primarily an airframe engineering and manufac-
2. There must be a means of limiting the
maximum pressure differential to which the cabin
walls will be subjected. This is provided by the
cabin safety valve.
3. The aircraft must have an adequate supply
of compressed air. This is provided through the
compressor section of the jet engine. A separate
compressor or supercharger is used on aircraft
having reciprocating engines. On all jet aircraft,
the air is taken directly from the compressor
section of the jet engine. This is generally referred
to as bleed air.
4. There must be a means of cooling the bleed
air before it enters the cabin. This is provided by
an aircraft refrigeration unit.
5. There must be a means of controlling the
cabin pressure. This is provided by the cabin
pressure regulator, which regulates the outflow
of air from the cabin.
In addition to the major components, various
valves, controls, and other allied units are
necessary to complete an aircraft pressurization
and air-conditioning system. The design, construc-
tion, and use of these components may vary
somewhat with different manufacturers; however,
the systems on all jet aircraft operate on the same
principles. The system used as an example in this
text is in the F-18 aircraft.
The environmental control systems of most
aircraft include cabin air conditioning and
pressurization, equipment cooling, defogging,
windshield washing and rain removal, and equip-
ment pressurization subsystems.
Coverage in this section is limited to air cycle
cabin and equipment pressurization and air
Bleed air is supplied by the last compressor
section of each engine (fig. 3-1, a foldout at the
end of this chapter). This bleed air flows from
the engines through two engine bleed-air pressure
regulation and shutoff valves. The valves are
spring-loaded closed when the system is not in use.
When air conditioning is selected, the valves open
and regulate bleed air to a predetermined pressure.
The bleed air then passes through two engine
bleed-air check valves, which prevent reverse flow
from one engine to the other. At this point the
bleed air from both engines enters a common duct
and flows through the engine bleed-air secondary
pressure-regulating and shutoff valve. This valve
is spring-loaded open and regulates the pressure
of the combined flow of bleed air from both
engines. The regulated bleed air then flows into
the primary heat exchanger of the ACS.
There are two overpressure switches (primary
and secondary) incorporated in the system to
prevent overpressure damage to system com-
ponents in case of a pressure regulator mal-
function. An air isolation valve is located in the
system to provide a means of providing bleed air
to the ACS when required and during cross
starting of engines. These bleed-air components
are discussed in the following paragraphs.
Engine Bleed-Air Pressure Regulation
and Shutoff Valve
These two valves (fig. 3-1) act as system
shutoff valves when air conditioning is not
required. They are spring-loaded closed. When
air conditioning is selected, an electric solenoid
is energized, which unseats a poppet from the vent
line. As air flows from the engine, a line
downstream of the butterfly valve routes a small
amount of the bleed air to the butterfly
diaphragm. This air is called control air since its
action on the diaphragm is the controlling force
for the valve. As pressure builds on the
diaphragm, it overcomes spring pressure holding
the butterfly closed and the valve opens. As the
bleed air passes through the valve, another line
upstream of the butterfly routes bleed air to the
regulator portion of the valve. As pressure builds
and overcomes spring pressure, a poppet is
reseated, allowing some of the control air pressure
from the open side of the butterfly diaphragm to
bleed off. Spring pressure can now start closing
the butterfly, thus lowering the bleed-air pressure
downstream of the butterfly. In this manner
bleed-air pressure is controlled to 75 ± 15 psi.
Engine Bleed-Air Check Valve
These dual-flapper check valves are located
downstream of the pressure regulator and shutoff
valves (fig. 3-1). They prevent cross-flow of bleed
air from one engine to the opposite engine in the
event of single engine operation.
Engine Bleed-Air Secondary
Pressure-Regulating And Shutoff Valve
This valve is located in the common ducting
upstream of the bleed-air check valves (fig. 3-1).