Introduction to Avionics Systems PDF

Introduction to Avionics Systems Introduction to Avionics Systems Third Edition by R.P.G. Collinson BScEng(Hons)., CEng., FIET., FRAeS Formerly Manager of the Flight Automation Research Laboratory of GEC Avionics, Rochester, Kent, UK (now part of BAE Systems) R.P.G. Collinson Formerly of GEC Avionics (now part of BAE Systems) Maidstone, Kent United Kingdom ISBN 978-94-007-0707-8 e-ISBN 978-94-007-0708-5 DOI 10.1007/978-94-007-0708-5 Springer Dordrecht Heidelberg London New York Library of Congress Control Number: 2011931528 © Springer Science+Business Media B.V. 2011 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microﬁlming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied speciﬁcally for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Cover design: SPi Publisher Services Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Contents Foreword.......................................................... xi Preface............................................................ xiii Acknowledgements................................................. xv 1 Introduction................................................... 1 1.1 Importance and Role of Avionics............................. 1 1.1.1 Systems Which Interface Directly with the Pilot.......... 3 1.1.2 Aircraft State Sensor Systems.......................... 5 1.1.3 Navigation Systems.................................. 6 1.1.4 Outside World Sensor Systems......................... 7 1.1.5 Task Automation Systems............................. 8 1.2 The Avionic Environment.................................... 11 1.2.1 Minimum Weight.................................... 14 1.2.2 Environmental Requirements.......................... 14 1.2.3 Reliability.......................................... 15 1.3 Choice of Units............................................ 16 2 Displays and Man–Machine Interaction.......................... 19 2.1 Introduction............................................... 19 2.2 Head Up Displays.......................................... 20 2.2.1 Introduction......................................... 20 2.2.2 Basic Principles..................................... 23 2.2.3 Holographic HUDs................................... 30 2.2.4 HUD Electronics.................................... 36 2.2.5 Worked Example on HUD Design and Display Generation. 39 2.2.6 Civil Aircraft HUDs.................................. 42 2.3 Helmet Mounted Displays................................... 47 2.3.1 Introduction......................................... 47 2.3.2 Helmet Design Factors................................ 48 v vi Contents 2.3.3 Helmet Mounted Sights............................... 50 2.3.4 Helmet Mounted Displays............................. 51 2.3.5 Head Tracking Systems.............................. 56 2.3.6 HMDs and the Virtual Cockpit......................... 58 2.4 Computer Aided Optical Design.............................. 61 2.4.1 Introduction......................................... 61 2.5 Discussion of HUDs versus HMDs............................ 62 2.5.1 Introduction......................................... 62 2.5.2 Military Aircraft HUDs and HMDs..................... 62 2.6 Head Down Displays....................................... 66 2.6.1 Introduction......................................... 66 2.6.2 Civil Cockpit Head Down Displays..................... 67 2.6.3 Military Head Down Displays.......................... 69 2.6.4 Display Symbology Generation........................ 70 2.6.5 Digitally Generated Moving Colour Map Displays........ 71 2.6.6 Solid State Standby Display Instruments................. 74 2.7 Data Fusion............................................... 77 2.8 Intelligent Displays Management............................. 79 2.9 Displays Technology........................................ 80 2.9.1 Replacing the HUD CRT.............................. 80 2.9.2 HMD/HUD Optical System Technology................. 84 2.9.3 The Q Sight HMD................................... 88 2.9.4 The Q HUD......................................... 91 2.10 Control and Data Entry...................................... 92 2.10.1 Introduction......................................... 92 2.10.2 Tactile Control Panels................................ 93 2.10.3 Direct Voice Input................................... 94 2.10.4 Speech Output Systems............................... 97 2.10.5 Display Integration with Audio/Tactile Inputs............ 97 2.10.6 Eye Trackers........................................ 97 3 Aerodynamics and Aircraft Control.............................. 101 3.1 Introduction............................................... 101 3.2 Basic Aerodynamics........................................ 101 3.2.1 Lift and Drag....................................... 101 3.2.2 Angle of Incidence/Angle of Attack..................... 103 3.2.3 Lift Coefficient and Drag Coefficient.................... 104 3.2.4 Illustrative Example on Basic Aerodynamics............. 105 3.2.5 Pitching Moment and Aerodynamic Centre.............. 107 3.2.6 Tailplane Contribution................................ 108 3.3 Aircraft Stability........................................... 110 3.3.1 Longitudinal Stability................................ 110 3.3.2 Aerodynamically Unstable Aircraft..................... 112 3.3.3 Body Lift Contributions............................... 114 3.4 Aircraft Dynamics.......................................... 115 Contents vii 3.4.1 Aircraft Axes – Velocity and Acceleration Components.... 115 3.4.2 Euler Angles – Definition of Angles of Pitch, Bank and Yaw............................................ 118 3.4.3 Equations of Motion for Small Disturbances............. 119 3.4.4 Aerodynamic Force and Moment Derivatives............ 122 3.4.5 Equations of Longitudinal and Lateral Motion........... 131 3.5 Longitudinal Control and Response........................... 134 3.5.1 Longitudinal Control................................. 134 3.5.2 Stick Force/g........................................ 135 3.5.3 Pitch Rate Response to Tailplane/Elevator Angle......... 136 3.5.4 Pitch Response Assuming Constant Forward Speed....... 138 3.5.5 Worked Example on q/η Transfer Function and Pitch Response........................................... 145 3.6 Lateral Control............................................. 148 3.6.1 Aileron Control and Bank to Turn...................... 148 3.6.2 Rudder Control..................................... 150 3.6.3 Short Period Yawing Motion........................... 152 3.6.4 Combined Roll-Yaw-Sideslip Motion................... 153 3.7 Powered Flying Controls.................................... 154 3.7.1 Introduction......................................... 154 3.7.2 PCU Transfer Functions.............................. 155 3.8 Stability Augmentation Systems.............................. 157 3.8.1 Limited Authority Stability Augmentation Systems....... 157 3.8.2 Full Authority Stability Augmentation Systems.......... 162 3.9 Helicopter Flight Control.................................... 163 3.9.1 Introduction......................................... 163 3.9.2 Control of the Helicopter in Flight...................... 168 3.9.3 Stability Augmentation............................... 172 4 Fly-by-Wire Flight Control...................................... 179 4.1 Introduction............................................... 179 4.2 FBW Flight Control Features and Advantages................... 180 4.2.1 FBW System Basic Concepts and Features............... 180 4.2.2 Advantages of FBW Control........................... 186 4.3 Control Laws.............................................. 194 4.3.1 Pitch Rate Command Control.......................... 197 4.3.2 Lags in the Control Loop.............................. 207 4.3.3 Roll Rate Command Control.......................... 212 4.3.4 Handling Qualities and PIOs........................... 212 4.3.5 Modern Control Theory............................... 216 4.4 Redundancy and Failure Survival............................. 217 4.4.1 Safety and Integrity.................................. 217 4.4.2 Redundant Configurations............................. 217 4.4.3 Voting and Consolidation............................. 219 4.4.4 Quadruplex System Architecture....................... 223 viii Contents 4.4.5 Common Mode Failures.............................. 224 4.4.6 Dissimilar Redundancy............................... 225 4.5 Digital Implementation...................................... 230 4.5.1 Advantages of Digital Implementation.................. 230 4.5.2 Digital Data Problems................................ 232 4.5.3 Software........................................... 234 4.5.4 Failure Modes and Effects Analysis.................... 242 4.6 Helicopter FBW Flight Control Systems....................... 242 4.7 Active FBW Inceptors...................................... 244 4.8 Fly-by-Light Flight Control.................................. 248 4.8.1 Introduction......................................... 248 4.8.2 Fly-by-Light Flight Control Systems.................... 250 4.8.3 Optical Sensors...................................... 251 5 Inertial Sensors and Attitude Derivation.......................... 255 5.1 Introduction............................................... 255 5.2 Gyros and Accelerometers................................... 255 5.2.1 Introduction......................................... 255 5.2.2 Micro Electro-Mechanical Systems (MEMS) Technology Rate Gyros......................................... 257 5.2.3 Optical Gyroscopes.................................. 260 5.2.4 Accelerometers...................................... 276 5.2.5 Skewed Axes Sensor Configurations.................... 282 5.3 Attitude Derivation......................................... 283 5.3.1 Introduction......................................... 283 5.3.2 Strap-Down Systems................................. 284 5.3.3 Coning Motion...................................... 293 5.3.4 Attitude with Respect to Local North, East, Down Axes.... 294 5.3.5 Vehicle Rate Corrections.............................. 296 5.3.6 Introduction to Complementary Filtering................ 299 6 Navigation Systems............................................. 303 6.1 Introduction and Basic Principles............................. 303 6.1.1 Introduction......................................... 303 6.1.2 Basic Navigation Definitions........................... 309 6.1.3 Basic DR Navigation Systems......................... 310 6.2 Inertial Navigation......................................... 313 6.2.1 Introduction......................................... 313 6.2.2 Basic Principles and Schuler Tuning.................... 314 6.2.3 Platform Axes....................................... 324 6.2.4 Initial Alignment and Gyro Compassing................. 327 6.2.5 Effect of Azimuth Gyro Drift.......................... 330 6.2.6 Vertical Navigation Channel........................... 330 6.2.7 Choice of Navigation Co-ordinates..................... 334 6.2.8 Strap-down IN System Computing...................... 335 Contents ix 6.3 Aided IN Systems and Kalman Filters......................... 337 6.4 Attitude Heading Reference Systems.......................... 345 6.4.1 Introduction......................................... 345 6.4.2 Azimuth Monitoring Using a Magnetic Heading Reference. 