternator or generator ! battery

! master switch or battery switch ! busbar, fuses, and circuit breakers ! voltage regulator

! ammeter ! starter motor

! associated electrical wiring ! accessories.

A schematic of a typical system found in General Aviation aircraft is shown in Figure 2-1. While the engine is running, an engine-driven generator or alternator provides electrical power. A battery provides the power for engine starting. It can also give a limited electrical supply for emergency use if the alternator (or generator) fails. Some aircraft have

receptacles for external units to provide power for starting. This is very useful, especially for cold-weather starts. When starting engines with auxiliary power units (when the battery is dead) care should be taken that electrical energy is not forced into the dead battery. This would overheat the battery, which might explode.

DEFINITION - CURRENT AND VOLTAGE

! Current is the flow of electrical charges (usually electrons) in an electrical circuit. The same effect as water flowing down a pipe.

! There are two kinds of electrical current – Direct Current produced by a DC Generator and Alternating Current produced by an Alternator. It is represented by the letter ‘I’.

" When you switch on a torch the current flows from the negative terminal to the positive terminal, always in the same direction. This is called DIRECT

CURRENT.

" Electrical current from a standard household changes direction 60 time each second. Because the current flows first in one direction and then the other this is called ALTERNATING CURRENT. AC is used by power companies because it can be transformed to higher and lower voltages through transformers allowing them to transmit and distribute power with lower losses.

! Voltage on the other hand is a measure of pressure – Like how many pounds per square inch of air are in your tyres! It is represented by the letter ‘V’. The basic unit of electrical pressure is called the volt. The definition of a volt is that 1 volt is the amount of pressure required to force 1 amp of current to flow through 1 ohm of resistance.

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ELECTRICITY AND MAGNETISM

If you move a wire through a magnetic field a small current (electricity) is created in the wire. This is called electromagnetic induction.

Conversely, if you put electricity down a wire (conductor), the wire will have a magnetic field around it.

The more wires you use and/or the greater the strength of the magnetic field the greater the effect becomes. These two principles are the basis for electric motors, generators and alternators. If you have one item, either movement or electricity, you can convert it into the other. To create a stronger effect you can use more turns of wire or windings (the term used for the wire in an armature)

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AIRCRAFT GENERATING SYSTEMS AND ASSOCIATED

EQUIPMEN

T

INTRODUCTION

Generators and Alternators are the two prime means of producing electrical energy on an aircraft. They provide power for example:

! lights

! radios and other services

! a very important function is to recharge the batteries.

Each converts mechanical energy into electrical energy by moving a conductor through a magnetic field.

A generator produces direct current (DC)

An alternator produces alternating current (AC). AC must be rectified or converted into

direct current before being supplied to equipment (such as the battery) which needs DC. The following factors are required in order to generate a current of electricity by induction:

! A magnetic field ! A closed conductor

! Movement of the conductor at right angles across the magnetic field

The principle on which generators and alternators depend for their operation is shown in Figure 2-2a below. A conductor, which is moved through a magnetic field, will experience an electromotive force (EMF). The EMF will cause a current to flow if the circuit is closed as shown.

Figure 2.2b illustrates the electrical rule used to find the direction of flow on induced current in a generator. The ‘Right Hand Generator Rule’ says “with the index finger of the right hand pointing in the direction of the magnetic field (north to south) and the thumb indicating the direction of the movement of the conductor across the magnetic field, the second finger will point in the direction of the flow induced current.

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POWER SUPPLIES

THE GENERATOR

The generator/alternator is the primary source of aircraft electrical power. The battery is the secondary source.

During normal engine operation the engine-driven alternator (or generator) supplies power for all aircraft electrical equipment. The battery is kept fully charged for use as an auxiliary source when the generator is not operating ("off line") or the engine is not developing power. The number of alternators or generators in a system depends on power requirements. Single-engine aircraft usually have one alternator or generator. Multi-engine aircraft may have two or more, depending on requirements and the number of engines.

If AC current is required (e.g. for AC driven instruments) an inverter can be incorporated. An inverter converts DC to AC.

This method is used in the Beechcraft King Air. A generator (which doubles as a starter motor) is used to produce electrical energy. If the main power generator is AC (an alternator), a rectifier must be incorporated to convert AC to DC for battery charging and other DC supplies.

If the alternator voltage is higher than the battery or electrical system voltage, a "stepdown transformer" must also be incorporated. The combination of transformer and rectifier is known as a transformer-rectifier unit (TRU).

REMEMBER: RAD= RECTIFIER CONVERTS ACTODC

IDA = INVERTS DC TO AC REQUIREMENTS OF A GENERATOR SYSTEM

! A generator or alternator must be very reliable over a wide range of climatic conditions.

! It must be able to maintain a continuous power supply for all power requirements. ! The supply must kept be at a specific voltage (phase also, in the case of an alternator)

regardless of engine speed and loads on the system.

