Aircraft Systems
Description
In-depth reference on fixed wing aircraft electrical and mechanical systems, covering electronics, hydraulics, and other essential components
Aircraft Systems delivers comprehensive material on the design and development of fixed wing aircraft electrical and mechanical systems, providing broad coverage of aircraft systems including electronics, hydraulics, pneumatics, flight control actuation, and landing gear. The book features many practical examples of existing commercial and military aircraft. Academic design studies and methods are presented, along with information on technical and mathematical methods of design. The book also features design guides for each system, including equations where appropriate to size sub-systems and major components, and discusses environmental concerns.
Topics discussed in Aircraft Systems include:
- Aircraft systems design philosophy and safety, covering systems design approaches, safety assessments, requirement captures, and component reliability
- Secondary power sources including main engines, auxiliary and ground power units, batteries, ram air turbines, and fuel cells
- Hydraulic system components, covering hydraulic fluids, pumps, and piping, reservoirs, filters, accumulators, and relief valves
- Aircraft icing and rain protection, covering ice protection systems such as electro-impulse de-icing and chemical ice protection systems
Aircraft Systems is an essential resource for undergraduate and postgraduate students studying the design of fixed wing aircraft systems. The book is also valuable to professionals in aerospace engineering due to its broad view of aircraft systems development and integration.
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Persons
Craig Lawson, PhD, is Professor of Aircraft Design at Cranfield University, UK. He is a Chartered Engineer, Fellow of the Royal Aeronautical Society, and Fellow of the UK Higher Education Academy. He is an expert in aircraft design focusing on assessment of the impact of novel systems on aircraft design, performance, and assembly. He manages industry and government funded research projects, supervises doctoral candidates, and teaches aircraft conceptual, preliminary, and detailed design to postgraduate students.
David Judt is the Head of Airworthiness at Aircraft Completion Engineering, France. He is a Member of the Royal Aeronautical Society and Fellow of the UK Higher Education Academy. Educated at Glasgow University and Cranfield University, he was Senior Lecturer and Course Director for the Aerospace Vehicle Design MSc. program, teaching and researching in the airframe systems field. He now leads the certification and continued airworthiness activities for cabin modifications and oxygen system products.
Content
Series Preface xiii
About the Authors xiv
Preface xv
1 Introduction 1
1.1 Which Are the Airframe Systems? 2
1.2 Aircraft Systems Design Processes and Aircraft- Level Considerations 2
1.3 Systems Integration 2
Bibliography 5
2 Airframe Systems Design Process 6
2.1 Introduction 6
2.1.1 Systems Design Approach 6
2.1.2 Safety Assessment Philosophy 7
2.1.3 Aircraft Systems Design Methods 8
2.1.4 Sae Arp4754a 8
2.1.5 Sae Arp4761 9
2.2 Requirements Capture 9
2.3 System Safety Assessment 11
2.3.1 Functional Hazard Assessment 11
2.3.2 Development Assurance Process and Levels 13
2.3.3 Fault Tree Analysis 14
2.3.4 Failure Modes and Effects Analysis 15
2.4 System Architectures 16
2.4.1 Representing Architectures 17
2.4.2 Architectures for Analysis 18
2.5 Systems Design for Maintainability 19
