Fly-by-wire
Fly-by-wire is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals, and flight control computers determine how to move the actuators at each control surface to provide the ordered response. Implementations either use mechanical flight control backup systems or else are fully electronic.
Improved fully fly-by-wire systems interpret the pilot's control inputs as a desired outcome and calculate the control surface positions required to achieve that outcome; this results in various combinations of rudder, elevator, aileron, flaps and engine controls in different situations using a closed feedback loop. The pilot may not be fully aware of all the control outputs acting to affect the outcome, only that the aircraft is reacting as expected. The fly-by-wire computers act to stabilize the aircraft and adjust the flying characteristics without the pilot's involvement, and to prevent the pilot from operating outside of the aircraft's safe performance envelope.
Rationale
Mechanical and hydro-mechanical flight control systems are relatively heavy and require careful routing of flight control cables through the aircraft by systems of pulleys, cranks, tension cables and hydraulic pipes. Both systems often require redundant backup to deal with failures, which increases weight. Both have limited ability to compensate for changing aerodynamic conditions. Dangerous characteristics such as stalling, spinning and pilot-induced oscillation, which depend mainly on the stability and structure of the aircraft rather than the control system itself, are dependent on the pilot's actions.The term "fly-by-wire" implies a purely electrically signaled control system. It is used in the general sense of computer-configured controls, where a computer system is interposed between the operator and the final control actuators or surfaces. This modifies the manual inputs of the pilot in accordance with control parameters.
Side-sticks or conventional flight control yokes can be used to fly fly-by-wire aircraft.
Weight saving
A fly-by-wire aircraft can be lighter than a similar design with conventional controls. This is partly due to the lower overall weight of the system components and partly because the natural stability of the aircraft can be relaxed, which means that the stability surfaces that are part of the aircraft structure can therefore be made smaller. These include the vertical and horizontal stabilizers that are at the rear of the fuselage. If these structures can be reduced in size, airframe weight is reduced. The advantages of fly-by-wire controls were first exploited by the military and then in the commercial airline market. The Airbus series of airliners used full-authority fly-by-wire controls beginning with their A320 series, see A320 flight control. Boeing followed with their 777 and later designs.Basic operation
Closed-loop feedback control
A pilot commands the flight control computer to make the aircraft perform a certain action, such as pitch the aircraft up, or roll to one side, by moving the control column or sidestick. The flight control computer then calculates what control surface movements will cause the plane to perform that action and issues those commands to the electronic controllers for each surface. The controllers at each surface receive these commands and then move actuators attached to the control surface until it has moved to where the flight control computer commanded it to. The controllers measure the position of the flight control surface with sensors such as LVDTs.Automatic stability systems
Fly-by-wire control systems allow aircraft computers to perform tasks without pilot input. Automatic stability systems operate in this way. Gyroscopes and sensors such as accelerometers are mounted in an aircraft to sense rotation on the pitch, roll and yaw axes. Any movement results in signals to the computer, which can automatically move control actuators to stabilize the aircraft.Safety and redundancy
While traditional mechanical or hydraulic control systems usually fail gradually, the loss of all flight control computers immediately renders the aircraft uncontrollable. For this reason, most fly-by-wire systems incorporate either redundant computers, some kind of mechanical or hydraulic backup or a combination of both. A "mixed" control system with mechanical backup feeds any rudder elevation directly back to the pilot and therefore makes closed loop systems senseless.Aircraft systems may be quadruplexed to prevent loss of signals in the case of failure of one or even two channels. High performance aircraft that have fly-by-wire controls may be deliberately designed to have low or even negative stability in some flight regimes rapid-reacting CCV controls can electronically stabilize the lack of natural stability.
Pre-flight safety checks of a fly-by-wire system are often performed using built-in test equipment. A number of control movement steps can be automatically performed, reducing workload of the pilot or groundcrew and speeding up flight-checks.
Some aircraft, the Panavia Tornado for example, retain a very basic hydro-mechanical backup system for limited flight control capability on losing electrical power; in the case of the Tornado this allows rudimentary control of the stabilators only for pitch and roll axis movements.
History
Servo-electrically operated control surfaces were first tested in the 1930s on the Soviet Tupolev ANT-20. Long runs of mechanical and hydraulic connections were replaced with wires and electric servos.In 1934, filed a patent about the automatic-electronic system, which flared the aircraft, when it was close to the ground. In 1941, while being an engineer at Siemens, developed and tested the first fly-by-wire system for the Heinkel He 111, in which the aircraft was fully controlled by electronic impulses.
