Chapter Objective: Upon completion of this chapter, you will have a working
knowledge of the operating principles and components of bleed-air utility
The utility systems of an aircraft provide an
additional measure of flight safety, pilot comfort
and convenience, and contribute to the overall
mission capability of the aircraft.
BLEED-AIR UTILITY SYSTEMS
Learning Objective: Recognize the
operating principles and components for
systems within the bleed-air utility system.
Many aircraft have utility systems that rely on
a bleed-air system to function. The P-3C deicing
system and the A-6E rain removal system are
examples of such systems and are discussed in this
chapter. This material will increase your
proficiency in troubleshooting and maintaining
these and similar systems.
An anti-icing system is designed to prevent ice
from forming on the aircraft. A deicing system
is designed to remove ice after it has formed. An
aircraft deice system removes ice from propellers
and the leading edges of wings and stabilizers.
These systems may use electrical heaters, hot air,
or a combination of both to remove the ice
formation. As an AME, you are primarily
concerned with hot air as a method to remove the
formation of ice on wings and stabilizers. The
P-3C wing deice system is used as an example in
this chapter to describe a hot-air system.
Description and Components
The P-3C wing deice system uses hot com-
pressed bleed air from the engines. The air is
ducted from the 14th stage of each engine
compressor, as shown in figure 3-1. The bleed air
is maintained at a fixed percentage of engine
airflow for all altitudes and flight speeds.
The hot bleed air is directed and regulated to
the leading edge ejector manifold through shutoff
valves, modulating valves, thermostats, skin
temperature sensors, and overheat warning
SHUTOFF VALVES. The wing deice
system contains several shutoff valves. The
fuselage bleed-air shutoff valves, installed in the
cross-ship manifold on the right and left wings,
isolate the wings from the fuselage duct section.
In addition, they maybe used to isolate one wing
duct from the other wing duct. Each valve is
individually controlled by a guarded toggle switch
mounted on the bleed-air section of the ice control
A bleed-air shutoff valve is also installed in
each engine nacelle. These shutoff valves are
physically identical. They are of the butterfly-type,
and they are actuated by an electric motor.
An indicator, located on top of the valve
housing, shows the position of the valveopen
or closed. This indicator enables you to visually
check the operation of the valve while it is still
installed in the deice system.
MODULATING VALVES. The P-3C de-
icing system has three modulating valves installed
in each wing. These valves are thermostatically
controlled and pneumatically operated. They
maintain the constant engine compressor bleed-
air temperature required for the wing leading
edge. When deicing is not required, the valves
operate as shutoff valves.
The modulating valves, shown in figure 3-2,
have pilot solenoid valves that are electrically