Airbus A320 Flight Controls Laws

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AutopilotAutopilot(Master FMGC)(Master FMGC)

Fly-Through-ComputerFly-Through-ComputerFly-Through-ComputerFly-Through-Computer

FAC 2FAC 2

FAC 1FAC 1

Flight Augmentation Computers

ELAC 2ELAC 2

ELAC 1ELAC 1

ELevator Aileron Computers

SEC 3SEC 3

SEC 2SEC 2

SEC 1SEC 1 Spoiler Elevator Computers

OR

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ELAC 2ELAC 2

ELAC 1ELAC 1

FAC 2FAC 2

FAC 1FAC 1

SEC 3SEC 3

SEC 2SEC 2

SEC 1SEC 1

Fly-Through-ComputerFly-Through-ComputerFly-Through-ComputerFly-Through-Computer

SEC 1 & 2 provide backup THSSEC 1 & 2 provide backup THSand elevator control.and elevator control.

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The flight control laws are the manufacturer’s built in computer instructions defining flight control movement, flight characteristics, and aircraft limitations.

The flight control laws are a series of instructions that are executed by the flight control computers.

For safety and redundancy there are three flight control laws:

• NORMAL LAW

• ALTERNATE LAW

• DIRECT LAW

Mechanical backup control of the THS and rudder is always available.

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Flight Control LawsFlight Control LawsFlight Control LawsFlight Control Laws

PITCHPITCH ROLLROLL YAWYAW

FLIGHT CHARACTERISTICS


PROTECTIONSPROTECTIONS

LAW: A predetermined set of rules which govern flight characteristicsLAW: A predetermined set of rules which govern flight characteristics

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NORMAL LAWThe flight control law in use during normal operations when

all, or nearly all, aircraft systems are operational.

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=NORMAL==NORMAL= PITCHPITCH ROLLROLL YAWYAW





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=NORMAL==NORMAL= PITCHPITCH

Flight CharacteristicsFlight Characteristics

ProtectionsProtections

Load Factor Demand

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LOAD FACTOR DEMAND

Neutral Stick – Requests NO CHANGE in Gs

The flight control computers maintain approximately 1GPitch attitude remains relatively constant.

Aft Stick – Requests a positive G CHANGE

The flight control computers order the elevators to move the required amount to provide the requested G change, resulting in the nose pitching up.

Forward Stick - Requests a negative G CHANGE

The flight control computers order the elevators to move therequired amount to provide the requested G change, resulting in the nose pitching down.

A specific pitch input results in the same G change regardless of the current airspeed.

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Load Factor Demand

Automatic Pitch Trim

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The Trimmable Horizontal Stabilizer (THS) is trimmed up or down to provide pitch trim.

The THS is a much more active part of the pitch control system on the Airbus than on many other aircraft.

When a G change (pitch change) is commanded, the elevatorsinitially move to provide the commanded change.

If the desired pitch attitude requires the elevators to remaindisplaced, THS movement is commanded until the elevatorsare centered with the THS. Pilot manual pitch trim inputs are not required.

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Load Factor Demand


Ground ModeFlight ModeLanding Mode

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GROUND MODE OF NORMAL LAW

A direct stick to flight control relationship exists.

This enables the pilot to:• Check the flight controls • Rotate the aircraft for takeoff

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FLIGHT MODE OF NORMAL LAW

• Load factor demand in pitch • All protections are available

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LANDING MODE OF NORMAL LAW

The landing mode was created to give the flare and touchdown a “conventional feel”.

At 50 feet AGL (measured by the RAs) the ELACs memorize the pitch attitude

At 30 feet AGL the ELACs add a gentle nose down command to the memorized pitch attitude, which the pilot counters with an aft stick input.

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Load Factor Demand



Maneuver Protection

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Maneuver protection prevents aircraft damage due to overstressing the aircraft. The flight control computers prevent flight control surface movement that would cause the aircraft to exceed preset G limits.

NOTE: MAX = +2.0G IF FLAPS EXTENDED

NOTE: MAX = 0.0G IF FLAPS EXTENDED

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Load Factor Demand



Maneuver ProtectionPitch Protection

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NO FLY

NO FLY

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Load Factor Demand




High Speed Protection

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High speed protection prevents aircraft damage High speed protection prevents aircraft damage due to excessive speed.due to excessive speed.

A gentle pitch up is produced by the flight control computers to limit further acceleration

Acceleration is limited even if full forward stick is applied.

