Flight controls and high-lift devices

The 747's flight controls do two jobs at once: change the aircraft's attitude and change the wing's shape for different phases of flight. A turn, a descent with speedbrakes, and an approach with flaps extended can involve several of the same surfaces, but they are different commands with different purposes.

This guide distinguishes the 747-400 from the redesigned 747-8 and explains the engineering principles behind both. It is a systems explanation, not an operating procedure; surface schedules, limitations and fault responses depend on the aircraft's approved configuration.

The three axes: roll, pitch and yaw

Motion	Principal surfaces	What changes
Roll: one wing rises while the other falls	Ailerons, assisted by spoilers	The lift distribution differs between the wings
Pitch: the nose moves up or down	Elevators; horizontal stabilizer for trim	The aerodynamic moment about the lateral axis changes
Yaw: the nose moves left or right	Rudder	Side force at the tail changes the aircraft's direction

These motions interact. Banking tilts the lift vector, which helps turn the flight path; moving the rudder alone is not the normal way to make a coordinated turn. The distinction between primary controls and devices such as flaps, spoilers and trim is explained in the FAA's flight-controls chapter, chapter 6.

A command is not the power that moves a surface

Moving a control wheel does not mean the pilot's muscles supply the force needed to move a loaded 747 control surface. In a hydromechanical system, mechanical linkages transmit a command to a control valve. Hydraulic pressure and flow supply the energy at the actuator. The actuator moves the surface; feedback makes its response follow the commanded movement.

That separation explains an apparent contradiction: an aircraft can have conventional mechanical signalling and still depend on hydraulic power. Likewise, fly-by-wire describes the command path, not necessarily an electric motor at the surface. Electrical signals can command a valve controlling a hydraulic actuator.

Original illustration: Boeing-747.com. Conceptual hydromechanical control chain, not a 747 plumbing or wiring schematic. Blue arrows follow the control chain; the amber panel identifies its energy supply.

The FAA Airframe Handbook, chapter 2, describes this distinction and the need for artificial feel. Hydraulic assistance isolates the pilot from much of the actual surface load, so a feel mechanism provides useful resistance at the cockpit controls. Control force is therefore not a direct measurement of the force acting on the wing.

Roll control: why split the ailerons?

The 747 has inboard and outboard ailerons. A surface farther from the fuselage has a longer lever arm, making it useful for producing a rolling moment. But a flexible swept wing also twists under load. The useful movement and loading of an outboard surface therefore depend on speed and configuration.

On the 747-400, outboard aileron availability is restricted at high speed, while the inboard ailerons and roll spoilers continue to provide roll control. This is a scheduled system, not a pilot choosing between two separate steering wheels. Do not carry that simplified description straight across to the 747-8: its outboard ailerons have electronic control and additional functions.

For a passenger looking out of the window, an aileron moving with the wing's other devices is not necessarily a fault. The controls can combine commands. Roll, flap-related droop and load-management functions must be distinguished by model and flight condition.

Spoilers have three different jobs

A spoiler rises from the upper wing surface and disturbs the airflow, reducing lift and adding drag. Its effect depends on which panels move and how they are commanded.

• Roll assistance: asymmetric deployment helps produce a rolling moment.
• In-flight speedbraking: deployment on both wings increases drag and helps manage energy during a descent or deceleration.
• Ground lift reduction: deployment after landing reduces the lift still being carried by the wing, increasing the load available on the wheels for braking.

The last point connects flight controls directly to the landing gear. Brakes act through the tyres against the runway; residual wing lift reduces tyre loading. Ground spoilers help transfer support from the wing to the wheels, but they do not replace wheel brakes or reverse thrust. The general aerodynamic roles are covered in the FAA flight-controls chapter.

Panel schedules are aircraft-specific. A diagram showing twelve spoiler panels should not be read as a promise that every panel moves equally for every speedbrake or roll command.

Pitch: elevator movement and stabilizer trim

Elevators provide pitch control by changing the aerodynamic force at the horizontal tail. Stabilizer trim changes the incidence of the horizontal stabilizer, allowing the aircraft to balance a different combination of speed, loading and configuration without requiring a persistent elevator command.

The two functions are related but not interchangeable. Extending flaps changes the aerodynamic balance; burning or transferring fuel changes weight and potentially centre of gravity. The pitch system has to accommodate those changes while the aircraft follows its intended flight path.

