TM 55-1520-240-23-78-1ENGINE INSTRUMENTS(Continued)8-1POWER TURBINE INLET TEMPERATURE ANDCONTINGENCY POWER INDICATING SYSTEM WITH74DescriptionExcept for component locations, the power turbine inlettemperature (PTIT) and contingency power indicatingcircuits are the same for both engines. The PTIT systemuses ten T4.5 thermocouples and a harness to measurethe temperature at the inlet to each engine powerturbine section. The power turbine inlet temperaturesignal is sent to the digital electronic control unit(DECU). An output signal is sent from the DECU to thePTIT indicators on the center instrument panel. Theindicators are hermetically sealed and powered by 28vdc. The indicator has a high impedance so that thewiring harness resistance does not have an effect onthe indication of the PTIT.NO BREAK - WORK HARDERThe contingency power indicating system is operatedfrom the PTIT indicators. When the power turbine inletreaches a temperature of greater than 894ºC on eitherengine, a switched ground circuit is completed at theindicator. This causes the amber ENG CONT PWR lighton the Master Caution Panel to come on. When thePTIT drops below the contingency level, the ENG CONTPWR light goes out.NO BREAK - WORK HARDERTen T4.5 thermocouple probes are mounted in tubesaround the power turbine nozzle. The probes extendinto the gas stream between the vanes of the thirdturbine nozzle. The probes are connected to a harness.Heat at the power turbine inlet causes the probes togenerate a voltage proportional to the heat. The voltageis sent to the DECU and then to the PTIT indicator. Theindicator displays the temperature on a scale calibrated0 to 1,200ºC.8-3
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