Avionics data bus systems

As well as the most obvious form of information transmission, that of voice, a vast amount of electronic data is exchanged in modern aircraft, both within the aircraft and to stations outside.

External voice transmissions are by radio, using a variety of wavebands depending on the category of aircraft - military aircraft have separate bands available to them, for obvious reasons, but may also use civil frequencies, for instance when communicating with air traffic control.

The two types of radio in common use are VHF radio (very high frequency) and HF radio (high frequency). VHF radio generally provides better quality transmission, because of the higher frequencies used. HF transmissions can travel much further for a given transmitter power, because the longer wavelength suffers less from atmospheric absorption and is less directional, following the curvature of the earth more readily. So VHF is used for ranges of a few tens or hundreds of miles, and HF radio is used to allow aircraft to keep in touch with their base wherever they are in the world.

Internal voice transmissions, i.e. aircraft intercoms, are required so that the crew of a combat aircraft are kept in constant contact - the environment of a fast jet does not lend itself to normal speaking. In civil aircraft, the flight deck may be quiet enough for the crew to converse without resorting to using intercoms, but they may need to talk to cabin staff or passengers.

However, by far the greatest amount of information is exchanged between individual units of both analogue and digital systems.

Modern aircraft contain large amounts of avionics, and each function of each system usually involves gathering or providing information. Until quite recently, each unit would be linked directly with the other units with which it shared data, and with any system other than the most simple one the amount of wiring needed is considerable. One way of overcoming this problem, and allowing much greater flexibility in developing additional functions, is to use a data bus, which forms a central transmission route for all data.

The aircraft has a data bus which runs throughout the aircraft, and all systems are connected to the bus. Information is passed to the data bus by each system in turn, and all the data traffic is regulated by the bus controller, to a standard protocol. The military communication standard for current aircraft is one developed by the US Department of Defense, MIL-STD-I 553, and this is progressively being adopted by almost all military aircraft in the West, and by many civil aircraft as well. Any system requiring information can find it simply by receiving it from the bus at the correct time in the cycle.

Of necessity, the data transmission rate is extremely high, and priority is given to the most important information and that requiring the highest data rates. As with most systems on aircraft, fail-safety is an important consideration, and it is normal for the databus to have back-up systems. Many aircraft have duplicated data buses, which are synchronised to safeguard data integrity. Using data-bus systems allows some systems to be incorporated that would be ruled out in practical terms using the earlier methods.

Central maintenance computers. which take information from almost every other system on the aircraft, are one example. All system status information, particularly of failed systems and components, is gathered together into one computer, which can link with other systems to provide crew alerts, a print-out for ground engineers, or transmit the information ahead by airborne data link, as required. This provides a safer system, with improved reliability and shorter turn-round times.

Aircraft data processing units

Many avionics systems are based on a microprocessor, but do not process data at a constant rate. For much of the time the processor will be idle, but it must have the processing capacity to handle peak demands. Systems designers are now looking towards providing a centralised data-processing system, attached to the bus, which can handle most or all of the data processing for the entire aircraft. Using perhaps four processors for fail-safety, avionics would become smaller and lighter, since much of the redundancy of processors would be eliminated.

The power consumption could also be drastically reduced. Since each unit is currently unable to share its processor with other units, if their processor has failed for instance, overall reliability would be improved, and it would be easier to provide better and faster functions from most systems. Each system would carry out the necessary pre-processing of data, then pass it to the centralised processing units via the data bus. After processing, the resulting data is then fed directly onto the data bus, where it can be accessed by any system requiring it.

However, this is for the future, and there are many problems. It requires higher rates of data transfer than current systems, since there is more information to be transmitted, and it increases the already serious problem of software integrity. All software used in aircraft, particularly that associated with flight control systems, must be free of errors.

Errors are notoriously difficult to eradicate in software, since the smallest of errors may remain hidden for years, only coming to light when a specific set of conditions occurs. It is impossible to replicate every possible combination of conditions, since the number of inputs may be high, and the resulting error may range from minor to catastrophic. The amount of data code used is increasing exponentially, and this problem may never be satisfactorily overcome.

Data bus test equipment

The avionics data bus standard in commercial aviation today is ARINC 429. Often, any failure is outside the LRU line replaceable unit, in the wiring for example. ARINC 629, a more recent technology than ARINC 429, appears to be more robust, and ARINC 659, used in the B777, seems to be relatively problem-free.

Most ARINC 429 faults manifest themselves in the loss of an avionics function or from data bus test equipment built into the system. Handheld data bus analyzers and automatic test equipment (ATE) is commonplace. Although sometimes referred to as avionics test equipment, avionics faults are inceasingly referred to the manufacturer and avionics component repair is a shrinking industry.

ARINC 629 is more robust than ARINC 429 and failures are isolated to the LRU.

In the longer term, civil aviation appears to be moving toward faster Ethernet technology, such as the switched Ethernet communications system on the A380 super jumbo aircraft.