Pressurization Failure

The failure of the pressurization system of an aeroplane is potentially life threatening where the outside air pressure (partial pressure of oxygen) is inadequate to preserve life. Decompression of a pressurized cabin under any circumstances requires that the aeroplane is descended to a minimum of 10 000 ft or the lowest safe flight level whichever is the highest. The aeroplane should be flown during the descent, with regard to maintaining the flying integrity of the airframe. At the lower altitude sufficient oxygen should be present in the atmosphere to sustain life. During the descent supplemental oxygen is required for crew and passengers in accordance with the table below.

Decompressionisdefinedaseitherslow,orrapid(orexplosive).Rapidorexplosivedecompression is the result of a failure of the airframe to contain the cabin pressure. A slow decompression is the failure of the pressurization system to maintain the cabin pressure where there has not been a failure of the airframe.

Rapid or explosive decompression results in the cabin altitude quickly (or virtually instantaneously) decreasing to the ambient (outside) pressure. This will only occur due to a catastrophic failure of the pressure hull or the loss of a major door or hatch. In the case of an explosive decompression, major damage will have occurred as would be the case of a bomb exploding within the pressure hull or major fatigue failure. Where the flying integrity of the airframe is preserved the aeroplane may be landed safely. Rapid decompression results from a relatively minor rupture of the pressure hull or the loss of a small hatch (emergency escape or a window). In essence, if the size of the rupture is such that the cabin pressure and the outside air pressure are not equalized immediately. The decompression is therefore rapid and not explosive. In reality, the differentiation is academic where the flying integrity of the aeroplane is maintained.

A slow decompression occurs where the pressurization system cannot overcome the loss of pressure caused by a normally controllable vent/opening in the pressure hull; e.g. a leaking pressureseal,anotfullyclosedpressurereliefvalveoraninadequacy(failure)inthepressurization system. In a normal system, once the cabin pressure reaches 10 000 ft (700 mb) the altitude warning horn will sound. Prior to this the crew should notice the loss on gauging systems if fitted, cabin altimeter showing an increase or cabin differential pressure gauge showing a reduction. However physiological changes are often the first indication of a problem.

During a slow decompression, passengers and crew will be aware of barometric pressure changes on the ears. Other body cavities (teeth, sinuses and gut) may give rise to discomfort. If it is not possible to equalise the differential pressure by natural venting, serious damage may result. At night, night vision will be seriously impaired at relatively low cabin altitudes.

In extreme cases (rapid and explosive decompression), sinuses and teeth may explode, ear drums rupture, and severe abdominal distension may occur resulting in rupturing of internal organs. The effects especially in the head, may be pronounced where the person is suffering vent blockage due to a build of mucus with a cold. During prolonged periods of reduced oxygen, tunnel vision and sensorial depletion may result. The most obvious indication of a rapid or explosive decompression is white-out, where the moisture in the atmosphere vaporizes, causing instantaneous fog.

Pressurization Failure 12

12

Failure Pressurization 12

Pressurization Failure

Minimum Requirements for Supplemental Oxygen for Pressurized Aeroplanes

The following table describes the requirement for supplemental oxygen (Note 1) as required by EU-OPS 1.770.

Notes:

1.The supply provided must take account of the cabin pressure altitude descent profile for the routes concerned.

2.The required minimum supply is that quantity of oxygen necessary for a constant rate of descent from the aeroplane’s maximum certificated operating altitude to 10 000 ft in 10 minutes followed by 20 minutes at 10 000 ft.

3.The required minimum supply is that quantity of oxygen necessary for a constant rate of descent from the aeroplane’s maximum certificated operating altitude to 10 000 ft in 10 minutes followed by 110 minutes at 10 000 ft. The oxygen required in CS-OPS 1.780(a)

(1) may be included in determining the supply required.

4.The required minimum supply is that quantity of oxygen necessary for a constant rate of descent from the aeroplane’s maximum certificated operating altitude to 15 000 ft in 10 minutes.

5.For the purpose of this table ‘passengers’ means passengers actually carried and includes infants.