DC GENERATOR

A direct current generator supplies current that flows in one direction – from positive to negative. The purpose of the generator is to change mechanical energy into electrical

energy which will supply power to operate all the electrical devices and keep the battery fully charged. The mechanical energy required to rotate the generator, driving it by pulleys, belts or gears is supplied by the engine. The generator operates on the principle of

Electromagnetic Induction – electrons are made to move by magnetism. (See figure 2.1) In a DC generator the three factors necessary to produce a current of electricity by induction are present:

! It has a closed conductor called an armature ! A magnetic field

! Movement of the conductor or armature at right angles across the magnetic field In a DC generator the induced voltage which induces the current in the armature coils, alternates. The armature coils are connected to the commutator which is actually just an

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extension of the ends of the armature winding. Carbon brushes in contact with the commutator change the side of the coil to which they are connected every half revolution. This keeps the current flowing in the same direction and so supplies DC to the external field. Figure 2-3a shows DC generation and the current output from a typical DC generator.

MAIN COMPONENTS OF A GENERATOR

FIELD FRAME – The main body of the generator. It supports the drive end frame and the

commutator end frame and retains the residual magnetism which supplies the original magnetic field for the armature to cut in order to generate a current of electricity.

COMMUTATOR END FRAME – Supports the commutator end of the armature, and

provides a position for fastening the generator to the engine

DRIVE END FRAME – Supports the drive end of the armature, and provides the other point

Fig. 2-3a Current Output from DC Generator

Field Frame

Field Pole Shoes

Laminated armature core

Commutator

Armature

Brush holders and springs

Brushes

Drive pulley and fan

Through bolts

Field Coils Drive end frame

Armature windings Commutator end frame

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DRIVE PULLEY AND FAN – Used to drive and cool the generator

ARMATURE – is made up of armature windings, laminated iron core, commutator, armature

shaft and insulators. The armature shaft carries and drives the armature parts and the insulators insulate the armature windings from the laminated iron core of the armature.

COMMUTATOR – An electromagnetic device rectifies the AC within the generator into DC BRUSHES – usually made of carbon which is a good conductor with low friction necessary

when coming into contact with the commutator. It transfers current to and from the commutator and assists the commutator to change the alternating current to direct current.

NOISE SUPPRESSOR – The name describes its

function. In order to prevent interference to radio/radar equipment the suppressor must be situated as close as possible to the generator.

ALTERNATORS

Unlike a generator an alternator has no moving parts and as a result it is not only very reliable but also comparatively inexpensive to build and repair. An alternator can be considered as an AC generator. It produces alternating current (AC). AC current changes direction (alternates) in a regular wave-like manner. In AC two current reversals compromise a cycle. The frequency is the number of cycles per second. The principle of AC current and a schematic layout of an alternator are shown in Figures 2-4 and 2-5 below.

Fig. 2-3c Noise Suppressor

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! On an alternator a magnetic field is spun inside of windings of wire called a stator in order to generate electricity.

! Because the wires are fixed they can easily be connected directly to their outputs without the need for sliding contacts to carry the relatively high output current. ! The magnetic field is still generated via electro magnets mounted on a rotor and the

relatively small field current that powers them is supplied to the rotor by two small brushes that each ride on a separate and continuous slip ring.

! Because these slip rings and the fact that the relatively heavy windings are fixed instead of rotating allows the alternator to be spun to much higher speeds. As a result maximum output is reached much sooner even at engine idle speeds.

THE STATOR

Three separate windings of wire in the stator are all set so that the AC current generated is slightly out of phase in each one. The stationary stator of the alternator takes the place of the rotating armature used in the DC generator. It has a laminated iron core and the frame is laminated to prevent eddy currents which could create heat which could burn the insulation off the stator windings.

A DIODE

! In order to rectify the AC current into DC current in an alternator a diode is built into

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! A diode is the simplest possible semiconductor device. It is a ‘solid state’ device that allows current to flow in one direction but not the other in much the same way as a turnstile at a stadium lets people go through in only one direction.

! It has no moveable parts.

! It relies on the different electrical properties of the materials it is made of to act as a one-way valve for current.

! A smooth and stable DC output is obtained by arranging diodes so that current from each of the three stator wires is only allowed to pass in one direction and by

connecting the three outputs together.

COMPARISON OF ALTERNATORS AND GENERATORS

Several fundamental differences exist between generators and alternators.

! Unlike alternators generators can seldom produce sufficient electrical current at low engine rpm to operate the entire electrical system. This means that when engine rpm are low, power must be drawn from the battery – which has a limited life. This is particularly important during prolonged night ground operations. However, the alternator is capable of producing sufficient power for the increases in power requirements even when idling.

! Another advantage of the alternator is that the output is constant at most engine speeds.

! Alternators are lighter, cheaper to maintain and less prone to overload when loads are heavy.

SWITCHES

! A master switch controls electric power for all aircraft equipment.

! The master switch activates all electrical circuits except the ignition system.

! Some aircraft have a battery switch, which is used in a similar fashion to the master switch. Often an alternator switch is fitted. This can isolate the alternator from the main circuit in the event of alternator failure. When the alternator switch is off, the battery is the only source of power.

