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• Use the digital readout on the MPD when
When performing the fuel control overspeed adjusting/setting the NR/NP. The NR
check, holding the DIGITAL or ANALOG test vertical scale indicator and the MFD NR
switch continuously will cause a constant backup display may indicate 1% higher
fluctuation in engine RPM until the test than the actual NR, which is shown on the
switch is released. MPD readout.
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TM 1-1520-248-10
(5) RPM Trim Switch. The power turbine or NOTE
engine RPM (ENG RPM) trim switch is located on the
pilot collective head. The switch is marked +RPM-. The
trim switch, with throttle full open, adjusts ENG RPM Turning the FADEC circuit breaker switch to
within the range of 96 to 102%. The RPM switch is the OFF position at engine speeds below
effective only in the digital mode. 85% NP may result in intermittent activation
of the FADEC FAIL and FADEC MANUAL
warning messages and audio tones. With
2-24. ENGINE FUEL SYSTEM. NG speeds below approximately 70%
engine may flame out.
a. Full Authority Digital Electronic Control
(FADEC) System. (d) Compressor Inlet Temperature Probe
(CIT). The CIT probe is mounted on the forward wall
The FADEC system is an electronic control with a of the particle separator cowling near the engine water
hydromechanical backup (manual) mode. The FADEC wash line and provides air temperature input to the
provides rotor speed governing, engine torque limiting, ECU. This input allows the ECU to adjust fuel
temperature limiting, automatic start sequencing, schedules based on inlet air temperature.
automatic flameout detection/relight capability, surge
detection/alleviation, and NP and NG speed limiting (e) NR RPM Transducer. The NR RPM
capability while in automatic mode. NP overspeed transducer is located on the aft right side of the
protection is the only feature provided in both the transmission case. This input allows the ECU to adjust
automatic and manual mode. fuel schedules based on main rotor RPM.
(1) FADEC System Hardware.
(2) Automatic Mode. Upon power-up and
(a) Hydromechanical Unit (HMU). The HMU successful completion of BIT, the FADEC system will
(8, fig. 2-12) is engine mounted and driven by the NG operate in the automatic mode. In this mode, the
gear train. It incorporates a fuel metering unit and a FADEC control system is an NG rate of acceleration
fuel pump. controller that regulates the gas producer acceleration
rate and the engine power. The system provides for
(b) Electronic Control Unit (ECU). The ECU precise governing and consistent engine acceleration/
is mounted in the aft electrical compartment and deceleration rates regardless of engine condition.
controls the engine during automatic mode operation by
monitoring, controlling and limiting engine speed, fuel ( 3 ) Manual Mode. The FADEC system
flow, acceleration rate, temperature, torque and other incorporates a hydromechanical manual backup system
engine parameters. During the engine start sequence, (manual mode) to provide a get home capability in the
the ECU will drop off line if supplied power drops below event of a critical ECU failure (hard fault). Manual
10.3 VDC (at the ECU). The ECU will come back on- mode selection also energizes the igniters to a
line during the start sequence at a supplied power of 12 continuous ON status to aid in the prevention of
VDC (at the ECU). flameout. The only FADEC automatic feature available
in the manual mode is the NP overspeed protection. In
( c ) P e r m a n e n t M a g n e t A l t e r n a t o r t h i s mode, t h e p i l o t ’ s t h r o t t l e i n p u t i s t i e d
(PMA). During normal engine operation an engine- hydromechanically to the fuel flow metering window in
mounted, NP-driven PMA (17, fig. 2-12) provides all the HMU. The manual mode is engaged by pressing
required FADEC electrical power at engine speeds the FADEC AUTO/MAN switch (21, fig. 2-6), located
above 85% NP. The aircraft will supply ECU power above the standby airspeed indicator on the instrument
during engine starting and speeds below 85% NP. panel.
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TM 1-1520-248-10
(5) Automatic Start Sequence. The FADEC
system provides automatic sequencing of starter,
ignition, and fuel flow during engine starting while in
the automatic mode. Fuel flow is automatically adjusted
to control NG acceleration rate and limit engine start
temperature. The system also provides a hot start abort
feature that cuts fuel flow off to prevent an
overtemperature condition.
During an automatic start, the starter and igniter are
sequenced OFF by the FADEC system when the
engine reaches 50% NG speed. The system will cut off
fuel flow and re-engage the starter i f turbine
　
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