Fly-by-wire Control Systems

An Airbus A321 aircraft fly by wire cockpit.

Mechanical and hydro-mechanical flight control systems are heavy and require careful routing of flight control cables through the aircraft using systems of pulleys, cranks, wires and with hydraulically-assisted controls, hydraulic pipes. Both systems often require redundant backup to deal with failures, which again increases weight. Furthermore, both have limited ability to compensate for changing aerodynamic conditions. Dangerous characteristics such as stalling, spinning and pilot-induced oscillation (PIO), which depend mainly on the stability and structure of the aircraft concerned rather than the control system itself, can still occur with these systems.

A fly-by-wire system actually replaces manual control of the 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 expected response. The actuators are usually hydraulic, but electric actuators have also been used.

By using electrical control circuits combined with computers, designers can save weight, improve reliability, and use the computers to mitigate the undesirable characteristics mentioned above. Advanced modern fly-by-wire systems are also used to control otherwise unstable fighter aircraft.

The words "Fly-by-Wire" (FBW) imply an electrically-signaled only control system. However, the term is generally used in the 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. These are carefully developed and validated in order to produce maximum operational effect without compromising safety.
Safety and Redundancy

Aircraft systems may be quadruplexed (four independent channels) in order to prevent loss of signals in the case of failure of one or even two channels. High performance aircraft that have FBW controls (also called CCVs or Control-Configured Vehicles) may be deliberately designed to have low or even negative aerodynamic stability in some flight regimes, the rapid-reacting CCV controls compensating for the lack of natural stability.

Weight Saving

A FBW aircraft can be lighter than a similar design with conventional controls. Partly due to the lower overall weight of the system components; and partly because the natural aerodynamic stability of the aircraft can be relaxed, slightly for a transport aircraft and more for a maneuverable fighter, 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 (fin and tailplane) that are (normally) at the rear of the fuselage. If these structures can be reduced in size, airframe weight is reduced. The advantages of FBW controls were first exploited by the military and then in the commercial airline market. The Airbus series of airliners used full-authority FBW controls beginning with their A320 series, see A320 flight control (though some limited FBW functions existed on A310). Boeing followed with their 777 and later designs.
Electronic fly-by-wire systems can respond flexibly to changing aerodynamic conditions, by tailoring flight control surface movements so that aircraft response to control inputs is appropriate to flight conditions. Electronic systems require less maintenance, whereas mechanical and hydraulic systems require lubrication, tension adjustments, leak checks, fluid changes, etc. Furthermore, putting circuitry between pilot and aircraft can enhance safety; for example the control system can try to prevent a stall, or it can stop the pilot from over stressing the airframe.

The main concern with fly-by-wire systems is reliability. While traditional mechanical or hydraulic control systems usually fail gradually, the loss of all flight control computers could immediately render the aircraft uncontrollable. For this reason, most fly-by-wire systems incorporate either redundant computers (triplex, quadruplex etc), some kind of mechanical or hydraulic backup or a combination of both. A "mixed" control system such as the latter is not desirable and modern FBW aircraft normally avoid it by having more independent FBW channels, thereby reducing the possibility of overall failure to minuscule levels that are acceptable to the independent regulatory and safety authority responsible for aircraft design, testing and certification before operational service.

Analog

The fly-by-wire flight control system eliminates the complexity, fragility and weight of the mechanical circuit of the hydromechanical flight control systems and replaces it with an electrical circuit. The cockpit controls now operate signal transducers which generate the appropriate commands, that are in turn processed by an electronic controller. The autopilot is 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, as first fitted to the Avro Vulcan in the 1950s.

In this configuration, the flight control systems must simulate "feel". The electronic controller controls electrical feel devices that provide the appropriate "feel" forces on the manual controls. This is still used in the Embraer E-Jets family of aircraft and was used in Concorde, the first fly-by-wire airliner.

In more sophisticated versions, analog computers replaced the electronic controller. The cancelled 1950s supersonic Canadian fighter, the Avro CF-105 Arrow, employed this type of system. Analog computers also allowed some customization of flight control characteristics, including relaxed stability. This was exploited by the early versions of F-16, giving it impressive maneuverability.

Digital

F-8C Crusader digital fly-by-wire testbed

The Airbus A320, first airliner with digital fly-by-wire controls.

A Dassault Falcon 7X, the first business jet with digital fly-by-wire controls.

