Aircraft flight control systems

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Aircraft flight control systems consist of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight. Aircraft engine controls are also considered as flight controls as they change speed.

The fundamentals of aircraft controls are explained in flight dynamics. This article centers on the operating mechanisms of the flight controls.

Cockpit controls

Primary controls

Generally the primary cockpit controls are arranged as follows:

A typical aircraft's primary flight controls in motion

A control column or a control yoke attached to a column—for roll and pitch, which moves the ailerons when turned or deflected left and right, and moves the elevators when moved backwards or forwards
Rudder pedals to control yaw, which move the rudder; left foot forward will move the rudder left for instance.
Throttle controls to control engine speed or thrust for powered aircraft.

The image shows the basic principles and the correct sense of movement of the primary controls, also illustrating a simple mechanical primary flying control system.

Even when an aircraft uses different kinds of surfaces, such as a V-tail/ruddervator, flaperons, or elevons, to avoid pilot confusion the aircraft will still normally be designed so that the yoke or stick controls pitch and roll in the conventional way, as will the rudder pedals for yaw.

Secondary controls

In addition to the primary flight controls for roll, pitch, and yaw, there are often secondary controls available to give the pilot finer control over flight or to ease the workload. The most commonly-available control is a wheel or other device to control elevator trim, so that the pilot does not have to maintain constant backward or forward pressure to hold a specific pitch attitude(other types of trim, for rudder and ailerons, are common on larger aircraft but may also appear on smaller ones). Many aircraft have wing flaps, controlled by a switch or a mechanical lever, which alter the shape of the wing for improved control at the slower speeds used for takeoff and landing. Other secondary flight control systems may be available, including slats, spoilers, air brakes and variable-sweep wings.

Basic flight control systems

Mechanical

de Havilland Tiger Moth elevator and rudder cables

Mechanical or manually-operated flight control systems are the most basic method of controlling an aircraft. They were used in early aircraft and are currently used in small aircraft where the aerodynamic forces are not excessive. Very early aircraft used a system of wing warping where no control surfaces were used.A manual flight control system uses a collection of mechanical parts such as rods, cables, pulleys and sometimes chains to transmit the forces applied to the cockpit controls directly to the control surfaces. Turnbuckles are often used to adjust control cable tension. The Cessna Skyhawk is a typical example of an aircraft that uses this type of system. Gust locks are often used on parked aircraft with mechanical systems to protect the control surfaces and linkages from damage from wind. Some aircraft have gust locks fitted as part of the control system.

Increases in the control surface area required by large aircraft or higher loads caused by high airspeeds in small aircraft lead to a large increase in the forces needed to move them, consequently complicated mechanical gearing arrangements were developed to extract maximum mechanical advantage in order to reduce the forces required from the pilots.This arrangement can be 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. Servo tabs are small surfaces hinged to the control surfaces. The flight control mechanisms move these tabs, aerodynamic forces in turn move, or assist the movement of 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.The Boeing 737 incorporates a system, whereby in the unlikely event of total hydraulic system failure, it automatically and seamlessly reverts to being controlled via servo-tab.

Hydromechanical

The complexity and weight of mechanical flight control systems increase considerably with the size and performance of the aircraft. Hydraulic power overcomes these limitations. With hydraulic flight control systems, aircraft size and performance are limited by economics rather than a pilot's strength. Initially only partially boosted systems were used in which the pilot could still feel some of the aerodynamic loads on the surfaces (feedback).

A hydromechanical flight control system has two parts:

The mechanical circuit, which links the cockpit controls with the hydraulic circuits. Like the mechanical flight control system, it consists of rods, cables, pulleys, and sometimes chains.

The hydraulic circuit, which has hydraulic pumps, reservoirs, filters, 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 valve in the hydraulic circuit. The hydraulic circuit powers the actuators which then move the control surfaces. As the actuator moves the servo valve is closed by a mechanical feedback linkage which stops movement of the control surface at the desired position.

