Fly-By-Wire Flight Control Systems

• The fly-by-wire system is designed to be safe, cost-effective, and pleasant to fly
• Flight control surfaces are electrically-controlled and hydraulically-activated

Basic Principles

• Stabilizer and rudder can also be mechanically controlled
• Pilots use sidesticks for pitch and roll, and indirectly for yaw through turn coordination
• Computers interpret pilot input and move flight control surfaces
• In normal law, computers prevent excessive maneuvers in pitch and roll
• The rudder does not have the same computer protection as other control surfaces

Control Surfaces

• Elevators and Stabilizer control pitch
• Ailerons and Spoilers control roll
• Rudder controls yaw
• Speed brakes are electrically controlled
• All surfaces are hydraulically actuated
• The flight controls are electrically or mechanically controlled

Cockpit Controls

• Each pilot has a sidestick controller for manual control of pitch and roll
• The two sidestick controllers are not mechanically coupled, sending separate signals to the flight control computers
• Two pairs of pedals give the pilot mechanical control of the rudder
• Pilots control speed brakes with a lever on the center pedestal
• Mechanically interconnected handwheels on each side of the center pedestal control the trimmable horizontal stabilizer
• Pilots use a single switch on the center pedestal to set rudder trim
• There is no manual switch for trimming the ailerons

Computers

• Seven flight control computers process pilot and autopilot inputs according to normal, alternate, or direct flight control laws
• 2 ELACs (Elevator Aileron Computer) control normal elevator and stabilizer, and Aileron
• 3 SECs (Spoilers Elevator Computer) control Spoilers, standby elevator and stabilizer
• 2 FACs (Flight Augmentation Computer) control electrical rudder
• 2 FCDCs (Flight Control Data Concentrators) acquire data from ELACs and SECs and send it to the EIS and CFDS

Architecture - General architecture

• Arrows indicate the control reconfiguration priorities
• G, B, and Y indicate the hydraulic power source (green, blue, or yellow) for each servo control

Pitch Control

• Two elevators and the Trimmable Horizontal Stabilizer (THS) control the aircraft in pitch
• Maximum elevator deflection is 30° nose up, 17° nose down
• Maximum THS deflection is 13.5° nose up, 4° nose down

Electrical Control

• In normal operations, ELAC2 controls the elevators and horizontal stabilizer
• Green and yellow hydraulic jacks drive the left and right elevator surfaces
• The THS is driven by N°1 of three electric motors
• If ELAC2 fails, the system shifts pitch control to ELAC1, which controls the elevators via blue hydraulic jacks and THS via the N° 2 electric motor
• If neither ELAC1 nor ELAC2 is available, the system shifts pitch control either to SEC1 or to SEC2, and to THS motor N° 2 or N° 3

Mechanical Control

• Mechanical control of the THS is available from the pitch trim wheel at any time, if either the green or yellow hydraulic system is functioning
• Mechanical control from the pitch trim wheel has priority over electrical control

Actuation (Elevators)

• Two electrically-controlled hydraulic servojacks drive each elevator
• Each servojack has three control modes:
• Active: Jack position is electrically controlled
• Damping: Jack follows surface movement
• Centering: Jack is hydraulically retained in the neutral position
• In normal operation:
• One jack is in active mode, the other in damping mode
• Some maneuvers cause the second jack to become active
• If the active servo jack fails, the damped one becomes active, and the failed jack switches to damping mode
• If neither jack is being controlled electrically, both are automatically switched to centering mode
• If neither jack is being controlled hydraulically, both automatically switch to damping mode
• If one elevator fails, deflection of the remaining elevator is limited

Stabilizer

• A screwjack, driven by two hydraulic motors, drives the stabilizer
• The two hydraulic motors are controlled by:
• One of three electric motors
• The mechanical trim wheel

Roll Control

• One aileron and four spoilers on each wing control the aircraft about the roll axis
• Maximum deflection of the ailerons is 25°
• Ailerons extend 5° down when flaps are extended (aileron droop)
• Maximum deflection of the spoilers is 35°

