Hypoxia

Hypoxia—lack of oxygen—is the greatest single danger to a pilot at high altitudes. When a pilot inhales air at high altitudes, there isn't enough oxygen pressure to force adequate amounts of this vital gas through the membranes of the lungs into the bloodstream so that it can be carried to the tissues of the body.

At sea level, a healthy person can extract enough oxygen from the air to continue normal activities. Above 8,000–9,000 ft (2,400–2,700 m), however, problems related to hypoxia begin to appear. Because the air is less dense, it offers less oxygen per breath of air inhaled—although oxygen and nitrogen are still mixed in a 20:80 ratio.At about 10,000 ft (3,048 m), the blood can still carry oxygen at 90 percent of its capacity. At this altitude, the flight performance of a healthy pilot is impaired only after some time. At 14,000 ft (4,300 m), the pilot may become appreciably handicapped—forgetting to switch fuel tanks, flying off course, or disregarding hazardous situations. From 18,000 ft (5,500 m) and beyond, exposure to environmental air quickly causes total collapse and inability to control the aircraft.

Symptoms of Hypoxia

Regardless of acclimatization, endurance, or other attributes, every pilot suffers from hypoxia when exposed to inadequate oxygen pressure. Symptoms vary, but they often include:

1. A pleasant, mildly intoxicated feeling
2. Poor coordination
3. A false sense of security
4. Drowsiness or stupor

As hypoxia worsens, other symptoms appear, including:

1. Dizziness
2. Tingling of the skin
3. Dull headache
4. Racing heart
5. Cyanosis—a bluish discoloration of the skin, especially of the lips and the skin beneath the fingernails
6. Tunnel or blurry vision

Safety Rules

The FARs (FAR 91.211) and CAR's require pilots to use supplemental oxygen when flying at high altitudes. Whenever an unpressurized airplane is operated between 12,500 and 14,000 ft (3,800–4,300 m) for more than 30 minutes the crew must use supplemental oxygen. Between 14,000 and 15,000 ft (3,800–4,600 m) the crew must use oxygen at all times. Above 15,000 ft (4,600 m) the crew must use oxygen and make it available to all passengers.

Avoiding Hypoxia

Here are some general suggestions which apply to young healthy flyers:

1. Carry oxygen in your plane or don't fly above 12,500 ft (3,800 m). If bad weather lies ahead, go around it if you can't get over it.
2. Use oxygen on every flight above 12,500 ft (3,800 m). You'll probably need it, and when you do, you might not realize it.
3. Use oxygen on protracted flights near 12,500 ft (3,800 m).
4. Use oxygen on all night flights above 5,000 ft (1,500 m). To give your night vision the best protection, use oxygen from the ground up.
5. Breathe normally when using oxygen. Rapid or deep breathing can cause loss of consciousness from hyperventilation.
6. Pilots who are older, overweight, out of condition, or who smoke heavily should limit themselves to a ceiling of 8,000–10,000 ft (2,400–3,000 m) unless supplemental oxygen is available.