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surface of the high-pressure turbine blades. This temperature
signal is transmitted to the AFTC and is used
to regulate engine fuel flow, which maintains turbine
blade temperature within limits. Cockpit indications of
turbine blade temperature appear on the MFD.
2.2.1.8 Flame Sensor. The flame sensor is anuitmviolet
radiation sensing unit in the AB duct. During AB
operation, ultraviolet rays detected through a quartz
window activate a gas tilled sensor that electrically
transmits a flame-present signal to the AFTC. Without
this signal, only minimum AB fuel flow is available. AB
will be inhibited ifthe flame sensor fails on. A L/R AUG
acronym is displayed in the ENGINE FAULTS block of
the MFD engine page.
2.2.4.9 Asymmetric Thrust Limiting. The asymmetric
thrust limiting circuit is designed to hold both
engines to minimum AB until both ABs are lit off. The
AFTC releases the hold on the AB when both engine
AB pumps are on and both enginesf lame sensorsa reo n.
Selecting the ASYM LIMITER switch to OFF (guard
cover up) overrides the comparison of left and right AB
status and allows each AB to operate independently.
A malfunctioning or deselected ATLS can
greatly increase the magnitude of asymmetric
thrust because of engine stall or failure.
2.2.1.10 Reduced Arrestment Thrust System.
The RATS is a feature of the AFTC provided to
reducet hrustf or carrierl andingst o a level consistenwt ith
carrier( CV) wind-over-decko perationsW. hen activated,
the AFTC automatically reducest he military power core
speed (Nz) by approximately 4.5 percent. This results in
an approximate2 0- to 25-pementd ecreasein thrust.
RATS employs two enabling circuits: an engine circuit
incorp~mted within each engine’s AFTC, and an aircraft
circuit. The enginec ircuit is enabledb y the aircraftc ircuit
via switch closure. Since the engine circuit is a function of
the AFTC, it is not available in SEC mode and can be
overriddenin PRI mode with selectiono fAB. The aircraft
circuit is enabled when weight is placed on either or both
main landing gears with the hook handle down or the hook
out of the stowed position. The RATS light, located on the
pilot advisory panel, illuminates when the aircraft circuit
is activatedb ut it is not an indicationt hat the enginesa re
operating at reduced thrust.
Note
The RATS light will be illuminated anytime
the aircraft circuit is enabled, even if the engines
are operating in SEC mode or the engine
circuit has been overridden by selection
of AB.
2.2.2 Variable Exhaust Nozzle. Engine exhaust
gases at higher thrust settings are discharged through
the nozzle throat at sonic velocity and are acceleratedto
supersonic velocity by the controlled expansion of the
gases. Varying nozzle throat area controls fan stall margin,
which optimizes performance.
The variablee xhaustn ozzlei s a three-flap,c onvergentdivergent-
typen ozzle.N ozzle variation is accomplished
by axial movement of four hydraulic actuators mechanically
synchronized for geometric stability. These hydraulic
actuatorsu se oil from a separatec ompartmenti n
the engineo il storaget ank and areo peratedb y a hydraulic
pump that responds to AFTC signals. A failed open
nozzle may be caused by an oil leak, but if the leak is in
thenozzJesystem,onlyaportionofthemain enginelube
oil will be lost. During basic engine operation, the nozzle
areai s modulated to a near-closedp osition, and, in AB,
the nozzle area is infinitely variable to a full-open position.
The nozzle will go full open airborne with the
throttle at IDLE at low altitude and airspeeds (Figure
2-13). A gauge for each engine on the pilot instrument
panel next to the engine instruments indicates nozzle
position in percentage from 0 to 100. Normal indication
for maximum AB is approximately 70 percent.
Note
When AFTC is operating in secondary
mode, the nozzle is commanded closed and
the exhaust nozzle indicator is inoperative.
With the landing gear handle down, engine at
IDLE, and weight off wheels, the nozzle is restricted
to a near closed position (maximum 26 percent) to
prevent exhaust nozzle flap contact with the deck/hook
during landing. Five seconds afterweight on wheels, the
nozzle resets to full open to reduce idle power during
landing roliout and while taxiing. On deck in PRI mode
with throttle above IDLE detent, nozzle position varies
linearly with throttle position.
2.3 FATIGUE ENGINE MONITORING SYSTEM
2.3.1 FEMS Functional Description. The FEMS
is a solid-state electronic system that provides data acquisition,
　
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