350 6.5 GPS – Global Positioning System............................. 355 6.5.1 Introduction......................................... 355 6.5.2 GPS System Description.............................. 357 6.5.3 Basic Principles of GPS............................... 358 6.5.4 Solution of Navigation Equations....................... 363 6.5.5 Integration of GPS and INS............................ 365 6.5.6 Differential GPS..................................... 365 6.5.7 Future Augmented Satellite Navigation Systems.......... 370 6.6 Terrain Reference Navigation................................ 372 6.6.1 Introduction......................................... 372 6.6.2 Terrain Contour Navigation............................ 373 6.6.3 Terrain Characteristic Matching........................ 374 6.6.4 Civil Exploitation of TRN............................. 375 7 Air Data and Air Data Systems.................................. 377 7.1 Introduction............................................... 377 7.2 Air Data Information and Its Use............................. 377 7.2.1 Air Data Measurement................................ 377 7.2.2 The Air Data Quantities and Their Importance............ 379 7.3 Derivation of Air Data Laws and Relationships.................. 383 7.3.1 Altitude–Static Pressure Relationship................... 384 7.3.2 Variation of Ground Pressure.......................... 388 7.3.3 Air Density versus Altitude Relationship................ 389 7.3.4 Speed of Sound...................................... 390 7.3.5 Pressure–Speed Relationships.......................... 392 7.3.6 Mach Number....................................... 395 7.3.7 Calibrated Airspeed.................................. 396 7.3.8 Static Air Temperature................................ 398 7.3.9 True Airspeed....................................... 399 7.3.10 Pressure Error....................................... 399 7.4 Air Data Sensors and Computing............................. 401 7.4.1 Introduction......................................... 401 7.4.2 Air Data System Pressure Sensors...................... 401 7.4.3 Air Data Computation................................ 409 7.4.4 Angle of Incidence Sensors............................ 412 8 Autopilots and Flight Management Systems....................... 415 8.1 Introduction............................................... 415 8.2 Autopilots................................................. 417 8.2.1 Basic Principles..................................... 417 8.2.2 Height Control...................................... 418 x Contents 8.2.3 Heading Control Autopilot............................ 419 8.2.4 ILS/MLS Coupled Autopilot Control.................... 428 8.2.5 Automatic Landing................................... 433 8.2.6 Satellite Landing Guidance Systems.................... 439 8.2.7 Speed Control and Auto-Throttle Systems............... 440 8.3 Flight Management Systems................................. 442 8.3.1 Introduction......................................... 442 8.3.2 Radio Navigation Tuning............................. 445 8.3.3 Navigation......................................... 446 8.3.4 Flight Planning...................................... 448 8.3.5 Performance Prediction and Flight Path Optimisation...... 451 8.3.6 Control of the Vertical Flight Path Profile................ 454 8.3.7 Operational Modes................................... 455 8.3.8 4D Flight Management............................... 457 9 Avionics Systems Integration.................................... 459 9.1 Introduction and Background................................. 459 9.2 Data Bus Systems.......................................... 466 9.2.1 Electrical Data Bus Systems........................... 466 9.2.2 Optical Data Bus Systems............................. 472 9.2.3 Parallel Data Buses.................................. 480 9.3 Integrated Modular Avionics Architectures..................... 480 9.3.1 Civil Integrated Modular Avionic Systems............... 483 9.4 Commercial Off-the-Shelf (COTS)............................ 486 10 Unmanned Air Vehicles......................................... 489 10.1 Importance of Unmanned Air Vehicles......................... 489 10.2 UAV Avionics............................................. 490 10.3 Brief Overview of Some Current UAVs/UCAVs................. 493 Glossary of Terms.................................................. 499 List of Symbols.................................................... 509 List of Abbreviations............................................... 519 Index............................................................. 523 Foreword Earlier editions of this book have sold widely, and it has become a standard on avionics used by many universities and colleges, and also by engineering and de- velopment establishments and avionic companies. Although the basic principles of avionic systems as described are basically unchanged, the period since 2003 when the second edition was published, has seen many improvements in technology, and this third edition has been revised and updated to accommodate these changes. The extensive revisions include, on displays, the use of ‘commercial off-the- shelf’ graphics processor chips and drive processors for symbology generation, as well as the replacement of HUD CRTs, and the application of holographic optical waveguide technology to helmet mounted displays and HUDs. On Controls, the op- portunity has been taken to add a section on helicopter flight control, and Chapter 4 – Fly-by-Wire Flight Control, now includes a description of current Airbus fly by wire, helicopter fly by wire, and active pilot’s control sticks/inceptors incorpor- ating controlled force feedback, The coverage of inertial navigation has also been updated, and the section on flight management has been expanded to cover the latest AIRBUS systems. Chapter 9, on System Integration, now includes an overview of the modular avionic architectures in the new generation of civil aircraft, such as the A380, and finally the coverage of Unmanned Air Vehicles has been extended, in view of the increasing role they are now playing in both military and civil applica- tions. Although technological advances will continue to be made, I am confident that this updated third edition will ensure that this definitive work will continue to be widely used by both avionics practitioners and students for many years to come. I am sure that with this book, Dick Collinson, a life-long enthusiast of avionics engineering and aircraft systems, has made a lasting contribution to avionics knowledge and experience. Derek Jackson BSc, C Eng formerly Managing Director, Smiths Aerospace, Cheltenham xi Preface My aims in writing this book were to explain the basic principles underlying the key, or core, avionic systems in modern civil and military aircraft and their implement- ation using modern technology. Technology is continually advancing and this third edition incorporates the advances being made since the second edition was finished 8 years ago. The opportunity has been taken to extend the coverage of particular areas, where relevant, and helicopter flight control has been added. The core systems covered comprise pilot’s display systems and man machine interaction, fly-by-wire flight control systems, inertial sensor systems, navigation systems, air data systems, autopilots and flight management systems, and avionic system integration. Unmanned Air Vehicles (UAVs) are briefly discussed. The systems are analysed mathematically (where appropriate) from the phys- ical laws governing their behaviour so that the system design and response can be understood and the performance analysed. Worked examples are included to show how the theory can be applied to a representative system. Physical explanations are set out of the system behaviour and the text is structured so that readers can ‘fast forward’ through the maths and just accept the results, if they so wish. The systems covered are all ‘flight safety critical’. Their implementation using modern digital technology to meet the very high safety and integrity requirements is explained together with the overall integration of these systems. A particular aim, based on my experience over many years, is to meet the needs of graduates (or equivalent) entering the avionics industry who have been educated in a wide variety of disciplines, for example, electronic engineering, computer science, mathematics, physics, mechanical and aeronautical engineering. The book also aims to meet the needs of engineers at all levels working in particular areas of avionics who require an appreciation and understanding of other areas and disciplines. A further objective is to show the very wide range of disciplines which are involved in avionic systems, as this makes the subject an interesting and challenging field. Apart from the interest inherent in aircraft, the range of disciplines and tech- nologies which are exploited covers aerodynamics and aircraft control, satellite nav- igation, optical gyroscopes, man-machine interaction, speech recognition, advanced xiii xiv Preface display systems, holographic optics, intelligent knowledge based systems, closed- loop control systems, high integrity failure survival systems, high integrity software, integrated circuit design and data bus systems. Personally, I have found avionics to be a very interesting and challenging area to work in and I hope this book will help the reader to share this interest. Dick Collinson September 2010 Note: The illustration shows the visual impact of the avionic systems in a modern aircraft. Acknowledgements I would like to thank the management of BAE Systems Avionics Group, Rochester for their assistance in writing this book and their permission to use a number of the illustrations showing BAE Systems equipment and concepts. I would also like to express my appreciation to former senior Managers within the company who have given their support and encouragement over the years in producing the first and second editions of this book, and who have now retired. In particular, Sue Wood, Brian Tucker, Robin Sleight and Ron Howard. My thanks and appreciation to my former colleagues in the Company, Chris Bart- lett, Ted Lewis and Paul Wisely for their help and advice on displays technology and John Corney for his help and advice on all aspects of flight control, particularly heli- copters. The ‘teach-ins’ over a pint of real ale at the Tiger Moth pub have been a pleasant part of writing this book. Paul refers to them as ‘literary lunches’. I would like to express my appreciation and thanks to Derek Jackson, former Managing Director of Smiths Aerospace, Cheltenham for his helpful comments and for writing the Foreword to this third edition. I would also like to thank my former colleagues Gordon Belcher, Andrew Gib- son, Derek Hamlin, Robin Heaps, and Dave Jibb for their help in obtaining inform- ation, checking the draft chapters and providing helpful and constructive comments in the earlier editions of this book; most of the theoretical coverage remains un- changed. I would also like to thank Professor David Allerton, of Cranfield University and Professor John Roulston, Technical Director, BAE Systems, Avionics Group, for their help and support in producing the first and second editions. The truly