Flight control modes (electronic)
Encyclopedia
Aircraft with fly-by-wire
flight controls require computer controlled flight control modes that 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 providing yaw damping
.
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 Airbus
A320
-A380
.
Boeing
's fly-by-wire system is used in the Boeing 777
, Boeing 787 Dreamliner, and Boeing 747-8 Freighter
. Boeing also has a passenger variant of the 747-8, the 747-8 Intercontinental which is still under development, and will too 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.
In older aircraft, control is through the pilot's control column, rudder pedals, trim wheel or throttles that mechanically move cables, pulleys or hydraulic servo valves. These then move control surfaces
or change engine settings.
Many newer aircraft replace these mechanical controls with fly-by-wire
systems. These aircraft have flight control computers which operate control surfaces, inform the pilot and provide performance information. In older aircraft the pilot's mechanical controls are resisted by the forces acting on the control surface, but nothing prevents the aircraft from stalling
, over-speeding or an excessive bank angle at high speed. Fly-by-wire systems limit control surface movements to ensure that aircraft limits are not exceeded.
Another function of flight control laws is to assess the performance of the aircraft under various conditions, such as takeoff, landing or normal cruise when flight control computers partially or completely fail. Designers build in the ability to by-pass the computers or for the standby systems to operate without the computers.
/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 A330, 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 redundant
processing units known as Air Data Inertial Reference Unit
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
s: 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 Law.
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 Law.
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 high-AOA
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 to +30 to -15° which is reduced to 25° as the aircraft slows.
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 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 2 Law 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 disagree. This mode may also be entered with an all spoilers fault, certain ailerons fault, or pedal transducers fault.
DIR is entered if there is failure of three inertial reference units or the primary flight computers, faults in two elevators, flame out in two engines (on a two engine aircraft) or when the captain's primary flight computer is inoperable.
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.
Fly-by-wire
Fly-by-wire 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 computers determine how to move the actuators at each control...
flight controls require computer controlled flight control modes that 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
Aircraft flight control systems
A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight...
(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
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
and providing yaw damping
Yaw damper
A yaw damper is a device used on many aircraft to damp the rolling and yawing oscillations due to Dutch roll mode. It involves yaw rate sensors and a processor that provides a signal to an actuator connected to the rudder...
.
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 Airbus
Airbus
Airbus SAS is an aircraft manufacturing subsidiary of EADS, a European aerospace company. Based in Blagnac, France, surburb of Toulouse, and with significant activity across Europe, the company produces around half of the world's jet airliners....
A320
Airbus A320 family
The Airbus A320 family is a family of short- to medium-range, narrow-body, commercial passenger jet airliners manufactured by Airbus Industrie.Airbus was originally a consortium of European aerospace companies, and is now fully owned by EADS. Airbus's name has been Airbus SAS since 2001...
-A380
Airbus A380
The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the largest passenger airliner in the world. Due to its size, many airports had to modify and improve facilities to accommodate it...
.
Boeing
Boeing
The Boeing Company is an American multinational aerospace and defense corporation, founded in 1916 by William E. Boeing in Seattle, Washington. Boeing has expanded over the years, merging with McDonnell Douglas in 1997. Boeing Corporate headquarters has been in Chicago, Illinois since 2001...
's fly-by-wire system is used in the Boeing 777
Boeing 777
The Boeing 777 is a long-range, wide-body twin-engine jet airliner manufactured by Boeing Commercial Airplanes. It is the world's largest twinjet and is commonly referred to as the "Triple Seven". The aircraft has seating for over 300 passengers and has a range from , depending on model...
, Boeing 787 Dreamliner, and Boeing 747-8 Freighter
Boeing 747-8
The Boeing 747-8 is a wide-body jet airliner developed by Boeing Commercial Airplanes. Officially announced in 2005, the 747-8 is the fourth-generation Boeing 747 version, with lengthened fuselage, redesigned wings and improved efficiency...
. Boeing also has a passenger variant of the 747-8, the 747-8 Intercontinental which is still under development, and will too 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.
Philosophies of design
Aircraft designers have created a set of flight control modes that include redundant electronics to safeguard against system failures. Failures can occur singly or combined to render systems inoperable. Pilots must be able to control the aircraft with some, or even none, of the computational electronics functioning. In the case of Airbus the back-ups are the direct and mechanical modes. Boeing's direct mode removes many of the computational 'limitations'.In older aircraft, control is through the pilot's control column, rudder pedals, trim wheel or throttles that mechanically move cables, pulleys or hydraulic servo valves. These then move control surfaces
Aircraft flight control systems
A conventional fixed-wing aircraft flight control system consists of flight control surfaces, the respective cockpit controls, connecting linkages, and the necessary operating mechanisms to control an aircraft's direction in flight...
or change engine settings.