VOLTAGE REGULATORS

Voltage regulators continually adjust the field current so that generator output voltage remains constant under all loads.

Ohm's law states that the current in a closed circuit is directly proportional to the circuit voltage and is inversely proportional to the resistance of the circuit (V = IR).

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Generator output is directly proportional to generator voltage and inversely proportional to the circuit resistance.

VOLTAGE OUTPUT

The voltage output of a generator depends on three factors: ! the number of conductors, or armature windings

! the speed (rate) at which the magnetic lines of force are cut ! the strength of the magnetic field.

Output voltage is regulated by increasing or decreasing the number of lines of force. This is accomplished by varying current strength in the generator field windings by changing the number of ampere turns, so altering the resistance. The current strength of the field windings can altered in several ways:

! connecting a variable resistor (rheostat) in series with the shunt circuit of a shunt-wound generator. (In a series-shunt-wound generator the rheostat is connected in parallel with the field)

! full distortion, usually called third-brush regulation ! installing vibrating voltage regulators

! incorporating a carbon-pile voltage regulator.

CONSTANT SPEED DRIVE UNIT (CSD or CSU or CSDU)

! Many aircraft services depend on stabilised frequency.

! This is only possible when the alternator rotates at a constant speed.

! Most aircraft have a CSD fitted, which regulates alternator rpm through a variable ration hydraulic drive mechanism.

! The CSD keeps the generator rpm constant independently of engine rpm and so ensures a constant supply frequency.

! A CSD is not required on aircraft fitted with constant speed engines.

VIBRATING VOLTAGE REGULATORS

! Vibrating voltage regulators are used in light aircraft in conjunction with current regulators.

! The current and voltage from DC generators are regulated through interconnected coils.

! These are connected, so that only one of the regulators can operate at a time.

MONITORING DEVICES

REVERSE CURRENT RELAYS (RCR's)

! An RCR is an integral part of the voltage regulation system and prevents a storage battery from "motorising" a generator by reversing the current flow.

! When battery output is greater than generator output (especially at low generator operating speeds – low engine rpm), the RCR points open, thereby breaking the circuit.

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! This stops battery current flowing in the reverse direction to the generator and turning it into a motor.

! If the RCR fails, battery output will travel back into the generator field.

! This type of malfunction can be corrected by "flashing" the generator field. "Flashing" the field is passing a very brief current from the positive terminal to the negative terminal.

SENSOR AND WARNING LIGHT

! In the event of over-voltage condition occurs, the over-voltage sensor will automatically remove alternator field current and shut down the alternator.

! A red warning light will then come on indicating that the alternator is not operating and that the battery is supplying all electrical power.

AMMETER

An ammeter is an instrument used to monitor the performance of the aircraft electrical system. It indicates the flow of current, in amperes, from the alternator to the battery or from the battery to the aircraft electrical system. When the engine is operating and the master switch is turned on the ammeter indicates the charging rate applied to the battery. In the event the alternator is not functioning or the electrical load exceeds the output of the alternator, the ammeter indicates battery discharge rate.

ELECTRICAL DISTRIBUTION INTRODUCTION

A busbar is the central distribution point for the main electrical system. It routes power from the generating source (battery/alternator/generator) to the electrical equipment. It is the common point from which power can be distributed throughout the system.

Electricity is distributed in the aircraft to various services from busbars. Each generator supplies one or more busbars, from where current is supplied to various aircraft services. Each busbar serves various circuits, depending on the type of circuit and the number of generators. The busbar in DC circuits is usually a copper strip; in AC circuits it is simply three wires, one for each phase.

Busbars are classified according to their importance. A typical aircraft system might have the following busbars:

! vital services busbar ! essential services busbar ! main busbar

! synchronising busbar (AC only).

BUSBARS

VITAL SERVICE BUSBARS

The vital services busbar is a DC busbar. It is supplied with power at all times direct from the batteries or via the secondary power distribution network. All vital services (those concerned with aircraft safety) are connected to this busbar. It is normally isolated from the main

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! fire extinguisher circuits

! crash relay switches and battery isolation ! engine relighting

! emergency power supplies (RAT/APUs etc) ! RAT and APU control (AC only)

! 28v DC control ! feathering controls

! powered flying controls (if no manual or hydraulic reversion)

ESSENTIAL SERVICES BUSBAR

The essential services busbar supplies power to all services, which enable the aircraft to keep flying after an emergency (as well as under normal ops). These services may include:

! power flying control units (fly by wire – if reversion option available) ! radios and intercom.

! flight instruments

! fire detection, warning and extinguisher operation ! heaters and wipers.

Different manufacturers use different names for busbars. Where no provision is made for both essential and vital services busbars, the term "essential services" means "vital services".

This busbar is normally DC and is supplied either from the battery through manual switching or by emergency power plants through appropriate transformers and rectifiers. Some aircraft have AC and DC essential services busbars.

