Engine TGT Limiter Function

TM 1-1520-238-10 Change 9 2-26.1/(2-26.2 blank) a. Engine TGT Limiter Function . The DECU incorporates a steady state dual and single engine TGT limiting function which restricts fuel flow within the HMU to prevent an engine over–temperature. The limiting function has an inherent ą4 C variance. The resistance in the cab- ling and circuitry between the DECU and TGT gauge is enough to produce a ą5 C variance factor. Applying the sum of these two factors, the dual engine limiter setting is allowed a value of 866 ą9 C (857 – 875 C) and the single engine (contingency power) limiter setting is al- lowed a value of 896 ą9 C (887 – 905 C). The TGT lim- iter setting for a particular engine can change within these ranges over a period of time. b. Hot Start Prevention (HSP). The HSP system is a part of the DECU and prevents overtemperature during engine start, such as a compressor stall. The HSP system receives power turbine speed (N_p) signal, gas generator speed (N_G) signal, and TGT. When N_p and N_G are below their respective hot start reference, and TGT exceeds 900° C an output from the HSP system activates a sole- noid in the ODV valve. This shuts off fuel flow and causes the engine to shut down. The HSP system will not operate if aircraft 400 Hz power is not present at the DECU. The HSP system can be turned off by pressing and holding ei- ther OVSP TEST switch during the engine starting se- quence.