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11.5.1 Atmosphere Pressure Changes

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Trapped Gases

Any air or gas contained within the body will expand or contract with any change in pressure.

Climb The following problems can occur when there is an increase in altitude.

Lungs and Intestine Gas collects along the gastro-intestinal tract because of:

  • Eating: When we eat, air is swallowed with the food we eat.
  • Bacteria: Gas is formed in the intestines by the action of bacteria on food.

The gas in the stomach or intestines expands during a rapid decompression. If this gas is not released to the atmosphere, severe pain can be experienced. Damage to the lungs, or even rupturing (pneumothorax - air between the lung and chest wall) can occur if pressure changes are extreme. Normally the rib cage will protect the respiratory system.

Teeth Good dentistry ensures that teeth are filled correctly and the oral health of the pilot is maintained to a high standard. Poor oral hygiene can result in abscesses, poor dentistry can lead to air pockets being left in filled teeth; both can cause pain during a decompression due to the expansion of gases.

The ear and the sinuses are parts of the body that suffer most in the descent.

Sinuses Pressure

Sinuses are air filled cavities in the bones of the skull that form the upper part of the face. They help resonate the voice and make the skull lighter.

The frontal sinuses are in the brow of the forehead above the eyes. The maxillary sinuses are larger cavities in the cheek bones. Other sinuses are found in the deeper bones of the skull, separating the nasal passages and the floor of the skull.

The sinuses are lined with mucous membrane and are connected to the nasal cavity by small openings. These openings, sinus canals, allow the air pressure to be equalised to the atmosphere. The sinus canals vent air to the atmosphere as the altitude increases.

The lining of the canals is made up of a soft mucous membrane which expands when a person is suffering from colds or flu. Air can still vent to the atmosphere in the climb; but in the descent the inward passage of air is impossible. During the descent, severe pain and injury can result.

This is known as a sinotic barotrauma or Barosinusitis.

Ear Pressure

The ear has three main areas which are discussed in detail in a later chapter:

  • Outer ear
  • Middle ear
  • Inner ear

The outer ear is exposed to atmospheric pressure.

The middle ear is an air filled cavity bordered by the ear drum and the Cochlea. It is connected to the back of the throat by the Eustachian tube. The walls of the Eustachian tube are made of soft tissues, with the opening into the throat acting as a flap valve. During ascent, air can vent to the atmosphere.

This flap valve can stop air returning into the middle ear during a descent when the pilot suffers from an infection.

Colds or flu can cause the soft tissue, of the Eustachian tube, to expand. Therefore, in a descent the ear cannot equalise the middle ear pressure to the outside pressure. Severe pain and injury (possible rupturing of the ear drum) can occur. This is the otic barotrauma or Barotitis Media.

The best prevention is do not fly with any of the following:

  • Cold
  • Flu or
  • Hay fever.
Cabin Decompression

Cabin pressurisation failures can occur at any time during flight. The rate of loss of pressure can be:

  • Very slow which allows time for a pilot to recognise and deal with the problems promptly, or
  • Very rapid if the decompression is due to the loss of a door or window.

For a Public Transport aircraft like the B747:

Loss of a door:

  • The pressure will equalise in approximately 12 - 20 seconds.

Loss of a window:

  • The pressure equalises in approximately 60 - 90 seconds.

In smaller aircraft the pressure will equalise in a much shorter time.

During a rapid decompression there will be a sudden explosive bang and the cabin will fill with fog, dust and flying debris. The fog occurs due to the rapid drop in temperature and the change in Relative Humidity. Normally the ears will clear automatically. Belching and the passage of intestinal gas will occur. Air escapes from the lungs through the mouth and nose.

In such a case the crew are immediately exposed to the following problems:

  • Hypoxia
  • Cold
  • Decompression Sickness

Oxygen is needed to avoid Hypoxia and a descent is required to a safe altitude below 10 000 ft. Where structural damage has occurred, the descent must be made at a rate that the damage allows. Emergency descents are not normally made in Public Transport aircraft for a rapid decompression as supplementary Oxygen is provided.

During rapid decompression the cabin altitude may rise above aircraft altitude due to a venturi effect. Aerodynamic suction occurs where the air on the outside, passing over the defect in the hull pulls air out of the cabin. The difference between cabin and aircraft altitude can differ by as much as 5000 ft.

If flying in a pressurised aircraft, which has a rapid decompression, then the Time of Useful Consciousness will be reduced. The rapid reduction of pressure in the aircraft will affect the body. Oxygen is exhaled from the lungs due to this pressure change. The partial pressure of Oxygen in the blood is reduced and the Time of Useful Consciousness can be reduced by up to ½ the normal time