Par 2-7 Page 19 pressures. If heavy smokers are among the crew or passengers, a lower cabin altitude should be set because physiologic altitudes for smokers are generally much higher than actual altitudes. For example, a smoker’s physiological altitude at sea level is approximately 7,000 ft MSL.
Twenty thousand ft actual altitude for a nonsmoker would be equivalent to a physiological altitude of 22,000 ft MSL for a smoker. The smoker is thus more susceptible to hypoxia at lower altitudes than the nonsmoker.
2. Hypemic hypoxia is corrected by locating and eliminating the source of the contaminating gases. A careful preflight of heating systems and exhaust manifold equipment is mandatory. If symptoms persist, ventilate the cabin, land as soon as possible, and seek medical attention, because the symptoms may be indicative of carbon monoxide poisoning.
WARNING:
If hypoxia is suspected, immediately don oxygen mask and breathe 100 percent oxygen slowly. Descend to a safe altitude. If supplemental oxygen is not available, initiate an emergency descent to an altitude below 10,000 ft MSL. If symptoms persist, land as soon as possible.
(c) Stagnant Hypoxia. This is a less common form of hypoxia. It is due to inadequate blood flow to the tissues. Causes include heart failure, shock, prolonged positive pressure breathing, and excessive G-forces.
WARNING:
If hypoxia is suspected, immediately don oxygen mask and breathe 100 percent oxygen slowly. Descend to a safe altitude. If supplemental oxygen is not available, initiate an emergency descent to an altitude below 10,000 ft MSL. If symptoms persist, land as soon as possible.
(d) Histotoxic Hypoxia. This is the inability of the body’s cells to use oxygen because of impaired cellular respiration. Alcohol or drug use can cause this type of hypoxia. The only method of avoiding this type of hypoxia is to abstain, before flight, from alcohol or drugs that are not approved by a flight surgeon or an Aviation Medical Examiner (AME).
WARNING:
If hypoxia is suspected, immediately don oxygen mask and breathe 100 percent oxygen slowly. Descend to a safe altitude. If supplemental oxygen is not available, initiate an emergency descent to an altitude below 10,000 ft MSL. If symptoms persist, land as soon as possible.
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(e) Hypoxia Signs and Symptoms.
1. Signs of hypoxia can be detected in an individual by an observer. Signs aren’t a very effective tool for hypoxic individuals to use to recognize hypoxia in themselves.
Symptoms of hypoxia are the sensations a person can detect while in a hypoxic state. Personal symptoms of hypoxia are as individual as the person experiencing them. A group of people who are hypoxic will, a majority of the time, get the same symptoms. However, the symptoms will appear in a different order and in varying intensities. The greatest benefit in hypoxia training in an altitude chamber is that the order and the intensity of your symptoms will usually remain constant over the years. Therefore, familiarity with one’s own hypoxia “signature” will facilitate the recognition of a hypoxic state during flight. Table 2-5, Common Hypoxia Signs and Symptoms, lists some of the more common signs and symptoms of hypoxia.
TABLE 2-5. COMMON HYPOXIA SIGNS AND SYMPTOMS
SIGNS SYMPTOMS Rapid Breathing Cyanosis (bluing effect of the skin) Poor Coordination Lethargy Executing Poor Judgment Sweating Trembling Myoclonic (muscle) Spasms Air Hunger Fatigue Nausea Headache Dizziness Hot and Cold Flashes Tingling Visual Impairment Euphoria
2. Of the listed symptoms, visual impairment is probably the least reliable.
Your visual field will be affected, but at such a slow rate that it could go unnoticed. Generally, symptoms will appear before unconsciousness occurs. Except for possible headache and nausea, there are no other uncomfortable symptoms. Of all the symptoms, euphoria (a false sense of well-being) is probably the most dangerous. Furthermore, in most cases, hypoxia onset is very insidious. Any preoccupation with flying duties could be enough of a distraction to allow the hypoxia to progress beyond the point of self-help.