excellent work carried out by Bob Ellwood in producing this book is gratefully acknowledged. Producing the typed and formatted text including all the equations to the ‘camera ready copy’ stage was no mean feat. I would also like to thank Andy Poad for his excellent work in producing most of the computer-generated illustrations from hand drawn diagrams. My thanks also to “Pat” Paternoster for the design and creation of the illustration in the Preface. Grateful acknowledgement is made to the following companies and organisations for permission to use their illustrations and material: xv xvi Acknowledgements AgustaWestland Lockheed Martin Airbus Royal Aeronautical Society Library BAE Systems Schlumberger Industries BEI Systron Donner Inertial Division Smiths Industries Honeywell The Boeing Company Finally, I would like to thank my wife and family for their whole-hearted support and encouragement in writing this book. Chapter 1 Introduction 1.1 Importance and Role of Avionics ‘Avionics’ is a word derived from the combination of aviation and electronics. It was first used in the USA in the early 1950s and has since gained wide scale usage and acceptance although it must be said that it may still be necessary to explain what it means to the lay person on occasions. The term ‘avionic system’ or ‘avionic sub-system’ is used in this book to mean any system in the aircraft which is dependent on electronics for its operation, al- though the system may contain electro-mechanical elements. For example, a Fly- by-Wire (FBW) flight control system depends on electronic digital computers for its effective operation, but there are also other equally essential elements in the sys- tem. These include solid state rate gyroscopes and accelerometers to measure the angular and linear motion of the aircraft and air data sensors to measure the height, airspeed and incidence. There are also the pilot’s control stick and rudder sensor assemblies and electro-hydraulic servo actuators to control the angular positions of the control surfaces. The avionics industry is a major multi-billion dollar industry world-wide and the avionics equipment on a modern military or civil aircraft can account for around 30% of the total cost of the aircraft. This figure for the avionics content is more like 40% in the case of a maritime patrol/anti-submarine aircraft (or helicopter) and can be over 75% of the total cost in the case of an airborne early warning aircraft (AWACS). Modern general aviation aircraft also have a significant avionics content. For example, colour head down displays, GPS satellite navigation systems, radio com- munications equipment. Avionics can account for 10% of their total cost. The avionic systems are essential to enable the flight crew to carry out the aircraft mission safely and efficiently, whether the mission is carrying passengers to their destination in the case of a civil airliner, or, in the military case, intercepting a hostile aircraft, attacking a ground target, reconnaissance or maritime patrol. R.P.G. Collinson, Introduction to Avionics Systems, DOI 10.1007/978-94-007-0708-5_1, 1 © Springer Science+Business Media B.V., 2011 2 1 Introduction A major driver in the development and introduction of avionic systems has been the need to meet the mission requirements with the minimum flight crew. In the case of a modern civil airliner, this means a crew of two only, namely the First Pilot (or Captain) and the Second Pilot. This is only made possible by reducing the crew workload by automating the tasks which used to be carried out by the Navigator and Flight Engineer. The achievement of safe two crew operation has very considerable economic benefits for the airline in a highly competitive market with the consequent saving of crew salaries, expenses and training costs. The reduction in weight is also significant and can be translated into more passengers or longer range on less fuel. This is because unnecessary weight is geared up ten to one, as will be explained later. In the military case, a single seat fighter or strike (attack) aircraft is lighter and costs less than an equivalent two seat version. The elimination of the second crew member (Navigator/Observer/Radar Operator) has also significant economic benefits in terms of reduction in training costs. (The cost of training and selection of aircrew for fast jet operation is very high.) Other very important drivers for avionic systems are increased safety, air traffic control requirements, all weather operation, reduction in fuel consumption, im- proved aircraft performance and control and handling and reduction in maintenance costs. Military avionic systems are also being driven by a continuing increase in the threats posed by the defensive and offensive capabilities of potential aggressors. The role played by the avionic systems in a modern aircraft in enabling the crew to carry out the aircraft mission can be explained in terms of a hierarchical struc- ture comprising layers of specific tasks and avionic system functions as shown in Figure 1.1. This shows the prime, or ‘core’, functions which are mainly common to both military and civil aircraft. It must be pointed out, however, that some avionic systems have been left off this diagram for clarity. For example, the Air Traffic Con- trol (ATC) transponder system, the Ground Proximity Warning System (GPWS) and the Threat Alert/Collision Avoidance System (TCAS), all of which are mandatory equipment for civil airliners. GPWS provides warning by means of a visual display and audio signal (‘Pull up, Pull up... ’) that the aircraft is on a flight path that will result in flying into the terrain, and that action must be taken to change the flight path. TCAS provides an alerting and warning display of other aircraft in the vicinity in terms of their range, course and altitude together with advisory collision avoidance commands. Referring to Figure 1.1, it can be seen that the main avionic sub-systems have been grouped into five layers according to their role and function. These are briefly summarised below in order to provide an overall picture of the roles and functions of the avionic systems in an aircraft. It should be noted that unmanned aircraft (UMAs) are totally dependant on the avionic systems. These are briefly discussed in Chapter 10. 1.1 Importance and Role of Avionics 3 Fig. 1.1 Core avionic systems 1.1.1 Systems Which Interface Directly with the Pilot These comprise displays, communications, data entry and control and flight control. The Display Systems provide the visual interface between the pilot and the air- craft systems and comprise head up displays (HUDs), helmet mounted displays (HMDs) and head down displays (HDDs). Most combat aircraft are now equipped with a HUD. A small but growing number of civil aircraft have HUDs installed. The HMD is also an essential system in modern combat aircraft and helicopters. The prime advantages of the HUD and HMD are that they project the display in- formation into the pilot’s field of view so that the pilot can be head up and can concentrate on the outside world. 4 1 Introduction The HUD now provides the primary display for presenting the essential flight information to the pilot and in military aircraft has transformed weapon aiming accuracy. The HUD can also display a forward looking infrared (FLIR) video picture one to one with the outside world from a fixed FLIR imaging sensor installed in the aircraft. The infrared picture merges naturally with the visual scene enabling operations to be carried out at night or in conditions of poor visibility due to haze or clouds. The HMD enables the pilot to be presented with information while looking in any direction, as opposed to the limited forward field of view of the HUD. An essential element in the overall HMD system is the Helmet Tracker system to derive the direction of the pilot’s sight line relative to the aircraft axes. This enables the pilot to designate a target to the aircraft’s missiles. It also enables the pilot to be cued to look in the direction of a threat(s) detected by the aircraft’s Defensive Aids system. The HMD can also form part of an indirect viewing system by driving a gimballed infrared imaging sensor to follow the pilot’s line of sight. Image intensification devices can also be integrated into the HMD. These provide a complementary night vision capability enabling the aircraft (or helicopter) to oper- ate at night or in poor visibility. Colour head down displays have revolutionised the civil flight-deck with multi- function displays eliminating the inflexible and cluttered characteristics of 1970s generation flight-decks with their numerous dial type instrument displays dedicated to displaying one specific quantity only. The multi-function colour displays provide the primary flight displays (PFDs) of height, airspeed, Mach number, vertical speed, artificial horizon, pitch angle, bank angle and heading, and velocity vector. They provide the navigation displays, or horizontal situation indicator (HSI) displays, which show the aircraft position and track relative to the destination or waypoints together with the navigational inform- ation and distance and time to go. The weather radar display can also be super- imposed on the HSI display. Engine data are presented on multi-function colour displays so that the health of the engines can easily be monitored and divergences from the norm highlighted. The aircraft systems, for example, electrical power sup- ply system, hydraulic power supply system, cabin pressurisation system and fuel management system, can be shown in easy to understand line diagram format on the multi-function displays. The multi-function displays can also be reconfigured in the event of a failure in a particular display. The Communications Systems play a vital role; the need for reliable two way communication between the ground bases and the aircraft or between aircraft is self evident and is essential for air traffic control. A radio transmitter and receiver equipment was in fact the first avionic system to be installed in an aircraft and goes back as far as 1909 (Marconi Company). The communications radio suite on modern aircraft is a very comprehensive one and covers several operating frequency bands. Long range communication is provided by high frequency (HF) radios operating in the band 2–30 MHz. Near to medium range communication is provided in civil aircraft by very high frequency (VHF) radios operating in the band 30–100 MHz, and in military aircraft by ultra high frequency (UHF) radio operating in the band 250–400 MHz. (VHF and UHF are line of sight propagation systems.) Equipment 1.1 Importance and Role of Avionics 5 is usually at duplex level of redundancy; the VHF radios are generally at triplex level on a modern airliner. Satellite communications (SATCOM) systems are also installed in many modern aircraft and these are able to provide very reliable world wide communication. The Data Entry and Control Systems are essential for the crew to interact with the avionic systems. Such systems range from keyboards and touch panels to the use of direct voice input (DVI) control, exploiting speech recognition technology, and voice warning systems exploiting speech synthesisers. The Flight Control Systems exploit electronic system technology in two areas, namely auto-stabilisation (or stability augmentation) systems and FBW flight con- trol systems. Most swept wing jet aircraft exhibit a lightly damped short period os- cillatory motion about the yaw and roll axes at certain height and speed conditions, known as ‘Dutch roll’, and require at least a yaw auto-stabiliser system to damp and suppress this motion; a roll auto-stabiliser system may also be required. The short period motion about the pitch axis can also be insufficiently damped and a pitch auto-stabiliser system is necessary. Most combat aircraft and many civil aircraft in fact require three axis auto-stabilisation systems to achieve acceptable control and handling characteristics across the flight envelope. FBW flight control enables a lighter, higher performance aircraft to be produced compared with an equivalent conventional design by allowing the aircraft to be de- signed with a reduced or even negative natural aerodynamic stability. It does this by providing continuous automatic stabilisation of the aircraft by computer control of the control surfaces from appropriate motion sensors. The system can be designed to give the pilot a manoeuvre command control which provides excellent control and handling characteristics across the flight envelope. ‘Care free manoeuvring’ char- acteristics can also be achieved by automatically limiting the pilot’s commands ac- cording to the aircraft’s state. A very high integrity, failure survival system is of course essential for FBW flight control. 