Flight Crew Considerations for Response to Aircraft Depressurization

Flight Crew Considerations for Response to Aircraft Depressurization

1. OVERVIEW

Loss of pressurization at high altitudes requires an informed decision be made without delay. The following guidance discusses the various risks and the reasoning behind the suggested mitigating responses if a pressurization event occurs.

THE RISKS ARE;

1. Hypoxia

2. Decompression sickness (DCS), also known as Altitude sickness or The Bends, is the formation of bubbles from the gasses normally dissolved in body tissues and fluids

3. Trapped gas expansion in gastrointestinal system, lungs and other gas containing areas of the body

4. Possible traffic conflict or midair collision during emergency descent

The amount of risk to the flight crew with a loss of pressurization increases as any or all of the following increase:

* The amount of pressure change

* The rate of the pressure change, and

* The length of time a person is exposed to the decreased pressure

Aircraft decompression is divided into 3 categories;

* EXPLOSIVE-Cabin decompresses faster than lungs can expel air which will probably result in internal injuries and DCS; pilot incapacitation is very likely (Most likely caused by Structural Damage)

* RAPID-Air is forcibly expelled from lungs. With the possibly of pain from the expanding of trapped gasses in abdomen, ears and sinuses. DCS is also an very likely possibility (Most likely caused by Structural Damage)

* GRADUAL-Usually goes un-noticed unless aircraft altitude warnings are activated or possibly the "popping" of ear drums may alert the crew to the climbing cabin altitude. Loss of consciousness from hypoxia is the highest risk if the gradual pressure decrease goes un-noticed. (Most likely caused by failing or miss set pressurization, NOT structural damage)

2. MITIGATING THE RISKS

RISKS FROM HYPOXIA-

Mental and Physical Impairment

Time of Useful Consciousness, (TUC) is the amount of time available after encountering excessive cabin altitudes until an individual becomes incapacitated due to forgetfulness, incompetence, impaired reasoning and indifference to self preservation.

The onset of hypoxia will be different for each individual, but the following times are within the range of the FAA's published averages for an active crewmember and easy to remember;

* 18,000 ft - 18 minutes

* 25,000 ft - 2.5 minutes

* 30,000 ft - 30 seconds

* 40,000 ft - Just enough time to put on Quick Donning O2 mask plus about 2 seconds

MITIGATION, upon the discovery of ANY abnormality with aircraft pressurization, the first and immediate action must be for all crew don O2 masks set to 100%; this action promptly executed will virtually eliminate the risk of Hypoxia.

RISK OF DECOMPRESSION SICKNESS (BENDS) VERSUS ALTITUDE

There is no specific altitude below which it can be assured that no one will develop DCS, however, the following statistics have been collected;

* Almost no DCS have been reported among healthy individuals at pressure altitudes below 18,000 ft who have not been scuba diving

* About 13% of the reported DCS incidents occurred between 18,000 ft and 25,000 ft

* The majority 87% of reported DCS incidents occurred at 25,000 ft or higher

MITIGATION, Donning O2 masks does NOT mitigate DCS but, by descending without delay to increase the pressure on the body, the gas bubbles released from the body tissues and fluids will cease to form and will slowly re-dissolve.

RISKS FROM TRAPPED GAS (BAROTRAUMA)

There are numerous reports of the expansion of gasses in internal organs and body cavities having caused such immediate and severe pain that one or more crewmembers were incapacitated and unable to perform their flight duties.

MITIGATION, Donning O2 masks does NOT mitigate barotrauma but, descending without delay to increase the pressure on the body, the expansion of the gasses will be reversed and pain reduced or eliminated

RISK OF TRAFFIC CONFLICT-

The next risk, traffic conflict