Probably the single most important system in an aircraft, aside from those natural prerequisites for high performance flight, an airframe and powerplant, is the control system.

Aircraft control systems have undergone considerable development in the last seventy years and the end of that path of development is by no means in clear sight.

The primary function of any control system is to convey instructions, in an aircraft, from the pilot to the vehicle. Aside from several not so conventional instances, such as the Harrier in hover or the Space Shuttle in orbit, this is achieved aerodynamically, by means of control surfaces.

In a modern aircraft this becomes a very complex design problem, the system must enable the pilot to retain full control in a speed range starting around 100 kts and extending to Mach 2 plus, including the transonic region with all of its characteristic properties; a modern fighter must also be capable of flight at large (positive and negative) angles of attack.

Aside from the aerodynamic complexities involved, there are also the aspects of lifetime, reliability, maintainability and ability to withstand damage.

The lifetime and reliability of a system are factors which go hand-in-hand. Fatigue and, to a lesser extent, wear are the basic causes of the majority of failures experienced.

A control system's ability to withstand combat damage is one of the crucial considerations in current air warfare, as the Americans learned in Viet-Nam, the hard way. The problem, aside from the issue of surviving SAM/AAM hits, which tend to inflict heavier damage, can be the case of the after effects of insidious small arms fire.

SAM/radar sites can be spoofed or busted by Wild Weasels, just as flak emplacements can be, however it's improbable that anyone will ever find an absolute means of eliminating the omnipresent infantry-man with his AK47 (or M16, for that).

The obvious solution to this problem is to provide the aircraft with redundant backup systems, which is exactly what's being done. On the other hand, the more systems, the more maintenance and in effect, the lesser the reliability, as the probability of components failing is n-times the probability of one component failing.

Weight is another factor to bear in mind, the effect of multiplying systems is obvious here. Cost likewise.

The conclusions which can be drawn are:

1. The system must satisfy all of the aircraft's performance requirements. 
2. The system must be capable of absorbing damage. 
3. The system must have a high degree of reliability. 
4. The system must be maintainable. 
5. Cost and weight must be kept at a minimum.

Aircraft, such as the F-14, satisfy 1.,2.,4. however they lag in 3. and definitely fail in 5.. The basic cause is that they employ conventional mechanical/hydraulic systems, which, in spite of their conceptual simplicity, become enormously complex in these instances. The only current system which satisfies 1. to 5. adequately is fly-by-wire control.

Fly-by-wire Control Systems.

As the name implies, fly-by-wire employs electrical signals to transmit information from the cockpit to the control actuators.

Control elements, e.g. control stick, rudder pedals, are fitted with mechanical / electrical transducers - either force (F-16) or position sensing devices, which generate electrical signals corresponding to the given command. Here is where we must make the distinction between analogue and digital systems.

Analogue systems operate with electrical analogues to real quantities. An example would be a device transmitting a quantity from 0 to 100% with an electrical voltage output of 0 to 10 Volts. A value of 15% would generate an output of 1.5 Volts. The number of ways in which analog information can be encoded is virtually unlimited.

Information can be coded into voltage, frequency, phase or combinations of these, it can also be compressed prior to encoding, enabling more of it to be transmitted at once.

Digital systems operate in binary. The binary number system (as compared to the decimal system we use) has only two values, 1 and 0. 2 forms a unit analogous to 10, 4 to 100, 8 to 1000, hence we can express a number such as six (6) as 110.

1,2,3... corresponds to 1, 10, 11.... Any number can be converted into binary, a digital device can then generate an output with only two states, on and off, corresponding to 1 and 0. All digital computers employ binary.

Analog and digital systems both have advantages and disadvantages. Analog systems are, generally, simpler and less demanding in component parameters, such as speed. On the other hand, they are more susceptible to induced noise and interference, as the information content is carried within fine variations of some signal parameter.

A digital system need only discriminate between on and off, the information being carried by sequences of binary numbers.

Digital systems may be easily reconfigured by changes in software, whereas an analog system, hardwired, would require rebuilding.

When analog systems fail, they often merely degrade in performance, a failure of a similar type could completely disable a digital device.

The signals generated by the control elements are then used to control the control surface actuators. However, the raw output of a cockpit control element is hardly enough for that. It is modified by a stability augmentation computer. The computer compares the aircraft's actual motion, as sensed by gyros and accelerometers, and corrects it to a control law, improving the aircraft's handling.

This type of system is used by the Tornado GR.1/F.2, which employs triplex control output sensing and computing, and quadruple electric control of the hydraulic control surface actuators.

Further safety is provided by a mechanical backup for pitch/roll control.

A similar approach was used in the YC-15 AMST STOL transport, which uses blown flaps to enhance short field performance. Variations in engine thrust can cause additional moments in roll and pitch, which would complicate the handling of such an aircraft.

Aircraft configured for direct force control featuring variable incidence wings, beavertail elevator and control surfaces beneath nose. DSF - direct side force, VLF - vectored lift force. (Carlo Kopp)

These tiny robot-like units are actually intricate components - subminiature gyroscopes used to guide aircraft, ships, missiles and torpedoes. The finished GR-G5 rate gyros, built by Northrop, are only two inches long and weigh less than five ounces. The GR-G5 is the most widely used rate gyroscope in the world, and is used in a quadruple-redundant system in the F-16 fly-by-wire flight control system. (Northrop)

In either instance the fly-by-wire system, aside from improving reliability and its associated factors, is used to modify the response of the aircraft. This is, by no means, full utilization of the potential offered by electronic flight control.