2.5.1 Maintenance Cost Estimation 20
2.5.2 Accessibility 21
2.5.3 System Ageing and Obsolescence 23
2.6 Aircraft Level Trade- off Analysis 25
2.6.1 Fuel Penalties 25
2.7 Cost 31
2.7.1 Subsystem Cost Breakdown 31
2.7.2 Estimation Approaches 33
2.8 Summary 35
References 35
3 Aircraft Secondary Power Systems 37
3.1 Introduction 37
3.2 Secondary Power Forms 37
3.2.1 Pneumatic Power Systems 37
3.2.2 Hydraulic Power Systems 39
3.2.3 Electrical Power Systems 40
3.2.4 Variable- Frequency AC 41
3.2.5 Fixed- Frequency AC 41
3.2.6 High- Voltage dc 41
3.2.7 Low- Voltage dc 41
3.3 Secondary Power Sources 42
3.3.1 Aircraft- Level Effects 42
3.3.2 Advantages and Disadvantages 43
3.3.3 Trade Study 44
3.3.4 Main Engines 45
3.3.5 Auxiliary Power Units 46
3.3.6 Ground Power Units 47
3.3.7 Ram Air Turbines 48
3.3.8 Emergency Power Units 49
3.3.9 Stored Energy and Batteries 49
4 Aircraft Pneumatic Power Systems 51
4.1 Introduction 51
4.2 The Requirements for Aircraft Pneumatic Power Systems 51
4.2.1 Airworthiness Requirements 52
4.3 Bleed Air Systems Design 53
4.4 Bleed Air Systems Components 54
4.4.1 Pneumatic Valves 54
4.4.2 Pre- coolers 54
4.4.3 Ducting 55
4.5 The Use of Bleed Air 56
4.5.1 Engine Starting Systems 56
4.5.2 Engine Thrust Reversing and Variable Nozzle Geometry Systems 57
4.5.3 Pitot- Static System 57
4.6 Boeing 767 Pneumatic System 57
Bibliography 59
5 Hydraulic Power Systems 60
5.1 Introduction 60
5.2 Hydraulic System Components 61
5.2.1 Hydraulic Fluids 61
Table of Contents ix
5.2.2 Hydraulic Pumps 63
5.2.3 Hydraulic Pumps for Low- Pressure Applications 64
5.2.4 Hydraulic Pumps for High- Pressure Applications 65
5.2.5 Hydraulic Piping 68
5.2.6 Reservoirs 70
5.2.7 Filters 72
5.2.8 Accumulators 73
5.2.9 Pressure Control Valves 74
5.2.10 Heat Exchangers 75
5.2.11 Power Control Units 76
5.3 Hydraulic System Aircraft Applications 78
5.3.1 Boeing 737 Hydraulic System 78
5.3.2 BAE Systems Hawk Hydraulic System 80
5.4 Requirements, System Design, Analysis and Sizing 82
5.4.1 Requirements Definition 82
5.4.2 Performance Requirements Analysis 82
5.4.3 Architecture Definitions 85
5.4.4 System Simulation and Control 87
5.5 Summary 89
References 89
6 Aircraft Electrical Power Systems 90
6.1 Introduction 90
6.2 The Requirements for Aircraft Electrical Power Systems 91
6.2.1 Airworthiness Requirements 92
6.3 Electrical Power Generation 92
6.3.1 dc Power Generation 92
6.3.2 AC Power Generation 94
6.4 Electrical Power Conversion 98
6.4.1 AC- to- DC Conversion Using TRUs 99
6.4.2 AC- to- AC Conversion Autotransformers 99
6.4.3 DC- to- AC Conversion Using Inverters 99
6.5 Electrical Power Distribution 100
6.5.1 Busbar Systems 100
6.5.2 Circuit Overload Protection 100
6.5.3 Wires and Cablings 101
6.5.4 Connectors 102
6.5.5 Current Return Earthing and Grounding 102
6.5.6 Electrical Bonding and Static Electricity Discharge 103
6.6 Electrical Power System Architectures 104
6.6.1 Typical 28 VDC System 104
6.6.2 Airbus A320 115 VAC 400 Hz System 105
6.6.3 Boeing 787 235 VAC System 106
6.6.4 F- 22 Raptor 270 VDC System 107
References 107
7 Flight Control Actuation Systems 108
7.1 Introduction 108
7.2 Control Surfaces 109
7.3 Flight Control Linkage Systems 111
7.3.1 Push-Pull Control Rod Systems 111
7.3.2 Cable- and- Pulley Systems 112
7.4 Trim Systems 112
7.5 Feel Systems 113
7.5.1 Spring Feel 113
7.5.2 'Q' Feel 113
7.6 Actuation Systems Using Hydraulic Actuators 114
7.6.1 Hydraulic Actuation with Mechanical Control 114
7.6.2 Hydraulic Actuation with Electrical Control 114
7.6.3 Types of Hydraulic Actuator 115
