Aircraft flight control system

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A typical aircraft's primary flight controls in motion ControlSurfaces.gif
A typical aircraft's primary flight controls in motion

A conventional fixed-wing aircraft flight control system (AFCS) consists 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 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. The basic system in use on aircraft first appeared in a readily recognizable form as early as April 1908, on Louis Blériot's Blériot VIII pioneer-era monoplane design. [1]

Cockpit controls

Primary controls

Cockpit controls and instrument panel of a Cessna 182D Skylane Cessna 182D Skylane Cockpit.jpg
Cockpit controls and instrument panel of a Cessna 182D Skylane

Generally, the primary cockpit flight controls are arranged as follows: [2]

The control yokes also vary greatly among aircraft. There are yokes where roll is controlled by rotating the yoke clockwise/counterclockwise (like steering a car) and pitch is controlled by moving the control column towards or away from the pilot, but in others the pitch is controlled by sliding the yoke into and out of the instrument panel (like most Cessnas, such as the 152 and 172), and in some the roll is controlled by sliding the whole yoke to the left and right (like the Cessna 162). Centre sticks also vary between aircraft. Some are directly connected to the control surfaces using cables, [3] others (fly-by-wire airplanes) have a computer in between which then controls the electrical actuators.

Bleriot VIII at Issy-les-Moulineaux, the first flightworthy aircraft design to have the initial form of modern flight controls for the pilot Sports Aviation - Issy-les-Moulineaux (10 septembre) - Le Bleriot VIII ... (7843390842).jpg
Blériot VIII at Issy-les-Moulineaux, the first flightworthy aircraft design to have the initial form of modern flight controls for the pilot

Even when an aircraft uses variant flight control surfaces such as a V-tail ruddervator, flaperons, or elevons, because these various combined-purpose control surfaces control rotation about the same three axes in space, the aircraft's flight control system will still be designed so that the stick or yoke controls pitch and roll conventionally, as will the rudder pedals for yaw. [2] The basic pattern for modern flight controls was pioneered by French aviation figure Robert Esnault-Pelterie, with fellow French aviator Louis Blériot popularizing Esnault-Pelterie's control format initially on Louis' Blériot VIII monoplane in April 1908, and standardizing the format on the July 1909 Channel-crossing Blériot XI. Flight control has long been taught in such fashion for many decades, as popularized in ab initio instructional books such as the 1944 work Stick and Rudder.

In some aircraft, the control surfaces are not manipulated with a linkage. In ultralight aircraft and motorized hang gliders, for example, there is no mechanism at all. Instead, the pilot just grabs the lifting surface by hand (using a rigid frame that hangs from its underside) and moves it.[ citation needed ]

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 [4] (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 or in some cases are fully automatic by computer control, which alter the shape of the wing for improved control at the slower speeds used for take-off and landing. Other secondary flight control systems may include slats, spoilers, air brakes and variable-sweep wings.

Flight control systems


de Havilland Tiger Moth elevator and rudder cables Tiger cables.JPG
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, such as the Wright Flyer I, Blériot XI and Fokker Eindecker used a system of wing warping where no conventionally hinged control surfaces were used on the wing, and sometimes not even for pitch control as on the Wright Flyer I and original versions of the 1909 Etrich Taube, which only had a hinged/pivoting rudder in addition to the warping-operated pitch and roll controls. [5] A manual flight control system uses a collection of mechanical parts such as pushrods, tension cables, pulleys, counterweights, 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. [6]

Increases in the control surface area, and the higher airspeeds required by faster aircraft resulted in higher aerodynamic loads on the flight control systems. As a result, the forces required to move them also become significantly larger. Consequently, complicated mechanical gearing arrangements were developed to extract maximum mechanical advantage in order to reduce the forces required from the pilots. [7] 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. [8] 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 designs, consisting of a mechanical circuit and a hydraulic circuit, were used to reduce the complexity, weight, and limitations of mechanical flight controls systems. Hydromechanical flight control system.png
Hydromechanical designs, consisting of a mechanical circuit and a hydraulic circuit, were used to reduce the complexity, weight, and limitations of mechanical flight controls systems.

The complexity and weight of mechanical flight control systems increase considerably with the size and performance of the aircraft. Hydraulically powered control surfaces help to overcome these limitations. With hydraulic flight control systems, the aircraft's size and performance are limited by economics rather than a pilot's muscular strength. At first, only-partially boosted systems were used in which the pilot could still feel some of the aerodynamic loads on the control surfaces (feedback). [7]

A hydro-mechanical flight control system has two parts:

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 - one that stops movement of the control surface at the desired position.

