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(4) Temperature Modulation Mode:
If wing and cowl anti-icing are selected on, pneumatic signals from
the bleed air temperature sensor and the bleed air temperature
anticipator sensor (both installed upstream of the precooler)
coordinate with the 12th stage servo controller to reset the HP bleed
air pressure regulator and shutoff valve. The valve then modulates
mixed 7th and 12th stage air temperature to 520°-590° F (271°-310°
C), thus modulating due to temperature, not pressure. With wing
and cowl anti-icing are selected on, the dual port solenoid shutoff
valve is de-energized open.
(5) Single Pack Mode:
With only one refrigeration pack selected on, both A and B solenoids
of the 12th stage servo controller are energized. The HP bleed air
pressure regulator and shutoff valve then resets to regulate
pressure to 30 psi. This allows for increased performance due to
single pack operations. Because the HP bleed air pressure regulator
and shutoff valve is operating in single pack mode, the dual port
solenoid shutoff valve remains closed.
If anti-icing is selected on during single pack operations, the system
switches back to temperature control mode and the dual port
solenoid shutoff valve is de-energized open.
B. Bleed Air Pressure Regulator and Shutoff Valve:
The bleed air pressure regulator and shutoff valve (also referred to the “40
psi valve”) is an electrically controlled, pneumatically actuated, springloaded
closed butterfly-type valve. Located immediately downstream of the
7th and 12th stage mixing junction, this valve regulates air having passed
through the HP bleed air pressure regulator and shutoff valve to 40.5 psi
nominal in the bleed air manifold. The valves receive power from the
Essential 28V DC bus through the L BLEED AIR and R BLEED AIR circuit
breakers, respectively.
The bleed air pressure regulator and shutoff valve is opened or closed by
the flight crew using the L/R ENG BLEED AIR switches located in the
BLEED AIR section of the cockpit overhead panel. Depressing the switch
routes Essential 28V DC bus power to an electrically actuated solenoid,
opening the valve and allowing pressure regulated bleed air through the
precooler to the bleed air manifold. The OFF legend in the switch is
extinguished.
C. Fan Air and Fan Air Modulating Valve:
Each engine’s fan supplies low pressure bypass air (commonly referred to
as LP or fan air) for use by both the airplane and its engines. For the
engines, fan air is used to pressurize engine oil system components and
cool various other engine components. For the airplane, fan air is routed
OPERATING MANUAL
2A-36-00 PRODUCTION AIRCRAFT SYSTEMS
Page 4
October 11/02
Revision 8
through the precoolers to reduce the temperature of 7th and/or 12th stage
bleed air.
The precooler fan air modulating valve (installed in the engine pylon)
controls fan air airflow to the precooler in order to maintain a nominal 400°
F (204° C) bleed air temperature. Precooler outlet air temperature is
continuously monitored by a precooler outlet temperature sensor and a
precooler temperature control anticipator sensor. Outputs from these
sensors are transmitted to the precooler temperature controller. The
controller, in turn, will change the output voltage to the servo air pressure
regulator and torque motor. The servo air pressure regulator and torque
motor then changes the electrical signal to a filtered pneumatic signal,
positioning the precooler fan air modulating valve accordingly.
D. Precooler:
The precooler, located in the engine pylon, functions as a heat exchanger
for the pneumatic system. HP bleed air flows from the bleed air pressure
regulator and shutoff valve directly to the precooler, where fan air is
introduced by the fan air modulating valve. Fan air and bleed air do not mix,
rather, bleed air passes through the interior of the precooler while fan air
flows around the exterior and exits through louvers in the bottom of the
pylon.
E. Temperature and Pressure Sensors/Switches:
(1) Bleed Air Temperature Sensor:
The bleed air temperature sensor is located in the bleed air
manifold, upstream of the precooler. It senses the mixed 7th and
12th stage air temperature and then modulates the HP bleed air
pressure regulator and shutoff valve via the dual port solenoid
shutoff valve and the 12th stage servo controller (solenoid “A” and
“B” HP control valve) until a 520°-590° F (271°-310° C) temperature
is maintained downstream of the mixing point.
(2) Bleed Air Temperature Anticipator Sensor:
The bleed air temperature anticipator sensor is also located in the
　
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