1.1.2 Aircraft State Sensor Systems These comprise the air data systems and the inertial sensor systems. The Air Data Systems provide accurate information on the air data quantities, that is the altitude, calibrated airspeed, vertical speed, true airspeed, Mach num- ber and airstream incidence angle. This information is essential for the control and navigation of the aircraft. The air data computing system computes these quantities from the outputs of very accurate sensors which measure the static pressure, total pressure and the outside air temperature. The air-stream incidence angle is derived from air-stream incidence sensors. The Inertial Sensor Systems provide the information on aircraft attitude and the direction in which it is heading which is essential information for the pilot in ex- ecuting a manoeuvre or flying in conditions of poor visibility, flying in clouds or at night. Accurate attitude and heading information are also required by a number 6 1 Introduction of avionic sub-systems which are essential for the aircraft’s mission – for example, the autopilot and the navigation system and weapon aiming in the case of a military aircraft. The attitude and heading information is provided by the inertial sensor system(s). These comprise a set of gyros and accelerometers which measure the aircraft’s an- gular and linear motion about the aircraft axes, together with a computing system which derives the aircraft’s attitude and heading from the gyro and accelerometer outputs. Modern attitude and heading reference systems (AHRS) use a strapped down (or body mounted) configuration of gyros and accelerometers as opposed to the earlier gimballed systems. The use of very high accuracy gyros and accelerometers to measure the aircraft’s motion enables an inertial navigation system (INS) to be mechanised which provides very accurate attitude and heading information together with the aircraft’s velocity and position data (ground speed, track angle and latitude/longitude co-ordinates). The INS in conjunction with the air data system also provides the aircraft velocity vector information. The INS is thus a very important aircraft state sensor system – it is also completely self-contained and does not require any access to the outside world. 1.1.3 Navigation Systems Accurate navigation information, that is the aircraft’s position, ground speed and track angle (direction of motion of the aircraft relative to true North) is clearly es- sential for the aircraft’s mission, whether civil or military. Navigation systems can be divided into dead reckoning (DR) systems and position fixing systems; both types are required in the aircraft. The Dead Reckoning Navigation Systems derive the vehicle’s present position by estimating the distance travelled from a known position from a knowledge of the speed and direction of motion of the vehicle. They have the major advantages of being completely self contained and independent of external systems. The main types of DR navigation systems used in aircraft are: (a) Inertial navigation systems. The most accurate and widely used systems. (b) Doppler/heading reference systems. These are widely used in helicopters. (c) Air data/heading reference systems These systems are mainly used as a rever- sionary navigation system being of lower accuracy than (a) or (b). A characteristic of all DR navigation systems is that the position error builds up with time and it is, therefore, necessary to correct the DR position error and update the system from position fixes derived from a suitable position fixing system. The Position Fixing Systems used are now mainly radio navigation systems based on satellite or ground based transmitters. A suitable receiver in the aircraft with a supporting computer is then used to derive the aircraft’s position from the signals received from the transmitters. 1.1 Importance and Role of Avionics 7 The prime position fixing system is without doubt GPS (global positioning system). This is a satellite navigation system of outstanding accuracy which has provided a revolutionary advance in navigation capability since the system started to come into full operation in 1989. There are also radio navigation aids such as VOR/DME and TACAN which provide the range and bearing (R/θ ) of the aircraft from ground beacon transmitters located to provide coverage of the main air routes. Approach guidance to the airfield/airport in conditions of poor visibility is provided by the ILS (instrument landing system), or by the later MLS (microwave landing system). A full navigation suite on an aircraft is hence a very comprehensive one and can include INS, GPS, VOR/DME, ILS, MLS. Many of these systems are at duplex level and some may be at triplex level. 1.1.4 Outside World Sensor Systems These systems, which comprise both radar and infrared sensor, systems enable all weather and night time operation and transform the operational capability of the aircraft (or helicopter). A very brief description of the roles of these systems is given below. The Radar Systems installed in civil airliners and many general aviation aircraft aircraft provide weather warning. The radar looks ahead of the aircraft and is optim- ised to detect water droplets and provide warning of storms, cloud turbulence and severe precipitation so that the aircraft can alter course and avoid such conditions, if possible. It should be noted that in severe turbulence, the violence of the vertical gusts can subject the aircraft structure to very high loads and stresses. These radars can also generally operate in ground mapping and terrain avoidance modes. Modern fighter aircraft generally have a ground attack role as well as the prime interception role and carry very sophisticated multi-mode radars to enable them to fulfil these dual roles. In the airborne interception (AI) mode, the radar must be able to detect aircraft up to 100 miles away and track while scanning and keeping tabs on several aircraft simultaneously (typically at least 12 aircraft). The radar must also have a ‘look down’ capability and be able to track low flying aircraft below it. In the ground attack or mapping mode, the radar system is able to generate a map type display from the radar returns from the ground, enabling specific terrain features to be identified for position fixing and target acquisition. The Infrared Sensor Systems have the major advantage of being entirely passive systems. Infrared (IR) sensor systems can be used to provide a video picture of the thermal image scene of the outside world either using a fixed FLIR sensor, or alternatively, a gimballed IR imaging sensor. The thermal image picture at night looks very like the visual picture in daytime, but highlights heat sources, such as vehicle engines, enabling real targets to be discriminated from camouflaged decoys. An IR system can also be used in a search and track mode; the passive detection and 8 1 Introduction tracking of targets from their IR emissions is of high operational value as it confers an all important element of surprise. FLIR systems can also be installed in civil aircraft to provide enhanced vision in poor visibility conditions in conjunction with a HUD. 1.1.5 Task Automation Systems These comprise the systems which reduce the crew workload and enable minimum crew operation by automating and managing as many tasks as appropriate so that the crew role is a supervisory management one. The tasks and roles of these are very briefly summarised below. Navigation Management comprises the operation of all the radio navigation aid systems and the combination of the data from all the navigation sources, such as GPS and the INS systems, to provide the best possible estimate of the aircraft posi- tion, ground speed and track. The system then derives the steering commands for the autopilot so that the aircraft automatically follows the planned navigation route, in- cluding any changes in heading as particular waypoints are reached along the route to the destination. It should be noted that this function is carried out by the flight management system (FMS) (if installed). The Autopilots and Flight Management Systems have been grouped together. Be- cause of the very close degree of integration between these systems on modern civil aircraft. It should be noted, however, that the Autopilot is a ‘stand alone’ system and not all aircraft are equipped with an FMS. The autopilot relieves the pilot of the need to fly the aircraft continually with the consequent tedium and fatigue and so enables the pilot to concentrate on other tasks associated with the mission. Apart from basic modes, such as height hold and head- ing hold, a suitably designed high integrity autopilot system can also provide a very precise control of the aircraft flight path for such applications as automatic landing in poor or even zero visibility conditions. In military applications, the autopilot sys- tem in conjunction with a suitable guidance system can provide automatic terrain following, or terrain avoidance. This enables the aircraft to fly automatically at high speed at very low altitudes (100 to 200 ft) so that the aircraft can take advantage of terrain screening and stay below the radar horizon of enemy radars. Sophisticated FMS have come into wide scale use on civil aircraft since the early 1980s and have enabled two crew operation of the largest, long range civil jet air- liners. The tasks carried out by the FMS include: Flight planning. Navigation management. Engine control to maintain the planned speed or Mach number. Control of the aircraft flight path to follow the optimised planned route. Control of the vertical flight profile. Ensuring the aircraft is at the planned 3D position at the planned time slot; often referred to as 4D navigation. This is very important for air traffic control. 