Electric Control

• ELAC 1 normally controls the ailerons
• If ELAC1 fails, the system automatically transfers aileron control to ELAC2
• If both ELACs fail, the ailerons revert to damping mode
• SEC3 controls the N° 2 spoilers, SEC1 the N° 3 and 4 spoilers, and SEC2 the N° 5 spoilers
• If a SEC fails, the spoilers it controls are automatically retracted

Actuation (Ailerons)

• Each aileron has two electrically controlled hydraulic servo jacks
• One of these servo jacks per aileron operates at a time
• Each servo jack has two control modes:
• Active: Jack position is controlled electrically
• Damping: Jack follows surface movement

Spoilers

• A servojack positions each spoiler
• Each servojack receives hydraulic power from the green, yellow, or blue hydraulic system, controlled by SEC 1, 2, or 3
• The system automatically retracts the spoilers to their zero position if it detects a fault or loses electrical control
• If the system loses hydraulic pressure, the spoiler retains the deflection it had
• When a spoiler surface on one wing fails, the symmetric one on the other wing is inhibited

Speedbrakes and Ground Spoilers - Speed Brake Control

• The pilot controls the speed brakes with the speed brake lever. The speed brakes are spoilers 2, 3 and 4.
• Speed brake extension is inhibited if:
• SEC1 and SEC3 both have faults.
• An elevator (L or R) has a fault (in this case only spoilers 3 and 4 are inhibited).
• Angle-of-attack protection is active.
• Flaps are in configuration FULL.
• Thrust levers are above MCT position.
• Alpha floor activation.
• Inhibitions cause an auto retraction until conditions are normal, and lever is reset
• When one wing fails, the other wing's speedbrake is inhibited

Speedbrake Deflection

• Maximum speed brake deflection in manual flight: 40° for spoilers 3 and 4, 20° for spoiler 2.
• Maximum speed brake deflection with the autopilot engaged: 25° for spoilers 3 and 4, 12.5° for spoiler 2
• Maximum speed brake deflection with the autopilot engaged is achieved with half speed brake lever deflection.
• Roll function has priority; the other wing will retract until the difference between the two surfaces is equal to the roll order

Ground Spoiler Control

• Spoilers 1 to 5 act as ground spoilers
• When a ground spoiler surface on one wing fails, the symmetric one on the other wing is inhibited

Arming

• The pilot arms the ground spoilers by pulling the speedbrake control lever up into the armed position

Full extension

• The ground spoilers automatically extend during rejected takeoff (above 72 knots), or at landing (both main landing gears have touched down):
• Ground spoilers are armed and all thrust levers are at or near idle, or
• Reverse is selected on at least one engine (other thrust lever at or near idle) if ground spoilers were not armed

Partial extension

• The ground spoilers partially extend (10°) when reverse is selected on at least one engine, and one main landing gear strut is compressed

Retraction

• The ground spoilers retract:
• After landing or rejected takeoff when the ground spoilers are disarmed
• During a touch and go when at least one thrust lever is advanced above 20°

Yaw Control

• One rudder surface controls yaw
• The yaw damping and turn coordination functions are automatic

Electrical Rudder Control

• The ELACs compute yaw orders for coordinating turns and damping yaw oscillations, and transmit them to the FACS

Mechanical Rudder Control

• Pilots use conventional rudder pedals to control the rudder

Rudder Actuation

• Three independent hydraulic servojacks, operating in parallel, actuate the rudder
• In automatic operation, a green servo actuator drives all three servojacks. A yellow servo actuator remains synchronized and takes over if there is a failure
• There is no feedback to the rudder pedals from the yaw damping and turn coordination functions

Rudder Travel Limit

• The deflection of the rudder and pedals is limited as a function of speed
• Each channel of the limiter is controlled and monitored by its associated FAC

