Aircraft with fly-by-wire flight
controls require computer controlled
flight control modesthat are capable of determining the operational
mode (computational law) of the aircraft.
A reduction of electronic flight control can be caused by the failure of
a computational device, such as the flight control computer or an
information providing device, such as the ADIRU.
Electronic flight control systems (EFCS)
also provide augmentation in normal flight, such as increased protection
of the aircraft from overstress or providing a more comfortable flight
for passengers by recognizing and correcting for turbulence and
Two aircraft manufacturers produce commercial passenger aircraft with
primary flight computers that can perform under different flight control
modes (or laws). The most well-known are the Normal,
Alternate, Direct and Mechanical
Laws of the AirbusA320-A380.
Boeing's fly-by-wire system is used in the Boeing
also has two other commercial aircraft under development, the 787 and
which will use fly-by-wire controls.These newer generation of aircraft use the lighter weight electronic
systems to increase safety and performance while lowering aircraft
weight. Since these systems can also protect the aircraft from
overstress situations, the designers can therefore reduce
over-engineered components, further reducing weight.
Airbus aircraft designs after the A300/A310 are
almost completely controlled by fly-by-wire equipment. These newer aircraft,
including the A320, A330, A340, A350 and A380 operate
under Airbus flight control Laws. The
flight controls on the Airbus 330, for example, are all electronically
controlled and hydraulically activated. Some surfaces, such as the rudder, can
also be mechanically controlled. While in normal flight the computers act to
prevent excessive forces in the pitch and roll.
The aircraft is controlled by three primary control computers (Captain's, First
Officer's and Standby) and two secondary control computers (Captain's and First
Officer's). In addition there are two Flight Control Data Computers (FCDC) that
read information from the sensors, such as air data (airspeed, altitude). This
is fed along with GPS data, into three processing units known as ADIRUs (Air
data/inertial reference units) which act both as an air data reference and
inertial reference. ADIRUs are part of the air data inertial reference system,
which, on the Airbus is linked to eight air
data modules: three are linked to pitot tubes and five are linked to static
sources. Information from the ADIRU is fed into one of several flight control
computers (Primary and secondary flight control). The computers also receive
information from the control surfaces of the aircraft and from the pilots
aircraft control devices and autopilot. Information from these computers is sent
both to the pilot's primary flight display and also to the control surfaces.
There are four named flight control laws, however Alternate
Law consists of two modes, Alternate
Law 1 and Alternate
Law 2. Each of these modes have different sub modes: ground mode, flight
mode and flare, plus a back-up Mechanical
Normal Law differs depending
on the stage of flight. These include:
- Stationary at the gate
- Taxiing from the gate to a runway or from
a runway back to the gate
- Beginning the take-off roll
- Initial climb
- Cruise climb and cruise flight at
- Final descent, flare and landing.
Normal Law is different
depending on the stage of flight. During the transition from take-off to cruise
there is a 5 second transition, from descent to flare there is a two second
delay and from flare to ground there is another 2 second transition in Normal
The aircraft behaves as in direct mode: The autotrim feature is turned off and
there is a direct response of the elevators to control column (or sidestick on
Airbus) inputs. The horizontal stabilizer is set to 4° up but manual settings
(e.g. for center of gravity) override this setting. After the wheels leave the
ground, a 5 second transition occurs where Normal
Law - flight mode takes over
from ground mode.
The flight mode of Normal Law provides
five types of protection: Pitch attitude, load factor limitations, high speed,
high-AOA and bank
angle. Flight mode is operational from take-off to 100 feet above the
ground, but can be lost as a result of pilot commands or system failures. Loss
of Normal Law as
a result of a system failure results in Alternate
Law 1 or 2.
Unlike conventional controls, in Normal
Law flight mode the sidestick
provides a load factor proportional to stick deflection which is independent of
aircraft speed. When the stick is neutral and the load factor is 1g the aircraft
remains in level flight without the pilot changing the elevator trim. The
aircraft also maintains a proper pitch angle once a turn has been established,
up to 33° bank. The system prevents further trim up when the angle of attack is
excessive, the load factor exceeds 1.3g or when the bank angle exceeds 33°.
