OPERATING PRINCIPLES
nozzle (VEN) control system (fig. 5-17). This is
the control system for the F-18 aircraft, and
The VEN schedule is in response to movement
it is a converging-diverging nozzle system.
of the throttle. Throttle setting repositions the
When operating, this system varies the exhaust
power lever angle (PLA) cam in the MFC,
escape area size to obtain desired thrust, while
providing a linear variable differential transformer
maintaining safe operating conditions throughout
(LVDT) signal to the ECA. The ECA biases the
the engine. The VEN control system consists of
VEN area schedule according to inputs from the
electrical, hydraulic, and mechanical components
following sources:
that position the VEN while maintaining exhaust
gas temperature (EGT).
Fan inlet temperature transmitter
MAJOR COMPONENTS
Air data computer (ADC) ambient pres-
sure
The VEN control system has nine major
components. As you read about these com-
Fan/low-pressure turbine speed
ponents, locate each component in figure 5-17 as
you read about it in the text.
Compressor/high-pressure turbine speed
VEN power unit. The VEN power unit
Main fuel control metering valve
provides hydraulic power to actuators for posi-
tioning the VEN area.
LVDT
Main fuel control (MFC). The MFC provides
Afterburner control metering valve
a regulated flow of fuel to the fuel nozzles.
Afterburner flame sensor
Electrical control assembly (ECA). The ECA
computes, schedules, and controls engine opera-
Afterburner permission signal
tion.
Thermocouples
Fan speed transmitters. The transmitters are
eddy-current sensors mounted in line with the
VEN area position transmitter
second stage fan blades. A permanent magnet,
rotating at the RPM of the fan blades, induces
AB permission switch
a voltage into a coil indicative of the fan speed.
The VEN area closes when the engine shuts
A f t e r b u r n e r control (ABC). The ABC
down and the throttle is in the off position. As
schedules fuel to the afterburner pilot and main
the throttle is moved to the idle position, the VEN
spray bars.
area rapidly moves to an almost full open
position. This aids engine starting and lowers
Afterburner (AB) flame sensor. This sensor
engine thrust, allowing higher idle speeds and
provides an electrical signal to the ECA. This
reducing engine acceleration time. As the throttle
signal must coincide with the AB no-light/light
advances past the idle position, the VEN area
condition to start the afterburner.
schedule closes the VEN area, increasing thrust.
As the VEN area decreases, EGT increases.
Thermocouple harness. This device senses the
The ECA adjusts the VEN area for varying
exhaust gas temperature (EGT).
atmospheric conditions. When the ECA receives
VEN position transmitter. The transmitter
an ambient pressure signal from the air data
provides feedback to the ECA to ensure the VEN
computer, it means the aircraft is at a pressure
altitude of 9,000 feet or greater. The ECA now
is in the correct position. It also gives feedback
i n c r e a s e s low-pressure turbine discharge
to the engine monitor indicator (EMI) to indicate
temperature and fan/low-pressure turbine speed
percent of nozzle position.
limits, providing more thrust.
VEN actuators. These actuators hydraulically
Below 4,000 feet pressure altitude, the
schedules trim back to reduce fuel consumption
operate to position the VEN.