Rudder Trim

• Two electric motors that position the artificial feel unit also trim the rudder
• In normal operation, motor N° 1, drives the trim, and FAC2 with motor N° 2 remains synchronized as back-up
• In manual flight, the pilot can apply rudder trim with the rotary RUD TRIM switch on the pedestal
• Maximum deflection is ± 20°
• Rudder trim speed is one degree per second
• In addition to limitation by TLU, if rudder trim is applied, maximum rudder deflection may be reduced in the opposite direction

Normal Law - General

• Flight control normal law covers:
• Three-axis control
• Flight envelope protection
• Alleviation of maneuver loads

Pitch Control

• Ground mode is active when the aircraft is on the ground
• Direct relationship between sidestick deflection and elevator deflection, without auto trim
• Automatically sets THS at 0° (inside the green band)
• A setting that the pilot enters manually to adjust for CG has priority for takeoff
• When the aircraft reaches 70 knots, the system reduces the maximum up elevator deflection from 30° to 20°, the aircraft performs the rotation maneuver in direct law
• As soon as the aircraft becomes airborne, the system blends in the flight mode. The reverse process occurs after touchdown

Flight Mode

• The normal-law flight mode is a load-factor-demand mode with automatic trim and protection throughout the flight envelope
• Sidestick controllers set the elevator and THS to maintain load factor proportional to stick deflection
• With the sidestick at neutral, wings level, the system maintains 1 g in pitch, and there is no need for the pilot to trim
• Pitch trim is automatic both in manual mode and when the autopilot is engaged
• Automatic pitch trim freezes:
• The pilot enters a manual trim order.
• The radio altitude is below 50 feet (100 feet with autopilot engaged).
• The load factor goes below 0.5 g.
• The aircraft is under high-speed or high-Mach protection
• When angle-of-attack protection is active, the THS setting is limited between the setting at the aircraft's entry into this protection and 3.5° nose down
• Similarly, when the load factor is higher than 1.25 g or when the aircraft exceeds 33° of bank, the THS setting is limited to values between the actual setting and 3.5° nose down

Control with Autopilot Engaged

• The ELACs and SECs limit what the autopilot can order.
• The pilot has to overcome a restraining force in order to move the sidestick when the autopilot is engaged
• The pilot can also disconnect the autopilot by pushing on the rudder pedals (10° out of trim), or by moving the pitch trim wheel beyond a certain threshold
• All protections of normal laws remain effective

Flare Mode

• The flight mode changes to flare mode when the aircraft passes 50 feet RA
• The system memorizes the attitude at 50 feet, which becomes the initial reference for pitch attitude control
• As the aircraft descends through 30 feet, the system begins to reduce the pitch attitude, reducing it to 2° nose down over 8 seconds

Protections

• Normal law protects the aircraft throughout flight envelope
• Load factor limitation
• Pitch attitude protection
• High-angle-of-attack (AOA) protection
• High-speed protection

Load Factor Limitation

• The load factor is automatically limited to:
• +2.5 g to -1 g for clean configuration
• +2 g to 0 for other configurations

Pitch Attitude Protection

• Pitch attitude is limited to:
• 30° nose up in conf 0 to 3 (progressively reduced to 25° at low speed)
• 25° nose up in conf FULL (progressively reduced to 20° at low speed)
• 15° nose down (indicated by green symbols "=" on the PFD's pitch scale)
• The flight director bars disappear when the pitch attitude exceeds 25° up or 13° down

High Angle of Attack Protection

• When the angle of attack becomes greater than αprot, the system switches to a protection mode
• the angle of attack is proportional to sidestick deflection
• The angle of attack will not exceed αmax, even if the pilot gently pulls the sidestick all the way back
• This protection has priority over all other protections. The autopilot disconnects at α prot + 1°
• Va prot, Va floor, Va max vary according to the weight and the configuration
• To deactivate the angle of attack protection, the pilot must push the sidestick:
• More than 8° forward, or
• More than 0.5° forward for at least 0.5 seconds, when a < a max
• In addition, below 200 feet, the angle of attack protection is also deactivated, when:
• Sidestick deflection is less than half nose-up, and
• Actual a is less than a prot – 2°
• o.floor is activated through the A/THR system.

