The switch is located near the point of stagnation on the wing  leading edge. A small lightly sprung tab activates the switch.  At normal AOA, the tab is held down by air that diverges  at the point of stagnation and flows under the wing. This  holds the switch open so the horn does not sound nor the  warning light illuminate. As the AOA increases, the point of  stagnation moves down. The divergent air that flows up and  over the wing now pushes the tab upward to close the switch  and complete the circuit to the horn or light.  [Figure 10-62]  A true AOA indicating system detects the local AOA of the  aircraft and displays the information on a cockpit indicator.  It also may be designed to furnish reference information to  other systems on high-performance aircraft. The sensing  mechanism and transmitter are usually located on the  forward side of the fuselage. It typically contains a heating  element to ensure ice-free operation. Signals are sent from  the sensor to the cockpit or computer(s) as required. An  AOA indicator may be calibrated in actual angle degrees,  arbitrary units, percentage of lift used, symbols, or even fast/ slow.  [Figure 10-63] There are two main types of AOA sensors in common use.  Both detect the angular difference between the relative wind  and the fuselage, which is used as a reference plane. One  uses a vane, known as an alpha vane, externally mounted to  the outside of the fuselage. It is free to rotate in the wind.  As the AOA changes, air flowing over the vane changes its  angle. The other uses two slots in a probe that extends out  of the side of the fuselage into the airflow. The slots lead  to different sides of movable paddles in a chamber of the  unit just inside the fuselage skin. As the AOA varies, the air  pressure ported by each of the slots changes and the paddles  rotate to neutralize the pressures. The shaft upon which the  paddles rotate connects to a potentiometer wiper contact  that is part of the unit. The same is true of the shaft of the  alpha vane. The changing resistance of the potentiometer  is used in a balanced bridge circuit to signal a motor in the  indicator to move the pointer proportional to the AOA.  [Figures 10-64  and  10-65] 10-35 Figure 10-63.  Angle of attack indicator.  Figure 10-62.  A popular stall warning switch located in the wing leading edge.  Switc h t a b h e l d o w n High angle of attack near stall point Low/normal angle of attack Direction of travel Relative wind Relative wind Point of stagnation S w i t c h t a b p u s h ed  up Point of stagnation 0 10 30 A N G L E O F  A T A C K O F F