Avionics Developments Transforming Aircraft Design

The· building blocks that fit together to make up avionics systems are modular and largely electronic. Powerful digital data processors operating at rates in excess of one million instructions per second (MIPS) can be identical and provide multiple redundancies and are available from manufacturers’ standard ranges. Only the software under which each one operates needs to be different.

Through linkages, they can provide, even under damaged conditions, a viable and adequate flight control system. Dedicated mechanical and hydraulic linkages have been replaced by electric cables, fibre optics and digital databus components which are the arteries of so-called fly-by-wire and fly-by-light systems. Samarium cobalt or other ‘rare earth’ motors at the end of these arteries can sustain heavy transient overloads, while supplying the drive for local hydraulic actuators or when integrated with the control surfaces themselves.

Narrow Selection

Display devices, which may be multi-purpose, cathode ray tube (CRT) or solid state light emitting diode (LED), combine the functions of various formerly independent indicators. More and more the pilot can see only what he needs to know, when he needs to know it. Unnecessary distraction is removed. Ergonomic considerations dictate the layout of controls in terms of importance and usage and knobs are replaced by soft-touch keys.

For such systems powerful and comprehensive software derives expert data from extensive databases so optimum and economical operational solutions can be provided for flight commands and operational limits for various manoeuvres cannot be exceeded. Much of the critical thinking necessary to control a high speed aircraft is thus delegated from the pilot to the computer.

The aircraft will be provided with a comprehensive electronic flight control system (EFCS). When the pilot moves his control (probably a sidestick), thereby creating a trajectory order, the system computes that trajectory order with database-held aerodynamic data to stabilise the aircraft. The EFCS, by reference to the database, will prevent excessive speeds, stalling or overstressing. With such control the aircraft is inherently safe, as it will keep to a logical track and altitude in the absence of pilot input, or with pilot input that is unacceptable to the logic. The flight management system (FMS) will be largely concerned with one of the major goals of aircraft technology optimisation of flight programmes to provide cost-effective flying. Navigational management systems fitted in aircraft have been concerned until recently, only with lateral steering to conform to a track over the ground. Information for this has been derived from an on-board database, combined with automatic sensor tuning and input of positional data from inertial and radio sources. In its onward march, microcomputery has not been content to order flight in two dimensions but has now also gained control of the vertical axis to provide a true flight management system in three dimensions.

Expanding Programme

An example of an FMS system subject to continuous enhancement is one developed by Smiths Industries. When work started on the design of a system for the Airbus in 1979 the standard for commercial flight management (ARINC 702) specified a computer programme size of 70k 16 bit words. The certification of lateral performance was received in 1983 and in April 1986 certification was given for the first stage performance and vertical guidance.

By this time the computer program size had risen to over 250k words, while the navigation and performance database was also over 250k words. Upwards of 200 sets of FMS equipment have now been produced by Smiths Industries, each consisting of two flight management computer (FMC) units and two control and display units (CDU).

The pilots use the flight deck pedestal mounted CDUs to enter data into the system via the alphanumeric and function keypad. and observe the text display providing flight plan information. The display provides 14 lines each of 24 sharply defined characters in two font sizes and has flicker free operation. A set of eight annunciators is included to give information on flight phase of pilot warnings. The CDU operating procedures have been optimised to give clarity of information with a minimum of key strokes, providing a very pilot friendly interface.

The FMS receives inputs from inertial references, very high frequency omnidirectional radio-range (VOR) and distance measuring equipment (DME) receivers, engine parameters, autopilot glare shield controls and from the flight control computer. Data outputs from the system are sent to the autopilot’s thrust control computer and to its CDU, as well as the electronic flight instrument system (EFIS). By performing routine calculation at a speed far beyond human effort and providing guidance computations for autopilot and autothrottle, the system substantially reduces the pilot’s workload. The aircraft’s position is determined by the FMS using the best sensor information available and data is stored on all significant navigational beacons relevant to the operator’s preferred routes.

The FMS establishes the most economical path by computing the optimum speed and height for a particular flight mode. It also computes the fuel and route times to destination and alternative airfields. The system guides the aircraft vertically from just after takeoff to just before landing, taking account of all altitude constraints necessitated by terrain and air traffic control along the route. Performance predictions are computed for the selected flight plan, assuming a climb from takeoff to cruise altitude, a level cruise, an idle descent and a level decelerating approach to the glide slope intercept point.

Modified Flight Plan

Before takeoff the FMS computes fuel planning predictions and during flight the fuel management predictions, including fuel remaining on board at destination, are computed from the aircraft’s position and are displayed on the CDU. The pilots have the facility to interrogate any performance pages on the CDU or to modify the flight plan on the ground or in the air.

Where no route is stored, the pilots can select a key origin or destination from the menu and explicitly define the route by entering the waypoints. The flight plan can be modified on the ground or in the air by changing the destination, the terminal area procedure, or any en route fix or airway sector.

FMS has been introduced into the aircraft of a number of airlines. As facilities expand, retrofit programmes are being mounted. The Smiths Industries’ FMS has also been selected for its first United States military role in the E-6A Tacamo – particularly important for operation over vast oceans and polar regions where conventional navaid coverage is sketchy or non-existent.

The nature of digital systems is such that software can be developed and new circuit boards produced relatively rapidly. For aircraft already flying, updating a system to provide additional facilities or enhancements can therefore be done in a short retrofit. For prototype aircraft it means that avionics development can continue long after the airframe and engine design are frozen and right up to the time of production. As computing devices become faster and their physical density greater, systems such as FMS can move ever nearer to the comprehensive knowledge based systems forecast for the 2000s.

Nowhere has the avionics revolution been more evident than in the cockpit. Its ergonomic design has an uncluttered air of purposefulness. A formidable array of knobs and dials has mostly disappeared, to be replaced by multi-function colour displays or LED indicator displays controlled by soft-touch keys. An indicator that, in pre-revolution days, might have had a vital function, for only a small percentage of the flying time is now incorporated with others in an integrated display, to be called on only when needed. Special displays, such as a digital colour map for en route navigation or tactical displays for attack aircraft or helicopter navigation, make route-following tasks less demanding.

Head-up Displays

GEC Avionics is the world’s major supplier of head-up displays (HUDs) using diffraction (holographic) optics. This type of display, employed by the air forces of most countries, is also proving invaluable in commercial aircraft for all weather usage. An advanced GEC Avionics’ HUD has been selected by McDonnell Douglas for the C-17 heavy lift short take off and landing (STOL) transport, capable of operating from short, rugged fields.

A HUD unit will be fitted in the cockpit coaming in front of each pilot to project holographically into his forward view an image of electronically generated symbols representing instrument readings and other pertinent flight information. This enables pilots, without looking down, to view critical flight data such as attitude and airspeed superimposed over their view of the out-side world, whether in level flight, takeoff, or landing. Wide angle-of-view, optics are combined with advanced electronics to provide a compact unit. The optical combiner, the screen through which the pilot sees the symbols in focus with the outside view, folds away when not required.

In the military application, GEC Avionics, as a result of considerable experience gained in the United States Air Force low altitude navigation and targeting by infra-red at night (LANTIRN) programme, has received a $72 million production order for a new type of HUD for General Dynamics F-16C Fighting Falcons. The pilot is presented with a night scene produced by a forward looking infra-red (FLIR) imager using a raster (television) type display, showing also stroke-written symbols. As the field of view (30 degrees azimuth by 17 degrees vertical) is exact in scale and orientation, the pilot now literally sees in the dark.