Introduction to Avionics Systems

This chapter explains the important role played by avionic systems in a modern civil or military aircraft. It describes how avionic systems can reduce crew workload, reduce fuel consumption, improve aircraft performance and improve safety. The functions covered are systems that interface directly with the pilot; aircraft state sensors; navigation systems that provide very accurate aircraft position, speed and track angle data; outside world sensor systems; and task automation systems. To illustrate the subject, some real-world examples of avionic systems are provided, as well as an overview of the avionics environment. Finally, the choice of SI units for worked examples is stated.

This chapter describes in detail the cockpit display systems and man–machine interaction facilities that enable the pilot to fly an aircraft safely and carry out the mission, for both civil and military aircrafts. It focusses on head up displays (HUDs), helmet mounted displays (HMDs), computer aided optical modelling, head down displays (HDDs), data fusion, intelligent displays management, advances in display technology and control and data entry.

HUDs enable the pilot to view and assimilate essential flight data whilst head up and maintaining full concentration on the outside world. HMDs enable pilots to see visual information heads up when looking in any direction. In its most sophisticated form, the HMD can provide, in effect, an ‘HUD on the helmet’. The concept of a ‘virtual cockpit’ is discussed, where information is presented visually to the pilot by means of computer-generated three-dimensional (3D) imagery.

For HDDs, the move from colour cathode ray tube (CRT) displays to high-resolution, active matrix colour liquid crystal displays (AMLCDs) or organic light-emitting diode (OLED) displays is described and examples provided.

The principles of data fusion are explained and overviews are given of intelligent knowledge-based system (IKBS), tactile control panels, direct voice input and eye trackers.

This chapter covers basic aerodynamics, aircraft stability, dynamic response, longitudinal and lateral control and response, powered flying controls, stability augmentation systems and helicopter flight control. This is to establish the background to fly-by-wire (FBW) flight control (Chap. 4) and autopilots (Chap. 8).

Basic aerodynamics and aircraft stability are explained. An aircraft’s dynamic response to a disturbance is analysed and equations of motions derived. The principles of longitudinal and lateral control are described. It is stated that an aircraft is said to be stable if it tends to return to its original position after being subjected to a disturbance without any control action by the pilot. It is explained that unstable aircraft have advantages including higher performance, reduced weight and increased agility, but they need a high-speed automatic control, known as a ‘fly-by-wire’ flight control system, to correct the tendency to divergence. The possible need to improve damping and stability in three axes by a stability augmentation system is described. It is stated that for high-speed aircraft, the forces needed to move the controls are too much for the pilot and that powered flying controls are needed, which are analysed in detail. This chapter concludes by describing the principles of helicopter flight control.

The introduction of fly-by-wire (FBW) flight control systems has been a watershed development in aircraft evolution as it has enabled technical advances to be made, which were not possible before. It is explained how an FBW system provides high-integrity automatic stabilisation of the aircraft to compensate for the loss of natural stability and thus enables a lighter aircraft with a better overall performance to be produced. It also provides the pilot with very good control and handling characteristics, including ‘carefree manoeuvring’, which are consistent and safe over the whole flight envelope. While FBW concepts are not new (guided missiles use them), what has taken the time is the development of failure survival technologies to enable high-integrity systems to be implemented economically with the required safety level, reliability and availability. A major factor has been the development of failure survival digital flight control systems with the associated software development complexity. Most modern fighter aircraft, such as the Eurofighter Typhoon and many modern civil aircraft from Airbus and Boeing exploit FBW control. Further, FBW flight control can also be applied to helicopters to produce a flight control system of very high integrity. The application of active sticks (inceptors) and throttles is described as a means of providing feedback to the pilot.

Gyroscopes (gyros) and accelerometers are known as inertial sensors. They are fundamental to the control and guidance of aircraft. This chapter covers gyros and accelerometers and attitude derivation from strap-down gyros and accelerometers. It provides the basic background to inertial navigation and AHRS systems, which are covered in Chap. 6.

The derivation of attitude from inertial information is described. The two types of system are illustrated: the stable platform system from which the Euler angles are output directly, and the strap-down systems in which they are computed. Mathematical derivation of the Euler angles is provided and the computation of aircraft heading is also explained. An introduction to complementary filtering is provided where the data from two independent sources are combined to improve accuracy.