MAIN BUSBAR

The main busbar is also called a load busbar or non-essential busbar. It may be either an AC or DC busbar. The generator supply is fed to the main busbar which is part of the main electrical system. Any secondary system is supplied from the main busbar via transformers and inverters etc.

SYNCHRONISING BUSBAR

The synchronising busbar is an AC busbar. It is used on aircraft fitted with alternators. It can be considered as the AC equivalent of the DC essential services busbar. It is always "on line". When a generator is on line to the synchronising busbar power is supplied to the essential services busbar via transformer rectifier units. In the event of total alternator failure, the RAT or APU can supply the synchronising busbar, after appropriate manual or automatic selection.

CIRCUIT DIAGRAM

Aircraft with complex electrical systems have circuit diagrams etched on the electrical control and indicator panels. This enables the Flight Engineer or pilot to monitor the operation of the various circuits, in particular when any fault-finding is required. Magnetic indicators show whenever a particular circuit has been made or broken and voltmeter/ammeters and frequency meters are included in the diagram. These are sometimes called mimic diagrams.

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TERMINAL, WIRING, FUSES AND SWITCHES

TERMINALS

Terminals facilitate the connection of electrical wire to junction boxes, terminal strips, or items of equipment. The tensile strength of the wire and the resistance of the terminal joint should be negligible compared with the resistance of the "cable run". Terminals can be soldered or solderless.

WIRING – ELECTRICAL CABLE AND FLAMMABLE FLUID LINES

When electric cables and flammable-fluid lines are installed along the same route, or run, they must be kept separate and the cable/s must always be above the fluid line/s.

The following terminology is often used in conjunction with aircraft wiring systems: ! OPEN WIRING - Electric wire is often installed in aircraft without being carried in a

conduit to facilitate maintenance and reduce weight.

! "SPAGHETTI" - Spaghetti is a soft slender tube of insulating material around wire or cabling.

SWITCHES

ELECTRICAL SWITCHES

Switches used in aircraft are: ! snap-action switches ! toggle switches ! relay switches

! rotary selector switches ! push-button switches ! knife switches.

Snap-action switches are best for rapid opening and closing of contacts, irrespective of the

speed of the operating toggle or plunger. This minimises arcing.

Switches are designated by the number of poles, throws and positions:

! The pole of a switch is its movable blade or contactor and the number of poles is the same as the number of terminals through which current can enter or leave the switch. ! The throw of a switch indicates the number of circuits which each pole can complete

through the switch.

! The number of positions a switch has is the number of places at which the operating device (toggle, plunger, etc) will come to rest.

Single-pole, single-throw (SPST) Switches are switches through which only one circuit

can run.

single-pole, double-throw (SPDT) switches are single-pole switches through which two

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Fig. 2-8

double-pole, single-throw (DPST) switches are Switches with two poles through each of

which one circuit can be completed.

double-pole, double-throw (DPDT) switches are those with two poles through each of

which two circuits can be completed.

A toggle switch that will come to rest at either of two positions (to open the circuit in one

position and complete it in another) is a two-position switch. A toggle switch that is spring-loaded to the "off" position and must be held in the "on" position to complete the circuit is a single-position switch.

Micro switches are switches that can open or close a circuit with a very small movement of

the tripping device. Micro switches are usually the push-button type and are used as limit switches for automatic control of landing gears, actuator motors, etc.

FUSES AND CIRCUIT BREAKERS

A circuit breaker is a protective device which can close or open a circuit. Circuit breakers are often used in place of fuses to protect a circuit as, unlike fuses, they can be reset. Some circuit breakers must be reset by hand; others are reset automatically. C/Bs are rated in amps and can be used as, or incorporated in, switches (Fig 2-7).

MAGNETIC CIRCUIT BREAKER

When a current flows in a circuit at a higher rate than desired, a magnetic circuit breaker generates an electromagnetic force through induction. This actuates a small solenoid which opens the circuit breaker. The circuit is interrupted and protected. Thus the circuit is being protected against the restraining force of a sprung detent.

CIRCUIT PROTECTORS (THERMAL OVERLOAD SWITCH)

The circuit protector is an automatic protective device that opens a circuit whenever the temperature of the protected unit rises above a certain value. It has two positions, automatic "ON" and automatic "OFF". The switch is often found in electric motor circuits. A bimetallic strip opens the switch when the temperature rises and re-makes the switch when it cools.

FUSE TYPES

CARTRIDGE FUSES

The cartridge type of fuse is a wire strand passing between two terminals enclosed in a glass tube. The fuse forms part of the circuit and the wire burns out if the circuit overloads. This fuse usually fits into a cap that is screwed into the fuse mounting. Most fuse strips are made of an alloy of tin and bismuth (Fig2-8).

CURRENT LIMITERS

Current limiters normally have a copper fuse element which can withstand a higher than normal temperature for a short period. It melts and opens the current when a sustained overcurrent condition (as opposed to a transient overcurrent) occurs. The melting point is much higher than regular fuses. They can withstand a considerable overload for a short period before breaking the circuit.

