The forerunners of aerospace transistors came along in the late 1950s and 1960s and supplanted thermionic valves for many applications. The improved cost-effectiveness of transistors led to the development of digital aircraft systems throughout the 1960s and 1970s, initially in the military combat aircraft where it was used for Nav/Attack systems.
For many years, the application of electronics to airborne systems was limited to analogue devices and systems with signal levels and voltages generally being related in some linear or predictive way. This type of system was generally prone to heat soak, drift and other non-linearities. The principles of digital computing had been understood for a number of years before the techniques were applied to aircraft. Size was the main barrier.
The first aircraft to be developed in the US using digital techniques was the North American A-5 Vigilante, a US Navy carrier-borne bomber which became operational in the 1960s. The first aircraft to be developed in the UK, intended to use digital techniques on any meaningful scale was the ill-fated TSR 2 which was cancelled by the UK Government in 1965. The technology employed by the TSR 2 was largely based upon solid-state transistors, then in comparative infancy. In the UK, it was not until the development of the Anglo-French Jaguar and the Hawker Siddeley Nimrod in the 1960s that weapon systems began to seriously embody digital computing, albeit on a meagre scale compared to the 1980s.
Since the late 1970s/early 1980s, digital technology has become increasingly used in the control of aircraft systems as well as just for mission related systems. A key driver in this application has been the availability of cost-effective digital data buses such as ARINC 429, Mil-Std-155311 and ARINC 629. This technology, coupled with the availability of cheap microprocessors and more advanced software development tools, has led to the widespread application of avionics technology throughout the aircraft.
This has advanced to the point that virtually no aircraft system – including the toilet system – has been left untouched.
The evolution and increasing use of avionics technology for civil applications of engine controls and flight controls has been since the 1950s. Engine analogue controls were introduced by Ultra in the 1950s which comprised electrical throttle signalling used on aircraft such as the Bristol Britannia. Full
authority digital engine control became commonly used in the 1980s. Digital primary flight control with a mechanical backup has been used on the Airbus A320 and A330/A340 families using side-stick controllers and on the Boeing 777, using a conventional control yoke. Aircraft such as the Dornier 728 family and the A380 appear to be adopting flight control without any mechanical backup, but with electrically signalled backup.
The application of digital techniques to other aircraft systems – utilities systems – began later. Today, avionics technology is firmly embedded in the control of virtually all aircraft systems. Therefore an understanding of the nature of avionics technology is crucial in understanding how the control of aircraft systems is achieved.
The nature of micro-electronic devices
The extent of the explosion in ICs developments can be judged by a ten-fold increase per decade in the number of transistors per chip. Another factor to consider is the increase in the speed of device switching. The speed of operation is referred to as gate delay; gate delay for a thermionic valve is of the order of 1,000 nanoseconds (1 nanosecond is 10-9 or one thousandth of one millionth of a second); transistors are
about ten times quicker at 100 nanoseconds. Silicon chips are faster again at approximately 1 nanosecond). This gives an indication of how powerful these devices are and why they have had such an impact upon our daily life.
Another area of major impact for ICs relates to power consumption. ICs consume minuscule amounts. Consumption is related to the technology type and speed of operation. The quicker the speed of operation then the greater the power required and vice versa. The main areas where avionics component technology have developed are:
Aerospace semiconductors transisitors and capacitors
Manufacturing and reliability progress has increased the use of electronic components in aircraft generally, from aerospace power applications to radar and defence.
Processors, Memory and Data buses
Digital processor devices became available in the early 1970s as 4-bit devices. By the late 1970s, 8-bit processors had been superceded by 16-bit devices; these led in turn to 32-bit devices such as the Motorola 68000 which have been widely used on the Eurofighter and Boeing 777. The pace of evolution of processor devices does present a significant concern due to the risk of the chips becoming obsolescent, leading to the
prospect of an expensive re-design. Following adverse experiences with its initial ownership of microprocessor based systems, the US Air Force pressed strong standardization initiatives based upon the
MILSTD-1750A microprocessor with a standardized instruction set architecture (ISA) though this found few applications in aircraft systems computing. For these types of application, starting with the adoption of the Motorola 68020 on Eurofighter, the industry is making extensive use of commercially developed microprocessor or microcontroller products.
Memory devices have experienced a similar explosion in capability. Memory devices comprise two main categories: Read-Only Memory (ROM) represents the memory used to host the application software for a particular function; as the term suggests this type of memory may only be read but not written to. A particular version of ROM used frequently was Electrically Programmable Read-Only Memory (EPROM), however this suffered the disadvantage that memory could only be erased by irradiating the device with ultra-violet (UV) light. For the last few years EPROM has been superseded by the more user-friendly Electrically Erasable Programmable Read-Only Memory (E2PROM). This type of memory may be re-programmed electrically with the memory module still resident within the LRU; using this capability it is now possible to reprogram many units in situ on the aircraft via the aircraft digital data buses.
Random-Access Memory (RAM) is read-write memory that is used as program working memory, storing variable data. Early versions required a power backup in case the aircraft power supply was lost. More recent devices are less demanding in this regard.
Digital data buses
The advent of standard digital data buses began in 1974 with the specification by the US Air Force of MIL-STD-1553. The ARINC 429 data bus became the first standard data bus to be specified and widely used for civil aircraft being widely used on the Boeing 757 and 767 and Airbus A300/A310 in the late 1970s and early 1980s. ARINC 429 (A429) is widely used on a range of civil aircraft today.
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