7.7 Actuation Systems Using Electro- Hydrostatic Actuators 116
7.8 Actuation Systems Using Electro- Mechanical Actuators 117
7.8.1 EMA Components and Operation 118
7.8.2 Types of EMA 118
7.8.3 EMA Advantages and Challenges 119
7.9 Fly- By- Wire Systems 120
7.9.1 A320 FBW Actuation System 120
7.9.2 A380 FBW Actuation System 121
7.10 Actuator Design Requirements and Sizing 122
7.11 Basic Actuator Design Considerations and 'Rules of Thumb' for Sizing 123
7.12 Summary 125
Bibliography 126
8 Aircraft Icing, Ice and Rain Protection Systems 127
8.1 Introduction 127
8.2 Aircraft Icing Conditions in Flight 128
8.3 Airworthiness Requirements 130
8.4 Ice Build- up in Flight 135
8.5 Estimation of Ice Accretion on an Aerofoil Surface in Flight 136
8.6 Aircraft Ice Protection Systems 140
8.6.1 Thermal Ice Protection Systems 140
8.6.2 Mechanical Ice Protection Systems 144
8.6.3 Chemical Ice Protection Systems 146
8.6.4 Hydrophobic and Ice- phobic Anti- icing 147
8.7 Aircraft Ground Icing 147
8.8 Ground Anti- icing and De- icing 147
8.9 Rain Protection 148
Bibliography 149
9 Aircraft Environmental Control Systems 150
9.1 Introduction 150
9.2 The Requirements for Aircraft ECS 150
9.2.1 Temperature Drivers 150
9.2.2 Humidity 152
Table of Contents xi
9.2.3 Pressure and Oxygen Drivers 153
9.2.4 Aircraft Equipment Requirements 154
9.2.5 Human Requirements 154
9.2.6 Human Fresh Airflow Requirements 156
9.2.7 Airworthiness Requirements 157
9.3 Cabin Heat Balance Calculations 158
9.3.1 Kinetic Heating 158
9.3.2 Solar Heating 158
9.3.3 Avionics and Airframe Systems Heat Loads 159
9.3.4 Aircraft Occupants Heat Loads 159
9.3.5 Conduction Through the Aircraft Skin 159
9.3.6 Overall Heat Balance Equation 160
9.4 ECS Designs 160
9.4.1 Ram Air Coolant-Based Systems 161
9.4.2 Engine Bleed Powered Air- Cycle Refrigeration 161
9.4.3 Electrically Powered Air- Cycle Refrigerators 166
9.4.4 Vapour Cycle Refrigeration 167
9.5 Environmental Control Subsystems 168
9.5.1 Cabin Pressurisation and Control 168
9.5.2 Humidity Control 169
9.5.3 Ozone Conversion 170
9.5.4 Filtration of Re- circulated Cabin Air 171
9.5.5 ACM Heat Exchangers 171
9.5.6 ACM Compressor and Turbine 172
9.5.7 Air Distribution 173
9.6 Oxygen Systems 173
9.6.1 Liquid Storage of Oxygen 174
9.6.2 Oxygen Concentration of Outside Air 174
9.6.3 High- Pressure Gaseous Storage of Oxygen 175
9.6.4 Oxygen Generation by Chemical Reaction 176
9.7 Example Complete ECS 176
Bibliography 178
10 Fuel and Fuel Systems 179
10.1 Introduction 179
10.2 Aviation Fuels 179
10.2.1 Fuels for Piston- Engine Aircraft 179
10.2.2 Fuels for Turbine- Engine Aircraft 181
10.3 Fuel System Components 182
10.3.1 Fuel Tanks 182
10.3.2 Pumps 185
10.3.3 Valves 189
10.3.4 Level Sensors and Gauging Probes 190
10.4 Engine Feed Systems 192
10.5 Fuel Transfer Systems 193
10.6 Refuel/Defuel Systems 194
10.7 Vent Systems 196
10.8 Fuel Quantity Measurement Systems 197
10.9 Fuel System Contamination 198
10.10 Fuel Jettison 198
10.11 Design Process Considerations 198
10.11.1 Identify Requirements, System Architecting and Safety Analysis 199
10.11.2 Available Fuel Volume Estimation Methods 200
10.11.3 Tank Architecture Definition 201
10.11.4 Feed, Transfer and Refuel System Sizing 204
10.11.5 Vent System Sizing 206
10.12 Summary 209
References 209
Index 210
2
Airframe Systems Design Process
2.1 Introduction
2.1.1 Systems Design Approach
There are many approaches to systems design, and an experienced design engineer will develop and refine their own approach over time. In this section, a common generic approach to systems design is presented. While the details of a design task will vary greatly depending on the situation, the following universal stages can usefully be considered for airframe systems design:
