Airbus A320 Flight Controls Laws

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Autopilot

(Master FMGC)

Fly-Through-Computer

FAC 2

FAC 1

Flight Augmentation Computers

ELAC 2

ELAC 1

ELevator Aileron Computers

SEC 3

SEC 2

SEC 1 Spoiler Elevator Computers

OR

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ELAC 2

ELAC 1

FAC 2

FAC 1

SEC 3

SEC 2

SEC 1

Fly-Through-Computer

SEC 1 & 2 provide backup THS

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 Laws

PITCH ROLL YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS

LAW: A predetermined set of rules which govern flight characteristics

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NORMAL LAW

The flight control law in use during normal operations when

all, or nearly all, aircraft systems are operational.

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Flight Control Laws

=NORMAL= PITCH ROLL YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


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=NORMAL= PITCH

Flight Characteristics

Protections

Load Factor Demand

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LOAD FACTOR DEMAND

Neutral Stick – Requests NO CHANGE in Gs

The flight control computers maintain approximately 1G

Pitch 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 the

required 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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=NORMAL= PITCH


Protections

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 elevators

initially move to provide the commanded change.

If the desired pitch attitude requires the elevators to remain

displaced, THS movement is commanded until the elevators

are centered with the THS.

Pilot manual pitch trim inputs are not required.

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=NORMAL= PITCH


Protections

Load Factor Demand


Ground Mode

Flight Mode

Landing 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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=NORMAL= PITCH


Protections

Load Factor Demand


Ground Mode

Flight Mode

Landing Mode

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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=NORMAL= PITCH


Protections

Load Factor Demand


Ground Mode

Flight Mode

Landing Mode

Maneuver Protection

Pitch Protection

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NO FLY

NO FLY

30

15

20

=NORMAL= PITCH


Protections

Load Factor Demand


Ground Mode

Flight Mode

Landing Mode

Maneuver Protection

Pitch Protection

High Speed Protection

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High speed protection prevents aircraft damage

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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=NORMAL= PITCH


Protections

Load Factor Demand


Ground Mode

Flight Mode

Landing Mode

Maneuver Protection

Pitch Protection


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 from

reaching 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, eventually

the 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 attack Indirectly 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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Flight Control Laws


FLIGHT

CHARACTERISTICS

PROTECTIONS


Load Factor

Demand

Automatic Pitch

Trim

Ground Mode

Flight Mode

Landing Mode

Maneuver Prot

Pitch Protection

Hi Speed Prot

of Attack Prot

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=NORMAL= ROLL


Protections

Roll Rate

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Neutral Stick – Requests zero roll rate.

The aircraft essentially maintains the current

bank 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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=NORMAL= ROLL


Protections

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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=NORMAL= ROLL


Protections

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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=NORMAL= ROLL


Protections

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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=NORMAL= ROLL


Protections

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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Flight Control Laws


FLIGHT

CHARACTERISTICS

PROTECTIONS


Load Factor

Demand

Automatic Pitch

Trim

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= YAW


Protections

Turn Coordination

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Yaw orders associated with

bank are…

ELAC 2

ELAC 1

…processed by the ELACs then...

The FACs direct the rudder to rudder’s

hydraulic 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 2

FAC 1

…transmitted to the FACs.

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=NORMAL= YAW


Protections

Turn Coordination

Yaw Damping

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YAW 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 Damp

Actuators

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=NORMAL= YAW


Protections

Turn Coordination

Yaw Damping

Rudder Trim

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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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=NORMAL= YAW


Protections

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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=NORMAL= YAW


Protections

Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder (HYD)

Rudder Limiting

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RUDDER LIMITING The 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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Flight Control Laws


FLIGHT

CHARACTERISTICS

PROTECTIONS


Load Factor

Demand

Automatic Pitch

Trim

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

Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

x ALTERNATE x

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ALTERNATE LAW A 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 3 ELAC 2

ELAC 1

FAC 2

FAC 1 SEC 2

SEC 1

Hydraulic Systems

A

D

I

R

A

D

I

R

A

D

I

R

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 3 ELAC 2

ELAC 1

FAC 2

FAC 1 SEC 2

SEC 1

ALTERNATE LAW x x

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F / CTL ALTN LAW

(PROT LOST)

MAX SPEED . . . . . . . . . . . . . . 320 KT

This statement is

misleading.

