Aircraft flight control systems

A flight control system consists of the flight control surfaces, the respective cockpit controls, connecting linkage, and necessary operating mechanisms to control aircraft in flight.

The fundamentals of aircraft controls have been explained in aeronautics. Discussion here centers on the underlying mechanisms of the flight controls. Generally the cockpit controls are arranged like this:

Control yoke for roll which moves the ailerons
Control column for pitch which moves the elevators
Rudder pedals for yaw which moves the rudder

Many aircraft use a control stick for both roll and pitch, and the rudder pedals for yaw.

Flight control systems

Mechanical

Mechanical flight control systems are the most basic designs. They were used in early aircraft and currently in small aeroplanes where the aerodynamic forces are not excessive. The flight control systems uses a collection of mechanical parts such as rods, cables, pulleys and sometimes chains to transmit the forces of the cockpit controls to the control surfaces. The Cessna Skyhawk is a typical example.

Since an increase in control surface area in bigger and faster aircraft leads to a large increase in the forces needed to move them, complicated mechanical arrangements are used to extract maximum mechanical advantage in order to make the forces required bearable to the pilots. This arrangement is found on bigger or higher performance propeller aircraft such as the Fokker 50.

Some mechanical flight control systems use servo tabs that provide aerodynamic assistance to reduce complexity. Servo tabs are small surfaces hinged to the control surfaces. The mechanisms move these tabs, aerodynamic forces in turn move the control surfaces reducing the amount of mechanical forces needed. This arrangement was used in early piston-engined transport aircraft and in early jet transports such as the mostly mechanical Boeing 707.

Hydromechanical

The complexity and weight of a mechanical flight control systems increases considerably with size and performance of the airplane. Hydraulic power overcomes these limitations. With hydraulic flight control systems aircraft size and performance are limited by economics rather than a pilot's strength.

A hydraulic flight control systems has 2 parts:

The mechanical circuit
The hydraulic circuit

The mechanical circuit links the cockpit controls with the hydraulic circuits. Like the mechanical flight control systems, it is made of rods, cables, pulleys, and sometimes chains.

The hydraulic circuit has hydraulic pumps, pipes, valves and actuators. The actuators are powered by the hydraulic pressure generated by the pumps in the hydraulic circuit. The actuators convert hydraulic pressure into control surface movements. The servo valves control the movement of the actuators.

The pilot's movement of a control causes the mechanical circuit to open the matching servo valves in the hydraulic circuit. The hydraulic circuit powers the actuators which then move the control surfaces.

This arrangement is found in older jet transports and high performance aircraft. Examples include the Antonov An-225 and the Lockheed SR-71.

Artificial feel devices

In mechanical flight control systems, the aerodynamic forces on the control surfaces are transmitted through the mechanisms and can be felt by the pilot. This gives tactile feedback of airspeed and aids flight safety.

Hydromechanical flight control systems lack this "feel". The aerodynamic forces are only felt by the actuators. Artificial feel devices are fitted to the mechanical circuit of the hydromechanical flight control systems to simulate this "feel". They increase resistance with airspeed and vice-versa. The pilots feel as if they are flying an aircraft with a mechanical flight control systems.

Fly-by-wire

Mechanical and hydraulic flight control systems are heavy and require careful routing of flight control cables through the airplane using systems of pulley and cranks. Both systems often require redundant backup, which further increases weight. Furthermore, both have limited ability to compensate for changing aerodynamic conditions.

By using computers and electrical linkages, designers can save weight and improve reliability. Electronic fly-by-wire systems can respond more flexibly to changing aerodynamic conditions, by tailoring flight control surface movements so that airplane response to control inputs is consistent for all flight conditions. Electronic systems require less maintenance, whereas mechanical and hydraulic system 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 prevent a stall, or can stop the pilot from overstressing the airframe.

A fly-by-wire system literally replaces physical control of the aircraft with an electrical interface. The pilot's commands are converted to electronic signals, and flight control computers determine how best to move the actuators at each control surface to provide the desired response. Those actuators initially are usually hydraulic, but electric actuators have been investigated.

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 will immediately render the airplane uncontrollable. For this reason, most fly-by-wire systems incorporated redundant computers and some kind of mechanical or hydraulic backup. This may seem to negate some advantages of fly-by-wire, but the redundant systems can be simpler, lighter, and offer only limited capability since they are for emergency use only.

Analog

The fly-by-wire flight control systems 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. The commands are 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. The valves are operated by the electronic controller. This is the simplest and earliest configuration, an analog fly-by-wire flight control systems, first fitted to the Avro Vulcan in the 1940s.