High-Speed Protection

• The aircraft automatically recovers following a high speed upset
• activated at/or above VMO/MMO
• When activated, the pitch trim is frozen
• Positive spiral static stability is introduced to 0° bank angle
• the aircraft always returns to a bank angle of 0°
• The bank angle limit is reduced from 67° to 45°
• As the speed increases above VMO/MMO, the sidestick nose-down authority is progressively reduced, and a permanent nose-up order is applied

Lateral Control

• When the aircraft is on the ground, the sidestick commands the aileron and roll spoiler surface deflection
• When the aircraft is in flight, normal law combines control of the ailerons, spoilers, and rudder in the sidestick
• The roll rate requested by the pilot during flight is proportional to the sidestick deflection, with a maximum rate of 15° per second
• When the aircraft is in flare mode, the lateral control is the same as in flight mode

Bank Angle Protection

• The system maintains positive spiral static stability for bank angles above 33°
• If the pilot releases the sidestick at a bank angle greater than 33°, the bank angle automatically reduces to 33°
• Up to 33°, the system holds the roll attitude constant when the sidestick is at neutral
• If the pilot holds full lateral sidestick deflection, the bank angle goes to 67° and no further
• If the angle-of-attack protection or high speed protection is operative, the bank angle goes to 45° and no further, if the pilot holds full lateral sidestick deflection
• If high speed protection is operative, the system maintains positive spiral static stability from a bank angle of 0°
• When bank angle protection is active, auto trim is inoperative If the bank angle exceeds 45°, the autopilot disconnects and the FD bars disappear

Sideslip Target

• If one engine fails, the FAC modifies the sideslip indication slightly to show the pilot how much rudder to use to get the best climb performance

Reconfiguration Control Laws - General

• Depending on failures, there are 3 levels of reconfiguration:
• Alternate law
• Direct law, and
• Mechanical
• They are two levels of alternate law: with and without reduced protections

Alternate Law

• Under alternate law the ground mode becomes active on the ground five seconds after touchdown; Is identical to the ground mode of the normal law
• In flight, the alternate law pitch mode follows a load-factor demand law much as the normal law pitch mode does, but it has less built-in protection (reduced protections)
• the flight mode changes to the flare mode when the pilot selects landing gear down
• The flare mode is a direct stick-to-elevator relationship

Lateral Control

• When the aircraft flying in pitch alternate law, lateral control follows the roll direct law associated with yaw alternate or mechanical

Yaw Alternate Law

• Only the yaw damping function is available
• Damper authority is limited to ± 5° of rudder deflection

Reduced Protections

• Load factor limitation is similar to that under normal law
• There is no pitch attitude protection. Amber Xs replace the green double bars " = " on the PFD

Low Speed Stability

• An artificial low speed stability replaces the normal angle-of-attack protection
• Available for all slat/flap configurations
• Active from about 5 knots up to about 10 knots above stall warning speed
• A gentle progressive nose down signal is introduced, which tends to keep the speed from falling below these values
• System also injects bank-angle compensation
• Audio stall warnings (crickets + "STALL" synthetic voice message)
• The PFD speed scale is modified to show a black/red barber pole below the stall warning

High Speed Stability

• Above VMO or MMO, a nose-up demand is introduced to avoid an excessive increase in speed. The pilot can override this demand. Aural overspeed warning remains available
• Bank angle protection is not provided
• AP will disconnect if speed exceeds VMO/MMO, or if the bank angle exceeds 45°

Alternate Law without Reduced Protection

• This is identical to alternate law except that it does not include the low-speed stability or the high-speed stability; it includes only the load factor limitation

Direct Law - Pitch Control

• Pitch direct law is a direct stick-to-elevator relationship
• In all configurations the maximum elevator deflection varies as a function of CG
• There is no automatic trim: the pilot must trim manually
• PFD displays "USE MAN PITCH TRIM" in amber
• No protections are operative
• The a floor function is inoperative
• Overspeed and stall warnings are available as for alternate law