1.1 Importance and Role of Avionics 9 Flight envelope monitoring. Minimising fuel consumption. The Engine Control and Management Systems carry out the task of control and the efficient management and monitoring of the engines. The electronic equipment involved in a modern jet engine is very considerable: it forms an integral part of the engine and is essential for its operation. In many cases some of the engine control electronics is physically mounted on the engine. Many modern jet engines have a full authority digital engine control system (FADEC). This automatically controls the flow of fuel to the engine combustion chambers by the fuel control unit so as to provide a closed-loop control of engine thrust in response to the throttle command. The control system ensures the engine limits in terms of temperatures, engine speeds and accelerations are not exceeded and the engine responds in an optimum manner to the throttle command. The system has what is known as full authority in terms of the control it can exercise on the engine and a high integrity failure survival control system is essential. Otherwise a failure in the system could seriously damage the engine and hazard the safety of the aircraft. A FADEC engine control system is thus similar in many ways to a FBW flight control system. Other very important engine avionic systems include engine health monitoring systems which measure, process and record a very wide range of parameters asso- ciated with the performance and health of the engines. These give early warning of engine performance deterioration, excessive wear, fatigue damage, high vibration levels, excessive temperature levels, etc. House Keeping Management is the term used to cover the automation of the background tasks which are essential for the aircraft’s safe and efficient operation. Such tasks include: Fuel management. This embraces fuel flow and fuel quantity measurement and control of fuel transfer from the appropriate fuel tanks to minimise changes in the aircraft trim. Electrical power supply system management. Hydraulic power supply system management. Cabin/cockpit pressurisation systems. Environmental control system. Warning systems. Maintenance and monitoring systems. These comprise monitoring and record- ing systems which are integrated into an on-board maintenance computer sys- tem. This provides the information to enable speedy diagnosis and rectification of equipment and system failures by pin-pointing faulty units and providing all the information, such as part numbers, etc., for replacement units down to mod- ule level in some cases. The above brief summaries cover the roles and importance of the avionic systems shown in Figure 1.1. It should be pointed out, however, that there are several major systems, particularly on military aircraft, which have not been mentioned. Space constraints limit the coverage of the systems shown in Figure 1.1 to Dis- plays, Data Entry and Control, Flight Control, Inertial Sensor Systems, Naviga- 10 1 Introduction tion Systems, Air Data Systems, Autopilots and Flight Management Systems, Data Buses and avionic systems integration. It is not possible to cover Communications, Radar, Infrared Systems, Engine Control and House Keeping Management. The visible impact of the avionic systems on a modern aircraft can best be seen in the cockpit where the outputs of the various systems just described are displayed on the HUD, HMD and the colour HDDs. Figure 1.2 shows the Eurofighter Typhoon fighter aircraft which is in service with the air forces of the UK, Germany, Italy and Spain. The Typhoon is a high agility single seat fighter/strike aircraft with an outstanding performance. The foreplane and elevons provide positive trim lift as it is highly unstable in pitch at subsonic speeds; time to double pitch amplitude following a disturbance is around 0.2 s. It is also unstable in yaw at supersonic speeds because of the reduced fin size. A high integrity FBW flight control system compensates for the lack of nat- ural stability and provides excellent control and handling across the flight envelope and under all loading conditions. Figure 1.3 shows the Typhoon cockpit which is designed to deliver optimum levels of tactical and functional information to the pilot without overloading him. The wide field of view holographic HUD and the HDDs, including the centrally located video colour moving map display, can be seen together with the small cent- rally mounted pilot’s control stick. The unobstructed view of the displays resulting from the use of a small control stick is apparent. The Typhoon pilot is equipped with a sophisticated binocular HMD and Sighting system which is shown in Figure 1.4. Miniature night vision cameras are mounted on each side of the helmet and the binocular video images are displayed on the HMD. The infrared video image scene from the gimballed IR sensor can also be displayed on the HMD to provide a complementary night vision system. (Gimballed IR Sensor is mounted on left side of aircraft nose in front of the windscreen – refer Figure 1.2.) Figure 1.5 shows the Airbus A380 long range airliner which entered airline ser- vice in 2008. The A380 is the largest airliner in the world and can carry up to 800 passengers. The advanced flight deck of the A380 can be seen in Figure 1.6 and features eight multi-function high resolution colour LCD flat panel displays. An optimised layout of the multi-function displays ensures that each of the two crew members can easily assimilate all relevant data, the screens being sized so that no eye scanning is necessary. In front of each pilot is a Primary Flight Display and a Navigation Display; the remaining two screens display engine and systems data. The pilot’s side stick controllers can be seen at the sides of the flight deck; the FBW flight control system eliminates the bulky control column between the pilots and instruments of earlier generation aircraft. This ensures an unobstructed view of the displays. There are also three multi-purpose control and display units which in addition to accessing the flight management system, are also used to give systems maintenance data in the air and on the ground. The two large screen displays at the sides of the A380 flight deck are able to dis- play, for example, comprehensive airport data including the airport layout, runways, etc. They can also be used to display relevant information from the flight manuals, 1.2 The Avionic Environment 11 Fig. 1.2 Eurofighter Typhoon (by courtesy of BAE Systems) emergency procedures etc. The central multi-function displays on the flight deck can also be reconfigured to display video images from monitoring cameras installed in the aircraft. The flight decks of the other members of the Airbus range of airliners, the A320 and its derivatives the A318, A319 and A321, the A330 and the A340 are basically very similar to that of the A380 but lack the large screen displays at each side of the A380 flight deck. 1.2 The Avionic Environment Avionic systems equipment is very different in many ways from ground based equipment carrying out similar functions. The reasons for these differences are briefly explained in view of their fundamental importance. 1. The importance of achieving minimum weight. 2. The adverse operating environment particularly in military aircraft in terms of operating temperature range, acceleration, shock, vibration, humidity range and electro-magnetic interference. 3. The importance of very high reliability, safety and integrity. 12 1 Introduction Fig. 1.3 Eurofighter Typhoon cockpit (by courtesy of BAE Systems) Fig. 1.4 Typhoon pilot with binocular HMD (by courtesy of BAE Systems) 1.2 The Avionic Environment 13 Fig. 1.5 Airbus A380 airliner (by courtesy of Airbus) Fig. 1.6 Airbus A380 flight deck (by courtesy of Airbus) 14 1 Introduction 4. Space constraints particularly in military aircraft requiring an emphasis on mini- aturisation and high packaging densities. The effects on the design of avionic equipment to meet these requirements can result in the equipment costing up to ten times as much as equivalent ground based electronic equipment. The aircraft environmental requirements are briefly discussed below. 1.2.1 Minimum Weight There is a gearing effect on unnecessary weight which is of the order of 10:1. For ex- ample a weight saving of 10 kg enables an increase in the payload capability of the order of 100 kg. The process of the effect of additional weight is a vicious circle. An increase in the aircraft weight due to, say, an increase in the weight of the avionics equipment, requires the aircraft structure to be increased in strength, and therefore made heavier, in order to withstand the increased loads during manoeuvres. (Assum- ing the same maximum normal acceleration, or g, and the same safety margins on maximum stress levels are maintained). This increase in aircraft weight means that more lift is required from the wings and the accompanying drag is thus increased. An increase in engine thrust is therefore required to counter the increase in drag and the fuel consumption is thus increased. For the same range it is thus necessary to carry more fuel and the payload has to be correspondingly reduced, or, if the pay- load is kept the same, the range is reduced. For these reasons tremendous efforts are made to reduce the equipment weight to a minimum and weight penalties can be imposed if equipment exceeds the specified weight. 1.2.2 Environmental Requirements The environment in which avionic equipment has to operate can be a very severe and adverse one in military aircraft; the civil aircraft environment is generally much more benign but is still an exacting one. Considering just the military cockpit environment alone, such as that experienced by the HUD and HDD. The operating temperature range is usually specified from −40◦ C to +70◦ C. Clearly, the pilot will not survive at these extremes but if the air- craft is left out in the Arctic cold or soaking in the Middle-East sun, for example, the equipment may well reach such temperatures. A typical specification can demand full performance at 20,000 ft within two minutes of take-off at any temperature within the range. Vibration is usually quite severe and, in particular, airframe manufacturers tend to locate the gun right under the displays. Power spectral energy levels of 0.04g 2 per Hz are encountered in aircraft designed in the 1970s and levels of 0.7g 2 per Hz 1.2 The Avionic Environment 15 at very low frequencies are anticipated in future installations. It is worth noting that driving over cobblestones will give about 0.001g 2 per Hz. The equipment must also operate under the maximum acceleration or g to which the aircraft is subjected during manoeuvres. This can be 9g in a modern fighter aircraft and the specification for the equipment would call up at least 20g. The electromagnetic compatibility (EMC) requirements are also very demand- ing. The equipment must not exceed the specified emission levels for a very wide range of radio frequencies and must not be susceptible to external sources of very high levels of RF energy over a very wide frequency band. The equipment must also be able to withstand lightning strikes and the very high electromagnetic pulses (EMP) which can be encountered during such strikes. Design of electronic equipment to meet the EMC requirements is in fact a very exacting discipline and requires very careful attention to detail design. 