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 170 and EMBRAER 190 and was used in Concorde, the first fly-by-wire airliner.

On more sophisticated versions, analog computers replaced the electronic controller. The cancelled supersonic Canadian fighter, the Avro CF-105 Arrow, was built this way in the 1950s. 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

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

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 operation 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 than analog systems. This is because a computer running software is the only control path between pilot and control surfaces. If the computer software crashes, the pilot cannot 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 computers read positions and forces from the pilot's controls and aircraft sensors. They calculate differential equations that move the flight controls to carry out the intentions of the pilot.

The program in the digital computers let aircraft designers tailor an aircraft's handling characteristics precisely. For example the software can prevent the aircraft from being handled dangerously by preventing pilots from exceeding preset limits (the aircraft's envelope). Software can also be used to filter control inputs to avoid pilot induced oscillation.

Sidesticks 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.

As the computers continuously fly the aircraft, pilot workload is reduced. It is now possible to fly aircraft with relaxed stability. The primary benefit for military aircraft is more responsive flight performance. Digital flight control systems enabled 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. The US Space Shuttle (first flown in 1982) has digital fly-by-wire controls. 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.

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 computer always retains ultimate control and will not permit the pilot to fly outside the normal flight envelope. In a Boeing 777, the pilot can override the system, allowing the plane to be flown outside this envelope in emergencies. The pattern started by Airbus A320 has been continued with the Airbus family and every new Boeing design since the Boeing 777.

=Aircraft-engine integration

= The advent of FADEC (full authority digital engine control) engines permits operation of the flight control systems and autothrottles for the engines to be fully integrated. On modern military aircraft other systems such as autostabilization, navigation, radar and weapons system are all integrated with the flight control systems.

FADEC allows maximum performance to be extracted from the aircraft without fear of engine misoperation, airplane damage or high pilot workloads.

In the civil field, the integration increases flight safety and economy. The Airbus A320 and its fly-by-wire brethren are protected from low-speed stall. In such conditions, the flight control systems commands the engines to increase thrust without pilot intervention. In economy cruise modes, the flight control systems adjusts the throttles and fuel tank selections more precisely than all but the most skillful pilots. FADEC reduces rudder drag needed to compensate for sideways flight from unbalanced engine thrust. The fuel management controls keep the aircraft's attitude accurately trimmed with fuel weight, rather than draggy aerodynamic trims in the elevators.

Cars

Fly by wire has now become mainstream enough to be used in mass production motor cars. The Toyota Prius Hybrid takes account of pedal action and gear changes to work out how much petrol is required, what CVT gearing to use, and how to apply the electric motor/generator.

Fly-by-optics

Fly-by-optics is sometimes used instead of fly-by-wire because it can transfer data at higher speeds, and it is immune to electromagnetic interference. In most cases, the cables are just changed from electrical to fiber optic cables. The data generated by the software and interpreted by the controller remain the same.

Power-by-wire

Having eliminated the mechanical circuits in fly-by-wire flight control systems, the next step is to eliminate the bulky and heavy hydraulic circuits. The hydraulic circuit is replaced by an electrical power circuit. The power circuits power electrical or self-contained electrohydraulic actuators that are controlled by the digital flight control computers. All benefits of digital fly-by-wire are retained.

The biggest benefits are weight savings, the possibility of redundant power circuits and tighter integration between the aircraft flight control systems and its avionics systems. The absence of hydraulics greatly reduces maintenance costs. This system is used in the Lockheed Martin F-35 and in Airbus A380 backup flight controls.

Intelligent

A newer flight control system, called Intelligent Flight Control System, is an extension of modern digital fly-by-wire flight control systems. The aim is to intelligently compensate for aircraft damage and failure during flight, such as automatically using engine thrust and other avionics to compensate for severe failures such as loss of hydraulics, loss of rudder, loss of ailerons, loss of an engine, etc. Several demonstrations were made on a flight simulator where a Cessna-trained small-aircraft pilot successfully landed a heavily-damaged full-size concept jet, without prior experience with large-body jet aircraft. This development is being spearheaded by NASA Dryden Flight Research Center^*. It is reported that enhancement is mostly a software upgrade to an existing fully computerized digital fly-by-wire flight control systems.

External links

Airbus A380 cockpit

Bibliography

Cary R. Spitzer, Ed., The Avionics Handbook, CRC Press, ISBN 0-8493-8348-X

R. F. Stengel, "Toward Intelligent Flight Control," IEEE Trans. Systems, Man, and Cybernetics,

Vol. 23, No. 6, Nov-Dec 1993, pp. 1699-1717.

Aircraft instruments

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