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Flight Control Laws

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Article Information
Category: Flight Technical Flight Technical
Content source: SKYbrary About SKYbrary
Content control: EUROCONTROL EUROCONTROL


Description

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 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.[2] 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.

Design Philosophy

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.

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