WARNING:
A common misconception among pilots is that it is easy to recognize the symptoms of hypoxia and to take corrective action before becoming seriously impaired. While this concept may be appealing in theory, it is both misleading and dangerous for crewmembers.
3. While other significant effects of hypoxia usually do not occur in a healthy pilot in an unpressurized aircraft below 12,000 ft MSL, there is no assurance that this will always be the case. Furthermore, the altitude range of impairment due to hypoxia is best described as a continuum; there is no definitive altitude at which the effects of hypoxia begin or
Par 2-7 Page 21 end. To mitigate the risk associated with these variations, if hypoxia is suspected, a descent to altitudes below 10,000 ft MSL is recommended.
(f) Preventative Measures Against Hypoxia.
1. A common technique for aviators to monitor themselves for hypoxia is the use of finger pulse oximeters that report the degree of hemoglobin oxygen saturation as a percent. This value is useful in predicting the partial pressure of oxygen available to drive oxygen into the cells.
CAUTION:
The FAA cautions against relying on pulse oximeters as the sole indicator of hypoxia because:
• Waiting for the percent oxygen saturation levels reported by the pulse oximeter to fall before taking action may result in a level of hypoxia sufficient to cause impairment.
• Hemoglobin oxygen saturation in blood passing through the finger may not reflect true oxygen availability to the brain because of reductions in brain blood flow resulting from hyperventilation (a response to hypoxia).
2. The following are recommended actions to reduce the chance of hypoxia.
• Fly at an altitude where oxygen is not required.
• Fly in a pressurized cabin.
• Fly in accordance with current regulations in reference to the use of supplemental oxygen.
NOTE: The FAA recommends that on any unpressurized flight to or above 10,000 ft in the day, or above 5,000 ft at night, supplemental oxygen should be used.
(g) Treatment for Hypoxia. Hypoxia, under most situations, will be insidious in its onset, which gives it its dangerous nature. Because of the rapid breathing associated with hypoxia, you must slow your breathing rate to prevent hyperventilation. Recognition and the immediate execution of emergency procedures for decompression are the most important factors that dictate the crew’s ability to survive. If hypoxia is suspected, immediately don an oxygen mask and breathe 100 percent oxygen slowly. Descend to a safe altitude. Once hypoxia is detected and 100 percent oxygen is administered, recovery usually occurs in a matter of seconds.
If supplemental oxygen is not available, initiate an emergency descent to an altitude below 10,000 ft MSL. If symptoms persist, land as soon as possible. The common emergency procedures follow in Table 2-6, Common Emergency Procedures for Hypoxia.
Page 22 Par 2-7 TABLE 2-6. COMMON EMERGENCY PROCEDURES FOR HYPOXIA DON MASK In 5 seconds or less. Check for flow. Breathe 100 percent oxygen.
DESCEND Preferably below 10,000 ft.
LAND A.S.A.P At nearest suitable installation where appropriate medical help can be found.
e. Decompression in Pressurized Aircraft. Even though there are numerous advantages to pressurized flight, there will always be one major disadvantage—possible loss of pressurization.
(1) Rate of Decompression. Factors that dictate how fast an aircraft will decompress are:
(a) Size of Opening. With all other factors equal, the larger the opening, the faster the pressure will travel out of the aircraft.
(b) Size of Cabin. With all other factors equal, the smaller the cabin, the faster the pressure will depart.
(c) Pressure Differential. With all other factors equal, greater pressure differential means slower decompression.
(d) Pressure Ratio. When cabin pressurization is lost, aircraft compressors will continue to operate. The rate at which the compressed air comes in dictates how fast pressure is lost.
(e) Altitude. Higher altitude results in a faster decompression due to less resistance to the air leaving the aircraft.
(2) Phenomena Associated with Decompressions. These decompression phenomena are very common to rapid and explosive decompressions. However, during a slow decompression, the phenomena below may not be evident. Furthermore, the insidious nature of hypoxia and depression of mental function decreases the ability to recognize the emergency and undertake appropriate recovery procedures. For this reason, slow decompressions are the most dangerous, and the aviator must always be on guard against this insidious threat.