1.2.3 Reliability The over-riding importance of avionic equipment reliability can be appreciated in view of the essential roles of this equipment in the operation of the aircraft. It is clearly not possible to repair equipment in flight so that equipment failure can mean aborting the mission or a significant loss of performance or effectiveness in carrying out the mission. The cost of equipment failures in airline operation can be very high – interrupted schedules, loss of income during ‘aircraft on the ground’ situations etc. In military operations, aircraft availability is lowered and operational capability lost. Every possible care is taken in the design of avionic equipment to achieve max- imum reliability. The quality assurance (QA) aspects are very stringent during the manufacturing processes and also very frequently call for what is referred to as ‘re- liability shake-down testing’, or RST, before the equipment is accepted for delivery. RST is intended to duplicate the most severe environmental conditions to which the equipment could be subjected, in order to try to eliminate the early failure phase of the equipment life cycle. A typical RST cycle requires the equipment to operate satisfactorily through the cycle described below. Soaking in an environmental chamber at a temperature of +70◦C for a given period. Rapidly cooling the equipment to −55◦C in 20 minutes and soaking at that temperature for a given period. Subjecting the equipment to vibration, for example 0.5g amplitude at 50 Hz, for periods during the hot and cold soaking phases. A typical specification would call for twenty RST cycles without a failure before acceptance of the equipment. If a failure should occur at the nth cycle, the failure must be rectified and the remaining (20 − n) cycles repeated. 16 1 Introduction Fig. 1.7 Young Isaac Newton about to experience a force of 1 Newton All failures in service (and in testing) are investigated by the QA team and re- medial action taken, if necessary. The overall cost differential in meeting all the factors imposed by the avionic equipment environment can thus be appreciated. 1.3 Choice of Units It is appropriate at this point to explain the units which are used in this book. All quantities in this book are defined in terms of the SI system of units (Le Système Internationale d’Unités). However, some other units, notably the foot for altitude and the knot (1 nautical mile per hour) for speed are quoted in parallel for reasons which will be explained. The SI unit of mass is the kilogram (kg), the unit of length is the metre (m) and the unit of time is the second (s). The SI unit of temperature is the Kelvin (K). Zero degrees Kelvin corresponding to the absolutely lowest temperature possible at which almost all molecular translational motion theoretically stops. To convert temperature in degrees Celsius (◦ C) to degrees Kelvin it is necessary to add 273.15. Thus 0◦ C = 273.15◦K and 30◦C = 303.15◦K and conversely 0◦ K = −273.15◦C. The SI unit of force is the Newton (N), one Newton being the force required to accelerate a mass of one kilogram by 1 metre per second per second. Converting to pound, foot, second units, one Newton is approximately equal to a force of 0.22 pound (lb) weight. The apocryphal story of ‘an apple falling on Newton’s head triggering off his theory of gravitation’ gives an appropriate twist to the unit of the Newton, as an average apple weighs about 0.22 lb! Figure 1.7 illustrates the event! 1.3 Choice of Units 17 Pressure in the SI system is measured in N/m2 or Pascals (Pa). The millibar (mb) which is equal to 10−3 bar is also widely used, the bar being derived from BARometric. One bar corresponds to the atmospheric pressure under standard sea level conditions and is equal to 105 N/m2 or 100 kPa, and 1 millibar is equal to 100 N/m2 or 100 Pa. Altitude is quoted in feet as this is the unit used by the Air Traffic Control au- thorities, and altimeters in the USA and the UK are calibrated in feet. Speed is also quoted in knots as this unit is widely used in navigation. One knot is one nautical mile (NM) per hour and one nautical mile is equal to the length of the arc on the Earth’s surface subtended by an angle of one minute of arc measured from the Earth’s centre and can be related directly to latitude and longitude. 1 NM = 6076.1155 ft (or 1852 m exactly). The conversion from knots to metres/second is given by 1 knot = 0.5144 m/s and conversely 1 m/s = 1.9438 knots. A useful approximate conversion is 1 knot ≈ 0.5 m/s, or 1 m/s ≈ 2 knots. Chapter 2 Displays and Man–Machine Interaction 2.1 Introduction The cockpit display systems provide a visual presentation of the information and data from the aircraft sensors and systems to the pilot (and crew) to enable the pilot to fly the aircraft safely and carry out the mission. They are thus vital to the operation of any aircraft as they provide the pilot, whether civil or military, with: Primary flight information, Navigation information, Engine data, Airframe data, Warning information. The military pilot has also a wide array of additional information to view, such as: Infrared imaging sensors, Radar, Tactical mission data, Weapon aiming, Threat warnings. The pilot is able to rapidly absorb and process substantial amounts of visual inform- ation but it is clear that the information must be displayed in a way which can be readily assimilated, and unnecessary information must be eliminated to ease the pi- lot’s task in high work load situations. A number of developments have taken place to improve the pilot–display interaction and this is a continuing activity as new tech- nology and components become available. Examples of these developments are: Head up displays, Helmet mounted displays, Multi-function colour displays, Digitally generated colour moving map displays, Synthetic pictorial imagery, R.P.G. Collinson, Introduction to Avionics Systems, DOI 10.1007/978-94-007-0708-5_2, 19 © Springer Science+Business Media B.V., 2011 20 2 Displays and Man–Machine Interaction Displays management using intelligent knowledge based system (IKBS) tech- nology, Improved understanding of human factors and involvement of human factors specialists from the initial cockpit design stage. Equally important and complementary to the cockpit display systems in the ‘man machine interaction’ are the means provided for the pilot to control the operation of the avionic systems and to enter data. Again, this is a field where continual de- velopment is taking place. Multi-function keyboards and multi-function touch panel displays are now widely used. Speech recognition technology has now reached suf- ficient maturity for ‘direct voice input’ control to be installed in the new generation of military aircraft. Audio warning systems are now well established in both milit- ary and civil aircraft. The integration and management of all the display surfaces by audio/tactile inputs enables a very significant reduction in the pilot’s workload to be achieved in the new generation of single seat fighter/strike aircraft. Other methods of data entry which are being evaluated include the use of eye trackers. It is not possible in the space of one chapter to cover all aspects of this subject which can readily fill several books. Attention has, therefore, been concentrated on providing an overview and explanation of the basic principles involved in the following topics: Head up displays (Section 2.2) Helmet mounted displays (Section 2.3) Computer aided optical design (Section 2.4) Discussion of HUDs versus HMDs (Section 2.5) Head down displays (Section 2.6) Data fusion (Section 2.7) Intelligent displays management (Section 2.8) Display technology (Section 2.9) Control and data entry (Section 2.10) 2.2 Head Up Displays 2.2.1 Introduction Without doubt the most important advance to date in the visual presentation of data to the pilot has been the introduction and progressive development of the Head Up Display or HUD. (The first production HUDs, in fact, went into service in 1962 in the Buccaneer strike aircraft in the UK.) The HUD has enabled a major improvement in man–machine interaction (MMI) to be achieved as the pilot is able to view and assimilate the essential flight data generated by the sensors and systems in the aircraft whilst head up and maintaining full visual concentration on the outside world scene. 2.2 Head Up Displays 21 Fig. 2.1 Head-up presentation of primary flight information (by courtesy of BAE Systems). The display shows the artificial horizon with the aircraft making a 3.6◦ descent, on a heading of 00◦. The left hand scale shows an airspeed of 137 knots and the right hand scale an altitude of 880 ft. The flight path vector symbol shows where the aircraft’s CG is moving relative to the horizon – a conventional blind flying planel only shows where the aircraft is pointing relative to the horizon; if the aircraft flight path was maintained, it would be the impact point with the ground. A head up display basically projects a collimated display in the pilot’s head up forward line of sight so that he can view both the display information and the outside world scene at the same time. The fundamental importance of collimating the dis- play cannot be overemphasised and will be explained in more detail later. Because the display is collimated, that is focused at infinity (or a long distance ahead), the pilot’s gaze angle of the display symbology does not change with head movement so that the overlaid symbology remains conformal, or stabilised, with the outside world scene.The pilot is thus able to observe both distant outside world objects and display data at the same time without having to change the direction of gaze or re- focus the eyes. There are no parallax errors and aiming symbols for either a flight path director, or for weapon aiming in the case of a combat aircraft, remain overlaid on a distant ‘target’ irrespective of the pilot’s head movement. (Try sighting on a landmark using a mark on a window. The aiming mark moves off the ‘target’ if the head is moved sideways – this effect is parallax.) The advantages of head up presentation of essential flight data such as the artifi- cial horizon, pitch angle, bank angle, flight path vector, height, airspeed and heading can be seen in Figure 2.1 which shows a typical head up display as viewed by the pilot during the landing phase. The pilot is thus free to concentrate on the outside world during manoeuvres and does not need to look down at the cockpit instruments or head down displays. It should be noted that there is a transition time of one second or more to re-focus the eyes from viewing distant objects to viewing near objects a metre or less away, such as the cockpit instruments and displays and adapt to the cockpit light environ- ment. In combat situations, it is essential for survival that the pilot is head up and scanning for possible threats from any direction. The very high accuracy which can be achieved by a HUD and computerised weapon aiming system together with the 22 2 Displays and Man–Machine Interaction Fig. 2.2 Typical weapon aiming display (by courtesy of BAE Systems) ability to remain head up in combat have made the HUD an essential system on all modern combat aircraft. They have also been widely retro-fitted to earlier genera- tion fighters and strike aircraft as part of a cost effective avionic system up date. Figure 2.2 illustrates a typical weapon aiming display. By using a Forward Looking InfraRed (FLIR) sensor, an electro-optical image of the scene in front of the aircraft can be overlaid on the real world scene with a raster mode HUD. The TV raster picture generated from the FLIR sensor video is projected on to the HUD and scaled one to one with the outside world enabling the pilot to fly at low level by night in fair weather. This provides a realistic night attack capability to relatively simple day ground attack fighters. A wide field of view HUD is required, however, as will be explained later. HUDs are now being installed in civil aircraft for reasons such as: 1. Inherent advantages of head-up presentation of primary flight information in- cluding depiction of the aircraft’s flight path vector, resulting in improved situ- ational awareness and increased safety in circumstances such as wind shear or terrain/traffic avoidance manoeuvres. 