Figure 2-18.  Canard wings on a Rutan VariEze. Figure 2-19. The  Beechcraft 2000 Starship has canard wings. These systems incorporate cable assemblies, cable guides,  linkage, adjustable stops, and control surface snubber or  mechanical locking devices. These surface locking devices,  usually referred to as a gust lock, limits the external wind forces  from damaging the aircraft while it is parked or tied down.  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 the aircraft operating in the lowest of the high speed  ranges (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.  Wing fences are flat metal vertical plates fixed to the upper  surface of the wing. They obstruct spanwise airflow along  the wing, and prevent the entire wing from stalling at once.  They are often attached on swept-wing aircraft to prevent the  spanwise movement of air at high AOA. Their purpose is to  provide better slow speed handling and stall characteristics.  [Figure 2-20] The fly-by-wire (FBW) control system employs electrical  signals that transmit the pilot’s actions from the flight deck  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.  Control Systems for Large Aircraft Mechanical Control Wing Fences  Fly-By-Wire Control  Hydromechanical Control Figure 2-20.  Aircraft stall fence. 2-14