(a) Noise. Noise can come from a leaky door seal, a window departing, a breach of structural integrity, or from the aircraft’s alarm system.
(b) Fog. On smaller airframes (Learjets, Citations, Falcons) the fog could fill the aircraft.
(c) Flying Debris, Dust, and Dirt. If there is a large opening, anything not secured down will move towards the opening. These items could cause injury.
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(d) Wind Blast. The loss of pressure in the aircraft may be felt by crew and passengers as a wind blast.
(e) Cooler Temperatures. When the pressure (air) departs the cabin, the temperature will drop.
(f) Gas Expansion. As the gas expands with decreasing pressure, it will be most noticeable in the ears and GI tract of the body.
(3) Time of Useful Consciousness (TUC) or Effective Performance Time (EPT).
This 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. The faster the rate of ascent, the worse the impairment and the faster it happens. TUC also decreases with increasing altitude. Figure 2-3, Times of Useful Consciousness versus Altitude, shows the trend of TUC as a function of altitude. However, slow decompression is as dangerous as or more dangerous than a rapid decompression. By its nature, a rapid decompression commands attention. In contrast, a slow decompression may go unnoticed and the resultant hypoxia may be unrecognized by the pilot.
WARNING:
The TUC does not mean the onset of unconsciousness. Impaired performance may be immediate. Prompt use of 100 percent oxygen is critical.
WARNING:
Slow decompression is as dangerous as or more dangerous than a rapid or explosive decompression.
Page 24 Par 2-7 FIGURE 2-3. TIMES OF USEFUL CONSCIOUSNESS VERSUS ALTITUDE
ALTITUDE TUC/EPT Following Rapid Decompression 18,000 20–30 min 10–15 min 22,000 10 min 5–6 min 25,000 3–5 min 1.5–2.5 min 28,000 2.5–3 min 1–1.5 min 30,000 1–2 min 30 s–1 min 35,000 30 s–1 min 15–30 s 40,000 15–20 s Nominal 43,000 9–12 s Nominal 50,000 9–12 s Nominal
CAUTION:
TUCs are based on data that represent average values and reflect wide variation among individuals in time to incapacitation. This variation results from differences in an individual’s total surface area for gas exchange in the lungs, total amount of hemoglobin available in the blood to bind oxygen, and oxygen consumption rate at rest (related to body mass index). Other sources of variation are the extent to which hypoxia stimulates increases in depth and rate of breathing, and increases in the amount of blood the heart pumps (faster heart rate). Finally, individuals able to increase the amount of oxygen they can extract from the blood in muscle and brain tissue are more hypoxia tolerant. This is largely genetically determined, but can be enhanced by physical conditioning resulting from a regular aerobic exercise program. As many of these factors will be unknown by the aviator, the safest approach would be to assume the lower TUC value is limiting.
(a) Factors Affecting Hypoxia Recognition and TUC: Slow Decompression and Rapid or Explosive Decompression. Rapid loss of aircraft pressurization dramatically reduces TUC. As a general rule, it can be assumed that the TUC following decompression to altitudes between 25,000 ft and 43,000 ft will be reduced by 50 percent. Above 43,000 ft, the TUC is reduced to the time it takes for the blood to circulate from the lung to the brain, plus any reserve oxygen stored in the brain. This is approximately 9–12 s from the start of a rapid decompression to the loss of functional capability. In some cases, hypoxia exposure can lead to a shock type reaction that could further reduce the TUC. In any case, the potential for impairment begins
Par 2-7 Page 25 almost immediately. For this reason, the donning of the oxygen mask should be practiced from time to time.
(b) Other Factors that Determine TUC.
1. TUC decreases with:
(i) Rate of ascent. The faster you ascend to altitude, the shorter your EPT/TUC becomes.
(ii) Rate of decompression. Rapid decompression decreases the TUC by at least 50 percent.
(iii) Physical activity at altitude. Any physical activity will reduce your EPT/TUC. For example, if you did 10 deep knee bends at 25,000 ft with your oxygen mask off, your EPT/TUC would be reduced by 50 percent.