This arrangement was found in the older-designed jet transports and in some 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, allowing tactile feedback of airspeed. With hydromechanical flight control systems, 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 can be used. For example, for the controls of the RAF's Avro Vulcan jet bomber and the RCAF's Avro Canada CF-105 Arrow supersonic interceptor (both 1950s-era designs), the required force feedback was achieved by a spring device. [10] The fulcrum of this device was moved in proportion to the square of the air speed (for the elevators) to give increased resistance at higher speeds. For the controls of the American Vought F-8 Crusader and the LTV A-7 Corsair II warplanes, a 'bob-weight' was used in the pitch axis of the control stick, giving force feedback that was proportional to the airplane's normal acceleration.[ citation needed ]

Stick shaker

A stick shaker is a device that is attached to the control column in some hydraulic aircraft. It shakes the control column when the aircraft is approaching stall conditions. Some aircraft such as the McDonnell Douglas DC-10 are equipped with a back-up electrical power supply that can be activated to enable the stick shaker in case of hydraulic failure. [11]


In most current systems the power is provided to the control actuators by high-pressure hydraulic systems. In fly-by-wire systems the valves, which control these systems, are activated by electrical signals. In power-by-wire systems, electrical actuators are used in favour of hydraulic pistons. The power is carried to the actuators by electrical cables. These are lighter than hydraulic pipes, easier to install and maintain, and more reliable. Elements of the F-35 flight control system are power-by-wire. [12] [13] [14] The actuators in such an electro-hydrostatic actuation (EHA) system are self-contained hydraulic devices, small closed-circuit hydraulic systems. The overall aim is towards more- or all-electric aircraft and an early example of the approach was the Avro Vulcan. Serious consideration was given to using the approach on the Airbus A380. [15]

Fly-by-wire control systems

A fly-by-wire (FBW) system replaces manual flight control of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires (hence the term fly-by-wire), and flight control computers determine how to move the actuators at each control surface to provide the expected response. Commands from the computers are also input without the pilot's knowledge to stabilize the aircraft and perform other tasks. Electronics for aircraft flight control systems are part of the field known as avionics.

Fly-by-optics, also known as fly-by-light, is a further development using fiber-optic cables.


Several technology research and development efforts exist to integrate the functions of flight control systems such as ailerons, elevators, elevons, flaps, and flaperons into wings to perform the aerodynamic purpose with the advantages of less: mass, cost, drag, inertia (for faster, stronger control response), complexity (mechanically simpler, fewer moving parts or surfaces, less maintenance), and radar cross section for stealth. These may be used in many unmanned aerial vehicles (UAVs) and 6th generation fighter aircraft. Two promising approaches are flexible wings, and fluidics.

Flexible wings

In flexible wings, also known as "morphing aerofoils", much or all of a wing surface can change shape in flight to deflect air flow much like an ornithopter. Adaptive compliant wings are a military and commercial effort. [16] [17] [18] The X-53 Active Aeroelastic Wing was a US Air Force, NASA, and Boeing effort. Notable efforts have also been made by FlexSys, who have conducted flight tests using flexible aerofoils retrofitted to a Gulf stream III aircraft. [19]

Active Flow Control

In active flow control systems, forces in vehicles occur via circulation control, in which larger and more complex mechanical parts are replaced by smaller, simpler fluidic systems (slots which emit air flows) where larger forces in fluids are diverted by smaller jets or flows of fluid intermittently, to change the direction of vehicles. [20] [21] In this use, active flow control promises simplicity and lower mass, costs (up to half less), and inertia and response times. This was demonstrated in the Demon UAV, which flew for the first time in the UK in September 2010. [22]

See also

Related Research Articles

<span class="mw-page-title-main">Fly-by-wire</span> Electronic flight control system

Fly-by-wire (FBW) 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 transmitted by wires, 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.

<span class="mw-page-title-main">Fixed-wing aircraft</span> Heavier-than-air aircraft with fixed wings generating aerodynamic lift

A fixed-wing aircraft is a heavier-than-air flying machine, such as an airplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinct from rotary-wing aircraft, and ornithopters. The wings of a fixed-wing aircraft are not necessarily rigid; kites, hang gliders, variable-sweep wing aircraft and airplanes that use wing morphing are all examples of fixed-wing aircraft.

<span class="mw-page-title-main">Aileron</span> Aircraft control surface used to induce roll

An aileron is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll, which normally results in a change in flight path due to the tilting of the lift vector. Movement around this axis is called 'rolling' or 'banking'.

<span class="mw-page-title-main">Stick shaker</span> Mechanical device in an aircraft cockpit to warn the pilot of an imminent stall

A stick shaker is a mechanical device designed to rapidly and noisily vibrate the control yoke of an aircraft, warning the flight crew that an imminent aerodynamic stall has been detected. It is typically present on the majority of large civil jet aircraft, as well as most large military planes.

<span class="mw-page-title-main">Flight control surfaces</span> Surface that allows a pilot to adjust and control an aircrafts flight attitude

Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.

<span class="mw-page-title-main">Elevon</span>

Elevons or tailerons are aircraft control surfaces that combine the functions of the elevator and the aileron, hence the name. They are frequently used on tailless aircraft such as flying wings. An elevon that is not part of the main wing, but instead is a separate tail surface, is a stabilator.

<span class="mw-page-title-main">Stabilator</span>

A stabilator is a fully movable aircraft horizontal stabilizer. It serves the usual functions of longitudinal stability, control and stick force requirements otherwise performed by the separate parts of a conventional horizontal stabilizer and elevator. Apart from reduced drag, particularly at high Mach numbers, it is a useful device for changing the aircraft balance within wide limits, and for reducing stick forces.

