Time of useful consciousness
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. 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.
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. Smoking drastically reduces oxygen intake efficiency, and can have the effect of reducing tolerance by 1,000-2,000 meters (approx. 3,000-6,000 feet). Hypoxia can be produced 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 for healthy, young military pilots:
|Altitude in Flight level||Time of Useful Consciousness||Altitude in meters||Altitude in feet|
|FL 150||30 min or more||4,572 m||15,000|
|FL 180||20 to 30 min||5,486 m||18,000|
|FL 220||5-10 min||6,705 m||22,000|
|FL 250||3 to 6 min||7,620 m||25,000|
|FL 280||2.5 to 3 mins||8,534 m||28,000|
|FL 300||1 to 3 mins||9,144 m||30,000|
|FL 350||30 sec to 60 sec||10,668 m||35,000|
|FL 400||15 to 20 sec||12,192 m||40,000|
|FL 430||9 to 15 sec||13,106 m||43,000|
|FL 500 and above||6 to 9 sec||15,240 m||50,000|
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.
For orbital altitudes and above, that is, direct exposure to space, 6-9 seconds of consciousness is expected.
- Cabin pressurization
- Pressure Suit
- Uncontrolled decompression
- Armstrong Limit maximum altitude that is survivable without pressurisation (about 62,000 feet)
- 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. p. 720. ISBN 978-0-7817-2898-0.
- 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.
- 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.
- Mark Wolff (2006-01-06). "Cabin Decompression and Hypoxia". theairlinepilots.com. Retrieved 2008-09-01. External link in
- Geoffrey A. Landis, Human Exposure to Vacuum web page