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O2 Issues

The whys and hows of oxygen use

Most pilots don't think too much about using portable oxygen. Sure, everyone knows that you have to use supplemental oxygen if you fly more than 30 minutes at cabin pressure altitudes of 12,500 feet or higher. That at cabin altitudes above 14,000 feet pilots must use oxygen at all times. And that above 15,000 feet each occupant of the aircraft must be provided supplemental oxygen. All of this is spelled out in Federal Aviation Regulations Part 91.211. So are the rules for oxygen use in pressurized airplanes, which are governed by the times necessary to descend to safe altitudes in the event of a cabin depressurization. (Above FL250, a 10-minute supply; between FL350 and FL410, one pilot must wear a mask if cabin pressures rise above 14,000 feet msl — unless there are two pilots at the controls and they have quick-donning masks available.)

From textbooks and stories of — or direct experience with — sessions in FAA-approved altitude chambers, pilots also know somthing about the dangers of hypoxia (insufficient oxygen) at altitude. Specifically, as the blood's oxygen saturation drops with altitude, a series of symptoms — all of them dangerous — can set in. We'll touch on these in a moment.

Apart from that, many pilots have but a vague understanding of the oxygen systems designed to keep them safe and alert at altitude. There's quite a wide selection of oxygen equipment out there, and knowing something about today's various regulators, masks, and flow meters can go a long way toward safer flying at altitude. Because most pilots in light aircraft don't fly with built-in systems, we'll emphasize portable systems when talking about oxygen equipment. Portable oxygen tanks sell for between $350 and $1,200, depending on size, capacity, construction (some are made of composites), type of regulator, and number of oxygen ports.


The air we breathe at the surface is roughly 79 percent nitrogen and other gases, and 21 percent oxygen. This proportion doesn't change until you reach an altitude of 70,000 feet or so.

What does change is the atmospheric pressure. There may be the same number of oxygen molecules at 20,000 feet as there are at sea level, but because of reduced pressure, those molecules are spaced farther apart. Consequently, the partial pressure of oxygen in the bloodstream is significantly reduced; there's not enough pressure to allow the oxygen to force its way into the blood, and you can't breathe deeply or fast enough to compensate. It's this reduced pressure that's at the heart of the hypoxia problem.

Lots of bad things happen when the blood's oxygen saturation drops. Night vision goes first, as the retina's function begins to deteriorate at altitudes as low as 5,000 feet. Nausea, apprehension, tunnel vision, headaches, fatigue, dizziness, blurred vision, tingling sensations, numbness, and mental confusion are some of the other symptoms. Symptoms vary from individual to individual. That's why a ride in the altitude chamber is essential for recognizing your particular response to hypoxia.


For pilots, hypoxia's adverse effects are described in terms of time of useful consciousness (TUC) and effective performance time (EPT). TUC is a measure of your ability to function in a meaningful way. In other words, it's a kind of threshold on the pathway to becoming, first, something like a drooling idiot, and second, unconscious and certifiably out of it. EPT is defined as the time from the loss of significant oxygen to the time when you are no longer able to perform tasks in a safe and efficient manner.

This is a dangerous condition, because hypoxia's onset is subtle. Pilots may think they're doing just fine — and in fact, may well have things under control — even though their EPT is dwindling away, and the countdown clock to unconsciousness is surely running. This false sense of well-being is, in itself, a symptom of hypoxia. But at this point the pilot doesn't care.

We're not the same

Even though we've just been talking in terms of EPT and TUC guidelines, it's time for a reminder: Not all pilots have the same EPT or TUC. If you're out of shape, smoke, or are under a great deal of stress, your EPTs and TUCs will be considerably shorter than the published guidelines. So for you, the published times may be meaningless.

A pack-a-day cigarette smoker is hypoxic at sea level. The smoker's lungs are so damaged that they're incapable of absorbing as much oxygen as those of a nonsmoker. At sea level, the smoker's blood-oxygen concentrations are already at the 7,000-foot level. For this reason, smokers and the sedentary conk out faster at altitude than the smoke-free and fit, and they should use oxygen at altitudes lower than required by theregulations.

Other day-to-day factors such as nutrition, alcohol use, and quality and amount of sleep can also affect your oxygen requirements. There's even evidence that poor air quality can lower your blood oxygen saturation level. Maybe that's why "oxygen bars" are beginning to pop up in Los Angeles, Mexico City, Tokyo, and other cities with especially polluted air.

Doctors and hospital staffs want to see your blood oxygen saturation level at 96 to 98 percent. That's considered normal. This level is measured by an oximeter that clips to the tip of a finger. A 100-percent level is as good as it gets, and 95 percent is considered a minimum. An oxygen saturation level below 90 percent is a warning sign. That's when patients — and pilots — begin to experience hypoxia.

