Effects Of Hypoxia
The common symptoms of hypoxia include increased breathing rate, dizziness, headache, sweating, reduced peripheral vision, and fatigue, but the most insidious symptom is a feeling of euphoria. Pilots suffering from hypoxia often experience a false sense of security rather than a sense of the danger inherent to this condition.
Hypoxia also impairs night vision. Because the rod cells in the eye, which give us night vision, require a lot of oxygen, a lack of oxygen causes visual impairment. One FAA reference suggests that without oxygen, a pilot flying at night is 24 percent blind at 8,000 feet, and 50 percent blind at 12,000 feet.
The onset of hypoxia, and its symptoms, varies from one person to another, and even from day to day in the same person. However, the Aeronautical Information Manual (AIM) tells us that at 15,000 feet pilot performance can deteriorate seriously within 15 minutes. Within 20 to 30 minutes at 18,000 feet, a pilot loses the ability to take corrective and protective action. It takes only five to 12 minutes at 20,000 feet.
Good physical fitness may raise the altitude at which hypoxia symptoms occur or delay its onset; but carbon monoxide from smoking or exhaust fumes, alcohol, or such medications as antihistamines and analgesics can increase a person's susceptibility. The effects of hypoxia are mind-numbing, but the use of supplemental oxygen provides a simple precaution to ensure high-altitude safety.
Regulations And More
FAR 91.211 requires all pilots to use supplemental oxygen when flying at cabin altitudes of 14,000 feet and higher, and for any portion of a flight at 12,500 to 14,000 feet that exceeds a duration of 30 minutes. Above 15,000 feet, all occupants must use oxygen. In an airplane without cabin pressurization, the altimeter indicates the cabin altitude.
Even if we follow these regulations, we can still be at risk. FAR 135.89 governs commercial operations such as on-demand charter. It requires pilots to use oxygen continuously when flying an unpressurized aircraft above 12,000 feet, and for any portion of a flight between 10,000 and 12,000 feet that exceeds 30 minutes.
If a professional, two-pilot crew needs oxygen to make good decisions above 10,000 feet, perhaps we should all follow Part 135's more stringent requirements. When flying at night and above 5,000 feet, it is recommended that pilots use supplemental oxygen to give them adequate night vision.
Supplemental Oxygen Systems
Three basic systems are available to feed pilots supplemental oxygen, and the altitudes at which you fly determine, in part, which system you should use. The most common oxygen system is called "continuous flow."
A continuous-flow oxygen system provides adequate oxygen up to 25,000 feet. It can be either portable or permanently installed in the aircraft. A typical continuous-flow system uses a high pressure cylinder and a regulator, which reduces the oxygen to a lower pressure. Plumbing carries the oxygen to the cabin outlets where pilots and passengers plug in their oxygen masks. A portable system doesn't have the plumbing. People plug their masks directly into the regulator.
An oxygen mask has a rebreather bag, flow indicator, and plug-in connector. Oxygen flows continuously and accumulates in the bag so the user has something to breath when inhaling. Some exhaled breath returns to the bag and mixes with the pure oxygen.
A calibrated orifice or fiberglass packing in the connector controls how much oxygen (usually measured in liters per minute) flows into the mask. Because pilots require more oxygen (a higher flow) than passengers, color-coded bands - usually red - denote a pilot's mask. Passenger masks usually are marked with a gold band.
The diluter-demand oxygen system uses virtually the same cylinder, regulator, and plumbing as a continuous-flow system, but the masks are different. Instead of a rebreather bag, the mask has a regulator that provides oxygen on demand when the pilot inhales. A lever on the mask allows the user to select a normal mixture (100 percent oxygen diluted with ambient air) - or 100 percent undiluted oxygen. The diluter-demand system wastes less oxygen than a continuous flow system, and with a tighter fitting mask, a pilot can use it up to 35,000 feet.
When you fly above 35,000 feet, 100 percent oxygen isn't enough because the low ambient pressure can't force the oxygen through the lungs' membranes and into the bloodstream. A pressure-demand oxygen system is necessary because it gives the user on-demand, 100-percent oxygen at a slightly positive pressure. In other words, the system inflates the lungs like a balloon, and the user must force the air from his lungs to exhale.
Safety Considerations
All supplemental oxygen systems must use aviation-grade breathing oxygen only, which is 99.5 percent pure and has no more than 0.005 milligrams of water per liter. Other grades, including medical-grade oxygen, contain too much moisture. If a pilot uses anything but aviation-grade oxygen, the extra moisture in the lesser grades might freeze at altitude and restrict the flow of oxygen or disable the regulator.
Properly maintaining your aircraft oxygen system is critical to high-altitude flight safety. Oxygen leaks and contaminated systems can cause a loss of oxygen in flight, and these discrepancies also are a fire or explosion hazard. Spontaneous combustion can occur when oxygen and petroleum products mix. Because oxygen is a prime ingredient of combustion, no one should smoke when oxygen systems are in use.
When you plan a flight that will require oxygen, computing how much oxygen you need is just as important as computing your fuel requirement. If the oxygen system is installed in the aircraft, the pilot operating handbook provides the data you need to find the correct cylinder pressure to meet the pilot's and passengers' needs.
The data are usually presented in charts or graphs, which make the calculations relatively simple. As with all POH charts and graphs, remember to read the fine print carefully. The charts and graphs are accurate only when the proper storage cylinder is installed, when the appropriate masks are used, and when the regulator is properly set. Considering the mind-numbing effects of hypoxia, running out of oxygen at altitude can be every bit as disastrous as running out of fuel.
Experience Hypoxia Firsthand
If you're skeptical about the effects of hypoxia, you can experience hypoxia firsthand. The FAA Civil Aeromedical Institute offers a Physiological Training Program at the Mike Monroney Aeronautical Center in Oklahoma City. Participants take a "flight" in a hypobaric chamber so they can experience hypoxia in a controlled setting. For more information, contact FAA Airman Education Program Branch, AAM-420, CAMI, P.O. Box 25082, Oklahoma City, OK 73125; 405/954-4837.
You can also get this training at selected U.S. military installations across the nation. FAA Form AC 3150-7, "Physiological Training Application," contains all the information about location, fees (often around $20), scheduling, procedures, course content, and individual requirements for "military training." You can get this form from the safety program manager at your nearest FAA Flight Standards District Office or the forms manager at your nearest FAA office.
You must send in the form and fee to be scheduled for the training, and some pilots have reported that it takes six months to a year after you submit the form to be scheduled. Apparently, scheduling depends on openings in the military classes because civilians get the same training as military aviators.
S.M. Spangler
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