If high speed protection activates, automatic pitch trim in the nose down direction is deactivated and the autopilot, if engaged, disengages.

Activates just above Vmo/Mmo

VMO / MMO

High speed protection activation

It is always possible to overspeed the aircraft. The fight control computers merely limit the maximum attainable speed to within safe limits.

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Load Factor Demand





Angle of Attack Protection

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(ALPHA) = Angle of Attack = the angular difference between the relative wind and the attitude of the aircraft

Relative Wind

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= The angle of attack where the flight control computers intervene to prevent alpha fromreaching a stall alpha.

At Alpha Prot:

• Pitch command logic changes from load factor demand to commanding an angle of attack.

• Additional nose up trim is inhibited - nose down trim is still available.

• The autopilot, if engaged, disengages.

• A lowering of pitch attitude occurs.

If the pilot persists and pulls the stick further aft, eventuallythe angle of attack will reach . . . . . . . . .

Alpha Prot

VLS

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= The highest angle of attack that the ELACs will allow.

ALPHA MAX is a lower alpha than ALPHA STALL; therefore, it is nearly impossible to stall the aircraft in normal law.

If autothrust is available, the airspeed is unlikely to reach Alpha Max because Alpha Floor will probably activate prior to reaching it.

NOTE: Although the FACs continuously compute the airspeeds corresponding with the various alpha protection values and display them on the airspeed tape, these speeds are for pilot reference only.

The actual angles of attack that correspond with Alpha Prot and Alpha Max are computed by the ELACs based on angle of attack, not airspeed.

Alpha Prot

Alpha Max

VLS

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Alpha Floor:

Is a predictive function of the autothrust system. It activates based on the current trend if it predicts thrust will be required.

Is normally available from immediately after takeoff throughout the flight down 100 feet RA in configuration 1 or greater.

Is only available in normal law.

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Excessive high angles of attackIndirectly as a result of

windshear

Excessive high nose up attitudes combined with specific

sidestick inputs

Alpha floor uses the autothrust system to automatically provide TOGA thrust if any of the following conditions occur:

• Excessive high angles of attack.

• Indirectly as a result of windshear.

• Excessive nose up attitudes combined with specific sidestick inputs.

Alpha Floor is predictive and can be triggered at any airspeed; therefore it is NOT depicted on the airspeed tape.

It is independent of Alpha Prot and Alpha Max. In other words, Alpha Floor can be triggered at airspeeds significantly higher than Alpha Prot.

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A. FLOOR

If Alpha Floor activates, TOGA thrust is automatically applied, regardless of thrust lever position.

Alpha Floor activation DOES NOT require that autothrust be ENGAGED, but it must be AVAILABLE (i.e., operational).

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Load Factor Demand


Ground Mode

Flight Mode

Landing Mode

Maneuver Prot

Pitch Protection

Hi Speed Prot

of Attack Prot

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=NORMAL==NORMAL= ROLLROLL



Roll Rate

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Neutral Stick – Requests zero roll rate.

The aircraft essentially maintains the currentbank angle.

ROLL RATE

Full Stick Deflection – Requests 15 / sec roll rate.

Stick Slightly Left / Right of Center- Requests a between 15 and 0 / sec roll rate.

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Roll Rate

Pitch Trim < 33

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Automatic pitch trim is available up to 33 of bank to assist the pilot in maintaining altitude during turns.

33 to 67

Beyond 67

NO FLY

33 to 67

0 to 33 0 to 33

Automatic pitch trim is available

Automatic pitch trim is available

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Roll Rate

Pitch Trim < 33

Bank Angle Hold < 33

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33 to 67

Beyond 67

NO FLY

33 to 67

0 to 33 0 to 33

If the stick is released to neutral, a 0 / second roll rate is commanded.

The aircraft essentially maintains a constant bank angle, unless commanded otherwise.

“Bank angle hold” 0-33°

“Bank angle hold” 0-33°

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Roll Rate

Pitch Trim < 33


Positive Spiral Stability > 33

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33 to 67

Beyond 67

NO FLY

33 to 67

0 to 33 0 to 33

If the bank angle exceeds 33 and the stick is released, bank angle returns to 33.

This called positive spiral stability.

Positive spiral stability

NOTE: Positive spiral stability returns the bank angle to 0° if high speed protection is active.

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Roll Rate

Pitch Trim < 33


Positive Spiral Stability > 33

BANK ANGLE PROTECTION

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33 to 67

Beyond 67

NO FLY

33 to 67

0 to 33 0 to 33

Bank angle protection

In normal, the flight control computes will not allow bank angles in excess of 67.

