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logic which will eliminate torque spike signals
during engine start-up and shutdown. The DECU contains
an automatic hot start preventer (HSP). The
DECU also provides signal validation for selected input
signals within the electrical control system. Signals are
continuously validated when the engine is operating at
flight idle and above. If a failure has occurred on a selected
input signal, the failed component or related circuit
will be identified by a pre-selected fault code. Fault
codes will be displayed on the engine torque meter (fig
2-18), which defines fault codes in terms of engine
torque. Fault codes will be displayed starting with the
lowest code for four seconds on/two seconds off, rotating
through all codes and then repeating the cycle. The
fault codes will be displayed on the engine torque meter
only when all of the following conditions are met:
l NG less than 20%
l Np less than 35%
l Other engine shutdown
l Aircraft 400 Hz power available
The fault codes can be suppressed by pressing either
OVSP TEST switch. The fault codes can be recalled by
again pressing either OVSP TEST switch. Once a failure
has been identified, the fault code will remain
available for diagnostic indication until starter dropout
on the next engine start.
a. 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 solenoid in the ODV valve. This shuts
off fuel flow and causes the engine to shut down. The
HSP system will not operate if the aircraft 400 Hz power
is not present at the DECU. The HSP system can be
turned off by pressing and holding either OVSP TEST
switch during the engine starting sequence.
2-26 Change 2
TM 1-1520-238-10
Figure 2-18. Signal Validation - Fault Codes
2-27
TM 1-1520-238-10
2.26.7 Fuel Pressure Warning System. The engine
fuel pressure warning system for each engine consists
of a pressure switch that illuminates the FUEL PSI
segment on the pilot caution/warning panel. The
FUEL PSI ENG 1 and FUEL PSI ENG 2 segments
will illuminate when fuel pressure drops below 9 psi.
2.27 ENGINE ALTERNATOR.
2.27.1 Engine Alternator 701. The engine alternator
(fig 2-16 ) supplies AC power to the ignition circuitry
and the ECU. It also supplies a signal to the NG speed
indicator. All essential engine electrical functions are
powered by the alternator. When the alternator power
to the ECU is interrupted, a loss of Np and torque indications
will occur on the affected engine(s), and the
engine(s) will increase to the maximum power. Percent
Np/Nrr will increase above 100% and T4.5 limiting will
be inoperative. When the alternator power providing
the NG signal is interrupted, a loss of NG indications
will occur with a corresponding engine out audio signal
and warning light being activated. Actual engine operation
is unaffected. A complete loss of engine alternator
power results in affected engine(s) increasing to
maximum power with a loss of of Np, NG indication,
torque and engine out audio signal and warning light
being activated, and by an inability to start the engine.
2.27.2 Engine Alternator 701C. When the engine alternator
(fig 2-16) power supply to the DECU is interrupted,
400 Hz, 115 vac aircraft power is used to power
the DECU, therefore preventing an engine (high side)
failure. Np/torque indications will not be affected.
When alternator power supply for the NG signal is interrupted,
a loss of the associated engine NG indication
and an engine out audio signal and warning light will
occur. Actual engine operation is unaffected. Complete
failure of the alternator will cause loss of NG indication,
activation of an engine out audio and warning light,
and inability to start the engine. Operation of the engine
and all other indications will be normal.
2.28 ENGINE OIL SYSTEM.
Each engine is lubricated by a self-contained, pressurized,
recirculating, dry sump system. Included are oil
supply and scavenge pumps, an emergency oil system,
an integral oil tank, a filter, an oil cooler, and seal pressurization
and venting. A chip detector that illuminates
the CHIPS ENG 1 or CHIPS ENG 2 segment on
the pilot caution/warning panel is in line downstream
of the scavenge pump.
2.28.1 Emergency Oil System. Small oil reservoirs,
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