Time of Useful Consciousness
Encyclopedia
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.
, 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 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.
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.
For orbital altitudes and above, that is, direct exposure to space, 6-9 seconds of consciousness is expected.
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
duties efficiently in an environment of inadequate oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
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
Unconsciousness
Unconsciousness is the condition of being not conscious—in a mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is a type of unconsciousness. Fainting due to a drop in blood pressure and a...
. 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 hypoxiaHypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
, even within the same person. Generally, old age
Old age
Old age consists of ages nearing or surpassing the average life span of human beings, and thus the end of the human life cycle...
tends to reduce the efficiency of the pulmonary system, and can cause the onset of hypoxia symptoms sooner. Smoking
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...
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 simulated in an altitude chamber
Altitude chamber
A hypobaric chamber, or altitude chamber, is a chamber used during aerospace or high terrestrial altitude research or training to simulate the effects of high altitude on the human body, especially hypoxia and hypobaria...
. 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:
| Altitude in Flight level Flight level A Flight Level is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa , the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either... |
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
Flight level
A Flight Level is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa , the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either...
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.
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.
For orbital altitudes and above, that is, direct exposure to space, 6-9 seconds of consciousness is expected.
See also
- Hypoxia (medical)Hypoxia (medical)Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
- Cabin pressurizationCabin pressurizationCabin pressurization is the pumping of compressed air into an aircraft cabin to maintain a safe and comfortable environment for crew and passengers when flying at altitude.-Need for cabin pressurization:...
- Pressure SuitPressure suitA pressure suit is a protective suit worn by high-altitude pilots who may fly at altitudes where the air pressure is too low for an unprotected person to survive, even breathing pure oxygen at positive pressure. Such suits may be either full-pressure or partial-pressure...
- Uncontrolled decompression
- Armstrong LimitArmstrong LimitThe Armstrong limit, often called Armstrong's line, is the altitude that produces an atmospheric pressure so low that water boils at the normal temperature of the human body: 37 °C . It is named after Harry George Armstrong, who founded the U.S. Air Force’s Department of Space Medicine in...
maximum altitude that is survivable without pressurisation (about 62,000 feet)