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

Artificial feel devices

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

With hydromechanical flight control systems however, the load on the surfaces cannot be felt and there is a risk of overstressing the aircraft through excessive control surface movement. To overcome this problem artificial feel systems are used; for example: with the controls of the Avro Vulcan jet bomber, the requisite force feedback was achieved by a spring device. The fulcrum of the device was moved in proportion to the square of the airspeed (for the elevators) to give increased resistance at higher speeds. In the controls of the Vought Crusader and Corsair II, a "bob-weight" was used in the pitch axis of the control stick, giving a force feedback proportional to the aircraft's normal acceleration.

Stick shaker

A stick shaker is a device fitted to provide artificial stall warning in some aircraft with hydraulically-powered control systems.

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.

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.

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 been used.

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 [1], 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 normal 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.

Engine digital control

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, aircraft 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 by flight envelope protection. As a result, in such conditions, the flight control systems commands the engines to increase thrust without pilot intervention. In economy cruise modes, the flight control systems adjust 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. On the A330/A340 family, fuel is transferred between the main (wing and center fuselage) tanks and a fuel tank in the horizontal stabilizer, in order to optimize the aircraft's center of gravity during cruise flight. The fuel management controls keep the aircraft's center of gravity accurately trimmed with fuel weight, rather than drag-inducing aerodynamic trims in the elevators.

Further developments

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. Sometimes it is referred to as "Fly-by-light" due to its use of Fiber Optics. 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. The Boeing 787 will also incorporate some electrically operated flight controls (spoilers and horizontal stabilizer), which will remain operational with either a total hydraulics failure and/or flight control computer failure.

Intelligent Flight Control System

A newer flight control system, called Intelligent Flight Control System (IFCS), 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 enhancements are mostly software upgrades to existing fully computerized digital fly-by-wire flight control systems.

Flying with disabled flight controls

Several aviation incidents have occurred in which the control surfaces of the aircraft became disabled, frequently due to loss of hydraulic systems. An aircraft's loss of control surfaces may result in its speed and direction being uncontrollable via conventional methods. Aircraft are not designed to be flown in such circumstances (which is why they have redundant hydraulics), but a few pilots have had some success in controlling such aircraft.

Technique

The basic means of controlling the aircraft is by making use of the position of the engine(s). If the engines are mounted under the centre of gravity, as is the case in most passenger jets, then increasing the thrust will raise the nose, while decreasing the thrust will lower it. This control method may call for control inputs that go against the pilot's instinct: when the aircraft is in a dive, adding thrust will raise the nose and vice versa.

Additionally, asymmetrical thrust may be used for directional control: if the left engine is idled and power is increased on the right side this will result in a yaw to the left, and vice versa. If throttle settings allow the throttles to be shifted without affecting the total amount of power, then yaw control can be combined with pitch control. If the plane is yawing, then the wing on the outside of this yaw movement will go faster than the inner wing. This creates higher lift on the faster wing, resulting in a rolling movement, which helps to make a turn.

Controlling speed is very difficult with engine control only, and will most likely result in a fast landing. A fast landing would be required anyway if the flaps can not be extended due to loss of hydraulics (often the source of the loss of control surfaces). Only jet aircraft with an engine mounted on the vertical tail in addition to wing-mounted powerplants, such as the DC-10, MD-11 or Lockheed Tristar trijet configurations, will be able to control the speed to a higher degree, as this engine is on the fuselage centreline and above the centre of gravity.

Aircraft that have two or four engines mounted on the sides of the empennage (as is the case with most business jets) will only have limited benefit from asymmetrical thrust.

The biggest challenge for a pilot forced to fly an aircraft without control surfaces is to avoid the phugoid instability mode (a cycle in which the aircraft repeatedly climbs and then dives), which requires careful use of the throttle.