Many newer aircraft replace these mechanical controls with fly-by-wire
Fly-by-wire
Fly-by-wire 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 computers determine how to move the actuators at each control...
systems. These aircraft have flight control computers which operate control surfaces, inform the pilot and provide performance information. In older aircraft the pilot's mechanical controls are resisted by the forces acting on the control surface, but nothing prevents the aircraft from stalling
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
, over-speeding or an excessive bank angle at high speed. Fly-by-wire systems limit control surface movements to ensure that aircraft limits are not exceeded.
Another function of flight control laws is to assess the performance of the aircraft under various conditions, such as takeoff, landing or normal cruise when flight control computers partially or completely fail. Designers build in the ability to by-pass the computers or for the standby systems to operate without the computers.
Flight control laws (Airbus)
Airbus aircraft designs after the A300Airbus A300
The Airbus A300 is a short- to medium-range widebody jet airliner. Launched in 1972 as the world's first twin-engined widebody, it was the first product of Airbus Industrie, a consortium of European aerospace companies, wholly owned today by EADS...
/A310
Airbus A310
The Airbus A310 is a medium- to long-range twin-engine widebody jet airliner. Launched in July 1978, it was the second aircraft created by Airbus Industrie,a consortium of European aerospace companies, Airbus is now fully owned by EADS and since 2001 has been known as Airbus SAS. the consortium of...
are almost completely controlled by fly-by-wire equipment. These newer aircraft, including the A320
Airbus A320 family
The Airbus A320 family is a family of short- to medium-range, narrow-body, commercial passenger jet airliners manufactured by Airbus Industrie.Airbus was originally a consortium of European aerospace companies, and is now fully owned by EADS. Airbus's name has been Airbus SAS since 2001...
, A330
Airbus A330
The Airbus A330 is a wide-body twin-engine jet airliner made by Airbus, a division of EADS. Versions of the A330 have a range of and can accommodate up to 335 passengers in a two-class layout or carry of cargo....
, A340
Airbus A340
The Airbus A340 is a long-range four-engine wide-body commercial passenger jet airliner. Developed by Airbus Industrie,A consortium of European aerospace companies, Airbus is now fully owned by EADS and since 2001 has been known as Airbus SAS. a consortium of European aerospace companies, which is...
, A350
Airbus A350
The Airbus A350 is a family of long-range, wide-body jet airliners under development by European aircraft manufacturer Airbus.A consortium originally comprising European aerospace companies from the UK, France, Spain and West Germany, Airbus is now fully owned by EADS and since 2001 has been known...
and A380
Airbus A380
The Airbus A380 is a double-deck, wide-body, four-engine jet airliner manufactured by the European corporation Airbus, a subsidiary of EADS. It is the largest passenger airliner in the world. Due to its size, many airports had to modify and improve facilities to accommodate it...
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. While in normal flight the computers act to prevent excessive forces in the pitch and roll.
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
processing units known as Air Data Inertial Reference Unit
Air Data Inertial Reference Unit
An Air Data Inertial Reference Unit is a key component of the integrated Air Data Inertial Reference System , that supplies air data and inertial reference information to the pilots' Electronic Flight Instrument System displays as well as other systems on the aircraft such as the engines,...
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
Air data module
Air Data Module is a component of the navigation system on fly-by-wire aircraft, such as Airbus A320 or later Airbus. Each unit converts pneumatic information from a Pitot tube or a static port in to numerical information which is sent on a data bus...
s: 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 Law.
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.
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 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-AOAAngle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
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.
Flare mode
Radar altimeter
A radar altimeter, radio altimeter, low range radio altimeter or simply RA measures altitude above the terrain presently beneath an aircraft or spacecraft...
indicates 100 feet above ground. At 50 feet the aircraft trims the nose slightly down. During the flare, Normal Law provides high-AOA
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
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 to +30 to -15° which is reduced to 25° as the aircraft slows.
Alternate law
There 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 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 2 Law 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 disagree. 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 stabilisator can only be controlled by the manual trim wheel. All protections are lost, but the maximum deflection of the elevators is changed as a function of the aircraft current centre of gravity.DIR is entered if there is failure of three inertial reference units or the primary flight computers, faults in two elevators, flame out in two engines (on a two engine aircraft) or when the captain's primary flight computer is inoperable.
Mechanical law
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.Boeing 777 Primary Flight Control System
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.