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Flight Control Laws
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Modern large commercial transport aircraft designs rely on sophisticated flight computers to aid and protect the aircraft in flight. These are governed by computational laws which assign flight control modes during flight.
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 Air Data Inertial Reference Unit (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 also has two other commercial aircraft under development, the 787 and the 747-8, 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.
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 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.
Flight Control Laws (Airbus)
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 redundant processing units known as Air Data Inertial Reference Units (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 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 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.
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.
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 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.
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 mode (DIR) loses normal lateral mode and all protections, the aircraft assumes Alternate Law yaw mode and Direct Law roll mode. Elevator can then only be controlled by the manual trim. Control surface motion is directly related to the sidestick and rudder pedal motion. 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.
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 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 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).
Direct mode In Direct mode each ACE decodes pilot commands directly from the pilot controller transducers. This mode can be entered automatically or manually. Automatic entry occurs when all PFCs fail, all ACEs fail, or a control data bus is lost.