NOTE: Bank angle is limited to 45° if high speed protection or angle of attack protection is active (the limit marks do not move).

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Load Factor Demand


Ground Mode

Flight Mode

Landing Mode

Maneuver Prot

Pitch Protection

Hi Speed Prot

of Attack Prot

Roll Rate

Pitch Trim

Bank Angle Hold

Pos Spiral Stab

Bank Angle Protection

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=NORMAL==NORMAL= YAWYAW



Turn Coordination

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Yaw orders associated with bank are…

ELAC 2ELAC 2

ELAC 1ELAC 1

…processed by the ELACs then...

The FACs direct the rudder to rudder’shydraulic servos to move the rudder.

Turn coordination is automatic, requiring no pilot rudder input.

There is no rudder pedal movement resulting from turn coordination.

FAC 2FAC 2

FAC 1FAC 1

…transmitted to the FACs.

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Turn Coordination

Yaw Damping

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YAW DAMPINGYAW DAMPING

Two yaw dampers are installed.

A yaw damper moves the entire rudder as necessary to dampen yaw oscillations.

Either yaw damper is capable of providing full yaw damping authority.

There is no rudder pedal movement resulting from yaw damping.

Yaw DampActuators

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Turn Coordination

Yaw Damping

Rudder Trim

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RUDDER TRIM RUDDER TRIM

Rudder trim commands are sent from the FACs to one of two rudder trim motors which move the entire rudder surface. Trim tabs are not used.

When an autopilot is engaged, rudder trim needs are computed by the FACs and automatically carried out. Manual rudder trim is deactivated.

With the autopilots off, the rudder may be trimmed using the RUD TRIM knob on the pedestal.

As the rudder is trimmed (either manually or automatically), the rudder pedals ARE symmetrically displaced.

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Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder (HYD)

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Backup mechanical control of the rudder is always available via cables from the rudder pedals to the rudder servos if at least one hydraulic system is available.

Rudder Pedals

Mechanical Connection

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Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder (HYD)

Rudder Limiting

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RUDDER LIMITINGRUDDER LIMITINGThe FACs, using airspeed inputs from the ADRs, limit rudder surface movement at high speeds to prevent excessive airframe loads and yaw responses.

The two rectangular symbols on the rudder travel indication represent the unrestricted rudder travel limit at low speeds.

NOTE: On some Spirit aircraft the high speed rudder travel limit depiction on the F/CTL page is fixed and does not move with changes in the rudder travel limit.

The maximum rudder travel available at the current speed is indicated by small L shaped indications next to the rudder travel arc.

Rudder PEDAL travel is never restricted. Only rudder SURFACE travel.

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Load Factor Demand


Ground Mode

Flight Mode

Landing Mode

Maneuver Prot

Pitch Protection

Hi Speed Prot

of Attack Prot

Roll Rate

Pitch Trim

Bank Angle Hold

Pos Spiral Stab


Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

x ALTERNATE x

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ALTERNATE LAWA single failure WILL NOT cause the flight control system to degrade

from NORMAL to ALTERNATE LAW.

A minimum of TWO or MORE failures must occur for the flight controls to degrade from NORMAL LAW, usually in the following systems:

x x

Flight Control Computers

SEC 3ELAC 2

ELAC 1

FAC 2

FAC 1SEC 2

SEC 1

Hydraulic Systems

AADD

IIRR

AADD

IIRR

AADD

IIRR

ADIRUs

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Two failures may not cause the flight control system to degrade.

For example:

The aircraft remains in normal law and the autopilots are operational.

SEC 3ELAC 2

ELAC 1

FAC 2

FAC 1SEC 2

SEC 1

ALTERNATE LAWx x

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F / CTL ALTN LAW (PROT LOST) MAX SPEED . . . . . . . . . . . . . . 320 KT

This statement ismisleading.

X X

ALTERNATE LAWx x

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Load Factor Demand


Ground Mode

Flight Mode

Landing Mode







Maneuver Prot

Pitch Protection

Hi Speed Prot

of Attack Prot

Roll Rate

Pitch Trim

Bank Angle Hold

Pos Spiral Stab


Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

x ALTERNATE x

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x ALTERNATE x PITCHPITCH



Load Factor Demand


Flight Mode

Maneuver Protection


of Attack Protection

Stability

Lo Speed Stability

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Stabilities are flight control commands that smoothly attempt to change pitch to prevent an overspeed or a low speed (stall) condition.