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EXTERNAL POWER SUPPLIES RAM AIR TURBINE (RAT)

The RAT is a temporary power supply which gives the crew time to rectify the generator faults. If this is not possible, an alternative source of power is used such as the APU (or AAPP).

The RAT is an AC generator driven by ram air. Pulling a handle releases a turbine (fan) into the air stream, which drives it by ram air. It can supply a limited 200v, 400 Hz current in the event of total generator failure in flight. The RAT output is fed direct to the synchronising busbar (Essential AC busbar).

The advantage of the RAT is that it can supply full output within two seconds.

Usually the RAT will automatically shed the non-essential electrical loads when selected. A high IAS is required to maintain full output. This makes the RAT unsuitable for use at low airspeeds or during an emergency landing.

AUXILIARY POWER UNIT (APU)

The APU is a gas turbine, which drives an AC generator. It provides the synchronising busbar with a 200v, 400 Hz AC supply for use in emergency or on the ground when no external power unit is available. Gas turbines are difficult to start at high altitude so the APU is normally lit at lower altitudes, while utilising the supply from the RAT.

GROUND POWER UNIT (GPU)

The GPU provides power for aircraft servicing, for aircraft systems and engine starting. Protection circuits ensure that the GPU is not paralleled with either the APU or the generators. As each generator is brought on line, the GPU supply is automatically isolated from that generator busbar.

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ENERGY STORAGE - BATTERIES

INTRODUCTION

During normal aircraft operation, electrical power is supplied by the alternator or generator. However when power is required to:

! start the engines or

! cope with the failure of the generating system ! act as a stabilizer in the charging circuit

some stored form of energy is required. The most common storage system is to use a storage battery. The most popular types in use are the lead-acid and the Nickel Cadmium batteries.

The light aircraft electrical system normally uses a 12 volt lead-acid battery and large or modern aircraft use 24 volt Nickel Cadmium (Nicad) batteries. These two types of batteries are very different and cannot be interchanged. The tools, storage facilities and maintenance methods in each case are totally different.

The battery is continually charged by the generator/alternator. The charging rate is

controlled by a voltage regulator, which stabilises output from the generator/alternator. The generator/alternator voltage output is usually slightly higher than the battery voltage. For example, a 12v-battery system would be fed by a generator/alternator system of

approximately 14v. This voltage difference keeps the battery charged.

LEAD-ACID BATTERIES

Most light aircraft have a Lead-acid battery. This battery creates an electrical current (amps) by a chemical reaction between lead plates immersed in weak sulphuric acid that acts as an electrolyte.

CONSTRUCTION OF THE BATTERY

The lead-acid battery consists of several separate cells, with each cell containing a number of lead plates.

! The negative plate is made of pure lead (Pb) (in a sponge form) and

! The positive plate is made of lead peroxide (PbO2).

! The plates are separated by porous insulators which keep the plates from touching one another while allowing free circulation of the electrolyte.

! The number of negative plates in a cell is always uneven. This ensures that there is always a negative plate either side of a positive plate. Separators Negative Plate Positive Plate

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! All similar plates are connected to a common terminal.

! The plates and separators are immersed in an electrolytic solution of dilute sulphuric acid (H2SO4).

The battery is housed in its own case in order to prevent corrosion from any spillage of the acid. This case is made of hard rubber and an element is placed in each cell. Hard rubber covers are used on each cell and lead cell connectors connect the elements in series. A twelve-volt battery has six separate cells – two volts per cell. A bituminous sealing compound is poured around the covers and over the connectors after the cell covers and cell connectors are in place in order to prevent the electrolyte from leaking. When the battery is charging a vent cap on each cell cover allows the hydrogen and oxygen to escape.

DC current charges lead-acid batteries. The current causes chemical changes in the plates and electrical energy is converted into chemical energy. During the process, lead sulphate is removed from both the plates and the sulphuric acid content of the electrolyte is again increased.

When the battery is in use, the stored chemical energy is converted to electrical energy and current flows when the circuit is made. The rated capacity of a battery varies with the number, area, and thickness of the plates in each cell. Batteries are rated in ampere-hours, which indicate the number of amps the battery can deliver over a period of hours.

! A 45 ampere-hour battery for example, can deliver 1 amp for 45 hours or ! 5 amps for 9 hours.

! The voltage of each cell is 2v (2.2v when charged). A 12v battery has 6 cells and a ! 24v battery has 12 cells.

INDICATIONS OF BATTERY STATE

! Excessive discharging of a lead acid battery is detrimental as it leads to heavy sulphation (lead sulphate forms on the outside of the plates as a result of the discharge chemical reaction).

! This deposit raises the internal resistance so that excessive heat is generated during rapid charging or further discharge.

! This in turn causes shedding of active material, which can short circuit the plates and result in internal discharge.

! In addition, the loss of active material reduces the capacity of the battery. The drain on a battery when starting large engines is very heavy. It is better to use a ground power unit.

