![Aeroflot Boeing 777 inflight](https://upload.wikimedia.org/wikipedia/commons/0/0f/Aeroflot_Boeing_777_inflight.jpg)
A flight control mode or flight control law is a computer software algorithm that transforms the movement of the
yoke
A yoke is a wooden beam sometimes used between a pair of oxen or other animals to enable them to pull together on a load when working in pairs, as oxen usually do; some yokes are fitted to individual animals. There are several types of yoke, us ...
or
joystick
A joystick, sometimes called a flight stick, is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. A joystick, also known as the control column, is the principal cont ...
, made by an aircraft pilot, into movements of the aircraft control surfaces. The control surface movements depend on which of several modes the flight computer is in. In aircraft in which the
flight control system is
fly-by-wire
Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control co ...
, the movements the pilot makes to the yoke or joystick in the
cockpit
A cockpit or flight deck is the area, usually near the front of an aircraft or spacecraft, from which a Pilot in command, pilot controls the aircraft.
The cockpit of an aircraft contains flight instruments on an instrument panel, and the ...
, to control the flight, are converted to electronic signals, which are transmitted to the flight control computers that determine how to move each control surface to provide the aircraft movement the pilot ordered.
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
Air Data Inertial Reference Unit An Air Data Inertial Reference Unit (ADIRU) is a key component of the integrated Air Data Inertial Reference System (ADIRS), which supplies air data (airspeed, angle of attack and altitude) and inertial reference (position and attitude) informatio ...
(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
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
and providing
yaw damping.
Two aircraft manufacturers produce commercial passenger aircraft with primary flight computers that can perform under different flight control modes. The most well-known is the system of ''normal'', ''alternate'', ''direct laws'' and ''mechanical alternate control'' laws of the
Airbus
Airbus SE (; ; ; ) is a European Multinational corporation, multinational aerospace corporation. Airbus designs, manufactures and sells civil and military aerospace manufacturer, aerospace products worldwide and manufactures aircraft througho ...
A320
The Airbus A320 family is a series of narrow-body airliners developed and produced by Airbus.
The A320 was launched in March 1984, first flew on 22 February 1987, and was introduced in April 1988 by Air France.
The first member of the famil ...
-
A380
The Airbus A380 is a large wide-body airliner that was developed and produced by Airbus. It is the world's largest passenger airliner and only full-length double-deck jet airliner.
Airbus studies started in 1988, and the project was annou ...
.
The other is
Boeing
The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and product ...
's fly-by-wire system, used in the
Boeing 777
The Boeing 777, commonly referred to as the Triple Seven, is an American long-range wide-body airliner developed and manufactured by Boeing Commercial Airplanes. It is the world's largest twinjet.
The 777 was designed to bridge the gap bet ...
,
Boeing 787 Dreamliner
The Boeing 787 Dreamliner is an American wide-body jet airliner developed and manufactured by Boeing Commercial Airplanes.
After dropping its unconventional Sonic Cruiser project, Boeing announced the conventional 7E7 on January 29, 2003, ...
and
Boeing 747-8
The Boeing 747-8 is a wide-body airliner formerly developed by Boeing Commercial Airplanes, and the largest variant of the 747.
After introducing the 747-400, Boeing considered larger 747 versions as alternatives to the proposed double-deck ...
.
These newer aircraft use electronic control systems to increase safety and performance while saving aircraft weight. These electronic systems are lighter than the old mechanical systems and can also protect the aircraft from overstress situations, allowing designers to reduce over-engineered components, which further reduces the aircraft's weight.
Flight control laws (Airbus)
![Gulfair](https://upload.wikimedia.org/wikipedia/commons/6/6f/Gulfair.a330-200.a40-kc.arp.jpg)
Airbus aircraft designs after the
A300/
A310 are almost completely controlled by fly-by-wire equipment. These newer aircraft, including the
A320
The Airbus A320 family is a series of narrow-body airliners developed and produced by Airbus.
The A320 was launched in March 1984, first flew on 22 February 1987, and was introduced in April 1988 by Air France.
The first member of the famil ...
,
A330
The Airbus A330 is a wide-body aircraft developed and produced by Airbus.
Airbus conceived several derivatives of the A300, its first airliner in the mid-1970s. Then the company began development on the A330 twinjet in parallel with the A340 ...
,
A340
The Airbus A340 is a long-range, wide-body passenger airliner that was developed and produced by Airbus.
In the mid-1970s, Airbus conceived several derivatives of the A300, its first airliner, and developed the A340 quadjet in parallel with ...
,
A350
The Airbus A350 is a long-range, wide-body twin-engine jet airliner developed and produced by Airbus.
The first A350 design proposed by Airbus in 2004, in response to the Boeing 787 Dreamliner, would have been a development of the A330 w ...
and
A380
The Airbus A380 is a large wide-body airliner that was developed and produced by Airbus. It is the world's largest passenger airliner and only full-length double-deck jet airliner.
Airbus studies started in 1988, and the project was annou ...
operate under Airbus flight control laws.
The flight controls on the Airbus A330, for example, are all electronically controlled and hydraulically activated. Some surfaces, such as the rudder, can also be mechanically controlled. In normal flight, the computers act to prevent excessive forces in pitch and roll.
