Many aircraft have stall warning indicators to warn the pilot of an impending aerodynamic stall. In the past, stall warning indicators were of a pneumatic control type. These devices activated either warning horns or flashing lights. Later, research found that a stall relates directly to the angle of attack, regardless of airspeed, power setting, or aircraft loading. The stall warning devices of most aircraft now in the fleet operate at a specified angle of attack. The devices operate through cams in the AOA indicator. The camdriven switch activates a vibrator motor connected to either a rudder pedal or the control stick. Figure 6-25 shows a simplified schematic of the rudder shaker system. When the aircraft reaches stall angle of attack, the AOA indicator cam-actuated switch completes the rudder shaker motor circuit to ground. When the angle of attack returns below stall conditions, the cam deactuates the switch. The switch action removes the ground from the rudder shaker motor.