A digital fly-by-wire flight control system is similar to its analog counterpart. However, the signal processing is done by digital computers and the pilot literally can "fly-via-computer". This increases flexibility as the digital computers can receive input from any aircraft sensor. It also increases electronic stability, because the system is less dependent on the values of critical electrical components in an analog controller.

The computers "read" position and force inputs from the pilot's controls and aircraft sensors. They solve differential equations to determine the appropriate command signals that move the flight controls in order to carry out the intentions of the pilot.

The programming of the digital computers enable flight envelope protection. In this aircraft designers precisely tailor an aircraft's handling characteristics, to stay within the overall limits of what is possible given the aerodynamics and structure 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 (the aircraft's envelope) such as the stall, spin or limiting G. Software can also be used to filter control inputs to avoid pilot-induced oscillation.

Side-sticks, center sticks, or conventional control yokes can be used to fly such an aircraft. While the side-stick offers the advantages of being lighter, mechanically simpler, and unobtrusive, Boeing considered the lack of visual feedback from the side-stick a problem, and so uses conventional yokes in the 777 and the upcoming 787. The Airbus series have used side-sticks extensively, and the new Airbus A380 super-jumbo uses them. In fighter aircraft, such the F-16 Falcon, the side-stick is smaller.

As the computers continuously "fly" the aircraft, pilot workload can be reduced. It is now possible to fly aircraft that have relaxed stability. The primary benefit for military aircraft is more maneuverable flight performance and so-called "carefree handling" because stalling, spinning and other undesirables can be prevented. Digital flight control systems enable inherently unstable aircraft such as Lockheed Martin F-117 Nighthawk to fly. A modified NASA F-8C Crusader was the first digital fly-by-wire aircraft, in 1972, mirrored in the USSR by the Sukhoi T-4. At about the same time, in the UK a trainer version of the Hawker Hunter fighter was modified at the Farnborough research center with FBW controls in the right seat, the left seat being for a safety pilot with conventional controls and an FBW cut-out. The US Space Shuttle has digital fly-by-wire controls, first used in free-flight Approach and Landing Tests in 1977. In 1984, the Airbus A320 was the first airliner with digital fly-by-wire controls. In 2005, the Dassault Falcon 7X was the first business jet with fly-by-wire controls.

On military aircraft, fly-by-wire improves combat survivability because it avoids hydraulic failure. A common reason behind the loss of military aircraft in combat is damage causing hydraulic leaks leading to loss of control. Most military aircraft have several completely redundant hydraulic systems, but hydraulic lines are often routed together, and can be damaged together. With a fly-by-wire system, wires can be more flexibly routed, are easier to protect and less susceptible to damage than hydraulic lines.

The Federal Aviation Administration (FAA) of the United States adopted the RTCA/DO-178B, titled "Software Considerations in Airborne Systems and Equipment Certification", as the certification standard for aviation software. Any safety-critical component in a digital fly-by-wire system including control laws and the operating system will have to be certified to DO-178B Level A, which is applicable for potentially catastrophic failures.

Nonetheless the top concern for computerized, digital fly-by-wire systems is reliability, even more so than for analog systems. This is because a computer running software is often the only control path between the pilot and control surfaces. If the computer software crashes, the pilot may not be able to control the aircraft. Therefore virtually all fly-by-wire systems are triply or quadruply redundant: they have three or four computers in parallel, and three or four separate wires to each control surface. If one or two computers crash, the others continue working. In addition most early digital fly-by-wire aircraft also had an analog electric, mechanical or hydraulic backup control system. The Space Shuttle has, in addition to the redundant set of computers running the primary software, a backup computer running a separately developed, reduced function system that can take over in the event of a fault that affects all of the computers in the redundant set. This is intended to reduce the risk of total failure due to a generic software fault.

For airliners, redundancy improves safety, but fly-by-wire also improves economy because the elimination of heavy mechanical items reduces weight.

Boeing and Airbus differ in their FBW philosophies. In Airbus aircraft, the flight envelope protection always retains ultimate control and will not permit the pilot to fly outside the limit flight envelope. In a Boeing 777, the pilot can override the system, allowing the aircraft to be flown outside this envelope in emergencies. The pattern started by the Airbus A320 has been continued with the Airbus family and the Boeing 777.

Source: Wikipedia.