<span class="mw-page-title-main">Aero Boero AB-115</span> Type of aircraft

The Aero Boero AB-115 is an Argentine civil utility aircraft. It was developed from the AB-95-115, a refined AB-95 with a more powerful engine and improved aerodynamics. Specific differences included wheel spats, a redesigned engine cowling molded of fiberglass, and aluminum ailerons and flaps.

<span class="mw-page-title-main">Empennage</span> Tail section of an aircraft containing stabilizers

The empennage, also known as the tail or tail assembly, is a structure at the rear of an aircraft that provides stability during flight, in a way similar to the feathers on an arrow. The term derives from the French language verb empenner which means "to feather an arrow". Most aircraft feature an empennage incorporating vertical and horizontal stabilising surfaces which stabilise the flight dynamics of yaw and pitch, as well as housing control surfaces.

Aircraft flight mechanics are relevant to fixed wing and rotary wing (helicopters) aircraft. An aeroplane, is defined in ICAO Document 9110 as, "a power-driven heavier than air aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight".

<span class="mw-page-title-main">Yoke (aeronautics)</span> Aircraft controls

A yoke, alternatively known as a control wheel or a control column, is a device used for piloting some fixed-wing aircraft.

<span class="mw-page-title-main">Trim tab</span> Boat or aircraft component

Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft, used to control the trim of the controls, i.e. to counteract hydro- or aerodynamic forces and stabilise the boat or aircraft in a particular desired attitude without the need for the operator to constantly apply a control force. This is done by adjusting the angle of the tab relative to the larger surface.

<span class="mw-page-title-main">Vertical stabilizer</span> Aircraft component

A vertical stabilizer or tail fin is the static part of the vertical tail of an aircraft. The term is commonly applied to the assembly of both this fixed surface and one or more movable rudders hinged to it. Their role is to provide control, stability and trim in yaw. It is part of the aircraft empennage, specifically of its stabilizers.

<span class="mw-page-title-main">Helicopter flight controls</span> Instruments used in helicopter flight

Helicopter flight controls are used to achieve and maintain controlled aerodynamic helicopter flight. Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor blades that make the helicopter move in a desired way. To tilt forward and back (pitch) or sideways (roll) requires that the controls alter the angle of attack of the main rotor blades cyclically during rotation, creating differing amounts of lift at different points in the cycle. To increase or decrease overall lift requires that the controls alter the angle of attack for all blades collectively by equal amounts at the same time, resulting in ascent, descent, acceleration and deceleration.

<span class="mw-page-title-main">Side-stick</span> Aircraft control

A side-stick or sidestick controller is an aircraft control stick that is located on the side console of the pilot, usually on the righthand side, or outboard on a two-seat flightdeck. Typically this is found in aircraft that are equipped with fly-by-wire control systems.

<span class="mw-page-title-main">USAir Flight 427</span> Aviation accident in 1994

USAir Flight 427 was a scheduled flight from Chicago's O'Hare International Airport to Palm Beach International Airport, Florida, with a stopover at Pittsburgh International Airport. On Thursday, September 8, 1994, the Boeing 737 flying this route crashed in Hopewell Township, Pennsylvania while approaching Runway 28R at Pittsburgh, which was USAir's largest hub at the time.

<span class="mw-page-title-main">Flight control modes</span> Aircraft control computer software

A flight control mode or flight control law is a computer software algorithm that transforms the movement of the yoke or joystick, made by an aircraft pilot, into movements of the aircraft control surfaces. The control surface movements depend on which of several modes the flight computer is in. In aircraft in which the flight control system is fly-by-wire, the movements the pilot makes to the yoke or joystick in the cockpit, to control the flight, are converted to electronic signals, which are transmitted to the flight control computers that determine how to move each control surface to provide the aircraft movement the pilot ordered.

Electro-hydraulic actuators (EHAs), replace hydraulic systems with self-contained actuators operated solely by electrical power. EHAs eliminate the need for separate hydraulic pumps and tubing, because they include their own pump, simplifying system architectures and improving safety and reliability. This technology originally was developed for the aerospace industry but has since expanded into many other industries where hydraulic power is commonly used.

Several aviation incidents and accidents have occurred in which the control surfaces of an aircraft became disabled, often due to failure of hydraulic systems or the flight control system. Other incidents have occurred where controls were not functioning correctly prior to take-off, either due to maintenance or pilot error, and controls can become inoperative from extreme weather conditions. Aircraft are not designed to be flown in such circumstances; however, a small number of pilots have had some success in flying and landing aircraft with disabled controls.

<span class="mw-page-title-main">2016 Malta Fairchild Merlin crash</span> 2016 aviation disaster near Kirkop, Malta

On 24 October 2016, a twin turboprop Fairchild SA227-AT Merlin IVC operated by CAE Aviation crashed near Kirkop, Malta, shortly after take-off from Malta International Airport. The aircraft was to operate in the vicinity of Misrata in Libya on a surveillance mission by the French Ministry of Defence. All five people on board the aircraft died in the crash, making it the deadliest aviation accident in Malta since 1975.



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