Aircraft Control Systems

What is an Aircraft Control System? A control system is a collection of mechanical and electronic equipment that allows an aircraft to be flown with exceptional precision and reliability. A control system consists of cockpit controls, sensors, actuators (hydraulic, mechanical or electrical) and computers.

Increases in the control surface area required by large aircraft or higher loads caused by high airspeeds in small aircraft lead to a large increase in the forces needed to move them; consequently complicated mechanical gearing arrangements were developed to extract maximum mechanical advantage in order to reduce the forces required from the pilots. This arrangement can be found on bigger or higher performance propeller aircraft.

Some mechanical flight control systems use Servo tabs that provide aerodynamic assistance. Servo tabs are small surfaces hinged to the control surfaces. The flight control mechanisms move these tabs, aerodynamic forces in turn move, or assist the movement of the control surfaces reducing the amount of mechanical forces needed. This arrangement was used in early piston engine transport aircraft and in early jet transports. The Boeing 737 incorporates a system, whereby in the unlikely event of total hydraulic system failure, it automatically and seamlessly reverts to being controlled via servo-tab.

In large aircraft, the control surfaces are operated by power operated hydraulic actuators controlled by valves moved by control yoke and rudder pedals. An artificial feel system gives the pilot resistance that is proportional to the flight loads on the surfaces. In the event of hydraulic system failure, the control surfaces are controlled by servo tabs in a process known as manual reversion. In the manual mode the flight control column moves the tab on the control surface and the aerodynamic forces caused by the deflected tab moves the main control surface.

Need for Powered Control System: complexity and Weight of the system (Mechanical) increased with Size and Performance of the aircraft. When the pilot’s action is not directly sufficient for the control, the main option is a powered system that assists the pilot. The hydraulic system has demonstrated to be a more suitable solution for actuation in terms of reliability, safety, weight per unit power and flexibility, with respect to the electrical system.

Fly by wire (FBW) control system replaced the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals and transmitted by wires (hence the fly by wire term). Flight control computers determine how to move the actuators at each control surface to provide the ordered response. The fly by wire system also allows automatic signals sent by the aircraft's computers to perform functions without the pilot's input, as in systems that automatically help stabilise the aircraft. The whole system in FBW acts as a closed feedback loop. The current generation of civil airliners exploit FBW control. (E.g. Airbus A319, A320, A330, A340, A380 and Boeing 777 and 787, A320 is the pioneering aircraft of Fly by Wire technology. Millions of flying hours have now been accumulated by aircraft with digital FBW flight control system and their safety and integrity have been established.

We can use an aircraft control system example such as the Auto-Throttle System and explain it's fundamental principle workings.

This invention relates to an auto throttle system for aircraft which are equipped with full authority digital engine controls and more particularly to a system in which a remote friction element provides the "feel" of a cable operated throttle system and at the same time allows a pilot to manually override an autopilot without a significant change in the torque required for advancing or retarding the throttle of the engines.

In modern multi jet engine aircraft, a pilot may manually control the engine power by means of the throttle control levers or quadrants. In such aircraft, the throttle control is integrated with the flight control system and under normal flight conditions controlled thereby. It is essential, however, that in an emergency situation, such as a wind shear warning or tower command, that the pilot immediately take over control of the aircraft. It is also essential that the manual control lever be coupled to the flight control system when the system is in automatic operation so that the lever assumes and maintains the proper setting for any future manual take-over.

An auto throttle system according to the present invention has been designed for aircraft which are equipped with full authority digital engine control, i.e., the so-called "fly by wire" aircraft. The system includes automatic means such as a speed control computer for controlling the power output of one or more engines. A manually operable means such as one or more throttle levers is also provided for controlling the power output of each of the engines. The system also includes a remote friction element or elements which are separated from the manually operable means, which is separated from the pilot's control console and/or throttle lever.

The auto throttle system also includes mechanical means for coupling the automatic means for controlling the power output of an engine and the manually operable means for controlling the power output of an engine through the friction element. The mechanical coupling of the automatic and manual control means through the friction element provides the feel of a cable based throttle system and allows the pilot to manually override the automatic means without significantly increasing the torque needed to advance or retard the throttle.

In a preferred embodiment of the invention, an auto throttle controls the output of a plurality of engines and the manually operable means includes a pilot console and a plurality of throttle control arms, one for each engine. A remote friction element such as a friction clutch is provided for each engine and is separated from the pilot's control console and/or throttle arms. A mechanical coupling connects a shaft which is driven by a servo motor in response to a signal from the automatic speed control to a rotary component which is connected to each of the throttle arms.

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