![Airspeed indication system - fly by wire](https://upload.wikimedia.org/wikipedia/commons/f/ff/Airspeed_indication_system_-_fly_by_wire.png)
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
redundant processing units known as
air data inertial reference unit An Air Data Inertial Reference Unit (ADIRU) is a key component of the integrated Air Data Inertial Reference System (ADIRS), which supplies air data (airspeed, angle of attack and altitude) and inertial reference (position and attitude) informatio ...
s (ADIRUs) that 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 module
An air data module is a component of the navigation system. Each unit converts pneumatic (air pressure) information from a pitot tube or a static port into numerical information which is sent on a data bus. This pressure information is received and ...
s: three are linked to
pitot tubes
A pitot ( ) tube (pitot probe) measures fluid flow velocity. It was invented by a French engineer, Henri Pitot, in the early 18th century, and was modified to its modern form in the mid-19th century by a French scientist, Henry Darcy. It is ...
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 pilot's 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 control''.
Normal law
''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 altitude
*Final descent, flare and landing.
During the transition from take-off to cruise there is a 5-second transition, from descent to flare there is a two-second transition, and from flare to ground there is another 2 second transition in ''normal law''.
Ground mode
The aircraft behaves as in direct mode: the autotrim feature is turned off and there is a direct response of the elevators to the sidestick 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''.
Flight mode
The flight mode of ''normal law'' provides five types of protection: pitch attitude, load factor limitations, high speed, high-
AOA and
bank angle
A banked turn (or banking turn) is a turn or change of direction in which the vehicle banks or inclines, usually towards the inside of the turn. For a road or railroad this is usually due to the roadbed having a transverse down-slope towards the ...
. ''Flight mode'' is operational from take-off, until shortly before the aircraft lands, around 100 feet above ground level. It can be lost prematurely 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'' vertical side stick movement corresponds to a load factor proportional to stick deflection 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. Horizontal side stick movement commands a roll rate, and the aircraft 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
In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
is excessive, the load factor exceeds 1.3g, or when the bank angle exceeds 33°.
Alpha protection (α-Prot) prevents stalling and guards against 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, V
MO (maximum operational velocity) and M
MO (maximum operational Mach) the two speeds are the same at approximately 31,000 feet, below which overspeed is determined by V
MO and above which by M
MO.
Flare mode
![Airbus A380](https://upload.wikimedia.org/wikipedia/commons/d/de/Airbus_A380.jpg)
This mode is automatically engaged when the
radar altimeter
A radar altimeter (RA), also called a radio altimeter (RALT), electronic altimeter, reflection altimeter, or low-range radio altimeter (LRRA), measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it t ...
indicates 100 feet above ground. At 50 feet the aircraft trims the nose slightly down. During the
landing flare
The landing flare, also referred to as the round out, is a maneuver or stage during the landing of an aircraft.
The flare follows the final approach phase and precedes the touchdown and roll-out phases of landing. In the flare, the nose of ...
, ''normal law'' provides high-
angle of attack
In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
protection and bank angle protection. The load factor is permitted to be from 2.5g to −1g, or 2.0g to 0g when slats are extended. Pitch attitude is limited from −15° to +30°, and upper limit is further reduced to +25° as the aircraft slows.
Alternate law
There are four reconfiguration modes for the Airbus fly-by-wire aircraft: ''alternate law 1'', ''alternate law 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 alternate law mode.
ALT1 may be entered if there are faults in the horizontal stabilizer, an elevator, yaw-damper actuation, slat or flap sensor, or a single air data reference fault.
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 law 2'' mode is the failure of two air-data references or if the two remaining air data references disagree.
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 disagreement. This mode may also be entered with an all spoilers fault, certain ailerons fault, or pedal transducers fault.
Direct law
''Direct law'' (DIR) introduces a direct stick-to-control surfaces relationship:
control surface motion is directly related to the sidestick and rudder pedal motion.
The trimmable horizontal stabilizer can only be controlled by the manual trim wheel. All protections are lost, and the maximum deflection of the elevators is limited for each configuration as a function of the current aircraft centre of gravity. This aims to create a compromise between adequate pitch control with a forward C.G. and not-too-sensitive control with an aft C.G.
[Airbus A320 AFM (requires page number, publisher, etc)]
DIR is entered if there is failure of three inertial reference units or the primary flight computers, faults in two elevators, or flame-out in two engines (on a two-engine aircraft) when the captain's primary flight computer is also inoperable.
Mechanical control
In the ''mechanical control'' back-up mode, pitch is controlled by the mechanical trim system and lateral direction is controlled by the rudder pedals operating the rudder mechanically.
Boeing 777 primary flight control system
![Boeing 777-200ER cockpit](https://upload.wikimedia.org/wikipedia/commons/d/db/Boeing_777-200ER_cockpit.jpg)
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
In aerodynamics, the flight envelope, service envelope, or performance envelope of an aircraft or spacecraft refers to the capabilities of a design in terms of airspeed and load factor or atmospheric density, often simplified to altitude. The t ...
protection.
The electronic system is subdivided between two levels, the four actuator control electronics (ACE) and the three 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
A backdrive is a component used in reverse to obtain its input from its output. This extends to many concepts and systems from thought based to practical mechanical applications.
Not every system can be backdriven. A DC electrical generator can be ...
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 not precluded."
Normal mode
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.
Secondary mode
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). Moreover, in case of a complete failure of all PFCs and ACEs, the ailerons and selected roll spoilers are connected to the pilot controls by control cable, permitting mechanical control on a temporary basis.
References
{{Aircraft components
Aerospace engineering
Aircraft instruments
Flight control systems
Technology systems