X X

ALTERNATE LAW x x

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Load Factor

Demand

Automatic Pitch

Trim

Ground Mode

Flight Mode

Landing Mode

Flight Control Laws

PITCH ROLL YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


Maneuver Prot

Pitch Protection

Hi Speed Prot

of Attack Prot

Roll Rate

Pitch Trim

Bank Angle Hold

Pos Spiral Stab

Bank Angle

Protection

Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

x ALTERNATE x

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x ALTERNATE x PITCH


Protections

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

Automatic Pitch

Trim

Flight Mode

Flight Control Laws

PITCH ROLL YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


Maneuver Prot

Hi Speed Stab

Lo Speed Stab

Roll Rate

Pitch Trim

Bank Angle Hold

Pos Spiral Stab

Bank Angle

Protection

Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

x ALTERNATE x

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Load Factor

Demand

Automatic Pitch

Trim

Flight Mode

Flight Control Laws

PITCH YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


Maneuver Prot

Hi Speed Stab

Lo Speed Stab

Turn Coordination

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

ROLL

ROLL

DIRECT

A direct stick to flight

control surface

relationship in roll

x ALTERNATE x

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Load Factor

Demand

Automatic Pitch

Trim

Flight Mode

Flight Control Laws

PITCH YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


Maneuver Prot

Hi Speed Stab

Lo Speed Stab

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

USE MAN PITCH TRIM

*

*Assuming a double FAC failure was

not the cause of the degradation.

x ALTERNATE x x DIRECT x ROLL

ROLL

DIRECT


control surface


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x 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.

x DIRECT LAW x

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USE MAN PITCH TRIM

F / CTL DIRECT LAW

(PROT LOST)

MAX SPEED . . . . . . . . 320 KT/ .77

MANEUVER WITH CARE

USE SPEED BRAKE WITH CARE

This statement is more

accurate.

No pitch or roll protections

are provided in DIRECT law.

x DIRECT LAW x

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Load Factor

Demand

Automatic Pitch

Trim

Flight Mode

Flight Control Laws

PITCH YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


Maneuver Prot

Hi Speed Stab

Lo Speed Stab

Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

USE MAN PITCH TRIM

x DIRECT x ROLL

ROLL

DIRECT


control surface


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Flight Control Laws

YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

USE MAN PITCH TRIM

x DIRECT x ROLL

ROLL

DIRECT


control surface


PITCH

PITCH

DIRECT


control relationship in

pitch.

Manual THS

Input

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Flight Control Laws

YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


Yaw Damping

Rudder Trim

Manual Rudder

Rudder Limiting

USE MAN PITCH TRIM

x DIRECT x ROLL

ROLL

DIRECT


control surface


PITCH

PITCH

DIRECT


control relationship in

pitch.

Manual THS

Input

Mech. Backup

MAN PITCH TRIM ONLY

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Mechanical 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 Backup

F/CTL L + R ELEV FAULT

MAX SPEED. . . . . . . . . . . 320/.77

- MAN PITCH TRIM . . . . . . . . USE

- SPD BRK . . . . . . . . DO NOT USE

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PITCH

Flight Control Laws

YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


Manual Rudder

ROLL

DIRECT

A Direct Stick to Flt

Control Surface

Relationship in Roll

Manual THS

Input

MAN PITCH TRIM ONLY

Mech. Backup ROLL

PITCH

MECHANICAL

PITCH

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Flight Control Laws

YAW

FLIGHT

CHARACTERISTICS

PROTECTIONS


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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Flight Control System Page

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Flight Control System Page

X X X X X X X X X X

1 2 2

1

3

X X X X

X X

X X

X X

X X

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Airbus A320 Flight Controls Laws

Documents

load factor demand

flight control lawslaw

flight control relationship

flight control laws

rollroll ratepitch trim

normal flight characteristics

flight control system

flight control computers