A useful observation is to separate the movement of an elevator's trailing edge from a change in the angle of the whole stabilizer. They answer different questions: what immediate pitch response is commanded, and what balance is required for the current condition? The FAA Airframe Handbook, chapters 1 - 2, explains elevator and adjustable-stabilizer principles.

Yaw control and damping

The rudder provides directional control, including the force needed to oppose asymmetric thrust after an engine failure. A yaw damper makes automatic corrective inputs to suppress unwanted oscillation; it is doing a stability job rather than selecting the aircraft's destination.

Swept-wing aircraft can exhibit coupled yaw and roll motion. Damping reduces the tendency for that motion to continue after a disturbance. The FAA Airframe Handbook, chapter 10, explains automatic rudder corrections and feedback. The precise sensors, channels and available authority belong to the particular aircraft installation.

Flaps reshape the wing for slower flight

Trailing-edge flaps increase wing camber, and Fowler-type motion also increases wing area as the assembly moves rearward. Slots allow air to pass between elements and help the flow remain attached at high lift. More extension also adds drag, so the most extended setting is not automatically the most efficient choice for every situation.

The familiar 747-400 arrangement uses triple-slotted trailing-edge flaps. Their tracks, drive mechanisms and supporting structure explain the prominent fairings beneath the wing. A flap is an assembly travelling along a controlled path, not simply a panel pivoting on a single hinge.

Leading-edge devices solve a related problem at the front of the wing. Krueger-type flaps unfold from below the leading edge; variable-camber versions change shape as they deploy. They should not be confused with a conventional slat sliding forward from the upper leading-edge region. The FAA Airframe Handbook, chapter 1, illustrates the different high-lift arrangements.

Selection, movement and indication are separate

The flap lever requests a configuration. Drive systems move the devices, while indications report their positions or system state. The selected position and actual position are different kinds of information: a lever can reach its detent before the moving surfaces finish travelling.

This distinction matters when understanding a fault. Loss of a power source, a jammed mechanism and an incorrect position signal can all prevent the expected result, but they are not the same failure. A useful diagram therefore follows three paths: the command, the supply of energy and the indication returned to the crew.

High-lift devices also rely on more than one aircraft system. Boeing described the 747-8's initial ground testing as hydraulic-system pressurisation alongside pneumatic operation of the leading-edge flaps. It is inaccurate to label every moving device on the wing "hydraulic." See Boeing's power-on report.

What changed on the 747-8?

Boeing's engineering account describes a coordinated redesign: double-slotted inboard and single-slotted outboard trailing-edge flaps, gapped inboard Krueger flaps, and revised variable-camber leading-edge devices. Outboard aileron droop helps low-speed performance.

Electronic control of the outboard ailerons and spoilers allows their response to be tuned and supports manoeuvre load alleviation. That function changes surface commands to reduce structural loading in defined conditions. It does not mean the wing never experiences high loads, nor that every 747 control became fly-by-wire.

Boeing also introduced a double-hinged lower rudder for increased directional authority. These changes belong to the 747-8 design, rather than being universal descriptions of all earlier 747s. See Chris Collisson's Boeing paper, Re-inventing the Legend: The Development of the 747-8, pages 5 - 6.

Redundancy protects functions, not just individual components

The 747's hydraulic systems distribute power to flight-control functions. Multiple circuits and divided surfaces reduce dependence on one supply, but redundant equipment cannot make common physical damage harmless. The FAA's Japan Airlines Flight 123 case study describes a classic 747 accident in which structural damage affected all four hydraulic systems.

That historical case is not a 747-400 pump schematic. Its wider lesson is to ask whether apparently independent systems share a vulnerable location or dependency. Read the companion hydraulic systems guide for the difference between losing a pump and losing a circuit.

Watch the mechanisms, then follow the system

Captain Joe's video below visits a Cargolux maintenance hangar and explains the 747 flap controls, indications and movement. Its chapter markers identify leading-edge flaps at 2:48, trailing-edge flaps at 3:51 and hydraulic power at 5:36. Use it to connect visible hardware with the concepts above; model-specific operating instructions still come from approved aircraft documentation.

Related

• How 747 systems work together
• Hydraulic systems
• Pneumatic and bleed-air system
• The 747-8 wing