Air flow Air flow Probe type Probe Vane Flush mounted in side of forward fuselage Air holes Paddle chamber Probe Paddle Separator Potentiometer Air passages Slots Orifice Temperature Measuring Instruments Non-Electric Temperature Indicators 10-36 A bourdon tube is also used as a direct reading non-electric  temperature gauge in simple, light aircraft. By calibrating  the dial face of a bourdon tube gauge with a temperature  scale, it can indicate temperature. The basis for operation is  the consistent expansion of the vapor produced by a volatile  liquid in an enclosed area. This vapor pressure changes  To incorporate a warning of an impending stall, many AOA  systems signal a stick shaker motor that literally shakes the  control column to warn the pilot as the aircraft approaches a  stall condition. Electrical switches are actuated in the AOA  indicator at various preset AOA to activate the motor that  drives an unbalanced weighted ring, causing the column to  shake. Some systems include a stick pusher actuator that  pushes the control yoke forward, lowering the nose of the  aircraft when the critical AOA is approached. Regardless of  the many existing variations for warning of an impending  stall, the AOA system triggers all stall warnings in high  performance aircraft. Direct reading bimetallic temperature gauges are often used  in light aircraft to measure free air temperature or outside air  temperature (OAT). In this application, a collecting probe  protrudes through the windshield of the aircraft to be exposed  to the atmospheric air. The coiled end of the bimetallic strip  in the instrument head is just inside the windshield where it  can be read by the pilot.  [Figures 10-66 and 10-67] Modern aircraft AOA sensor units send output signals  to the ADC. There, the AOA data is used to create an  AOA indication, usually on the primary flight display.  AOA information can also be integrated with flap and slat  position information to better determine the point of stall.  Additionally, AOA sensors of the type described are subject  to position error since airflow around the alpha vane and  slotted probe changes somewhat with airspeed and aircraft  attitude. The errors are small but can be corrected in the ADC. A bimetallic thermometer is very useful in aviation. The  temperature sensing element of a bimetallic thermometer  is made of two dissimilar metals strips bonded together.  Each metal expands and contracts at a different rate when  temperature changes. One end of the bimetallic strip is fixed,  the other end is coiled. A pointer is attached to the coiled end  which is set in the instrument housing. When the bimetallic  strip is heated, the two metals expand. Since their expansion  rates differ and they are attached to each other, the effect is  that the coiled end tries to uncoil as the one metal expands  faster than the other. This moves the pointer across the dial  face of the instrument. When the temperature drops, the  metals contract at different rates, which tends to tighten the  coil and move the pointer in the opposite direction. The physical characteristics of most materials change  when exposed to changes in temperature. The changes are  consistent, such as the expansion or contraction of solids,  liquids, and gases. The coefficient of expansion of different  materials varies and it is unique to each material. Most  everyone is familiar with the liquid mercury thermometer.  As the temperature of the mercury increases, it expands up  a narrow passage that has a graduated scale upon it to read  the temperature associated with that expansion. The mercury  thermometer has no application in aviation.  The temperature of numerous items must be known for  an aircraft to be operated properly. Engine oil, carburetor  mixture, inlet air, free air, engine cylinder heads, heater ducts,  and exhaust gas temperature of turbine engines are all items  requiring temperature monitoring. Many other temperatures  must also be known. Different types of thermometers are used  to collect and present temperature information. Figure 10-64.  A slotted AOA probe and an alpha vane.  Figure 10-65.  The internal structure of a slotted probe airstream  direction detector. 