(iv) Fatigue. If you enter the cockpit in a fatigued state, you are less resistant to hypoxia.
(v) Diet. The brain feeds exclusively from glucose (blood sugar), so if your glucose is low, as in hypoglycemia, you are more prone to hypoxia.
(vi) Alcohol. Alcohol brings about its own form of hypoxia. When altitude is coupled with alcohol, you are a strong candidate for a hypoxic episode.
(vii) Medications. Some drugs, including over-the-counter medications, will cause cells not to utilize oxygen properly and therefore will make you less altitude resistant.
(viii) Other factors. Smoking, poor physical condition, and illness also decrease the TUC.
2. Impairment increases with:
• Increasing altitude.
• Increasing difficulty and complexity of the task.
• New tasks compared to learned, practiced tasks.
(4) Altitude-Induced Decompression Sickness (DCS). According to Henry’s Law, when the partial pressure of a gas over a liquid is decreased, the amount of gas dissolved in that liquid will also decrease. One of the best practical demonstrations of this law is offered by opening a soft drink. When the cap is removed from the bottle, gas is heard escaping and bubbles can be seen forming in the soda. This is carbon dioxide gas coming out of solution as a result of sudden exposure to a lower partial pressure of carbon dioxide in the environment.
(a) Nitrogen Absorption. One of the more dangerous problems an aviator may face is the threat of DCS (nitrogen bubbles in body fluids and tissues) at high altitudes. Nitrogen we breathe is taken into the lungs at a pressure of 608 mmHg (80 percent of the total
Page 26 Par 2-7 atmospheric pressure 760 mmHg at sea level). The nitrogen is then distributed throughout the body by the circulatory system and stored at a pressure of about 608 mmHg. At sea level, the nitrogen pressure inside the body and outside of the body is in equilibrium. When atmospheric pressure is reduced as a result of ascent, the equilibrium is upset. This results in nitrogen leaving the body by passing from the cells, to the blood, and then out through the respiratory system. If the nitrogen is forced to leave the solution too rapidly because of a large partial pressure difference, bubbles may form, causing a variety of signs and symptoms. The symptoms result from the location of the bubbles. These evolving and expanding gases in the body are known as DCS. There are two groups.
1. Trapped Gas. Expanding or contracting gas in certain body cavities during altitude changes can result in abdominal pain, toothache, or pain in ears and sinuses if the person is unable to equalize the pressure changes. Above 25,000 ft MSL, distention can produce particularly severe GI pain.
2. Evolved Gas. When the pressure on the body drops sufficiently, nitrogen comes out of solution and forms bubbles, which can have adverse effects on some body tissues.
See Table 2-7, Types of Evolved Gas Decompression Sickness. Fatty tissue contains more nitrogen than other tissue, thus making overweight people more susceptible to evolved gas DCS.
Par 2-7 Page 27 TABLE 2-7. TYPES OF EVOLVED GAS DECOMPRESSION SICKNESS DCS Type Bubble Location Signs & Symptoms (Clinical Manifestations) Bends Mostly large joints of the body (shoulders, elbows, wrists, hips, knees, ankles) Localized deep pain, ranging from mild (a “niggle”) to excruciating. Sometimes a dull ache, but rarely a sharp pain. Active or passive motion of the joint aggravates the pain. Pain can occur at altitude, during the descent, or many hours later. The pain gradually becomes more severe, can become incapacitating, and can even result in collapse.
Neurologic Brain Confusion or memory loss. Headache. Spots in visual field (scotoma), tunnel vision, double vision (diplopia), or blurry vision. Unexplained extreme fatigue or behavior changes. Seizures, dizziness, vertigo, nausea, vomiting, and unconsciousness may occur.
Spinal Cord Abnormal sensations such as numbness, burning, stinging, and tingling around the lower chest and back.
Symptoms may spread from the feet up and may be accompanied by ascending weakness or paralysis.
Girdling abdominal or chest pain.
Peripheral Nerves Urinary and rectal incontinence. Abnormal sensations such as numbness, burning, stinging, and tingling (paresthesia). Muscle weakness or twitching.