Pilots can manually override either stability with stick movement, causing an or a to occur.

activates at a speed slightly below VMO/MMO.

activates if the speed decreases to within a few knots of the stall warning.

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Load Factor Demand


Flight Mode







Maneuver Prot

Hi Speed Stab

Lo Speed Stab

Roll Rate

Pitch Trim

Bank Angle Hold

Pos Spiral Stab


Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

x ALTERNATE x

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Load Factor Demand


Flight Mode


PITCHPITCH YAWYAW





Maneuver Prot

Hi Speed Stab

Lo Speed Stab

Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

ROLLROLL

ROLL DIRECTA direct stick to flight control surface relationship in roll

x ALTERNATE x

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Load Factor Demand


Flight Mode


PITCHPITCH YAWYAW





Maneuver Prot

Hi Speed Stab

Lo Speed Stab

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

USE MAN PITCH TRIM

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*Assuming a double FAC failure was not the cause of the degradation.

x ALTERNATE x x DIRECT x ROLLROLL


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x DIRECT LAW xx DIRECT LAW x

It is possible for the flight control system to degrade from

NORMAL law straight to DIRECT law (e.g., dual radar altimeter

failure).

The primary reason that DIRECT law is reached is because

the aircraft is in ALTERNATE law and the landing gear is

lowered in preparation for landing.

Why?

Because ALTERNATE law does not provide a landing mode.

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DIRECT law results in a direct stick to flight control surface

relationship. What does that really mean?

Stick inputs are still processed by the flight control computers

and transferred to the flight control surfaces.

The computers, however, carry out pilot orders exactly as they

are signaled.

Unlike NORMAL or ALTERNATE laws, the computers have no

authority to modify or override the stick inputs, thus NO

protections OR stabilities are provided.


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USE MAN PITCH TRIM

F / CTL DIRECT LAW (PROT LOST)MAX SPEED . . . . . . . . 320 KT/ .77MANEUVER WITH CAREUSE SPEED BRAKE WITH CARE

This statement is more accurate.

No pitch or roll protections are provided in DIRECT law.


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Load Factor Demand


Flight Mode


PITCHPITCH YAWYAW





Maneuver Prot

Hi Speed Stab

Lo Speed Stab

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

USE MAN PITCH TRIM

x DIRECT x ROLLROLL


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YAWYAW





Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

USE MAN PITCH TRIM

x DIRECT x ROLLROLL


PITCHPITCH

PITCH DIRECTA direct stick to flight control relationship in pitch.

Manual THS Input

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YAWYAW





Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

USE MAN PITCH TRIM

x DIRECT x ROLLROLL


PITCHPITCH

PITCH DIRECTA direct stick to flight control relationship in pitch.

Manual THS Input

Mech. Backup

MAN PITCH TRIM ONLY

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Mechanical BackupMechanical Backup

Mechanical backup control of the THS and rudder is always

available.

Mechanical backup is the only way to maintain aircraft control if

all the flight control computers fail or there is a TOTAL loss of

electrical power.

It either occurs, the sidesticks are inoperative and the aircraft is

controlled using the trim wheels, rudder pedals, and engine

thrust.

Mechanical backup provides a means of aircraft control until a

higher law can be restored.

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No ECAM message specifically indicates that mechanical backup is active.

X X

Mechanical BackupMechanical Backup

F/CTL L + R ELEV FAULT MAX SPEED. . . . . . . . . . . 320/.77 - MAN PITCH TRIM . . . . . . . . USE - SPD BRK . . . . . . . . DO NOT USE

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PITCHPITCH


YAWYAW





Manual Rudder

ROLL DIRECTA Direct Stick to Flt Control Surface Relationship in Roll

Manual THS Input

MAN PITCH TRIM ONLY

Mech. Backup ROLLROLL

PITCH MECHANICAL

PITCH

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YAW





Manual Rudder

Manual THS Input

YAW MECHANICAL

YAW

PITCH MECHANICAL

PITCH

MAN PITCH TRIM ONLY

Mech. Backup

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If extreme conditions cause the aircraft to leave the protected envelope (e.g. severe turbulence), and exceed a normal law limit, abnormal attitude law becomes active.

Abnormal attitude law is alternate law without protections and stabilities except for load factor protection.

This is a safety feature to ensure that the flight control computers never prevent the pilots from recovering from an abnormal attitude.

After recovery, the flight controls remain in alternate law without protections but with auto trim. There is no reversion to direct law when the gear is extended.

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Airbus A320 Flight Controls Laws