2. To display automatic landing guidance to enable the pilot to land the aircraft safely in conditions of very low visibility due to fog, as a back up and monitor for the automatic landing system. The display of taxi-way guidance is also being considered. 3. Enhanced vision using a raster mode HUD to project a FLIR video picture of the outside world from a FLIR sensor installed in the aircraft, or, a synthetic picture of the outside world generated from a forward looking millimetric radar sensor 2.2 Head Up Displays 23 Fig. 2.3 Civil HUD installation – BAE Systems VGS HUD installed in a Boeing 737-800 series airliner (by courtesy of BAE Systems). in the aircraft. These enhanced vision systems are being actively developed and will enable the pilot to land the aircraft in conditions of very low or zero visib- ility at airfields not equipped with adequate all weather guidance systems such as ILS (or MLS). Figure 2.3 illustrates a civil HUD installation. 2.2.2 Basic Principles The basic configuration of a HUD is shown schematically in Figure 2.4. The pilot views the outside world through the HUD combiner glass (and windscreen). The combiner glass is effectively a ‘see through’ mirror with a high optical transmission efficiency so that there is little loss of visibility looking through the combiner and windscreen. It is called a combiner as it optically combines the collimated display symbology with the outside world scene viewed through it. Referring to Figure 2.4, the display symbology generated from the aircraft sensors and systems (such as the INS and air data system) is displayed on the surface of a cathode ray tube (CRT). The display images are then relayed through a relay lens system which magnifies the display and corrects for some of the optical errors which are otherwise present in the system. The relayed display images are then reflected through an angle of near 90◦ by the fold mirror and thence to the collimating lens which collimates the display images which are then reflected from the combiner glass into the pilot’s forward field of view. The virtual images of the display symbology appear to the pilot to be at infinity and overlay the distant world scene, as they are collimated. The function 24 2 Displays and Man–Machine Interaction Fig. 2.4 HUD schematic. of the fold mirror is to enable a compact optical configuration to be achieved so that the HUD occupies the minimum possible space in the cockpit. The fundamental importance of collimation to any HUD system merits further explanation for the benefit of readers whose knowledge of optics needs refreshing. A collimator is defined as an optical system of finite focal length with an image source at the focal plane. Rays of light emanating from a particular point on the focal plane exit from the collimating system as a parallel bunch of rays, as if they came from a source at infinity. Figures 2.5(a) and (b) show a simple collimating lens system with the rays traced from a source at the centre, O, and a point, D, on the focal plane respectively. A ray from a point on the focal plane which goes through the centre of the lens is not refracted and is referred to as the ‘non-deviated ray’. The other rays emanating from the point are all parallel to the non-deviated ray after exiting the collimator. It should be noted that the collimating lens, in practice, would be made of several elements to minimise the unacceptable shortcomings of a single element. It can be seen from Figure 2.5(a) that an observer looking parallel to the optical axis will see point O at eye positions A, B and C, and the angle of gaze to view O is independent of the displacement of the eye from the optical axis. Similarly, it can be seen from Figure 2.5(b) that the angle of gaze from the ob- server to see point D is the same for eye positions A, B and C and is independent of translation. The refractive HUD optical system, in fact, is basically similar to the simple optical collimation system shown in Figure 2.6. The rays are traced for the observer to see points D, O and E on the display with eye positions at points A, B and C. It can be seen that the angles of gaze to see points D, O and E are the same from points A, B or C. The appearance of the collimated display is thus independent of the position 2.2 Head Up Displays 25 Fig. 2.5 Simple optical collimator. Fig. 2.6 Simple optical collimator ray trace. 26 2 Displays and Man–Machine Interaction (or translation) of the eye and is only dependent on the angle of gaze. Also because of collimation, the display appears to be at infinity as the rays emanating from any point on the display are all parallel after exiting the collimating system. It should be noted that the display images must be correctly collimated. De- collimation is the term used when the light from a given point on the display does not leave the optical system parallel over the entire lens surface. The light can converge, diverge or otherwise ‘misbehave’ resulting in ‘swimming’ of the display images when the pilot’s head moves. Sometimes this creates discomfort and in the case of convergence can even cause nausea. A very important parameter with any HUD is the field of view (FOV), which should be as large as possible within the severe space constraints imposed by the cockpit geometry. A large horizontal FOV is particularly important to enable the pilot to ‘look into turns’ when the HUD forms part of a night vision system and the only visibility the pilot has of the outside world is the FLIR image displayed on the HUD. It is important to distinguish between the instantaneous field of view (IFOV) and the total field of view (TFOV) of a HUD as the two are not the same in the case of the refractive type of HUD. The instantaneous field of view is the angular coverage of the imagery which can be seen by the observer at any specific instant and is shown in the simplified diagram in Figure 2.7(a). It is determined by the diameter of the collimating lens, D, and the distance, L, of the observer’s eyes from the collimating lens. IFOV = 2 tan−1 D/2L The total field of view is the total angular coverage of the CRT imagery which can be seen by moving the observer’s eye position around. TFOV is determined by the diameter of the display, A, and effective focal length of the collimating lens, F. TFOV = 2 tan−1 A/2F Reducing the value of L increases the IFOV as can be seen in Figure 2.7(b) which shows the observer’s normal eye position brought sufficiently close to the collim- ating lens for the IFOV to equal the TFOV. However, this is not practical with the conventional type of HUD using refractive optics. This is because of the cockpit geometry constraints on the pilot’s eye position and the space constraints on the dia- meter of the collimating lens. The IFOV is generally only about two thirds of the TFOV. It can be seen from Figure 2.7(c) that by moving the head up or down or side to side the observer can see a different part of the TFOV, although the IFOV is unchanged. The effect is like looking through and around a porthole formed by the virtual image of the collimating lens as can be seen in Figure 2.8. The diagram shows the IFOV seen by both eyes (cross hatched), the IFOV seen by the left and right eyes respectively and the TFOV. 2.2 Head Up Displays 27 Fig. 2.7 Instantaneous and total FOV. The analogy can be made of viewing a football match through a knot hole in the fence and this FOV characteristic of a HUD is often referred to as the ‘knot hole effect’. The constraints involved in the HUD design are briefly outlined below. For a given TFOV, the major variables are the CRT display diameter and the effective focal length of the collimating lens system. For minimum physical size and weight, a small diameter CRT and short focal length are desired. These parameters are usually balanced against the need for a large diameter collimating lens to give the maximum IFOV and a large focal length which allows maximum accuracy. The diameter of the collimating lens is generally limited to between 75 mm and 175 mm (3 inches and 7 inches approximately) by cockpit space constraints and practical considerations. Large lenses are very heavy and liable to break under thermal shock. The HUD combiner occupies the prime cockpit location right in the centre of the pilot’s forward line of view at the top of the glare shield. The size of the combiner is determined by the desired FOV and the cockpit geometry, especially the pilot’s seating position. The main body of the HUD containing the optics and electronics must be sunk down behind the instrument panel in order to give an unrestricted 28 2 Displays and Man–Machine Interaction Fig. 2.8 HUD installation constraints and field of view. view down over the nose of the aircraft during high attitude manoeuvres (refer to Figure 2.8). The pilot’s design eye position for the HUD is determined by pilot comfort and the ability to view the cockpit instruments and head down displays and at the same time achieve the maximum outside world visibility. In the case of a combat aircraft, there is also the ejection line clearance to avoid the pilot being ‘kneecapped’ by the HUD on ejecting, which further constrains the design eye position. Typical IFOVs range from about 13◦ to 18◦ with a corresponding TFOV of about 20 to 25◦. The total vertical FOV of a HUD can be increased to around 18◦ by the ◦ use of a dual combiner configuration rather like a venetian blind. Effectively two overlapping portholes are provided, displaced vertically. The effect of the cockpit space and geometry constraints is that the HUD design has to be ‘tailor made’ for each aircraft type and a ‘standard HUD’ which would be interchangeable across a range of different aircraft types is not a practical proposi- tion. The conventional combiner glass in a refractive HUD has multi-layer coatings which reflect a proportion of the collimated display imagery and transmit a large proportion of the outside world, so that the loss of visibility is fairly small. A pilot looking through the combiner of such a HUD sees the real world at 70% brightness upon which is superimposed the collimated display at 30% of the CRT brightness (taking typical transmission and reflection efficiencies). The situation is shown in Figure 2.9 and is essentially a rather lossy system with 30% of the real world bright- ness thrown away, (equivalent to wearing