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4th stage air. This controls the temperature of the air that goes
to the HPT shroud support.
The EEC calculates the demand HPT case support temperature
(TCC) from N2 speed T3 and altitude (P0). If TCC is too high the
EEC sends a signal to the HMU to cool the HPT shroud support.
If TCC is too low the EEC sends a signal to the HMU to cool the
shroud support less.
The actuator has two LVDTs. The EEC uses the LVDTs to
monitor the position of the HPTACC actuator. One LVDT sends
an electrical signal to channel A of the EEC. The other LVDT
sends an electrical signal to channel B.
Modes of Operation
These are the five HPTACC modes:
* No air - The actuator fully retracts. The HPC 4th stage and the
9th stage valves are closed. This is the actuator position
when the engine is off. This is the fail-safe position. The EEC
commands the HPTACC valve to this position if there is a
malfunction of the EEC or the HMU. The HPT blade tip
clearance is maximum when the HPTACC is in this position.
* Low flow 9th stage - The EEC sets the actuator to 8 percent of
extension. The 9th stage valve lets a low flow of 9th stage air
go to the HPT shroud support. The 4th stage butterfly valve is
fully closed. This cools the support shroud a small amount.
* High flow 9th stage - The EEC sets the actuator to 37 percent
of extension. The 9th stage valve fully opens. The 4th stage
butterfly valve is fully closed. This cools the shroud support
more.
ENGINE AIR - HPTACC - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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* Mixed - The EEC calculates an actuator position between 38
percent and 99 percent. This sets the 9th stage and 4th stage
air ratio to accurately adjust the HPT clearance. This cools
the shroud support even more.
* Full 4th stage - The actuator fully extends (100 percent). The
9th stage valve is fully closed. The 4th stage valve is fully
open. This gives the maximum shroud support cooling which
provides the minimum HPT clearance.
The typical HPTACC modes and the airplane condition are
listed in this table.
Engine Condition HPTACC MODEs
Cold Start Initially full 4th stage, then Transition
through mixed mode to full 9th stage
Warm Start 9th stage to minimize HPT rubs.
Take Off and Climb Initially 4th stage to minimize peak EGT,
then transition to mixed mode
Cruise 4th stage to minimize fuel consumption
Descent Low flow 9th stage to protect against
rubs
Training Information Point
You can see the HPTACC position in percent on the engine
maintenance pages of the control display units (CDUs).
See the engine fuel and control section for more information on
the engine maintenance pages in the CDU. (SECTION 73-21)
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ENGINE AIR - HPTACC - FUNCTIONAL DESCRIPTION
ENGINE AIR - HPTACC - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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General Description
The low pressure turbine active clearance control (LPTACC)
system controls the low pressure turbine (LPT) blade tip
clearance. The LPTACC increases or decreases the amount of
fan discharge air that goes to the LPT case. This air cools the
LPT case. Cooling the LPT case controls thermal expansion
which keeps the clearance with the LPT blade tips to a
minimum. This increases fuel efficiency.
The LPTACC system has these parts:
* LPTACC valve
* LPTACC air duct
* LPTACC manifold.
ENGINE AIR - LOW PRESSURE TURBINE ACTIVE CLEARANCE CONTROL (LPTACC) - GENERAL DESCRIPTION
EFFECTIVITY
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