When installed in the aircraft, there is sometimes only one  thermocouple lead and indicator for each engine installed  in an aircraft. Cylinder head temperatures are indicated by a gauge  connected to a thermocouple attached to the cylinder, that  tests show to be the hottest on an engine in a particular  installation. The thermocouple may be placed in a special  gasket located under a rear spark plug, or in a special well  in the top or rear of the cylinder head. The temperature recorded at either of these points is merely  a reference or control temperature; but as long as it is  kept within the prescribed limits, the temperatures inside  the cylinder dome, exhaust valve, and piston is within a  satisfactory range. Since the thermocouple is attached to only  one cylinder, it can do no more than give evidence of general  engine temperature. While normally it can be assumed that  the remaining cylinder temperatures are lower, conditions  such as detonation are not indicated unless they occur in the  cylinder that has the thermocouple attached. The cylinder head temperature gauge range marking is  similar to that of the manifold pressure and tachometer  indicator. The cylinder head temperature gauge is a dual  gauge that incorporates two separate temperature scales.  [Figure 10-39]  The scales are calibrated in increments of 10°,  with numerals at the 0°, 100°, 200°, and 300° graduations. The  space between any two graduation marks represents 10 °C. Most torque systems use an oil pressure output from a torque  valve to indicate actual engine power output at various power  settings. The torquemeter indicates the amount of torque being  produced at the propeller shaft. A helical gear moves back  and forth as the torque on the propeller shaft varies. This gear,  acting on a piston, positions a valve that meters the oil pressure  proportionally to the torque being produced. A change in  Turbine engines use percent rpm indicators due to the  high rpm that the engines generally operate. Each rotating  assembly in an engine has its own percent rpm indicator.  The 100 percent position on the indicator is the highest rpm  the engine can operate. Red lines and green arcs operate the  same as with reciprocating engines. During the engine test procedures, the cylinder head  temperatures of various cylinders on the reciprocating  engine are normally tested. Thermocouples are connected  to several cylinders and, by a selector switch, any cylinder  head temperature can be indicated on the indicators.  10-24 Figure 10-39.  Cylinder head temperature gauge.  Figure 10-38.  Tachometer.  Cylinder Head Temperature Indicator Figure 10-37.  Manifold pressure gauge.  Blue Green Red 300 200 0 100 300 200 0 100 CYL. TEMP. HEAD C° 300 200 0 100 300 200 0 100 Torquemeter Green Red MAN. IN. HG. ABS. 10 15 20 25 30 35 40 45 50 55 60 65 70 75 20 25 30 35 40 45 50 55