Because this type of aircraft control is difficult for humans to achieve, some researchers have attempted to integrate this control ability into the computers of fly-by-wire aircraft. Early attempts to add the ability to real aircraft were not very successful, the software having been based on experiments conducted in flight simulators where jet engines are usually modeled as "perfect" devices with exactly the same thrust on each engine, a linear relationship between throttle setting and thrust, and instantaneous response to input. Later, computer models were updated to account for these factors, and aircraft have been successfully flown with this software installed. However, it remains a rarity on commercial aircraft.

Incidents on commercial airliners

American Airlines Flight 96, a McDonnell Douglas DC-10, on June 12, 1972. The failure of the rear cargo door caused an explosive decompression, which in turn caused the rear main cabin floor to collapse and severed flight controls. The pilots had only limited ailerons and elevators; the rudder was jammed. The No. 2 engine also ran down to idle at the time of decompression. The aircraft landed safely at Detroit-Metropolitan Airport.

Turkish Airlines Flight 981, a McDonnell Douglas DC-10, on March 3, 1974. The failure of the rear cargo door caused an explosive decompression, which in turn caused the rear main cabin floor to collapse and severed flight controls. The No. 2 engine also flamed out at the time of decompression. Unlike American Airlines Flight 96, however, the pilots were left with no functioning controls whatsoever, and all aboard perished.

LOT Polish Airlines Flight 007, an Ilyushin Il-62M, on March 14, 1980. During go-around the low pressure turbine of Engine No. 2 disintegrated explosively. Debris from the engine severed the rudder and elevator control pushers and destroyed the number 1 and 3 engines, causing loss of control and an uncontrolled descent. The aircraft impacted the ground at a 20-degree down angle, 950 meters away from the runway and 100 meters from a dense residential area. At the last moment, Pawel Lipowczan, using nothing but the plane’s ailerons, managed to avoid hitting a hospice for schoolchildren located at Rozwojowa street. There were 87 casualties. Ilyushin denied existence of the design flaw that left the plane vulnerable to the kind of engine failures that could ultimately lead to the loss of control.

Japan Airlines Flight 123, a Boeing 747, on August 12, 1985. A faulty repair years earlier had weakened the aircraft's rear pressure bulkhead, which failed in flight. The vertical stabilizer and much of the aircraft's tail was blown off during the decompression. The pilots were unable to regain full control of the aircraft; all but four aboard perished.

LOT Polish Airlines Flight 5055, an Ilyushin Il-62M, on May 9, 1987. According to the Polish investigatory commission, the cause of the crash was the disintegration of an engine shaft due to faulty bearings inside engine No. 2, which seized, causing extensive heat. This in turn caused the consequent damage to engine No. 1, rapid decompression of the fuselage, and a fire in the cargo hold, as well as the loss of elevator controls and progressive electrical failures. Zygmunt Pawlaczyk decided to return to Warsaw Okecie Airport using only ailerons and engines to control the flight of the plane. He lost his struggle to land about 5km from the runway in the Kabacki Forest. All 172 passengers and 11 crew members perished. As with Polish Airlines Flight 007, the Soviet manufacturer of the plane denied existence of the design flaw responsible for the loss of control.

United Airlines Flight 232, a McDonnell Douglas DC-10, on July 19, 1989. A fan disk in the No. 2 engine fractured, severing most of the flight controls. Dennis Fitch, a deadheading DC-10 instructor who had studied the case of JAL Flight 123, was able to help the pilots steer the aircraft using throttle differential. Despite the break-up of the plane upon landing, 175 of 285 passengers and 10 of the 11 crew members survived.

Philippine Airlines Flight 434, a Boeing 747, on December 11, 1994. The hydraulics were damaged by a bomb in the passenger cabin.

DHL shootdown incident in Baghdad on November 22, 2003. The Airbus A300 DHL aircraft was the first jet airliner to land safely without any hydraulics using only engine controls.

Air Transat Flight 961 on March 6, 2005, catastrophic structural failure: the rudder detached from the aircraft with a loud bang. The pilots regained enough control to land the aircraft safely.

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Published in July 2009.

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