Modern aircraft employ a variety of flight control systems to transmit pilot inputs to the control surfaces. As aircraft became larger, faster, and more complex, control systems evolved from simple mechanical linkages to sophisticated hydraulic and electronic systems. Understanding these control system designs is essential for pilots, technicians, and aviation enthusiasts because they directly affect aircraft handling, performance, reliability, and safety.
Mechanical Control
This is the basic type of system that was used to control early aircraft and is currently used in smaller aircraft where aerodynamic forces are not excessive. The controls are mechanical and manually operated.
The mechanical system of controlling an aircraft can include cables, push-pull tubes, and torque tubes. The cable system is the most widely used because deflections of the structure to which it is attached do not affect its operation. Some aircraft incorporate control systems that are a combination of all three. These systems incorporate cable assemblies, cable guides, linkages, adjustable stops, and control surface snubbers or mechanical locking devices. These surface-locking devices, usually referred to as gust locks, limit the effects of external wind forces that could damage the aircraft while it is parked or tied down.
Hydromechanical Control
As the size, complexity, and speed of aircraft increased, actuation of controls in flight became more difficult. It soon became apparent that the pilot needed assistance to overcome the aerodynamic forces to control aircraft movement. Spring tabs, which were operated by the conventional control system, were moved so that the airflow over them actually moved the primary control surface. This was sufficient for aircraft operating in the lower end of the high-speed range (250–300 mph). For higher speeds, a power-assisted (hydraulic) control system was designed.
Conventional cable or push-pull tube systems link the flight deck controls with the hydraulic system. With the system activated, the pilot’s movement of a control causes the mechanical link to open servo valves, thereby directing hydraulic fluid to actuators, which convert hydraulic pressure into control surface movements.
Because of the efficiency of the hydromechanical flight control system, the aerodynamic forces on the control surfaces cannot be felt by the pilot, and there is a risk of overstressing the structure of the aircraft. To overcome this problem, aircraft designers incorporated artificial feel systems into the design that provided increased resistance to the controls at higher speeds. Additionally, some aircraft with hydraulically powered control systems are fitted with a device called a stick shaker, which provides an artificial stall warning to the pilot.
Fly-By-Wire Control
The fly-by-wire (FBW) control system employs electrical signals that transmit the pilot’s actions from the flight deck through a computer to the various flight control actuators. The FBW system evolved as a way to reduce the weight of flight control systems, lower maintenance costs, and improve reliability. Electronic FBW control systems can respond to changing aerodynamic conditions by adjusting flight control movements so that the aircraft response is consistent for all flight conditions. Additionally, the computers can be programmed to prevent undesirable and dangerous characteristics, such as stalling and spinning.
Many of the new military high-performance aircraft are not aerodynamically stable. This characteristic is designed into the aircraft for increased maneuverability and responsive performance. Without the computers reacting to the instability, the pilot would lose control of the aircraft.
The Airbus A320 was the first commercial airliner to employ a full digital fly-by-wire control system. Boeing subsequently incorporated fly-by-wire systems into the 777 and later commercial aircraft designs. The Dassault Falcon 7X was the first business jet to use a FBW control system.