The first non-experimental aircraft that was designed and flown with a fly-by-wire flight control system was the Avro Canada CF-105 Arrow, the North American A-5 Vigilante which flew later the same year would be the first aircraft to reach operational service with a fly by wire system. This system also included solid-state components and system redundancy, was designed to be integrated with a computerised navigation and automatic search and track radar, was flyable from ground control with data uplink and downlink, and provided artificial feel to the pilot.
The first electronic fly-by-wire testbed operated by the U.S. Air Force was a Boeing B-47E Stratojet
The first pure electronic fly-by-wire aircraft with no mechanical or hydraulic backup was the Apollo Lunar Landing Training Vehicle, first flown in 1968. This was preceded in 1964 by the Lunar Landing Research Vehicle which pioneered fly-by-wire flight with no mechanical backup. Control was through a digital computer with three analog redundant channels. In the USSR, the Sukhoi T-4 also flew. At about the same time in the United Kingdom a trainer variant of the British Hawker Hunter fighter was modified at the British Royal Aircraft Establishment with fly-by-wire flight controls for the right-seat pilot.
In the UK the two seater Avro 707C was flown with a Fairey system with mechanical backup in the early to mid-60s. The program was curtailed when the air-frame ran out of flight time.
In 1972, the first digital fly-by-wire fixed-wing aircraft without a mechanical backup to take to the air was an F-8 Crusader, which had been modified electronically by NASA of the United States as a test aircraft; the F-8 used the Apollo guidance, navigation and control hardware.
The Airbus A320 began service in 1988 as the first mass-produced airliner with digital fly-by-wire controls. As of June 2024, over 11,000 A320 family aircraft, variants included, are operational around the world, making it one of the best-selling commercial jets.
Boeing chose fly-by-wire flight controls for the 777 in 1994, departing from traditional cable and pulley systems. In addition to overseeing the aircraft's flight control, the FBW offered "envelope protection", which guaranteed that the system would step in to avoid accidental mishandling, stalls, or excessive structural stress on the aircraft. The 777 used ARINC 629 buses to connect primary flight computers with actuator-control electronics units. Every PFC housed three 32-bit microprocessors, including a Motorola 68040, an Intel 80486, and an AMD 29050, all programmed in Ada programming language.
Analog systems
All fly-by-wire flight control systems eliminate the complexity, fragility and weight of the mechanical circuit of the hydromechanical or electromechanical flight control systems – each being replaced with electronic circuits. The control mechanisms in the cockpit now operate signal transducers, which in turn generate the appropriate commands. These are next processed by an electronic controller—either an analog one, or a digital one. Aircraft and spacecraft autopilots are now part of the electronic controller.The hydraulic circuits are similar except that mechanical servo valves are replaced with electrically controlled servo valves, operated by the electronic controller. This is the simplest and earliest configuration of an analog fly-by-wire flight control system. In this configuration, the flight control systems must simulate "feel". The electronic controller controls electrical devices that provide the appropriate "feel" forces on the manual controls. This was used in Concorde, the first production fly-by-wire airliner.
Digital systems
A digital fly-by-wire flight control system can be extended from its analog counterpart. Digital signal processing can receive and interpret input from multiple sensors simultaneously and adjust the controls in real time. The computers sense position and force inputs from pilot controls and aircraft sensors. They then solve differential equations related to the aircraft's equations of motion to determine the appropriate command signals for the flight controls to execute the intentions of the pilot.The programming of the digital computers enable flight envelope protection. These protections are tailored to an aircraft's handling characteristics to stay within aerodynamic and structural limitations of the aircraft. For example, the computer in flight envelope protection mode can try to prevent the aircraft from being handled dangerously by preventing pilots from exceeding preset limits on the aircraft's flight-control envelope, such as those that prevent stalls and spins, and which limit airspeeds and g forces on the airplane. Software can also be included that stabilize the flight-control inputs to avoid pilot-induced oscillations.
Since the flight-control computers continuously feedback the environment, pilot's workloads can be reduced. This also enables military aircraft with relaxed stability. The primary benefit for such aircraft is more maneuverability during combat and training flights, and the so-called "carefree handling" because stalling, spinning and other undesirable performances are prevented automatically by the computers. Digital flight control systems enable inherently unstable combat aircraft, such as the Lockheed F-117 Nighthawk and the Northrop Grumman B-2 Spirit flying wing to fly in usable and safe manners.