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During starts, an absolute 816C limit is set and if exceeded
an engine out indication and shutdown will occur.
If compressor performance deteriorates for any reason,
surge detection automatically allows recovery from compressor
instability while protecting the engine from damage
due to overtemperature.
The FADEC system consists of:
q. The Digital Electronic Control Unit (DECU) includes
a primary mode and a reversionary section for
backup (fig. 2-3-7).
r. The Hydromechanical Metering Assembly
(HMA), includes Hydromechanical Fuel Metering Unit
(HMU) and fuel pump unit for all fuel metering to support
both primary and reversionary fuel metering, a self-contained
alternator for powering the FADEC electronics, a
primary and revisionary compressor bleed air control,
and redundant speed sensing.
s. ENG COND panel (fig. 2-3-5).
t. FADEC control panel (fig. 2-3-6).
u. RPM INC/DEC (Beep) switch THRUST CONT
Lever.
(1) On 714A engine installations, engine beep
switches are only active when in reversionary mode.
(2) Each switch is labeled NO. 1 or NO. 2 which
is used to adjust RRPM when in reversionary mode.
(3) Operation of the beep switches on the
714A in the reversionary mode are the same as for the
712 except that each switch operates respective engine
independently. If only one engine is in reversionary
mode, the RRPM will not change, as it is governed by the
engine in primary mode.
2-3-30. Reversionary System. 714A
NOTE
Aircrew should be alert to the possibility of
abrupt NR and engine power changes when
operating the FADEC in single or dual engine
REV mode (s).
The reversionary (backup mode) automatically takes
control of the engine if the primary mode fails or if selected
by the operator via the FADEC panel, REV switch.
When an engine is operating in reversionary mode,
FADEC provides engine and rotor control through N1
speed governor, beep control, and thrust pitch
compensator.
When both engines are in reversionary mode, RRPM will
require more pilot attention since proportional rotor
speed governor will not hold speed as accurately as the
primary systems. With large collective changes, the rotor
speed can change up to 3 percent from a nominal
setting.
The reversionary system provides the following control
functions:
a. Automatic start sequencing including over temperature
protection, but not start abort.
b. Pilot controlled start fuel enrichment/derichment,
if required, through ECL modulation.
c. Ground idle set 55 5 percent with ECL at GND.
TM 1-1520-240-10
2-3-8
d. RRPM droop compensation based on thrust lever
position.
e. Beep capability becomes active for load match to
other engine.
f. Full contingency power capability.
g. Over temperature protection throughout operation.
h. Engine shutdown in response to ECL being
placed at STOP
i. Tracking of primary mode during normal primary
mode operation allowing a smooth switchover when selected.
If in reversionary mode for any reason (training) and
there is a reversionary failure, the FADEC will not automatically
switch back to the primary mode. The pilot must
manually select PRI mode.
CAUTION
If both the primary and reversionary system
fail, the engine remains at the fuel flow
being used at the time of the failure. When
a failed fixed fuel flow condition exists, the
ECL and the beep trim switch for the affected
engine is inoperative, therefore
there is no proportional control through
ECL except under some conditions STOP.
Engine shutdown may be accomplished
by moving the ECL from its present position
to STOP. Under these conditions, the
ENG 1 (2) FAIL and/or FADEC 1 (2) cautions
are illuminated.
NOTE
If FADEC and/or REV and ENG FAIL lights
are illuminated (which signifies a fuel flow
fixed condition), toggling the PRI/REV switch
reboots the microprocessor in the DECU. If it
is a spurious fault, the lights extinguish except
the REV light. If the hard fault is real, the REV
and ENG FAIL lights illuminate again but not
the FADEC light.
When taking off with one engine in reversionary mode
the procedure is, before lift-off, the engine still in PRI
mode is used to set the correct rotor speed via the
FADEC NR% switch. The operator then uses the beep
switch of the engine in reversionary mode to match engine
torque.
2-3-31. ENG COND Panel. 714A
CAUTION
When adjusting controls or switches on
the overhead switch panel, make sure
gloves or sleeves do not catch and inadvertently
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