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regulating valve, mounted on each engine. The valve in turn, is
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controlled by an ON-OFF switch, one for each side, located on the
cockpit overhead panel and labeled L COWL and R COWL.
NOTE:
Ice accumulation on the front of the engine fan and
spinner can disrupt and restrict airflow into the engine.
Large build ups of ice could increase the risk of
structural and impact damage. The fan spinner uses a
soft rubber point that distorts during engine operation
to ensure ice accretion is shed centrifugally before it
builds up. The fan blades use centrifugal force to shed
ice and prevent accumulation during operation. This
use of centrifugal force is known as the passive form
of ice protection.
(2) Engine Stability, Surge Recovery and Flameout Protection:
The HP compressor control system allows for the stability of airflow
in all ranges of RPM through the use of Variable Inlet Guide Vanes
(VIGV’s), a 7th stage bleed air valve and the Airflow Control
Regulator (AFCR). The VIGV’s and the 7th stage bleed air valve are
operated together by the AFCR, which operates in response to input
based on HP compressor inlet temperature and HP rpm.
At a low RPM, the VIGV’s are at their maximum or “closed” angle
(relative to engine airflow). The bleed valve, which is in the open
position at this time, allows the bleed air to escape into the bypass
duct and exit the engine through the fan air exhaust.
As engine RPM increases (approximately 70 to 80% HP RPM), the
AFCR initiates movement of the VIGV’s toward their “open” position,
creating a minimized airflow angle of attack and closes the bleed air
valve. Closing the valve channels 7th stage bleed air into the
manifold the surrounds the HP compressor case. The VIGV’s
increase the angle of incidence (with decreased airflow) on the front
stage of the HP compressor as engine RPM decreases. This
prevents the flow of air back towards the front of the compressor
which can create blade stall and compressor surge, as a result of
the unstable airflow.
C. Accessory Gearbox:
(See Figure 8.)
(1) Low-Speed Gearbox:
Also referred to as the left gearbox, it is not considered a module
and is mounted on the left side of the Intermediate Pressure
Compressor (IPC) case. The gearbox is driven by the LP spool
through the intermediate gearbox and has mounting provisions for
the following:
• LP Shaft Governor
• LP Tach Generator
(2) High-Speed Gearbox:
Also referred to as the right gearbox, it is mounted on the lower right
side of the IPC case. The gearbox is driven by a shaft from the
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internal gearbox located between the IP and HP compressors and
transfers power from the HP spool to the gearbox. It has mounting
provisions for the following:
• Engine Alternator
• Centrifugal Breather Outlet
• Hydraulic Pumps
• LP Fuel Pump
• HP Fuel Pump
• HP RPM Tach Generator
• Fuel Flow Regulator
• Starter Motor
• Airflow Control Signal Transmitter
(3) Power For Starting And Cranking:
The high-speed gearbox transmits power from the starter motor to
the engine during normal start/crank procedures.
3. Controls and Indications:
The Mechanical Accessories system has no controls and indications as a standalone
system. See the following sections for specific controls and indications:
• Section 2A-21-00, Air Conditioning
• Section 2A-30-00, Ice and Rain Protection
• Section 2A-36-00, Pneumatics
• Section 2A-80-00, Engine Starting
4. Limitations:
There are no limitations established for the Mechanical Accessories system at the
time of this revision.
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Bleeds System Simplified
Block Diagram
Figure 7
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Accessory Gearbox Layout
Figure 8
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ENGINE FUEL AND CONTROL
2A-73-10: Engine Fuel System
1. General Description:
The engine fuel system is a mechanical, all-speed governing system which
　
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