! Batteries lose water through evaporation and in the charging process when water is split into its hydrogen and oxygen components. As a result, distilled water must be

Cell

Six Cells Lead connector Vent Cap Hard covering

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added to the electrolyte (battery fluid) to keep the dilute sulphuric acid at the correct concentration. Batteries should be kept in a warm place if temperatures are low. A discharged battery will freeze at -15°C, but will not freeze until around -65°C when fully charged.

! The gases liberated when a battery is being charged (or is discharging) require

adequate ventilation to reduce the fire hazard and to ensure that harmful fumes will not infiltrate the aircraft.

! The electrolyte level should be checked periodically to ensure that the plates are covered. If necessary, the battery should be topped up with distilled water. A battery with inadequate electrolyte will not hold its normal capacity and the ammeter will indicate an abnormally high charge rate in flight. Never use non-distilled water as impurities in the water can react with both the electrolyte and the plates which will reduce the battery capacity.

MEASUREMENT OF SPECIFIC GRAVITY

Battery charge condition can be determined by a hydrometer. This measures the specific gravity (SG) of the electrolyte.

Specific gravity of a fluid is the ratio of the density of a fluid relative to that of water

! Water is assumed to have a density of 1 kg per litre.

! The specific gravity of a fully charged battery is around 1.3. ! A flat battery will have an SG of between 1.2 and 1.24.

Testing the specific gravity of the electrolyte in the battery cells tells the state of charge of the battery.

! The more sulphuric acid there is in the electrolyte the higher the charge in the battery. ! The less sulphuric acid there is in the electrolyte solution the lower is the charge in the

battery.

Fig. 2-11 A typical lead acid Plate Sediment Chamber

Hard Rubber Case Terminal Post

Vent Cap

Cell Connector

Separator Cell Cover

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In order to take a specific gravity test hold the hydrometer steady in a vertical position so that the float in the hydrometer will move freely and not touch the sides. Draw enough electrolyte into the hydrometer to float the float. Do not allow the top of the float to touch the top of the float chamber as this will give an inaccurate reading. The reading should be taken at eye level.

In a fully charged condition the positive plate is lead peroxide, the negative plate active material is sponge lead and the electrolyte is a solution of sulphuric acid and water with a specific gravity of 1.260 to 1.280. The electrolyte solution should be heavy, dense and buoyant and it should float the hydrometer float high, showing a specific gravity of 1.280. In a fully discharged battery the float in the hydrometer sinks deep into the electrolyte solution indicating a specific gravity of approx. 1.150. (As the battery discharges the sulphate in the electrolyte combines with the active materials in the positive and negative plates and the active materials in both plates gradually change to lead sulphate (see above).The sulphate then becomes less dense and consequently less buoyant. Continued use of the battery will result in almost all of the sulphate leaving the electrolyte and combining with the active materials in the plates. This reduces the electrolyte solution to water which is not dense and buoyant enough to support the hydrometer float at a high level.)

The following table indicates the specific gravity of the solution at its ability to crank the engine at 80º(F) From 1.260 to 1.280 100% charged From 1.230 to 1.250 75% charged From 1.200 to 1220 50% charged From 1.170 to 1.190 25% charged

From 1.140 to 1.160 Very little useful capacity

From 1.110 to 1.130 Discharged More buoyant, more charge Less buoyant, less charge Fig. 2-12 Hydrometer

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Metal

Each cell produces around 1 Volt

Cells can be replaced individually Fig. 2-13 Construction of a NiCad Battery

NICKEL CADMIUM (NICAD) BATTERIES

A nickel cadmium battery has positive plates made of nickel hydroxide deposited on a fine nickel mesh screen. Negative plates are made of cadmium hydroxide and the electrolyte is dilute potassium hydroxide.

All positive plates are connected together and all negative plates are connected together.

The specific gravity remains between 1.24 and 1.3 at room temperature.

The plates are housed in a nickel-plated steel container. A vent is incorporated in the case so

gases can escape and distilled water can be added as required. Each cell produces a potential of about 1.9v when fully charged which falls to 0.9v when the cell is fully discharged. When the battery is charged, the negative plates lose oxygen leaving pure cadmium while the positive plates become more oxidised.

ADVANTAGES OF THE NICAD BATTERY

The NiCad battery has several advantages over the lead-acid battery: ! low maintenance, each cell is an individually unit

! the battery can remain in a low charge state without damage ! the freezing point is below operating temperatures

! it can tolerate a very high charge and discharge rate ! the battery is stronger and reliability is good

! the electrolyte undergoes little change ! it does not produce harmful fumes.

BATTERY HAZARDS AND SAFETY PRECAUTIONS WATER

During charging the water in the battery decomposes and leaves the cells as hydrogen and oxygen gases. The water must be replaced with distilled water.

GASSES

Charging batteries emit a mixture of hydrogen and oxygen. They are highly inflammable and will explode if a flame is brought to close to them. Lighted matches and torches should be kept away from charging batteries.