Alpha protection (α-Prot) prevents stalling and the effects of windshear. The
protection engages when the angle of attack is between α-Prot and α-Max and
limits the angle of attack commanded by the pilot's sidestick or, if autopilot
is engaged, it disengages the autopilot.
High speed protection will automatically recover from an overspeed. There are
two speed limitations for high altitude aircraft, VMO (Velocity
Maximum Operational) and MMO (Mach
Maximum Operational) the two speeds are the same at approximately 31,000 feet,
below which overspeed is determined by VMO and
above 31,000 feet by MMO.
This mode is automatically engaged when the radar
altimeter indicates 100 feet
above ground. At 50 feet the aircraft trims the nose slightly down. During the
flare, Normal Law provides
and bank angle protection. The load factor is permitted to be from 2.5g to -1g,
or 2.0g to 0g with slats are extended. Pitch attitude is limited to +30 to -15°
which is reduced to 25° as the aircraft slows.
The are four reconfiguration modes for the Airbus fly-by-wire aircraft, two Alternate
Law (1 and 2),Direct Law and Mechanical
Law. The ground mode and flare modes for Alternate
Law are identical to those modes
for Normal Law.
Alternate law 1 (ALT1) mode
combines a Normal Law lateral
mode with the load factor, bank angle protections retained. High angle of attack
protection may be lost and low energy (level flight stall) protection is lost.
High speed and high angle of attack protections enter alternative law mode.
ALT1 maybe entered if there are faults in the horizontal stabilizer, an
elevator, yaw-damper actuation, slat or flap sensor, or a single air data
Alternate law 2 (ALT2) loses Normal
Law lateral mode (replaced by
roll direct mode and yaw alternate mode) along with pitch attitude protection,
bank angle protection and low energy protection. Load factor protection is
retained. High angle of attack and high speed protections are retained unless
the reason for Alternate 2 Law mode
is the failure of two air-data references or if the two remaining air data
ALT2 mode is entered when 2 engines flame out (on dual engine aircraft), faults
in two inertial or air-data references, with the autopilot being lost, except
with an ADR disagree. This mode may also be entered with an all spoilers fault,
certain ailerons fault, or pedal transducers fault.
In the Mechanical Law back-up
mode pitch is controlled by the mechanical trim system and lateral direction is
controlled by the rudder pedals operating the rudder mechanically.
777 Primary Flight Control System
The fly-by-wire electronic flight control system of the Boeing 777 differs from
the Airbus EFCS. The design principle is to provide a system that responds
similarly to a mechanically controlled system. Because
the system is controlled electronically the flight control system can provide
flight envelope protection.
The electronic system is subdivided between 2 levels, the 4 actuator control
electronics (ACE) and the 3 primary flight computers (PFC). The ACEs control
actuators (from those on pilot controls to control surface controls and the PFC).
The role of the PFC is to calculate the control laws and provide feedback
forces, pilot information and warnings.[Standard
Protections and augmentations
The flight control system on the 777 is designed to restrict control authority
beyond certain range by increasing the back pressure once the desired limit is
reached. This is done via electronically controlled backdrive actuators
(controlled by ACE). The protections and augmentations are: bank angle
protection, turn compensation, stall protection, over-speed protection, pitch
control, stability augmentation and thrust asymmetry compensation. The design
philosophy is: "to inform the pilot that the command being given would put the
aircraft outside of its normal operating envelope, but the ability to do so is
In Normal mode the
PFCs transmit actuator commands to the ACEs, which convert them into analog
servo commands. Full functionality is provided, including all enhanced
performance, envelope protection and ride quality features.
Boeing Secondary mode is
comparable to the Airbus Alternate
Law, with the PFCs supplying commands to the ACEs. However, EFCS
functionality is reduced, including loss of flight envelope protection. Like the
Airbus system, this state is entered when a number of failures occur in the EFCS
or interfacing systems (e.g. ADIRU or SAARU).Direct
In Direct mode each
ACE decodes pilot commands directly from the pilot controller transducers. This
mode can be manually or automatically entered. Automatic entry occurs when there
is a failure of all PFCs, ACEs, and/or loss of a control data bus.