The Complete Guide To Aircraft Decompressions

| July 16, 2020 | By: Severe VFR
Cockpit view of southwest jet overflying a cessna skyhawk

An aircraft decompression occurs when an aircrafts pressurization systems fails to maintain the required pressure differential. 

Throughout a pilots career, they will tend to progress towards flying more advanced and capable aircraft. These aircraft typically have turbochargers, anti-ice systems, and constant speed props. Some pilots may even find themselves flying aircraft with pressurized cabins which grants the ability to fly at higher altitudes without the need for supplemental oxygen. 

However, if the aircraft's pressurization system where to fail the aircraft will experience a decompression.  While this definition is valid for all decompressions, not all decompressions are the same. 

Types Of Aircraft Decompressions

Aircraft decompressions fall into three categories:

  1. Explosive
  2. Rapid; or
  3. Gradual

Each type of aircraft decompression is classified by the speed at which the aircraft loses pressurization. The time it takes for an aircraft to lose pressurization can range from less than half a second upwards to a few minutes. 

The rate at which an aircraft loses pressurization results in different risk factors to both the crew and passengers on board and the potential cause of the decompression. 

Explosive Decompressions

Any decompression that occurs in less than .5 seconds is categorized as an explosive decompression.

When air is enclosed in a small space, such as a person's lungs, it will expand and compress according to the outside air pressure. During decompression, the air pressure inside the cabin falls to levels far below the pressure inside of a person's lungs.

Explosive decompressions are dangerous because the air inside the lungs expands faster than it can be expelled through the mouth and nose. The expanding air inside the lungs can potentially over-inflate the lungs, causing lung damage or rupture.

The risk of experiencing lung trauma is high with an explosive decompression. However, the human lungs require .2 seconds to decompress without an obstruction, such as a mask. Since explosive decompressions are defined as decompressions lasting less than .5 seconds, it is possible to avoid lung damage.

Explosive decompressions tend to cause individuals to become dazed for a brief moment, and escaping air may be felt as it rushes out of the mouth and nose.

Rapid Decompressions

Rapid decompressions are decompressions lasting greater than .5 seconds and take a matter of seconds to complete. A rapid decompression will have the aircraft cabin decompress at a rate greater than 7,000ft/min.

The risk of lung trauma with a rapid decompression is much lower than an explosive decompression. The lower risk is associated with the lungs being able to decompress faster than the cabin.

However, rapid decompressions still pose a risk to pilots. Rapid decompressions cause the partial pressure of oxygen in the blood to reduce. This reduction in partial pressure causes a pilot's time of useful consciousness to reduce by a third to a fourth than expected.
Time Of Useful Consciousness or Effective Performance Time is the period of time from interruption of the oxygen supply, or exposure to an oxygen-poor environment, to the time when an individual is no longer capable of taking proper corrective and protective action.

With a rapid decompression, a pilot will have an even smaller time window than expected to take proper action to a rapid decompression.

Gradual Decompressions

A Gradual, slow, or insidious decompression is a decompression that occurs over a long period of time.

A gradual decompression is as dangerous or more dangerous than a rapid decompression and is often regarded as the most dangerous type of decompression of the three different types.

Gradual decompressions are dangerous because they occur slowly without any sign a decompression has occurred, such as the case with a rapid or explosive decompression. Without any sign, a gradual decompression may go unnoticed by the flight crew until it is too late to take action due to the onset of hypoxia.

Hypoxia may also cause a flight crew to have a greater inability to recognize and react to a gradual decompression due to decreased mental function.

Identifying Decompressions

Explosive and Rapid decompressions can be identified by:

  • A long bang, thump or clap.
  • Cloud of fog or mist due to a drop in temperature and changing humidity. 
  • A rush of air exiting the cabin
  • Rapidly falling temperatures

If the cause of decompression is a structural failure, expect:

  • Unsecured items in the immediate area to be ejected from the aircraft.
  • Debris flying around the cabin
  • Loose items becoming projectiles
  • Dust particles limiting visibility

A decompression caused by structural damage gives persons and debris the potential to be thrown out of the aircraft. Structural damage can also subject the cabin to high wind gusts and frigid temperatures. Communication may also be difficult with high noise levels and limited visibility. 

Gradual decompressions can be identified by:

  • Unusual or whistling noises from doors and seals
  • Inner ear discomfort or popping
  • Joint pain
  • Stomach pain
  • Closely monitoring pressurization gauges

Recovery From Decompressions

Recovery from all types of decompressions is similar. Oxygen masks should be donned, and a rapid descent initiated to avoid the onset of hypoxia. Pilots should also be aware of cold-shock in piston engines, causing cracked cylinders and other engine damage.

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