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Students who have physical limitations, such as impaired vision, loss of a limb, or hearing impairment may be issued a medical certificate valid for “student pilot privileges only” while learning to fly. Pilots with disabilities may require special equipment installed in the aircraft, such as hand controls for pilots with paraplegia. Some disabilities necessitate a limitation on the individual’s certificate; for example, impaired hearing would require the limitation “not valid for flight requiring the use of radio.” When all the knowledge, experience, and proficiency requirements have been met and a student can demonstrate the ability to operate the aircraft with the normal level of safety, a “statement of demonstrated ability” (SODA) can be issued. This waiver, or SODA, is valid as long as the physical impairment does not worsen. Contact the local Flight Standards District Office (FSDO) for more information on this subject.
Health and Physiological Factors Affecting Pilot Performance
A number of health factors and physiological effects can be linked to flying. Some are minor, while others are important enough to require special attention to ensure safety of flight. In some cases, physiological factors can lead to inflight emergencies. Some important medical factors that a pilot should be aware of include hypoxia, hyperventilation, middle ear and sinus problems, spatial disorientation, motion sickness, carbon monoxide (CO) poisoning, stress and fatigue, dehydration, and heatstroke. Other subjects include the effects of alcohol and drugs, anxiety, and excess nitrogen in the blood after scuba diving.Hypoxia
Hypoxia means “reduced oxygen” or “not enough oxygen.” Although any tissue will die if deprived of oxygen long enough, usually the most concern is with getting enough oxygen to the brain, since it is particularly vulnerable to oxygen deprivation. Any reduction in mental function while flying can result in life-threatening errors. Hypoxia can be caused by several factors, including an insufficient supply of oxygen, inadequate transportation of oxygen, or the inability of the body tissues to use oxygen. The forms of hypoxia are based on their causes: hypoxic hypoxia, hypemic hypoxia, stagnant hypoxia, and histotoxic hypoxia.Hypoxic Hypoxia
Hypoxic hypoxia is a result of insufficient oxygen available to the body as a whole. A blocked airway and drowning are obvious examples of how the lungs can be deprived of oxygen, but the reduction in partial pressure of oxygen at high altitude is an appropriate example for pilots. Although the percentage of oxygen in the atmosphere is constant, its partial pressure decreases proportionately as atmospheric pressure decreases. As the airplane ascends during flight, the percentage of each gas in the atmosphere remains the same, but there are fewer molecules available at the pressure required for them to pass between the membranes in the respiratory system. This decrease in number of oxygen molecules at sufficient pressure can lead to hypoxic hypoxia.Hypemic Hypoxia
Hypemic hypoxia occurs when the blood is not able to take up and transport a sufficient amount of oxygen to the cells in the body. Hypemic means “not enough blood.” This type of hypoxia is a result of oxygen deficiency in the blood, rather than a lack of inhaled oxygen, and can be caused by a variety of factors. It may be due to reduced blood volume (due to severe bleeding), or it may result from certain blood diseases, such as anemia. More often hypemic hypoxia
16-3
Figure 16-1. Time of useful consciousness.
Altitude
Time of Useful Consciousness45,000 feet MSL40,000 feet MSL35,000 feet MSL30,000 feet MSL28,000 feet MSL25,000 feet MSL22,000 feet MSL20,000 feet MSL9 to 15 seconds15 to 20 seconds30 to 60 seconds1 to 2 minutes2½ to 3 minutes3 to 5 minutes5 to 10 minutes30 minutes or more
occurs because hemoglobin, the actual blood molecule that transports oxygen, is chemically unable to bind oxygen molecules. The most common form of hypemic hypoxia is CO poisoning. This is explained in greater detail on page 16-11. Hypemic hypoxia can also be caused by the loss of blood due to blood donation. Blood can require several weeks to return to normal following a donation. Although the effects of the blood loss are slight at ground level, there are risks when flying during this time.Stagnant Hypoxia
Stagnant means “not flowing,” and stagnant hypoxia, or ischemia, results when the oxygen-rich blood in the lungs is not moving, for one reason or another, to the tissues that need it. An arm or leg “going to sleep” because the blood flow has accidentally been shut off is one form of stagnant hypoxia. This kind of hypoxia can also result from shock, the heart failing to pump blood effectively, or a constricted artery. During flight, stagnant hypoxia can occur with excessive acceleration of gravity (Gs). Cold temperatures also can reduce circulation and decrease the blood supplied to extremities.Histotoxic Hypoxia
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Pilot's Handbook of Aeronautical Knowledge航空知识手册3(40)
