Airbus Flight Control Laws
AIRBUS
FLIGHT CONTROL LAWS |
High AOA Protection |
Load
Factor Limitation |
Pitch
Attitude Protection |
NORMAL
LAW |
High
Speed Protection |
Flight
Augmentation (Yaw) |
Bank
Angle Protection |
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Low
Speed Stability |
Load
Factor Limitation |
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ALTERNATE
LAW |
High
Speed Stability |
Yaw
Damping Only |
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Load
Factor Limitation |
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ABNORMAL
ALTERNATE LAW w/o Speed Stability |
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Yaw
Damping Only |
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FLIGHT
CONTROL LAWS SUMMARY |
NORMAL LAW |
Normal operating configuration
of the system. Failure of any single computer does not affect normal
law.
Covers 3-axis control, flight envelope protection, and load alleviation.
Has 3 modes according to phase of flight. |
Ground
Mode |
- Active when
aircraft is on the ground.
- Direct proportional
relationship between the sidestick deflection and deflection of the
flight controls.
- Is active
until shortly after liftoff.
- After touchdown,
ground mode is reactivated and resets the stabilizer trim to zero.
|
Flight
Mode |
- Becomes
active shortly after takeoff and remains active until shortly before
touchdown.
- Sidestick
deflection and load factor imposed on the aircraft are directly proportional,
regardless of airspeed.
- With sidestick
neutral and wings level, system maintains a 1 g load in pitch.
- No requirement
to change pitch trim for changes in airspeed, configuration, or bank
up to 33 degrees.
- At full
aft/fwd sidestick deflection system maintains maximum load factor for
flap position.
- Sidestick
roll input commands a roll rate request.
- Roll rate
is independent of airspeed.
- A given
sidestick deflection always results in the same roll rate response.
- Turn coordination
and yaw damping are computed by the ELACs and transmitted to the FACs.
- No rudder
pedal feedback for the yaw damping and turn coordination functions.
|
Flare
Mode |
- Transition
to flare mode occurs at 50' RA during landing.
- System memorizes
pitch attitude at 50' and begins to progressively reduce pitch, forcing
pilot to flare the aircraft
- In the event
of a go-around, transition to flight mode occurs again at 50' RA.
|
Protections |
Load
factor Limitation
- Prevents
pilot from overstressing the aircraft even if full sidestick deflections
are applied.
Attitude Protection
- Pitch limited
to 30 deg up, 15 deg down, and 67 deg of bank.
- These limits
are indicated by green = signs on the PFD.
- Bank angles
in excess of 33 deg require constant sidestick input.
- If input
is released the aircraft returns to and maintains 33 deg of bank.
High Angle of Attack
Protection (alpha):
- When alpha
exceeds alpha prot, elevator control switches to alpha protection mode
in which angle of attack is proportional to sidestick deflection.
- Alpha max
will not be exceeded even if the pilot applies full aft deflection
High Speed Protection:
- Prevents
exceeding VMO or MMO by introducing a pitch up
load factor demand.
- The pilot
can NOT override the pitch up command.
Low Energy Warning:
- Available
in CONF 2,3, or FULL between 100' and 2,000' RA when TOGA not selected.
- Produces
aural "SPEED SPEED SPEED" when change in flight path alone
is insufficient to regain a positive flight path (Thrust must be increased).
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ALTERNATE
LAW |
If
Multiple Failures of Redundant Systems occur, the flight controls
revert to Alternate Law.
The ECAM displays the message: ALTN
LAW: PROT LOST |
Ground
Mode |
The
ground mode is identical to Normal Law. |
Flight
Mode |
- In pitch
alternate law the flight mode is a load factor demand law similar to
the Normal Law flight mode, with reduced protections.
- Pitch alternate
law degrades to pitch direct law when the landing gear is extended to
provide feel for flare and landing, since there is no flare mode when
pitch normal law is lost.
- Automatic
pitch trim and yaw damping (with limited authority) is available.
- Turn coordination
is lost.
- When pitch
law degrades from normal law, roll degrades to Direct Law - roll rate
depends on airspeed.
|
Protections |
- All protections
except for load factor maneuvering protection are lost.
- The load
factor limitation is similar to to that under Normal Law.
- Amber XX's
replace the green = attitude limits on the PFD.
- A low speed
stability function replaces the normal angle-of-attack protection
- System introduces
a progressive nose down command which attempts to prevent the speed
from decaying further.
- This command
CAN be overridden by sidestick input.
- The airplane
CAN be stalled in Alternate Law.
- An audio
stall warning consisting of "crickets" and a "STALL"
aural message is activated.
- The Alpha
Floor function is inoperative.
- The PFD
airspeed scale is modified:
- VLS
remains displayed
- VALPHA
PROT and VALPHA MAX are removed
- They are
replaced by a red and black barber pole, the top indicating the stall
warning speed VSW
- A nose up
command is introduced any time the airplane exceeds VMO/MMO
to keep the speed from increasing further, which CAN be overridden by
the sidestick.
- Bank angle
protection is lost.
- Certain
failures cause the system to revert to Alternate Law without speed stability.
- Yaw damping
is lost if the fault is a triple ADR failure.
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ABNORMAL
ALTERNATE LAW |
Abnormal
Alternate Law is activated if the airplane enters an unusual attitude,
allowing recovery from the unusual attitude. |
- Pitch law
becomes Alternate (without autotrim or protection other than Load Factor
protection).
- Roll law
becomes Direct law with mechanical yaw control.
- After recovery
from the unusual attitude, the following laws are active for the remainder
of the flight:
- Pitch: Alternate
law without protections and with autotrim.
- Roll: Direct
law
- Yaw: Alternate
law
- There is
no reversion to Direct law when the landing gear is extended.
|
DIRECT
LAW |
Direct
law is the lowest level of computer flight control and occurs with certain
multiple failures. |
- Pilot control
inputs are transmitted unmodified to the control surfaces, providing
a direct relationship between sidestick and control surface.
- Control
sensitivity depends on airspeed and NO autotrimming is available.
- An amber
message USE
MAN PITCH TRIM
appears on the PFD.
- If the flight
controls degrade to Alternate Law, Direct Law automatically becomes
active when the landing gear is extended if no autopilots are engaged.
If an autopilot is engaged, the airplane will remain in Alternate Law
until the autopilot is disconnected.
- There are
no protections provided in Direct Law, however overspeed and stall aural
warnings are provided.
- The PFD
airspeed scale remains the same as in Alternate Law.
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MECHANICAL
BACKUP |
In
case of a complete loss of electrical flight control signals, the aircraft
can be temporarily controlled by mechanical mode. |
- Pitch control
is achieved through the horizontal stabilizer by using the manual trim
wheel.
- Lateral
control is accomplished using the rudder pedals.
- Both controls
require hydraulic power.
- A red MAN PITCH TRIM ONLY warning appears on the PFD.
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