sunglasses) as is 70% of the CRT display brightness. In order to achieve an adequate contrast ratio so that the display can be seen against the sky at high altitude or against sunlit cloud it is necessary to achieve a 2.2 Head Up Displays 29 Fig. 2.9 Conventional refractive HUD combiner operation. display brightness of 30,000 Cd/m2 (10,000 ft L) from the CRT. In fact, it is the brightness requirement in particular which assures the use of the CRT as the dis- play source for some considerable time to come, even with the much higher optical efficiencies which can be achieved by exploiting holographic optical elements. The use of holographically generated optical elements can enable the FOV to be increased by a factor of two or more, with the instantaneous FOV equal to the total FOV. Very much brighter displays together with a negligible loss in outside world visibility can also be achieved, as will be explained in the next section. High optical transmission through the combiner is required so as not to degrade the acquisition of small targets at long distances. It should be noted, however, that the development of ‘Rugate’ dielectric coatings applied to the combiners of conventional refractive HUDs has enabled very bright displays with high outside world transmission to be achieved, comparable, in fact, with holographic HUDs. A Rugate dielectric coating is a multi-layer coating having a sinusoidally varying refractive index with thickness which can synthesise a very sharply defined narrow wavelength band reflection coating, typically around 15 nm at the CRT phosphor peak emission. The coating exhibits similar high reflection and transmission values to holographic coatings but is not capable of generating optical power. The IFOV of a refractive HUD using a combiner with a Rugate dielectric coating still suffers from the same limitations and cannot be increased like a holographic HUD. It can, nevertheless, provide a very competitive solution for applications where a maximum IFOV of up to 20◦ is acceptable. 30 2 Displays and Man–Machine Interaction Fig. 2.10 Instantaneous FOV of conventional HUD. Fig. 2.11 Instantaneous FOV of holographic HUD. 2.2.3 Holographic HUDs The requirement for a large FOV is driven by the use of the HUD to display a col- limated TV picture of the FLIR sensor output to enable the pilot to ‘see’ through the HUD FOV in conditions of poor visibility, particularly night operations. It should be noted that the FLIR sensor can also penetrate through haze and many cloud con- ditions and provide ‘enhanced vision’ as the FLIR display is accurately overlaid one to one with the real world. The need for a wide FOV when manoeuvring at night at low level can be seen in Figures 2.10 and 2.11. The wider azimuth FOV is essential for the pilot to see into the turn. (The analogy has been made of trying to drive a 2.2 Head Up Displays 31 Fig. 2.12 Off-axis holographic combiner HUD configuration. Fig. 2.13 Collimation by a spherical reflecting surface. car round Hyde Park Corner with a shattered opaque windscreen with your vision restricted to a hole punched through the window.) In a modern wide FOV holographic HUD, the display collimation is carried out by the combiner which is given optical power (curvature) such that it performs the display image collimation. Figure 2.12 shows the basic configuration of a modern single combiner holographic HUD. The CRT display is focused by the relay lens system to form an intermediate image at the focus of the powered combiner. The intermediate image is then reflected from the fold mirror to the combiner. This acts as a collimator as the tuned holographic coating on the spherical surface of the com- biner reflects the green light from the CRT display and forms a collimated display image at the pilot’s design eye position. Figure 2.13 illustrates the collimating action of a spherical reflecting surface. The collimating action is, in fact, the optical reciprocal of Newton’s reflecting telescope. 32 2 Displays and Man–Machine Interaction Fig. 2.14 The head motion box concept. Because the collimating element is located in the combiner, the porthole is con- siderably nearer to the pilot than a comparable refractive HUD design. The collim- ating element can also be made much larger than the collimating lens of a refractive HUD, within the same cockpit constraints. The IFOV can thus be increased by a factor of two or more and the instantaneous and total FOVs are generally the same, as the pilot is effectively inside the viewing porthole. This type of HUD is sometimes referred to as a ‘Projected Porthole HUD’ and the image is what is known as pupil forming. The display imagery can, in fact, only be seen within the ‘head motion box’. If the eyes or head move outside a three dimensional box set in space around the pilot’s normal head position, then the display fades out. It literally becomes a case of ‘now you see it – now you don’t’ at the head motion box limits. Modern holographic HUDs are designed to have a reasonably sized head motion box so that the pilot is not unduly constrained. Figure 2.14 illustrates the head motion box concept. The combiner comprises a parallel-faced sandwich of plano-convex and plano- concave glass plates with a holographic coating on the spherical interface between them. The holographic coating is formed on the spherical surface of the plano- convex glass plate and the concave surface glass forms a cover plate so that the holographic coating can be hermetically sealed within the combiner. The holo- graphic coating is sharply tuned so that it will reflect the green light of one particular wavelength from the CRT display with over 80% reflectivity but transmit light at all other wavelengths with around 90% efficiency. (The CRT phosphors generally used are P43 or P53 phosphors emitting green light with a predominant wavelength of around 540 nm, and the hologram is tuned to this wavelength.) This gives extremely good transmission of the outside world through the combiner. (The outer faces of the combiner glass are parallel so that there is no optical distortion of the outside scene.) The outside world viewed through the combiner appears very slightly pink 2.2 Head Up Displays 33 as the green content of the outside world with a wavelength of around 540nm is not transmitted through the combiner. Holographic HUDs, in fact, are recognisable by the green tinge of the combiner. The spherical reflecting surface of the combiner collimates the display imagery but there are large optical aberration errors introduced which must be corrected. These aberration errors are due to the large off-axis angle between the pilot’s line of sight and the optical axis of the combiner which results from the HUD configuration. Some corrections can be made for these aberrations by the relay lens system but there is a practical limit to the amount of correction which can be achieved with conventional optical elements without resorting to aspherical surfaces. This is where a unique property of holographically generated coatings is used, namely the ability to introduce optical power within the coating so that it can correct the remaining aberration errors. The powered holographic coating produces an effect equivalent to local variations in the curvature of the spherical reflecting surface of the combiner to correct the aberration errors by diffracting the light at the appropriate points. The holographic coating is given optical power so that it behaves like a lens by using an auxiliary optical system to record a complex phase distribution on the combiner surface during the manufacturing process. This will be explained shortly. A very brief overview of holographic optical elements is set out below to give an appreciation of the basic principles and the technology. Holography was invented in 1947 by Denis Gabor, a Hungarian scientist working in the UK. Practical applications had to wait until the 1960s, when two American scientists, Emmet Leith and Joseph Upatnieks, used coherent light from the newly developed laser to record the first hologram. Holographic HUDs use reflection holograms which depend for their operation on refractive index variations produced within a thin gelatin film sandwiched between two sheets of glass. This is really a diffraction grating and hence a more accur- ate name for such HUDs is diffractive HUDs. A holographic reflection coating is formed by exposing a thin film of photo-sensitive dichromated gelatin to two beams of coherent laser light. Due to the coherent nature of the incident beams a series of interference fringes are formed throughout the depth of the gelatin film. During the developing process these fringes are converted into planes of high and low re- fractive index parallel to the film surface. To a first approximation, the refractive index change between adjacent planes is sinusoidal as opposed to the step func- tion associated with multi-layer coatings. During the developing process the gelatin swells, producing an increase in the tuned wavelength. Re-tuning the hologram is achieved by baking the film which reduces the thickness and hence the spacing between planes of constant refractive index. The designer therefore specifies a con- struction wavelength at a given angle of incidence after baking. Figure 2.15 illus- trates the planes or layers of varying refractive index formed in the holographic coating. The bandwidth of the angular reflection range is determined by the magnitude of the change in refractive index. This variable can be controlled during the developing process and is specified as the hologram modulation. 34 2 Displays and Man–Machine Interaction Fig. 2.15 Holographic coating. Fig. 2.16 Angularly selective reflection of monochromatic rays. At any point on the surface, the coating will only reflect a given wavelength over a small range of incidence angles. Outside this range of angles, the reflectivity drops off very rapidly and light of that wavelength will be transmitted through the coating. The effect is illustrated in Figures 2.16 and 2.17. Rays 1 and 3 are insufficiently close to the reflection angle, θ , for them to be reflected whereas Ray 2 is incident at the design angle and is reflected. There is another more subtle feature of holograms which gives particular ad- vantages in an optical system. That is the ability to change the tuned wavelength 2.2 Head Up Displays 35 Fig. 2.17 Holographic coat- ing performance. uniformly across the reflector surface. Figure 2.12 shows that the reflection coating must reflect the display wavelength at a different incident angle at the bottom of the combiner from that at the top. It is possible to achieve this effect with a hologram because it can be constructed from the design eye position. The process for producing the powered holographic combiner is very briefly out- lined below. The process has three key stages: Design and fabricate the Computer Generated Hologram (CGH). Produce master hologram. Replicate copies for the holographic combiner elements. The CGH and construction optics create the complex wavefront required to produce the master hologram. The CGH permits control of the power of the diffraction grat- ing over the combiner thus allowing correction of some level of optica

Introduction to Avionics Systems PDF

Document Details

Tags

Related

Summary

Full Transcript

Upgrade to continue