Going with the flow

We can't all be flying around with oximeters on our fingers (although it's possible, given the new, portable oximeters), so there must be a method of regulating the flow of oxygen from tank to mask. The higher you go, the higher the flow rate must be in order to maintain that ideal 96- to 98-percent saturation. That ideal flow rate turns out to be one liter of oxygen per minute per 10,000 feet of altitude.

There are five methods of regulating oxygen flow:

Continuous flow. This is the least expensive method of delivering oxygen. Here, oxygen flow is governed by a regulator set at a constant flow rate — usually about 2.5 liters per minute. This flow rate is a compromise. It's more than enough for lower altitudes, but not enough for flying above 25,000 feet. So you can end up wasting oxygen at lower altitudes — a problem that can be solved by the flow meters that we'll talk about shortly.

Altitude adjustable. With this system, there's an adjustable control on the oxygen tank's regulator. Flying at 20,000 feet? Then dial in the necessary oxygen flow until the indicator needle (also on the regulator) shows 20,000. Altitude-adjustable systems are more costly than the continuous-flow types, but they give you more assurance of a proper flow rate.

Altitude compensating. This type of system is typically used in built-in oxygen systems, not most portable ones. As the name indicates, oxygen flow changes automatically with changes in altitude. Some systems, however, don't turn on until reaching 8,000 to 10,000 feet, so if you want or need oxygen below those altitudes, it may not be available.

Demand systems. This is designed for airplanes capable of flying up to 35,000 feet. Oxygen is provided in bursts whenever the pilot inhales, and the tight-fitting, alien-face-grabbing masks (they have to fit tightly to avoid dangerous leaks) have switches that let you select between a normal and a 100-percent oxygen setting.

Pressure-demand systems. Now we're in fighter country. With this system, oxygen is pumped continuously to the mask under positive pressure. This makes it easy to inhale, but sometimes rather difficult to exhale. The whole idea is to make absolutely sure that the pilot has enough oxygen up to 45,000 feet — even when pulling Gs or performing other extreme maneuvers. At 45,000, TUC is a scant 10 seconds or so, making positive pressure and a well-designed mask absolutely essential.

Ascertaining the flow

Let's get this important fact out: Portable oxygen systems are not covered by the Federal Aviation Regulations. Built-in systems are. This means that some portable systems may not have been tested for adequate oxygen-flow rates in an altitude chamber and may not meet the safety and manufacturing criteria that certified, built-in systems do.

Yes, there is a pressure-activated, spring-driven shuttle valve that tells you if oxygen is being delivered to the pilot's mask. This valve is located right in the oxygen hose. If there's an oxygen flow to the mask, then the pressure forces the valve's green flow indicator into view. Turn off the flow — or put a kink in the oxygen line — and the loss of pressure lets the spring push a red warning indicator into view. It's simple. You look down, see if the flow indicator is green or red, and carry on accordingly.

With continuous-flow systems, however, what this ultra-simple indicator doesn't tell you is the exact flow rate.

Precise Flight, Incorporated has come to the rescue with a simple flow meter that fits into the oxygen line — or lines, in multiple installations. Twist a knob, and you can adjust the flow rate by watching a tiny ball that rises or falls in a graduated chamber as you increase or decrease the flow rate. Calibrated markings on the side of the chamber let you make the flow appropriate to your cabin altitude. Precise Flight claims that this lets you conserve oxygen at lower altitudes (remember that continuous-flow systems put out too much oxygen at lower altitudes), and customize flow rates for individuals who need more. The Precise Flight meters come in various models, and will work with all of the more popular oxygen systems. Suggested retail prices run from $23.95 to $84.95 per meter.

Mountain High Equipment and Supply Company also makes metering devices. Its $600 Model A-1 Analog Computer Electronic Oxygen Delivery System is a demand-type unit that automatically delivers pulses of oxygen with each inspiration of air. Pulses are limited to 20 inhalations per minute, which the company says eliminates the chance of hyperventilation. An altitude-compensating function sends the precise amount of oxygen needed for your altitude. The A-1, in effect, transforms continuous-flow systems into altitude-compensating ones. Because the A-1 delivers oxygen only at the moment of inspiration, Mountain High claims that their system lets you consume one-tenth the amount of oxygen you'd normally use.

The great promises of these flow-metering devices are conservation of oxygen and customization of flow rates. Both Precise Flight and Mountain High assert that they've tested their products to assure that the flow rates are safe. But, as mentioned earlier, portable oxygen units and their ancillary equipment are unregulated in the FAR Part 23 environment. "There are no TSOs [technical standard orders], no PMAs [parts manufacturer approvals], and there's a lot of surplus medical stuff out there and no one knows the difference," says Scott Aviation President James Kaletta. Scott Aviation manufactures a wide range of general aviation, military, and airline oxygen equipment, and Kaletta is quick to point out that Scott makes a lot of equipment for airframe manufacturers, and has many FAA approvals.

