ATA-27 Flight Controls

Fly-By-Wire SystemsA method of control used by some modern aircraft in which control movement or

pressures exerted by the pilot or directed into a digital computer where they are input into a program tailored to the flight characteristics of the aircraft. The computer output signals are sent to actuators at the control surfaces to move them the optimum amount for the desired maneuver.

Some state of the art modern airplane designs use fly-by-wire systems to connect the flight control surfaces in the cockpit controls with electrical wires rather than with steel cables, push-pull tubes, torque tubes, or other mechanical methods.

The cockpit controls are devices that convert the movements or pressures exerted by the pilot into electrical signals which are sent into a computer programmed with all of the flight characteristics of the airplane.

The computer output is directed through more wires to electro-hydraulic valves that convert the electrical signal into hydraulic fluid flow. This flow changes the position of a main control valve, which directs hydraulic fluid to the appropriate control actuators. Within the actuators, linear variable displacement transducers complete the loop and send feedback signals to the computer, informing it of the amount and speed of actuator movement.

Rather than using a control wheel or stick that actually moves, some fly-by-wire equipped airplanes have sidestick controllers to fly the airplane. Pressures exerted on the controller mounted on the cockpit side console are converted into electrical signals, just as are movements of conventional controls. The General Dynamics F-16 uses a sidestick controller.

Side stick controller is a cock pit flight control used on some of the fly by wire equipped airplanes. The stick is mounted rigidly on the side console of the cockpit and pressure exerted on the stick by the pilot produce electrical signals that are sent to the computer that flies the airplane

Fly-By-Light SystemsWhile fly-by-wire systems offer the significant benefits of reduced aircraft weight,

simplified control routing, and improved control consistency, they do have: one significant drawback—they are susceptible to electromagnetic magnetic interference (EMF). Fly-by-light systems use fiber optic cables rather than wires to transmit the control signals. Digital electrical signals from the computer are convened into light signals and sent through the aircraft via fiber optic cables to electro-optic converters. Here the light signals are changed back to electrical signals for the actuation of the hydraulic control valves.

The weight saving, freedom from EMI and capability of high-speed data transmission ensure that fly-by-light systems will be found on an increasing number of aircraft in the future.

Control Actuation Systems for Large AirplanesThe control forces required by large transport airplanes are too great for a pilot to fly

them manually, so the control surfaces are actually moved by hydraulic servos, or actuators. Figure identifies the Flight control surfaces on a Boeing 727 airplane. We will consider each of these surfaces and the way they are actuated.

The primary flight controls of this airplane consist of inboard and out-board ailerons, elevators, and upper and lower rudders. These controls are operated hydraulically from two independent hydraulic systems, the A system and the B system.

The ailerons and elevators typically are powered from both A and B systems, but either system can operate the controls, which also can be operated manually.

The upper rudder is operated by B system. The lower rudder is operated by A system, and also can be operated by the standby hydraulic system.

There are five flight spoilers on each wing to assist the ailerons in roll control. The three inboard spoilers are operated by B system and the two outboard spoilers are operated by A system. All the flight spoilers plus Iwo ground spoilers on each wing may be operated when the airplane is on the ground and weight is on the landing gear.

The leading edge of the horizontal stabilizer may be raised or lowered with an electrically operated jackscrew. If the electrical actuator should fail, the stabilizer may be positioned manually with a trim wheel.

Hydraulic actuators supplied from A system actuate the leading edge flaps and slats, but if A system should fail, these devices may be extended by the standby hydraulic system.

Roll ControlEach wing has two ailerons that are powered by a dual power unit supplied from both A

and B systems. Either system can give full actuation of the ailerons. Movement of the ailerons is assisted by internal balance panels and balance tabs.

When the flaps are up the outboard ailerons are locked in their faired position, but as the flaps extend, the outboard ailerons become progressively effective, and by the time the outboard flaps have extended 5°, 80% of the outboard aileron travel is available.

If all hydraulic pressure is lost, the tab on the inboard aileron is mechanically linked to the control wheel. This allows the pilot to move the lab in produce aerodynamic forces on the aileron, which deflects it to provide roll control.

The aileron trim control allows the pilots to center the hydraulic power units, which can provide aileron trim when the hydraulic systems are functioning.

