The breathable air. The problem was that not enough

re-circulation fan will draw air from the cabin area, through a check valve and
filter assembly to remove any smoke and noxious odours before passing it to the
mixer unit for re-distribution. The check valve prevents any reverse flow
through the fan and ducting when the fan is not in use.


Pressurisation Systems

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As aircraft
became capable of obtaining altitudes above that at which flight crews could
operate efficiently, a need developed for complete environmental systems to
allow these aircraft to carry passengers. Air conditioning could provide the
proper temperature and supplemental oxygen could provide sufficient breathable


The problem was
that not enough atmospheric pressure exists at high altitude to aid breathing
in and even at lower altitudes the body must work harder to absorb sufficient
oxygen, through the lungs, to operate at the same level of efficiency as at sea
level. This problem is overcome by pressurising the cockpit/ cabin area. Cabin pressurisation is a means of adding pressure to the
cabin of an aircraft to create an artificial atmosphere that when flying at
high altitudes it provides gives an environment equivalent to that below 10000
feet. The minimum quantity of fresh air supplied to each person on board must
be at least 0.5lb/ minute.


Aircraft are
pressurised by sealing off a strengthened portion of the fuselage. This is
usually called the pressure vessel and will normally include cabin, cockpit and
possibly cargo areas. Air is pumped into this pressure vessel and is controlled
by an outflow valve located at the rear of the vessel.


Sealing of the
pressure vessel is accomplished by the use of seals around tubing, ducting,
bolts, rivets, and other hardware that pass through or pierce the pressure
tight area. All panels and large structural components are assembled with
sealing compounds. Access and removable doors and hatches have integral seals.
Some have inflatable seals.


Pressurisation systems do not have to
move large volume of air. Their function is to raise the pressure inside the
vessel. Small reciprocating engine powered aircraft receive their
pressurisation air from the compressor of a coupled turbocharger. Larger
reciprocating engine powered aircraft receive air from engine driven
compressors and turbine powered aircraft use compressor bleed air


Small Reciprocating Engine Powered


Turbochargers are driven by the engine exhaust gases flowing through
a turbine. A centrifugal compressor is coupled to the turbine. The compressors
output is fed to the engine inlet manifold to increase manifold pressure which
allows the engine to develop its power at altitude. Part of this compressed air
is tapped off after the compressor and is used to pressurise the cabin. The air
passes through a flow limiter (or sonic venturi) and then through an
inter-cooler before being fed into the cabin. A typical system is shown at
Figure 22.


Sonic Venturi


A sonic venturi is fitted in line between the engine and the
pressurisation system. When the air flowing across the venturi reaches the
speed of sound a shock wave is formed which limits the flow of air to the
pressurisation system


Small Reciprocating Engine
Aircraft  Pressurisation System

Figure 22


Large Reciprocating Engine Powered


These aircraft use engine driven compressors driven through an
accessory drive or by an electric or hydraulic motor. Multi engine aircraft
have more than one air compressor. These are interconnected through ducting but
each have a check valve or isolation valve to prevent pressure loss when one
system is out of action.


Turbine Powered Aircraft


The air supplied from a gas turbine engine compressor
is contamination free and can be suitably used for cabin pressurisation (Figure
23). So