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aircraft, the cabin pressure at cruising level corresponds to an ambient altitude of 1 500 to 2 450 m (5 000
to 8 000 feet).
PROTECTIVE SYSTEMS
Cabin pressurization
Cabin pressurization is one of the examples of technological solutions to a physiological problem in
relation to aviation. In most modern commercial aircraft the problems of hypoxia and decompression
symptoms are overcome by pressurizing the aircraft cabin to maintain a pressure that is compatible with
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ICAO Preliminary Unedited Version — October 2008 II-1-9
normal physiological needs.
It would seem ideal to maintain sea-level pressure in an aircraft cabin at all times. This solution is
usually impractical due to weight penalties and technical considerations. For these reasons, aircraft cabins
are designed with pressure differentials which represent the compromise between the physiological ideal
and optimal technological design. The pressurization characteristics of different commercial aircraft types
are similar, with minor variations. In general, while the aircraft rate of climb might be in the order of
1000-3 000 ft/min (5-15 m/s) at lower altitudes, cabin altitude increases at a rate of about 500 ft/min (2.5
m/s) which represents an acceptable physiological compromise to equilibrate pressures within the body
and the surrounding environment with a minimum of discomfort. On descent, the usual rate is no more
than 300 ft/min (1.5 m/s).
The normal method of achieving cabin pressurization is by obtaining compressed air from the engine
compressor, cooling it and leading it into the cabin. The pressure level is then set by controlling the rate
of escape of the compressed air from the cabin by means of a barometrically operated relief valve.
Figure 4 indicates a typical pressure differential between the ambient altitude and cabin altitude for a
commercial aircraft.
Figure 4.— Aircraft and cabin altitudes for a commercial aircraft during a typical flight1
DECOMPRESSION
All gases present in the body, either in free form in the cavities of the viscera or in solution in the body
fluids, are in equilibrium with the external environment. Therefore, any changes in barometric pressure
will give rise to transient pressure gradients between gases within the body and the external environment,
and a gradient will persist until a new balance is reached. Depending upon the magnitude of the changing
pressure and the rate at which it takes place, mechanical deformation and structural damage may occur on
1 Adapted from Rainford, D.J., Gradwell, D.P. eds. (2006)
Time of flight (min)
Altitude (thousands of feet)
Cabin altitude
Aircraft
altitude
0 5 10 15 95 100 105 110 115 120
10
20
30
40
ICAO Preliminary Unedited Version — October 2008 II-1-10
decompression due to the relatively higher pressure of free gases trapped in body cavities.
In spite of all precautions, loss of cabin pressurization, including the remote event of rapid
decompression, remains a potential hazard in the operation of pressurized aircraft at high altitudes.
Rapid decompression is an uncommon event in civil aviation operations. It may be produced as a
result of structural failure or damage to the cabin wall (pressure hull). If it occurs, those on board might
be exposed to the sudden onset of hypoxia for which oxygen equipment will be required. If the rate of
decompression is of severe magnitude, organ and tissue damage may also ensue. Free gases in the body
will expand. Cavities containing such gases are:
a) those with distensible walls;
b) those with free communication with the external environment; and
c) rigid or semi-rigid closed cavities.
The gases present in the distensible cavities, i.e. gastrointestinal tract, will expand under hypobaric
conditions and may cause symptoms of discomfort and pain. Cavities with free communication will not
give rise to complications as long as the size and patency of the communicating orifice and/or anatomical
structure is adequate. Examples of these cavities are paranasal sinuses with open communication. The
third type of cavities are those formed when a blocked paranasal sinus ostia or blocked Eustachian tube
leading to the middle ear is present; they might give origin to pain of magnitude so severe as to be
incapacitating.
Other forms of decompression manifestations are those produced by the evolution of bubbles from
gases dissolved in blood and tissues - decompression sickness. In the context of civil aviation operations,
this might occur when a person has been exposed to a hyperbaric environment, which has
overcompressed inert gases in the body, prior to an ascent to altitude. Based on case studies and
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Manual of Civil Aviation Medicine 1(60)