Chokes Lungs Burning deep chest pain (under the sternum). Pain is aggravated by breathing. Dry constant cough. Possible cyanosis (a bluish color of the skin, nail beds, and the mucous membranes due to insufficient oxygen in the blood). Shortness of breath (dyspnea), sensation of suffocation, progressively shallower breathing, and, if a descent is not made immediately, collapse and unconsciousness.
Skin Manifestations Skin Itching usually around the ears, face, neck, arms, and upper torso. Sensation of tiny insects crawling over skin (formication). Mottled or marbled skin usually around the shoulders, upper chest, and abdomen, accompanied by itching. Swelling of the skin, accompanied by tiny scar-like skin depressions (pitting edema).
(b) Risk Trends. Most cases of altitude DCS occur among individuals exposed to altitudes of 25,000 ft or higher. A United States Air Force (USAF) study of altitude DCS cases reported that only 13 percent occurred below 25,000 ft. The risk of developing altitude DCS increases with altitude and time of exposure.
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(c) Predisposing Factors.
1. Repetitive Exposures. Repetitive exposures to altitudes above 18,000 ft within a short period of time (few hours) also increase the risk of developing altitude DCS.
2. Rate of Ascent. The faster the rate of ascent to altitude, the greater the risk of developing altitude DCS. An individual exposed to a rapid decompression (high rate of ascent) above 18,000 ft has a greater risk of altitude DCS than being exposed to the same altitude at a lower rate of ascent.
3. Time at Altitude. The longer the duration of the exposure to altitudes of 18,000 ft and above, the greater the risk of altitude DCS. Time at altitude has been shown to be the strongest predictor of DCS.
4. Age. There are some reports indicating a higher risk of DCS with increasing age.
5. Previous Injury. There is some indication that recent joint or limb injuries may predispose individuals to developing “the bends.”
6. Ambient Temperature. Exposure to very cold ambient temperatures may increase the risk of altitude DCS.
7. Body Type. A person who has a high body fat content may be at greater risk of altitude DCS, as nitrogen is stored in greater amounts in fat tissues.
8. Exercise. When a person is physically active while flying at altitudes above 18,000 ft, there is greater risk of altitude DCS.
9. Alcohol Consumption. The after-effects of alcohol consumption, such as dehydration, increase susceptibility to DCS.
(d) Self-Contained Underwater Breathing Apparatus (SCUBA) Diving Before Flying. SCUBA diving requires breathing air under high pressure. Under these conditions, there is a significant increase in the amount of nitrogen dissolved in the body (body nitrogen saturation). The deeper the SCUBA dive, the greater the body nitrogen saturation. Following SCUBA diving, if not enough time is allowed to eliminate the excess nitrogen stored in the body, altitude DCS can occur during exposure to altitudes as low as 5,000 ft or less. After SCUBA diving, a person who flies in an aircraft to a pressure altitude of 8,000 ft MSL would experience the same effects as a nondiver flying unpressurized at 40,000 ft MSL.
1. The recommended waiting period before going to flight altitudes up to 8,000 ft MSL is at least:
• Twelve hours after nondecompression stop diving (diving that does not require a controlled ascent), and
• Twenty-four hours after decompression stop diving (diving that requires a controlled ascent).
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2. For flight altitudes above 8,000 ft MSL, the recommended wait time is at least 24 hours after any SCUBA diving. (See AIM paragraph 8-1-2, subparagraph d2.)
(5) What to Do If You Suspect DCS in Flight.
(a) Put on your oxygen mask immediately and switch the regulator to 100 percent oxygen.
(b) Begin an emergency descent and land as soon as possible. Even if the symptoms disappear during descent, you should still land and seek medical evaluation while continuing to breathe oxygen.
(c) If one of your symptoms is joint pain, keep the affected area still; do not try to work pain out by moving the joint around.