Navigation involves control of an aircraft’s flight path and guidance for its mission. It is explained that accurate and high integrity navigation is essential for both civil and military aircraft. It is stated that there are two basic methods of navigation, namely, dead reckoning (DR) and position fixing systems. The various types within each method are detailed followed by an explanation of the principles and mathematics underlying them.

DR navigation systems are described. In order of increasing accuracy these are Air data-based DR navigation, Doppler/heading reference systems, inertial navigation (IN) systems and aided IN systems. For IN systems, the importance of Schuler tuning, gyro drift correction and initial alignment is explained. For aided IN systems, it is shown how accuracy is improved by combining outputs with another system such as Doppler radar or satellite navigation systems (GPS), using a Kalman filter.

The principles of Attitude Heading Reference Systems (AHRS) are explained along with the associated mathematics.

Position fixing systems are described. These include VOR/DME, TACAN, GPS and Terrain Reference Navigation (TRN).

For TRN systems, it is explained how they derive an aircraft’s position by correlating sensed terrain measurements with known terrain features, stored in a database, near the aircraft’s estimated position.

This chapter explains that air data systems provide accurate information on quantities including pressure altitude, vertical speed, calibrated airspeed, true airspeed, Mach number, static air temperature and air density ratio. This information is required by a number of key avionic sub-systems including navigation, flight instruments, flight management and autopilot which enable the pilot to carry out the mission and to fly the aircraft safely.

This chapter covers the following:

Autopilots and flight management systems (FMS) have been grouped together in one chapter as modern aircraft require a high degree of integration between the two systems. It is explained that:

The autopilot modes and their associated control and mathematics are explained with the aid of worked examples. These include height control, heading control, instrument landing system (ILS) and microwave landing system (MLS) coupled control and automatic landing systems. Also covered are satellite landing guidance systems, speed control and auto-throttle systems.

The FMS has become a key avionics system largely because it brings a significant reduction in pilot workload. Automatic navigation and flight path guidance allow the optimisation of the aircraft performance, civil air traffic density to be increased and flight costs reduced. Using an Airbus as an example, the main functions are described. These include navigation, flight planning, flight path optimisation, flight envelope monitoring, vertical flight path profile control and 4D flight management.

The combining of a number of avionic sub-systems is known as avionics systems integration (ASI). It is explained that key to ASI has been the development of data buses, standards and computing power.

Data bus systems can be divided into electrical and optical systems. As an example of an electrical system, MIL STD 1553 is explained in detail. The ARINC 629, STANAG 3910, Parallel and Avionics Full Duplex Switched Ethernet (AFDX) data buses are also mentioned. For optical data bus systems, fibre optic theory and the advantages and disadvantage of multi-mode and single mode optical fibres are explained.

It is described how the development of ‘weapon systems’ for military aircraft and the adoption of ARINC specifications for civil avionics facilitated the development of federated architectures. In these, a number of functionally independent sub-systems are interconnected with some degree of central computer control. However, the availability of affordable high-speed data buses, such as AFDX or STANAG 3910, and powerful computers has enabled the development of integrated modular avionics (IMA) architectures. These employ the ‘three-layer stack’, with hardware, operating system and application layers. It is described how they provide increased performance, capability and availability, but with less maintenance resulting in reduced costs. The final section discusses the application of commercial off-the-shelf hardware and software.

Unmanned air vehicles (UAVs) have become increasingly important in both civil and military roles. UAVs depend totally on avionic systems in order to function and carry out their mission, and this chapter provides an overview of the current situation.

It is explained that the advantage of UAVs is their ability to perform dangerous, sensitive or dull tasks in an efficient manner. They are widely used in a range of surveillance tasks from battlefield surveillance and target acquisition at fairly low altitudes to long range surveillance/reconnaissance missions carried out at very high altitude. UAVs also have potentially many civil uses, but it is explained that the main obstacles to their civilian use are resolving safety and legislative issues.

The avionic systems covered in this book are generally, equally applicable to both manned and unmanned aircraft. This chapter looks at the application of the following to UAVs:

Finally, a brief overview of the following UAVs/UCAVs is given: