Time of Useful Consciousness

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Time of useful consciousness (TUC) is defined as the amount of time an individual is able to perform flying duties efficiently in an environment of inadequate oxygen supply.[1] It is the period of time from the interruption of the oxygen supply or exposure to an oxygen-poor environment to the time when useful function is lost, and the individual is no longer capable of taking proper corrective and protective action. It is not the time to total unconsciousness. The TUC has also been called Effective Performance Time (EPT). At the higher altitudes, the TUC becomes very short; considering this danger, the emphasis is on prevention rather than cure.

[edit] Medical analysis and variations

There are many individual variations of hypoxia, even within the same person. Generally, old age tends to reduce the efficiency of the pulmonary system, and can cause the onset of hypoxia symptoms sooner.[2] Smoking drastically reduces oxygen intake efficiency, and can have the effect of reducing tolerance by 1,000-2,000 meters.[3] Hypoxia can be simulated in an altitude chamber. This can be useful for identifying individual symptoms of hypoxia, along with rough estimates of the altitude that causes problems for each person. Identifying symptoms is often helpful for self-diagnosis in order to realize when altitude should be reduced. Although the times in the table below are often called average TUCs, an average failure is meaningless to a person who has a shorter TUC.

The table below reflects various altitudes with the corresponding average TUC. These times have been established from observations over a period of years and are for an individual at rest. Any exercise will reduce the time considerably. For example, usually upon exposure to hypoxia at FL 250, an average individual has a TUC of 3 to 5 minutes. The same individual, after performing 10 deep knee bends, will have a TUC in the range of 1 to 1.5 minutes.

Altitude in Flight level Time of Useful Consciousness Altitude in meters Altitude in feet
FL 180 20 to 30 Min 5,486 m 18,000
FL 220 10 Min 6,706 m 22,000
FL 250  3 to 5 Min 7,620 m 25,000
FL 280 2.5 to 3 Min 8,534 m 28,000
FL 300 1 to 2 Min 9,144 m 30,000
FL 350 0.5 to 1 Min 10,668 m 35,000
FL 400 15 to 20 Sec  12,192 m 40,000
FL 430 9 to 12 Sec 13,106 m 43,000
FL 500 and above 9 to 12 Sec 15,240 m 50,000

A rapid decompression can reduce the TUC by up to 50 percent caused by the forced exhalation of the lungs during decompression and the extremely rapid rate of ascent.

[edit] See also

[edit] References

  1. ^ Dehart, R. L.; J. R. Davis (2002). Fundamentals Of Aerospace Medicine: Translating Research Into Clinical Applications, 3rd Rev Ed.. United States: Lippincott Williams And Wilkins, 720. ISBN 9780781728980. 
  2. ^ Yoneda I, Tomoda M, Tokumaru O, Sato T, Watanabe Y (January 2000). "Time of useful consciousness determination in aircrew members with reference to prior altitude chamber experience and age". Aviat Space Environ Med 71 (1): 72–6. PMID 10632134. 
  3. ^ Yoneda I, Watanabe Y (September 1997). "Comparisons of altitude tolerance and hypoxia symptoms between nonsmokers and habitual smokers". Aviat Space Environ Med 68 (9): 807–11. PMID 9293349.