Lateral Control

• When flying in direct law, the roll direct law associated with mechanical yaw control governs lateral control

Roll Direct Law

• Roll direct law is a direct stick-to-surface-position relationship
• System gains are set automatically to correspond to slat/flap configuration
• The maximum roll rate is about 30° per second with aircraft is clean
• With slats extended, it is about 25° per second
• To limit roll rate, the roll direct law uses only ailerons and spoilers N° 4 and 5

Yaw Mechanical Control

• Pilots control yaw with rudder pedals
• Yaw damping and turn coordination functions are lost

Abnormal Attitude Laws

• Applied if limits are exceeded
• Pitch attitude > 50° nose up or 30° nose down
• Bank angle > 125°
• Angle of attack > 30° or < -10° (-15° for A319 and A321)
• Speed > 440 knots or <
• The law in pitch is the alternate law with no protection except load-factor protection and without auto trim. In roll it is a full-authority direct law with yaw mechanical
• When the aircraft has recovered from its abnormal attitude, the flight control laws in effect are: in pitch: alternate law without protection with autotrim in roll: full authority direct law with yaw alternate law
• There is no reversion to the direct law when the pilot extends the landing gear

Mechanical Back-Up

• Permits pilots to control the aircraft during a temporary complete loss of electrical power
• in pitch by manually applying trim to the THS
• PFDs display “MAN PITCH TRIM ONLY” in red
• Laterally the pilot uses the rudder pedals

Controls and Indicators - Pedestal - RUD TRIM Rotary Switch

• Controls the rudder trim actuator, which moves the neutral point of the artificial feel by the equivalent of one degree of rudder travel per second
• RESET Pushbutton: by pushing, the zero trim position is ordered at 1.5° / second

Position Indicator

• Displays rudder trim direction (L or R) and value (0 to 20°)

SPEED BRAKE Lever

• The position of the speed brake surfaces
• The manual preselection of the ground spoilers

Pitch Trim Wheel

• Both pitch trim wheels provide mechanical control of the THS and have priority over electrical control
• A pilot action on the pitch trim wheel disconnects the autopilot
• The THS is manually controlled on ground for the THS setting, before takeoff and in flight, when in direct law
• In flight, when in direct law, the pilot uses the THS conventionally to fly in trim
• Following nosewheel touchdown, as the pitch attitude becomes less than 2.5° for more than 5 seconds, pitch trim is automatically reset to zero

Lateral Consoles - Sidesticks

• Each pilot has a sidestick to manually control pitch and roll; each sidestick is springloaded to neutral
• When the autopilot is engaged, a solenoid-operated detent locks both sidesticks in the neutral position; if the pilot applies enough force, the stick becomes free and the autopilot disengages
• The hand grip has two switches:
• Autopilot disconnect and sidestick takeover pushbutton Push-to-talk button

Sidestick Priority Logic

• When only one pilot operates the sidestick, it sends control signals to the computers
• When pilots move both side stick simultaneously in the same or opposite direction and neither takes priority, the system adds the signals of both pilots algebraically
• A pilot can take full control by pressing and keeping pressed his priority takeover pushbutton

In a priority situation

• A red light comes on in front of the pilot whose stick is deactivated
• A green light comes on in front of the pilot who has taken control, if the other stick is not in the neutral position (to indicate a potential and unwanted control demand)
• If the aircraft is on the ground and commencing its takeoff run and one stick is deactivated, this triggers the takeoff "CONFIG" warning

Overhead Panel

• ELAC 1(2) pushbutton controls the Elevator and Aileron Control (ELAC) Computer 1(2)
• It performs: normal pitch and roll, alternate pitch, direct pitch and roll, abnormal attitude, aileron droop, and acquisition of autopilot orders The corresponding computer is not active when switched OFF (resets the computer if switched OFF) If a failure is detected or has power issues the light comes along with an ECAM caution

