Carburetor Air Temperature (CAT) Indicator Fuel Pressure Indicator Measured at the carburetor entrance, carburetor air temperature  (CAT) is regarded by many as an indication of induction  system ice formation. Although it serves this purpose, it also  provides many other important items of information. The powerplant is a heat machine, and the temperature of  its components, or the fluids flowing through it, affects  the combustion process either directly or indirectly. The  temperature level of the induction air affects not only the  charge density, but also the vaporization of the fuel. CAT  is also useful for checking induction system condition.  Backfiring is indicated as a momentary rise on the gauge,  provided it is of sufficient severity for the heat to be sensed  at the carburetor air-measuring point. A sustained induction  system fire shows a continuous increase of CAT. The CAT should be noted before starting and just after  shutdown. The temperature before starting is the best  indication of the temperature of the fuel in the carburetor  body and tells whether vaporization is sufficient for the initial  firing, or whether the mixture must be augmented by priming.  If an engine has been shut down for only a short time, the  residual heat in the carburetor may make it possible to rely  on the vaporizing heat in the fuel and powerplant. Priming  would then be unnecessary. After shutdown, a high CAT is a warning that the fuel  trapped in the carburetor will expand, producing high internal  pressure. When a high temperature is present at this time,  the fuel line and manifold valves should be open so that the  pressure can be relieved by allowing fuel passage back to  the tank. The CAT gauge indicates the temperature of the air  before it enters the carburetor. The temperature reading is  sensed by a bulb or electric sensor. In the test cell, the sensor  is located in the air intake passage to the engine and, in an  aircraft, it is located in the ram-air intake duct. The CAT  gauge is calibrated in the centigrade scale.  [Figure 10-35] This gauge, like many other multi-engine aircraft instruments,  is a dual gauge; two gauges, each with a separate pointer and  scale, are used in the same case. Notice the range markings used. The yellow arc indicates a  range from –10 °C to +15 °C, since the danger of icing occurs  between these temperatures. The green range indicates the  normal operating range from +15 °C to +40 °C. The red line  indicates the maximum operating temperature of 40 °C; any  operation at a temperature over this value places the engine  in danger of detonation. The fuel pressure gauge is calibrated in pounds per square  inch (psi) of pressure. It is used during the test run-in to  measure engine fuel pressure at the carburetor inlet, the fuel  feed valve discharge nozzle, and the main fuel supply line.  Fuel gauges are located in the operator’s control room and  are connected by flexible lines to the different points at which  pressure readings are desired during the testing procedures. In some aircraft installations, the fuel pressure is sensed at the  carburetor or fuel injection unit inlet of each engine, and the  pressure is indicated on individual gauges on the instrument  panel.  [Figure 10-36]  The dial is calibrated in graduations  and is extended and numbered. The numbers range from  0 to 10 in this example. The red line on the dial at the 2  pounds psi graduation shows the minimum fuel pressure  allowed during flight. The green arc shows the desired range  of operation, which is 2 to 9 psi. The red line at the 9 psi  graduation indicates the maximum allowable fuel pressure.  Fuel pressures vary with the type of fuel system installation  Yellow Green Red 150 100 50 -50 0 150 100 50 -50 0 C° 50 0 Figure 10-36.  Engine instrument clusters.  Figure 10-35.  Carburetor air temperature gauge.  10-22