- Decide what the system must do.
- Consider what it might have to do in the future.
- Design a system to meet stages 1 and 2 and regulatory requirements.
- Refine the system to make it:
- Safe
- Low cost
- Low mass
- Reliable
- Easy to maintain
- Easy to understand
- Simplify whenever possible, as simplicity is likely to be beneficial in helping to satisfy stage 4.
While stage 1 may seem obvious, great care should be taken in defining the scope and boundaries of what the system has to do. It is also important not to be too heavily influenced by previous designs. While studying existing systems may provide extremely valuable information, it is important to return to first principles to address the problem of what is required for the particular application being designed.
Stage 2 involves consideration of what happens when an aircraft enters service. Whether it is a military or a civil aircraft, this usually involves the aircraft class's life being extended by enhancement. This can be achieved relatively inexpensively by making limited systems changes, provided that this provision has been considered at the initial design stage.
Stage 3 is the start of the practical design work. As well as considering the broad objectives set out in stages 1 and 2, the relevant airworthiness regulations must be consulted to define their implications for the system. Although regulations do not overly influence the permissible design of systems, they do often imply particular types of systems. In many ways, airworthiness requirements reflect a summation of current safe practice and thus reflect past and present certified designs. This naturally leads to a review of existing aircraft.
Stage 4 explicitly mentions reliability and also brings in safety. A distinction between reliability and safety is useful: Reliability requires few failures, whereas safety requires few catastrophic failures. Redundancy in systems usually improves the latter, but often at the expense of the former.
In the past, the design of airframe systems has, unfortunately, been treated as something of an afterthought. However, with the increasing sophistication of aircraft over time, for modern and future aircraft, this is no longer the case. Due to the increased complexity of the systems themselves, and the greater integration between airframe, engines and systems, airframe systems design is now considered at an early stage in the aircraft design process.
The cost implications associated with systems should also be considered. On small and medium-sized transport aircraft, the cost of the systems typically represents more than one third of the total initial cost of the aircraft. Furthermore, systems incur costs throughout the life of the aircraft due to increased fuel consumption (due to their weight, power requirements, and perhaps direct drag increases), maintenance requirements, and their less-than-100% reliability (causing delays and spares requirements).
These costs should all be considered during the systems design process to compare the overall effectiveness of one design against another.
2.1.2 Safety Assessment Philosophy
The safety assessment of aircraft systems can be divided into several stages, as follows:
- What can go wrong?
- What effect will this have? (stages 1 and 2 may be combined by asking: What happens if ..?)
- How often will this occur?
- Is this acceptable?
- If not, what changes should be made?
