Oxygen issues for General Aviation pilots

Most people I know (pilots included) agree that oxygen is an essential requirement for life.

But many pilot’s understanding of how their body uses oxygen is less than adequate when considering its importance to safe flight. This understanding is not overly complicated and so it is simply a matter of reviewing the basics from time to time.

This month we’ll do just that; we will look at the basic characteristics of oxygen and its use by our bodies. We will consider the impact of lack of oxygen on piloting, review the basics of onboard oxygen equipment and the regulatory requirements for its use in unpressurized aircraft.

First, a couple of words about oxygen before we get to the practical stuff. Oxygen is the eighth element by atomic number on the periodic table of elements. It is a colourless, odorless, tasteless gas with an atomic weight of about 16.

Pure oxygen is nonflammable, meaning that it will not burn (although it readily supports combustion of other flammable substances).

Oxygen derives its name from the Greek word "oxys" meaning "root of acid." The genesis of this meaning was that early scientists believed oxygen was an important ingredient in all acids. Of course this is not true, but the word stuck all the same.

Oxygen combines with most other elements and is instrumental is the existence of hundreds of thousands of organic compounds. It’s little wonder then that our survival also depends on such an important element.

While the portion of the atmosphere we fly in is a fairly homogenous conglomeration of gases (what we call air), its’ primary components are nitrogen (~78%) and oxygen (~21%).

All other gases comprise the remaining one per cent, along with water vapor which varies from one per cent to about five per cent on a given day.

These percentages remain relatively constant with changes in altitude, but as we should all recall, the pressure exerted by air decreases as we climb. Since the weight of air at sea level on a standard day is 14.7 PSI and oxygen content is 21 per cent, it will exert a partial pressure of about 3.1 PSI.

This pressure is sufficient to oxygenate the blood at sea level. However, as we climb, this partial pressure decreases even though the percent content of oxygen remains the same. At some point the pressure is so low that oxygen can no longer be effectively forced into the bloodstream and we begin to experience symptoms of hypoxia.

As a result, in an unpressurized aircraft, we need to have supplemental oxygen at an altitude around 10,000 MSL and higher during prolonged periods of flight.

Fly high enough and not even supplemental oxygen is sufficient; we must now receive that oxygen under pressure to force it into the blood stream.

Since the symptoms of hypoxia (lack of oxygen to the organs) are sometimes subtle, it’s important to review what to expect with increases in altitude.

It is generally accepted that the first organs to be affected by the lack of oxygen are the eyes. The pilot may not notice these symptoms during daylight hours, and below about 10,000 feet it presents no significant problem for safety. On the other hand, at night above 5,000 to 6,000 feet our eyes are more susceptible to the effects of oxygen deprivation.

It’s usually too dark to notice that we are not seeing as well as we should so this can present a real hazard if the pilot is not aware of the problem. For this reason, supplemental oxygen is recommended when flying at night above 5,000 feet.

Above 10,000 feet prolonged flight generally leads to fatigue and sluggishness, so pilots flying unpressurized aircraft from 10,000 feet up to 13,000 feet for more than 30 minutes are required to have and use supplemental oxygen.

Passengers must be supplied enough supplemental oxygen for 10 per cent of two or more passengers or enough for no less than one passenger in any case. Above 13,000 feet there must be a supplemental oxygen supply for everyone onboard for the aircraft.

From 10,000 to 13,000 feet passengers need not use the oxygen, but above 13,000 feet, CAR 605.32 (2) says that each person on board must wear an oxygen mask and use supplemental oxygen for the duration of the flight.

Bearing in mind that there is wide variability in susceptibility to hypoxia, above 15,000 feet the average pilot can add drowsiness, headaches, and poor judgment to the list of symptoms and also expect them to occur in as little as 30 minutes from arrival at those altitudes.

The first noticeable vision decrements starts around 18,000 feet with general blurriness. At 20,000 feet it only takes about 15 minutes for a loss of muscular control to set in. Judgment, reasoning and memory go right out the window as well.

Above around 22,000 feet it takes just minutes for the average person to lose consciousness. In an even worse case, there are about 15-20 seconds of useful consciousness at 40,000 feet. Keep in mind that above about 30,000 feet cabin altitude, the partial pressure of oxygen is too low for conventional supplemental oxygen systems, and so we must have a system that delivers oxygen under positive pressure to force the oxygen into the blood stream. One can think of a positive pressure system as effectively reducing the cabin altitude in which we are trying to breath.

