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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
prospective investigations, the Undersea and Hyperbaric Medical Society recommends the following
intervals between diving and flying:
Dive schedule
Minimum interval
1.
Non-decompression dives
a. Less than 2 hours accumulated dive time in the
48 hours preceding surfacing from the last dive
12 hours
b. Multi-day, unlimited diving
24 hours
2.
Dives requiring decompression stops (but not
including saturation dives)
24-48 hours
Table 4.— Recommended intervals between diving and flying
Further information concerning dive times and flying is available from the Professional Association
of Diving Instructors (PADI) and the National Association of Underwater Instructors (NAUI).
Another important consideration in civil aviation operations is the possibility of slow decompression,
including failure to pressurize during climb, which might occur as a result of failures of pressurization
equipment, such as failure of an outflow valve, or incorrect settings of the flight deck pressurization
controls by flight crew. If a slow loss of pressure occurs, the aircraft usually initiates a descent to a safer
ICAO Preliminary Unedited Version — October 2008 II-1-11
altitude; in some cases, on account of high ground, the aircraft is forced to continue flying at an altitude
requiring oxygen. In such cases, the availability of oxygen systems is mandatory and if the planned route
is over high ground that prevents an immediate descent to 10,000 feet or below, additional oxygen is
required to be carried. When cabin pressure is lost, a barometrically triggered valve opens at a given cabin
altitude - usually 10 000 - 14 000 ft (3 050 - 4 250 m) - and releases the masks for passengers. Passengers
are briefed, prior to the flight, about the procedures to be taken to start breathing oxygen when required.
Other forms of decompression symptoms (dysbarisms) such as barotitis, barosinusitis and barodontalgia
are further described in Part III, Chapter 12 of this manual.
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