The flight spoilers actuate with the ailerons in normal flight to provide roll control by deflecting to a maximum of 30°. They can also be actuated by movement of the speed brake control for deflections between 0° and 45°, depending upon the position of the speed brake handle. When the airplane is on the ground, the ground spoilers extend to their full 45° when the speed brake lever is moved through 10°.

The hydraulic pressure that actuates the flight spoilers will he relieved if the air loads on the spoilers become great enough to stall the actuator. This allows the spoilers to blow down until the airspeed is decreased.

Pitch ControlThe elevators are controlled by two dual hydraulic power units that are supplied by both

A and B systems and controlled by fore-and-aft movement of the control column. The elevator tabs act as balance tabs for normal flight when hydraulic pressure is available, but if hydraulic pressure should fail, the tabs can be moved from the cockpit so that they act as control tabs to produce aerodynamic forces that move the elevators.

A feel computer is incorporated in the elevator system. It senses airspeed, which gives the pilot a progressive restraint on the control column, and indicates the amount of control forces being used.

Pitch trim is provided by varying the angle of incidence of the horizontal stabilizer with a jackscrew that can be actuated electrically or manually.

Yaw ControlThe Boeing 727 has two separate, independent rudders. The upper rudder power unit is

supplied from B system and the lower rudder is operated from A system, or from the standby system. Both rudders have anti-balance tabs.

The rudder system is protected against structural damage in high-speed flight by automatically limiting the hydraulic pressure to the rudder power systems when the trailing edge flaps are retracted.

The rudder pedals, in addition to controlling the rudder, also steer the nose wheel through 8° of travel, but this control may be overridden by the nose wheel steering/wheel.

A yaw damper controls the rudder power systems all the time pressure is available from the main hydraulic systems. Yaw is sensed by the rate gyros in the two turn and slip indicators, and they provide rudder displacement proportional to, but opposite in direction to, the amount of yaw. One rate gyro controls the yaw damper for the upper rudder, and the other controls the yaw damper for the lower rudder. There is no yaw damper action for the lower rudder when it is being operated by the standby system.

Wing FlapsThe two triple-slotted Fowler flaps on each wing's trailing edge are operated by torque

tubes and jackscrews which are powered by separate hydraulic motors for the inboard and outboard flaps. The hydraulic motors are supplied by A system. When the outboard flaps extend 2° the leading edge flaps and slats extend.

In the event of loss of all hydraulic pressure, the flaps may be operated by electric motors which drive the torque tubes, while the hydraulic fluid circulates in the hydraulic motors without causing opposition.

FLIGHT CONTROL PANELFlight Control Switch

∆ OFF - Corresponding hydraulic system pressure ailerons, elevators and rudder shutoff.∆ STDBY RUD (either switch) - Corresponding hydraulic system pressure to ailerons,

elevators and rudder is shutoff. Turns on standby pump, opens standby rudder shutoff valve and pressurizes standby rudder power control unit.

∆ ON (guarded position) - Normal operation.

Flight Control Low Pressure Lights (amber) ON - Indicates low pressure of corresponding hydraulic system to ailerons, elevator and

rudder. MASTER CAUTION light and FLT CONT annunciator illuminate. Deactivated when corresponding flight control switch is positioned to STDBY RUD and the standby rudder shutoff valve is open.

The A system indicates a failure of the pressure reducer to switch back to full system pressure when commanded.

Flight Spoiler Switcho A - Controls inboard flight spoilers shutoff valve.o B - Controls outboard flight spoilers shutoff valve.

Alternate Flap Master Switch ARM - Closes trailing edge flap bypass valve, turns on standby pump, arms alternate

flaps position switch and arms standby hydraulic LOW PRESSURE light. OFF - (guarded position) - Normal operation

Alternate Flap Position Switch Functions only when alternate flaps master switch is in ARM position. DOWN (Momentary) - Extends leading edge devices fully using standby hydraulic

system. When held in DOWN, electrically extends trailing edge flaps. UP - Electrically retracts trailing edge flaps.

Elevator Feel Differential Pressure Light (amber)• ON - Indicates excessive differential pressure in the elevator feel computer with

the trailing edge flaps are up.