(6) Medical Treatment. A mild case of the bends may disappear during descent from high altitude. However, it still requires medical evaluation. If the signs and symptoms persist during descent or reappear at ground level, it may be necessary to receive hyperbaric oxygen treatment immediately. This involves 100 percent oxygen delivered in a high-pressure chamber.
WARNING:
Neurological DCS, “the chokes,” and skin manifestations (mottled or marbled skin lesions) are very serious and potentially fatal if untreated.
(7) Things to Remember about DCS.
(a) Altitude DCS is a potential risk every time you fly in an unpressurized aircraft above 18,000 ft, and at lower altitudes if you SCUBA dive prior to the flight.
(b) Be familiar with the signs and symptoms of altitude DCS and monitor all aircraft occupants, including yourself, any time you fly an unpressurized aircraft above 18,000 ft.
(c) Avoid unnecessary strenuous physical activity prior to flying an unpressurized aircraft above 18,000 ft and for 24 hours after the flight.
(d) Even if you are flying a pressurized aircraft, altitude DCS can occur as a result of sudden loss of cabin pressure (in-flight rapid decompression).
(e) Following exposure to an in-flight rapid decompression, do not fly for at least 24 hours. In the meantime, remain vigilant for the possible onset of delayed symptoms or signs of altitude DCS. If you experience delayed symptoms or signs of altitude DCS, seek medical attention immediately.
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(f) Keep in mind that breathing 100 percent oxygen during flight (ascent, en route, descent) without oxygen prebreathing prior to take off, does not prevent the occurrence of altitude DCS.
(g) Do not ignore any symptoms or signs that go away during the descent. In fact, this could confirm that you are actually suffering altitude DCS.
(h) Any case of altitude DCS should be medically evaluated as soon as possible, even if symptoms are mild or disappear on descent.
(i) If there is any indication that you may have experienced altitude DCS, do not fly again until you are cleared to do so by a physician familiar with and experienced with treatment of DCS. For questions about your continued eligibility for your airman medical certificate contact an FAA medical officer or your AME.
(j) Allow at least 24 hours to elapse between SCUBA diving or hypobaric (altitude) chamber exposure and flying.
(k) Be prepared for a future emergency by familiarizing yourself with the availability of hyperbaric chambers in your area of operations. However, keep in mind that not all of the available hyperbaric treatment facilities have personnel qualified to handle altitude DCS emergencies. To obtain information on location of hyperbaric treatment facilities capable of handling altitude DCS emergencies, call the Divers Alert Network at 919-684-9111.
f. Vision Deterioration. Vision has a tendency to deteriorate with altitude; smoking worsens this effect. A reversal of light distribution at high altitudes (bright clouds below the airplane and darker, blue sky above) can cause a glare inside the cockpit. Glare effects and deteriorated vision are enhanced at night when the body becomes more susceptible to hypoxia and can occur at altitudes as low as 5,000 ft MSL. In addition, the empty visual field caused by cloudless, blue skies during the day can cause inaccuracies when judging the speed, size, and distance of other aircraft. We recommend sunglasses to minimize the intensity of the sun’s ultraviolet rays at high altitudes.
2-8. FAA PHYSIOLOGICAL TRAINING. There are no specific requirements in 14 CFR part 91 or 125 for physiological training. However, § 61.31(g) requires the high-altitude training outlined in this AC. Also, 14 CFR parts 121 and 135 require flightcrew members that serve in operations above 25,000 ft MSL to receive training in specified subjects of aviation physiology.
None of the regulations include altitude chamber training. Although most of the subject material normally covered in physiological training concerns problems associated with reduced atmospheric pressure at high-flight altitudes, it covers other equally important subjects as well.
Such subjects of aviation physiology as vision, disorientation, physical fitness, stress, and survival affect flight safety and are normally present in a good training program.
a. FAA Civil Aerospace Medical Institute (CAMI) Aviation Physiology Course.
CAMI offers a 1-day aviation physiology course for FAA flightcrews, civil aviation pilots, and FAA AMEs. In addition to the basic academic contents, this course offers practical demonstrations of rapid decompression (8,000 to 18,000 ft MSL) and hypoxia (25,000 ft MSL)