This is a good place to say something about smoking and hypoxia symptoms. It is well known in the aviation physiology community that smoking reduces the oxygen carrying capability and so these pilots can expect symptoms anywhere from 2,000 to 7,000 feet lower than a nonsmoker. For night flying, this could mean that a smoker might need supplemental oxygen at any altitude above ground level to have a safe level of night vision.

As well, certain drugs, either in combination or alone, effectively increase the pilot’s susceptibility to hypoxia, so consult with your doctor prior to flying at altitude with any new medicines.

Individual symptoms to hypoxia vary from one person to another, so symptoms you hear about from a friend may or may not be the same as what you are likely to experience. Age also leads to changes in symptoms and the altitude of onset. The only good way to know exactly how you will respond to the onset of hypoxia is to experience it under controlled circumstances.

High altitude training, with an altitude chamber flight, is the best way to gain this knowledge about your own symptom set.

For most people, an additional symptom to those presented earlier is cyanosis. Cyanosis is generally noticed in the extremities and lips. So a bluish appearance of lips and fingernails is usually a good sign of hypoxia onset.

Sometimes the first signs of hypoxia are a general inability to solve simple math problems (that being the case, I have apparently been in a hypoxic state ever since college calculus).

Resolving hypoxia in a passenger or pilot is relatively easy if supplemental oxygen is available. Otherwise, a descent to lower altitude is warranted.

There are three basic types of oxygen systems for aircraft use: gaseous, liquid, and chemical. The basic light aircraft supplemental oxygen system is generally gaseous and uses the familiar bright green oxygen bottle (at least green at the top) to differentiate it from other less pure forms of the gas.

For instance, the usual medical oxygen is not nearly pure enough (too much water content) for use in aviation. Aviator’s oxygen will be in the neighborhood of 99.5 per cent pure to eliminate the possibility of water freezing within the delivery lines and potentially blocking the flow of oxygen to users.

This is a pressurized container so it must be hydrostatically tested every so often, the same as for many types of fire extinguishers. However, systems should be tested only by a maintenance facility approved to test aviator’s oxygen systems.

The advantages of a gaseous system make it ideal for small aircraft. They are easy to handle, and transport. They are easy to service and relatively low cost to install. Some systems are portable, so you need not take it on every flight!

The only disadvantages over other oxygen systems are that connections are prone to leaks and a delivery pressure regulator is required due to the high pressure under which the oxygen is stored.

Liquid oxygen (LOX) systems are not common in light aircraft; in fact I’ve never heard of it being used in light aircraft. In any event, LOX systems meet the same basic requirements as for gaseous systems, but the oxygen is kept very cold in order to keep it in liquid form. One clear disadvantage of a LOX system is that it needs to be replenished from time to time even when not in use.

Servicing is more difficult due to lack of facilities to deal with it and it costs more to install. Of course, both LOX and gaseous systems present fire hazards during a crash in that things burn hotter and quicker in the presence of high levels of oxygen.

The last system (chemical) is also not common on light aircraft but is available for one-time or emergency use. I say this because once you start a chemical oxygen system, you can’t stop it!

One such brand of portable oxygen generator that’s been around for many years is AVIOX. These portable units are also fairly expensive considering you only get one use out of them. On the other hand, advantages to this type of chemical generation of oxygen is that it will not deplete until actually used, there are no high pressures involved and they do not present nearly as much danger as the LOX or gaseous systems in case of a crash.

Bear in mind that there is no way to test a chemical system prior to use and that non-portable units have special installation requirements due to the high heat generated by the unit once activated. (For additional reading on how hot these units can get, see the NTSB report on the Valujet 592 crash).

Oxygen is critical to safe flight and knowing one’s own response to a lack of oxygen is also very useful information. In lieu of knowing how you’ll respond to hypoxia, remember the common symptoms and watch for them when flying at high altitude. Realize that you cannot always recognize the onset of symptoms so follow the recommendations and regulations for oxygen use.

Finally, if you use supplemental oxygen either as a safety precaution or as required by regulation, make sure your equipment is properly serviced and full prior to flight.

For additional information on hypoxia and oxygen regulations, consult CAR part VI, subpart 5, regulation 605.31 and 605.32, and the Aeronautical Information Manual (TP 14371). Both can be easily accessed through the Transport Canada website.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 180 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.