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 4 C variance. The resistance in the cab-
ling and circuitry between the DECU and TGT gauge is
enough to produce a 5 C variance factor. Applying the
sum of these two factors, the dual engine limiter setting is
allowed a value of 866 9 C (857 – 875 C) and the
single engine (contingency power) limiter setting is al-
lowed a value of 896 9 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 (Np) signal, gas generator
speed (NG) signal, and TGT. When Np and NG 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.
