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* Variable stator vanes (VSVs)
* Variable bleed valve (VBV).
The VSV system controls the high pressure compressor (HPC)
airflow. The VSV system makes sure the correct quantity of air
flows through to the HPC which prevents HPC stall. The VSV
system controls the HPC inlet guide vanes and the variable
stator vanes of the HPC. The first three stages of the HPC have
variable stator vanes.
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The VBV system controls the low pressure compressor (LPC)
discharge airflow. There are 12 variable bleed valves that let
some LPC discharge air bypass the engine and mix with the fan
discharge air. This airflow prevents LPC stall during fast
deceleration. The VBVs also keep water out of the HPC and
prevent foreign object damage (FOD) during low speed
operation and during reverse thrust operation.
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High Pressure Turbine Active Clearance Control (HPTACC)
The EEC calculates commanded high pressure turbine
clearance as a function of engine and P0 (altitude) data. P0 is
the ambient pressure. P0 data usually comes from the ADIRUs
through the display electronic units (DEUs). The HPTACC valve
controls the amount of HPC 9th stage and 4th stage air that goes
to the HPT shroud support. The EEC sends a command signal to
the HMU. The HMU sends servo fuel pressure to move the
actuator in the HPTACC valve. Two linear variable differential
transducers (LVDTs) send the actuator position data to the EEC
for closed-loop control.
See the engine fuel and control section for more information on
how the EEC gets P0 pressure. (SECTION 73-21)
Low Pressure Turbine Active Clearance Control (LPTACC)
The EEC calculates the commanded low pressure turbine blade
tip clearance as a function of engine and airplane data. The
airplane data usually comes from the ADIRUs through the
DEUs. The LPTACC valve controls the amount of fan discharge
air that goes to the LPT case. The EEC sends a command signal
to the HMU. The HMU sends servo fuel pressure to move the
actuator in the LPTACC valve. Two rotary variable differential
transducers (RVDTs) send the valve position data to the EEC for
closed-loop control.
Variable Stator Vanes (VSVs)
The EEC calculates the commanded VSV position as a function
of the engine and airplane data. The airplane data usually
comes from the ADIRUs through the DEUs. The EEC controls
the two VSV actuators to modulate the amount of air that goes
through the HPC. The EEC sends a command signal to the
HMU. The HMU sends servo fuel pressure to move the two
actuators. The actuators mechanically connect with the stator
vanes. Two LVDTs send the position data of the actuators to the
EEC for closed-loop control.
Variable Bleed Valve (VBV)
The EEC calculates the commanded VBV position as a function
of engine speed and airplane data. The airplane data usually
comes from the ADIRUs through the DEUs. The VBVs control
the amount of LPC discharge air that goes into the fan
discharge airflow. The EEC sends a command signal to the
HMU. The HMU sends servo fuel pressure to move the two
actuators. The actuators mechanically connect with the bleed
valves. Two LVDTs send the position of the actuators to the EEC
for closed-loop control.
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CFM56 ENGINES (CFM56-7) 737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
中国航空网 www.aero.cn
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