Auto Slat Fail Light (amber)• ON - Indicates failure of both auto slat computers

Standby Rudder ON Lighto ON - Indicates the standby hydraulic system is commanded on to pressurize the

standby rudder control unit

Leading Edge Devices Annunciator PanelLights out - Corresponding leading edge device retracted.

• Leading Edge Devices Transit Light (amber)• On - Corresponding leading edge device in transit.• Leading Edge Devices Extended Light (green)

- On - Corresponding leading edge slat in intermediate position.• Leading Edge Devices Full Extended Light (green)

- On - Corresponding leading edge device fully extended.• Annunciator Panel Test Switch

- Press - Tests all annunciator lights.

Speed Brake Armed Light (green)Light deactivated with speed brake lever in DOWN position.

ON - Indicates valid speed brake system inputs.

Speed Brake Do Not Arm Light (amber)Light deactivated with speed brake lever in DOWN position.

• On - Indicates invalid signals or test inputs to automatic speed brake system.• The SPEEDBRAKES EXTENDED light is on the P3 panel.

CENTER PANELHorizontal Stabilizer

∆ The electric trim switches are on the outboard side of each control wheel.

Flap Position Indicator Indicates position of left and right outboard trailing edge flaps and provides trailing

edge flaps asymmetry protection circuit.

Flap Load Relief Light (amber) Indicates activation of flap load relief system.

LE Flap Extended Light (green)o ON - All leading edge flaps extended and all leading edge slats in intermediate position

(Flap positions!, 2, and 5) or, all leading edge devices fully extended (Flap positions 10 through 40).

LE Flaps Transit Light (amber) ON - Any leading edge device in transit, or not in programmed position with respect to

trailing edge flaps.Note: Light is inhibited during auto slat operation in flight.

Stall Warning• The stall warning test switches are on the P5 aft overhead panel.

CENTER CONTROL STANDSpeed Brake Lever

o DOWN (detent) - All flight and ground spoiler panels are in faired position.o ARMED - Automatic speed brake system armed. All flight and ground spoiler panels

extend upon touchdown ( speed brake lever moves to UP position).o FLIGHT DETENT - All flight spoilers extended to their maximum position for flight use.o UP - All flight and ground spoilers are extended to their maximum position for ground

use.NOTE: All spoiler panels retract on the ground if either throttle is advanced for takeoff (speed brake lever moves to DOWN position).

All spoiler panels extend if takeoff is rejected and the reverse thrust levers are positioned for reverse thrust (speed brake lever moves to UP position).

Stabilizer Trim Handle Provided for manual operation of the stabilizer. Overrides any other stabilizer trim

inputs. Handle should be folded inside stab trim wheel for normal operation. Rotates when stabilizer is in motion.

Stabilizer Trim Wheel• Rotates when stabilizer is in motion.

Stabilizer Trim Indicator• Indicates units of airplane trim on the adjacent scale.

Stabilizer green Band Range• Corresponds to allowable range of trim settings for takeoff.

Stabilizer Trim Autopilot Cutout Switch• Cutout - Removes autopilot servo power to stabilizer drive. Autopilot disengages if

engaged.

Stabilizer Trim Main Electric Cutout Switch• Cutout - Removes power from stabilizer main electric trim motor.

Aileron Trim Switches• Movement of both switches repositions the aileron neutral control position. (Spring

loaded to neutral position)

Rudder Trim Control• Electrically trims the rudder in the desired direction. (Spring loaded to neutral position)

Rudder Trim Indicator• Indicates units of rudder trim

Rudder Trim Off Flag (amber)• Indicates loss of electrical power for rudder trim indicator.

Flap Lever• Selects position of flap control valve directing hydraulic pressure for flap drive unit.

Position of leading edge devices is determined by selected trailing edge flap position. At flap [ever position 40, the flap load relief system is armed. This causes automatic flap redaction to flap position 30 or prevents flap extension to flap position 40 in the event of excessive airspeed. The flap lever remains in position 40.

Flap Gates• Prevents inadvertent flap lever movement beyond:

- Position 1 - to check flap position for one engine inoperative go-around.- Position 15 - to check flap position for normal go-around.