TERMINALS

Make sure that battery terminals are switched to the ‘OFF’ position before removing them. Removing terminals with the heater switch ‘ON’ will cause a spark at the battery terminal which could ignite the hydrogen-oxygen gas mixture in the cells and thus cause an

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REMOVAL OF BATTERY

Remove the ground cable first and connect it last when removing or installing a battery as the use of battery pliers and wrenches may cause short circuits resulting in flashes which could cause an explosion.

BATTERY ACID

Battery acid is highly corrosive and great care must be taken to avoid getting it on the face or in the eyes. In the event of an accident wash the affected areas with large quantities of cold water. The acid will also eat holes in clothing so when carrying the battery use a battery strap and keep it level and well away from clothing.

IGNITION SYSTEMS

INTRODUCTION - IGNITION

Reciprocating engines require a very high voltage spark inside each cylinder for correct and complete combustion. The ignition system provides this high voltage spark to the spark plug. The high voltage is usually provided by a magneto. Two types of magneto ignition systems are in use :-

! high tension (HT) and ! low-tension (LT).

Modern aircraft have double magneto ignition systems for safety and better engine

performance. The double system may be two separate magnetos or a single "dual magneto" (two magnetos in one housing).

Aircraft engines have two spark plugs for each cylinder. Battery ignition systems with coils (similar to motor car systems) are sometimes found on older aircraft and are reappearing in newly certified engines.

A magneto system is superior to a battery system because the spark remains at a consistently high voltage.

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MAGNETOS

A magneto is a combination of AC generator (permanent magnet type) and

auto-transformer.

It uses a permanent magnet as the source of energy. Through gearing, the magneto rotates as the engine turns and a current is produced by induction. The resulting voltage is high but it is further boosted by a capacitor which is connected to the breaker points. The voltage is increased through various wiring and switching circuits and is fed to a distributor (which is often an integral part of the magneto system).

The distributor feeds the high voltage current to the spark plugs in sequence.

! C – Coil Shaft ! E – Pole Shoe ! D – Magnets

Magnetos developing less than 100v are classified as low tension (LT), those above 100v as high tension (HT). As a general rule, light aircraft use HT systems and larger aircraft use LT systems.

Magnetos produce ionised gas as a by-product. Holes allow cooling, ventilation and the products of ionisation to vent to

atmosphere.

! The H.T. magneto system is divided into three circuits, the magnetic circuit, the primary electrical circuit and the secondary electrical circuit.

! The L.T. magnetos give less trouble and are more efficient than the H.T. magnetos. Because of the lower voltage there is less electrical leakage and they are lighter due to the use of lightweight leads from the magneto to the plugs.

!

HT MAGNETO AND LT MAGNETO SYSTEMS

The L.T. magneto system operates on the same principle as the H.T. magneto as far as the principle current flow is induced.

The principal difference between an HT system and an LT system is the manner in which current from the magneto is distributed and raised to a higher voltage.

HT ignition systems are often used on light aircraft. The system is composed of two

magnetos and two distributors. The distributor is usually an integral part of the magneto. The switching sequence from the distributor current sequence is predetermined according to the firing order. The distributor consists of the rotor, and the distributor block. Embedded in the distributor block are terminals made from a non- conducting material. Each terminal is connected to the relevant spark plug. Long wires carry the HT electrical impulses in

sequence to the spark plugs. A hot spark is produced when the plugs receive a high voltage impulse from the magnetos, hence the term "impulse-type magnetos".

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High altitude aircraft often have an LT system. In an LT system, the magneto does not have a "secondary" coil. The output voltage is kept low until reaching a secondary coil near the plug. This means that only a very short HT lead runs from the secondary coil to the plug. The system greatly reduces insulation losses and breakdowns, as well as flashovers which are the greatest source of malfunctions found when using HT systems above 25 000 ft.

MAGNETO FAULTS/FAILURE

Even today the vast majority of aircraft with piston engines use magnetos as their sole ignition source. In spite of over 100 years of experience with magnetos, extensive certifications of aircraft magnetos and quality control requirements by the aviation

authorities, they still fail or require maintenance, more often than any other part of an aircraft engine. Primary Coil Grounded to primary coil Secondary coil Distributor

Fig. 2-16 High Tension Ignition System

Distributor

Transformer – voltage stepped up

Low voltage to the distributor

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The keys to improved flight efficiency below all reduce the reliability of the magneto and especially its distributor.

! higher compression ratios or turbo chargers, ! reduced pumping losses,

! flight at high altitude, ! larger spark plug gaps and ! higher ignition voltages ! High Tension Lead Failure

A common fault of magnetos is the break down of the high tension leads resulting in the failure of that particular lead to the spark plug. The high tension lead could also chafe against an engine part resulting in it shorting out. This will result in a loss of power.

Primary Coil Failure

The magnetos rely on a current passing through the primary coil to be activated. Any breakdown or fault in the primary coil will result in the magneto failing.

Condenser failure

The condenser is an integral part of the circuitry and should the condenser fail or the points either stick or not close will also result in the failure of the magnetos.

