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passage of fuel, a bypass valve permits fuel to bypass
the filter. The differential pressure initiating bypass actuates
the fuel-pressure bypass sensor, thus causing
the FUEL BYP ENG 1 or FUEL BYP ENG 2 segment
on the pilot caution/warning panel to illuminate (fig
2-44). An impending filter bypass button on the filter
housing pops out when filter element differential pressure
indicates impending bypass.
2.26.3 Hydromechanical Unit (HMU). The HMU provides
metered fuel to the combustor to control the gas
generator (NG) speed. The HMU contains a high pressure
fuel pump to supply fuel to the metering section.
The HMU responds to mechanical inputs from the
crewmembers through the power available spindle
(PAS) and the load demand spindle (LDS). The PAS is
mechanically connected to the pilot PWR levers while
the LDS is connected to a bellcrank attached to the collective
servo. The HMU regulates fuel flow and controls
positioning of the inlet guide vanes, variable compressor
stage 1 and 2 vanes as well as the anti-ice and start
bleed valve in response to engine inlet air temperature,
compressor discharge air pressure, NG speed, PAS and
LDS positioning, and the ECU 701 or DECU 701C. The
torque motor feedback signals from the HMU to the
ECU/DECU are provided by the linear variable displacement
transducer (LVDT) to complete the control
activated within the HMU at 100- 112% NG speed. The
HMU uses signals from the ECU/DECU to interpret
fuel requirements and to vary fuel flow for automatic
power control. The HMU will additionally provide NG
overspeed protection in the event the gas generator exceeds
108- 112% NG. The reaction of the HMU to an
NG overspeed is the same as for an Np overspeed. Overspeed
protection protects the gas generator turbine
from destructive overspeeds. When an NG overspeed is
sensed, fuel is directed to the MIN pressure valve of the
HMU which causes it to close and shut off fuel to the
engine.
2.26.4 Overspeed and Drain Valve (ODV). The ODV
responds to a signal from the ECU/DECU. Under normal
operation, fuel is routed from the HMU via the oil
cooler and through the ODV to the combustor. When an
overspeed condition is sensed, a signal from the ECU
701 or DECU 701 closes a solenoid in the ODV, thus
routing fuel back into the HMU. All residual fuel is
drained overboard. Fuel flow to the fuel manifold
ceases, and the engine flames out.
2-25
(fig 2-16)
TM 1-1520-238-10
NOTE
terchangeable between -701 and -701C
engines.
(fig 2-16) controls the engine and transmits operational
information to the crew stations. It is a solid-state device
mounted below the engine compressor casing. Powered
by the engine alternator, the ECU receives inputs
from the thermocouple harness, NP sensor, torque and
overspeed sensor, opposite engine torque for load sharing,
NG signal horn the alternator, NP reference signal
from the turbine speed control unit, and a feedback signal
from the HMU for system stabilization. The torquesharing
system increases power on the lower-output
engine to match it with the higher output engine. The
ECU also receives opposite engine torque inputs to enable
contingency power. When this input signal is 51%
torque or below, contingency power is automatically enabled.
However, contingency power is not applied until
the flight crew pulls in collective above 867°C TGT. The
ECU automatically allows the normally operating engine
to increase its TGT limit, thereby increasing its
torque output. The overspeed protection system senses
a separate NP signal independently of the governing
channel. ECU also provides signals to the NP indicator,
TORQUE indicator, and history recorder. In case of the
ECU malfunction, system operation maybe overridden
by momentarily advancing the engine PWR lever to
LOCKOUT and then retarding the lever past the FLY
position to manually control engine power. This locks
out the ECU from all control/limiting functions except
Np overspeed protection, which remains operational. To
remove the ECU horn lockout operation, the engine
PWR lever must be moved to IDLE, then back to FLY.
2.26.6 Digital Electronic Control Unit (DECU)
location as the ECU. The DECU can be overridden like
the ECU by momentarily advancing the engine PWR
lever to LOCKOUT. The DECU, which incorporates
improved technology, performs the same functions as
the ECU except for the following functional and control
improvements.
The DECU can be fully powered by either the engine altemator
or by 400 Hz, 120 vac aircraft power. It incorporates
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