When considering stage 1, it is important to keep in mind Murphy's Law1: 'Anything that can go wrong, will go wrong.' The systems engineer generally appreciates this and designs to prevent failures that are likely to cause a hazard at the aircraft level. An important part of the process of enabling this to be achieved is identifying all of the failure modes, their link to aircraft hazards and their criticality classification.
Accidents in modern aviation are commonly the result of failures in one system affecting another. Therefore, it is important for the systems designer to consider how a malfunction in their system will affect the operation of another system, which requires close coordination across the traditional systems boundaries. Model-based systems engineering philosophies can aid this process where requirements, functional failures and aircraft-level hazards can be traced across multiple systems.
2.1.3 Aircraft Systems Design Methods
Industry standards provide the designer with useful guidance, regardless of the particular system being designed and the type of aircraft to which it will be applied. The type becomes important to establish the aircraft's certification basis, whose requirements must be rigorously followed. Such documents are consulted at the very beginning of the design process. Design guidance documents relevant to specific systems are mentioned in subsequent chapters. Here is a list of documents relevant to the design process and aircraft systems in general:
- SAE ARP 4754 [1]
- SAE ARP 4761 [2]
- EASA CS-25.1309 and EASA AMC25.1309 (FAA AC 25.1309-1A) [3]
- ATA-100 (still widely used, but superseded by ATA iSpec 2200) [4, 5]
- RTCA DO-178C [6]
- RTCA DO-254 [7]
Figure 2.1, which has been simplified from ARP 4754, provides an overview of the interrelationships between the design standard documents listed earlier.
2.1.4 SAE ARP4754
This document, titled 'Guidelines for Development of Civil Aircraft and Systems', outlines a set of design processes for aircraft systems, employing a top-down approach beginning with aircraft-level functions. It provides guidance in developing design practices and in methods for showing compliance with safety regulations. Details to enable the design of particular aircraft systems are not covered in the document.
Figure 2.1 Relationships between guideline documents for aircraft and systems development
Figure 2.2 Simplified relationships between design phases and safety analysis
2.1.5 SAE ARP4761
This document, titled 'Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment', outlines a set of tools and techniques to enable showing compliance with EASA/FAR 25.1309. Aircraft-level safety assessment is covered and techniques outlined including Functional Hazard Assessment (FHA), Preliminary Systems Safety Assessment (PSSA), and Systems Safety Assessment (SSA) (Figure 2.2).
2.2 Requirements Capture
The early design-stage activity of establishing and recording the requirements for the system in question is probably the most important step in the process. This includes what has informally been referred to previously in this chapter as 'deciding what the system has to do'. It is therefore conducted in a structured and rigorous manner, resulting in a set of requirements that are interpretable, traceable back to the aircraft's needs and verifiable by the eventual matured design. This activity is not a discrete step, since new requirements will emerge as specific design architectures are developed, components selected and integration choices with the airframe made.
Requirements are often developed from the concept of operation provided by the aircraft's end-user. Here the most top-level design drivers are mission performance, operational economics and safety. In addition, the original equipment manufacturer (OEM) will need to consider its own economic drivers in relationship to the aircraft's development. Such higher-level drivers will eventually translate to the system drivers and requirements. These can again be roughly categorised by performance, cost, safety and function.
The earliest system architectures are functional and show how a set of provided functions in combination satisfy the top level required functions for the system as a whole. Hence, identifying these functional requirements of the system is key to establish a first solution architecture.
Requirements information, as shown in Table 2.1, is traditionally captured in text-based formats under version control guidelines. More recent approaches use software tools, either stand-alone for requirements capture or as part of a product lifecycle management (PLM) software framework. Such environments bring the benefit of identifying stakeholders in multi-disciplinary projects, tracing requirements to design solutions and showing compliance via linked test or analysis results. Efforts to standardise such model-based representation of requirements were led by Object Management Group (OMG's) Unified Modelling Language (UML)...