BATTERY IGNITION SYSTEM

The battery ignition system uses a battery instead of a magneto as the source of electrical energy.

The ignition system is in fact part of the ordinary electrical system.

The voltage is raised by a conventional ignition (induction) coil. The HT current then goes to a distributor for proper timing and distribution to the engine spark plugs.

Battery

Ignition Coil Distributor

Engine Mechanical Linkage Breaker contact Points (Open) Cam Breaker contact Points (Closed) Primary Coil Secondary Coil

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IGNITION TIMING

Ignition timing is important for efficient engine operation. If the spark is not produced at the optimum moment, incomplete combustion will take place. The time lapse between ignition and full combustion is affected by several factors :

! the engine (crankshaft) rpm

! the turbulence of the mixture in the combustion chamber ! the physical location of the spark plug

! the fuel quality ! the fuel/air ratio.

Of these, the fuel/air ratio and rpm are the most important factors.

SINGLE POINT AND DUAL POINT IGNITION

Most aircraft engines have duplicated ignition systems (dual ignition). Dual ignition affords added safety and improved combustion.

Dual ignition systems operate either with synchronised or staggered timing patterns. ! Staggered ignition timing is when the exhaust-side spark plug fires 4° – 8° before

the intake-side spark plug.

! Synchronised timing is when both spark plugs fire at the same time.

During normal combustion, the fuel-air mixture nearest the exhaust valve becomes contaminated with exhaust-gas residues. It burns more slowly than the mixture on the intake-valve side of the chamber.

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In staggered timing, the exhaust-side spark plug is timed to fire a few thousandths of a second before the other spark plug. This earlier start to combustion on the exhaust side compensates for the slower burning rate of the contaminated mixture.

Faster burning of the "clean" mixture (intake side) approaches the ideal burning pattern. Both flame fronts meet in the centre of the combustion chamber. This type of combustion gives the even expansion which pushes the piston back with great power.

IMPULSE COUPLING

A magneto needs to run at a minimum of 300 rpm in order to function. This is known as the "coming-in speed".

During engine start, the magneto speed is usually less than the coming-in speed. A booster magneto, or induction vibrator coil is incorporated which receives low voltage from the battery and steps up the voltage sufficiently to start the engine by boosting the spark produced by the magneto.

Some magnetos incorporate an impulse coupling for engine starting. The impulse coupling does the same job as the booster magneto, or induction vibrator coil. When the magneto reaches a predetermined speed the impulse coupling cuts out and the magneto continues to provide the necessary current.

INDUCTION VIBRATOR

An induction vibrator supplies an interrupted low voltage to the magneto primary coil. This induces a high voltage in the secondary coil of the magneto. The high voltage produced in the magneto secondary coil is distributed to the spark plugs in the same way as the magneto spark.

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ELECTRICAL – MISCELLANEOUS

ABNORMAL OPERATION – EMERGENCY HANDLING

Emergency drills for generator malfunctions are detailed in the appropriate Aircrew Manual and Flight Reference Cards.

On aircraft with only one generator, the drill is:

! to reduce electrical load to a minimum to conserve battery life. ! The generator should be brought back on line if possible.

! Load shedding should be carried out according to laid down procedures.

! Battery life will be limited and a safe landing should be made as soon as possible. ! Load shedding may include switching fuel pumps off so engine limitations must be

observed.

On aircraft with more than one generator the emergency drill will depend largely on whether alternative/emergency power is available.

! Where such an alternative power source is provided, this will be used and will support essential electrical loads if speeds and heights are flown as recommended.

! Where no emergency power supply is provided, the drill will be as for a single generator aircraft.

AC POWER SUPPLY

A rectifier is incorporated in the AC generator (alternator) to convert AC to DC for battery charging and other DC needs. When the alternator voltage is greater than battery or electrical system voltage, a step-down transformer must also be incorporated. The combination of transformer and rectifier is called a TR Unit or a Transformer-Rectifier Unit (TRU).

AMMETER

Electric current is measured in amperes (amps) by an ammeter. The ammeter displays the number of amps flowing in the circuit and also indicates if the battery is receiving a charge. Most ammeters have a zero datum in the upper centre of the dial with a positive indication to the right, and negative to the left.

A positive value indicates that the battery is being charged. After drawing power from the

battery for starting, the ammeter needle will indicate a positive value for approximately 30 minutes while the battery is re-charged. This gives a good indication of the drain on the battery during start. If the needle indicates a negative value, it shows that the output of the generator/alternator is inadequate; that is, current is being drawn from the battery to supply the electrical system. This could be caused by a defective alternator/generator or by an overload in the system, or both. Full scale discharge or rapid fluctuation of the needle usually means a serious generator/alternator malfunction. When this occurs the

generator/alternator should be tripped out of the system and battery power conserved by reducing electrical loads.

Not all aircraft have ammeters. Some have a generator discharge light. When this is lit, it indicates a system discharge because of generator/alternator malfunction.