SEC 1(2)(3) pushbutton

• Controls the spoiler and elevator (SEC) computers 1(2)(3)
  • performs: normal roll (by controlling the spoilers) Speedbrakes and ground spoilers, alternate pitch (SEC 1 and SEC 2 only), direct pitch (SEC 1 and SEC 2 only), direct roll, abnormal attitude
• Computer is not active when switched off (resets the computer if switched OFF) If a failure is present the FAULT light comes on along with an ECAM cautionary

FAC 1(2) pb sw

• Controls the flight augmentation computer (FAC) 1(2) Both FACs perform: normal roll (coordinating turns and damping dutch roll), rudder trim, rudder travel limit,alternate yaw The corresponding computer is not active

Controls and Indicators - Side Stick Indications on PFD

• On the ground, after the first engine start, sidestick position indications appear white on both PFDs
• The indications disappear when the aircraft goes from the ground into flight

ECAM F/CTL PAGE - Spoilers/speed brakes indication:

• SPOILER DEFLECTED BY MORE THAN 2.5° (GREEN) or spoiler retracted (green)
• SPOILER FAULT DEFLECTED (AMBER)
• Hydraylic system pressure indication (normally green) Changes to amber indicates a failure with the ECAM and other lights

Controls and Indicators - Aileron position indication

White scale and green index; It changes to amber, when neither (green nor blue) servojack is available.

Aileron and elevator actuator indication

• G and B are normally green
• The color changes to amber, in case of a green or blue hydraulic system low pressure
• The partial box also changes to amber, if the associated computer or actuator fails

ECAM Wheel Page - Spoilers/Speed Brakes Indication

• These indications show the same as the display on the FLT CTL page

Flaps and Slats - Description

Each wing has lift augmentation devices: two flap and five slat surfaces. Surfaces are electrically-controlled and hydraulically operated via the FLAPS lever.

Main Components

• Two slat flap control computers (SFCCs)
• A power control unit (PCU) with two independent hydraulic motors coupled by a differential gearbox
• Pressure-off brakes (POBs) that lock transmission when a position is reached
• Five slat surfaces and two flap surfaces per wing
• An asymmetry position pick-off unit (APPU)
• A flap disconnect detection system
• Wingtip brakes (WTBs), activated in asymmetry, mechanism overspeed, symmetrical runaway, The cannot be released in flight
• Feedback position pick-off units (FPPUs) that feed positions to the SFCCs
• Indication position pick-off unit (IPPU) data to ECAM
• If one SFCC is inoperative, speed is set to half
• If one hydraulic system is inoperative, slats and flaps operate similarly.

Flaps and Slats - Architecture

Displays information in chart and diagrammatic format

Configurations

• The FLAPS lever has five positions: 0, 1, 2, 3 and FULL
• Two configurations correspond to position 1: Configuration 1 and Configuration 1 + F

Alpha/Speed Lock Function

• This function inhibits slat retraction at high angles of attack and low speeds
• SFCCs use corrected angle-of-attack (alpha) or airspeed data
• If alpha exceeds 8.6° or airspeed falls below 148 knots, retraction from position 1 to position 0 is inhibited
• The inhibition is removed when alpha falls below 7.6° or the speed exceeds 154 knots
• Function is not active is alpha exceeds 8.6° or airspeed is below the limit, or craft is on the ground with a speed than 60 knots.

Controls and Indicators - Pedestal- Flaps Lever

• The lever selects simultaneous operation of the slats and flaps that align to surface positions
• A balk in position prevents over excessive travel with a single action

Takeoff

• Configuration 1 selects 1 + F (18°/10°); flap retract automatically is 210 KIAS
• Configurations 2 and 3 retract if gear is down, the speed brakes are not retracted the trim is aligned and etc.

ECAM Display

• Upper display contains points on where to select in the selection position
• This also is displayed if the Flaps has fault is under maintenance.

Electrical Supply

• Electrical supply relays to different parts of the aircraft