"This whole emphasis on saving oxygen is not right," Kaletta says. "I mean, so you get 10 hours — use out of a small portable tank. What you really ought to be concerned about is whether you're really getting the right amount of oxygen flow."


General aviation face masks come in two basic flavors: partial rebreathers and sequential breathers.

The partial rebreather is the most common. With these, there's an external plasic bag that inflates every time you exhale. The purpose of the bag is to store any unused oxygen, so that it can be inhaled with the next breath. These masks work fairly well up to 25,000 feet — as long as the mask seals well against the face. Excessive sun exposure and normal wear and tear can also make a mask less effective at altitude. The heat of a baking cabin can deform the face seal of any mask, as well as bring on cracks in the rebreather bag.

By the way, beards and moustaches don't go well with oxygen masks. Masks cannot seal properly, and for this reason, some manufacturers suggest that bearded pilots on oxygen fly no higher than 18,000 feet. Above that altitude, the risk of oxygen leakage is too great.

Rebreathers can be ordered with built-in microphones. This spares the pilot of the bother and risk of removing and replacing the mask every time a radio transmission is made. The down side is that the masks can impart an odd muffled sound to your transmissions, sometimes making them nearly undecipherable.

Partial rebreathers have sturdier masks with better molding to fit the face. There are a set of check valves and ports that let a mixture of oxygen and outside air into the mask, and allow the exhaled breath to be vented with each exhalation. Because they fit better, these masks can work up to 30,000 feet — but only if the oxygen flow is sufficient, and only if you are beard- and moustacheless.


Nasal cannulas have become increasingly popular alternatives to face masks. Cannulas free you to talk, eat, or drink without the hassle of a conventional face mask. In short, they're more comfortable.

Cannulas are only approved for use up to cabin altitudes of 18,000 feet. The reason for this restriction is that above that altitude there's too great a risk of blood oxygen saturation levels dropping to dangerous levels if the pilot breathes through his mouth — or talks too much. The rebreathing masks don't present a problem in this regard because more oxygen is stored in the bag after each exhalation.

Aerox Aviation Oxygen Systems and Precise Flight say that standard cannulas waste oxygen. That's why these companies endorse the use of a new, oxygen-conserving cannula called the Oxysaver (sold by Aerox) and Oxymizer (Precise Flight), both of which are manufactured by Chad Therapeutics, Inc. Aerox first pioneered the idea of oxygen-conserving cannulas 18 years ago.

The Oxysaver and Oxymizer incorporate an oxygen reservoir in their cannula face plates. Exhaled oxygen that would normally be wasted is captured in the reservoir, and is inhaled with the next breath. A check valve allows oxygen to flow into the cannula only when the user inhales. Because this type of cannula stores oxygen, Aerox and Precise Flight claim you can safely reduce the oxygen flow rate from the tank.

In one test, the Nelson Aircraft Company (which developed flow meters in the first place, and was bought by Precise Flight) took a 50-year-old, nonsmoking pilot of average health, climbed to 18,000 feet and, using a Nelson flow meter, dialed in a flow rate of 0.6 liters of oxygen per minute. The Oxymizer produced blood oxygen saturation levels of 89 to 90 percent. That's not good enough, so Nelson/Precise Flight meters designed for the Oxymizer now are set for flows of 0.7 liters per minute at 18,000 feet. This setting, Nelson said, produces saturation levels "in excess of 90 percent."

Similarly, Mountain High claims saturation levels of "well over 90 percent" using its Model A-1 electronic delivery system with standard cannulas up to 25,000 feet. But even so, all cannulas are still limited to 18,000 feet by regulation.

In addition to the 18,000-foot limitation, the FAA has something else to say about cannula use in Part 91 operations, and it suggests something about cannula effectiveness: for each cannula in use, you have to have a standby, conventional face mask at the ready. Here's something else: Cannulas aren't approved for flights operating under Part 135 (air taxis and charters) or Part 121 (airlines).

Yet another caveat comes from Precise Flight itself. The company says that in high-stress situations, such as single-pilot IFR operations in actual instrument conditions, you should limit cannula use to 15,000 feet. Above that altitude, they say, use a standard face mask. When the heat is on, oxygen demands increase. Apparently, the company thinks cannula flow rates — be they from a standard or Oxymizer design — aren't high enough to deal with high-stress demands.

Bottom line

Even if you are only an occasional high-altitude flyer, make sure you have supplemental oxygen aboard. Each individual has his or her own specific oxygen needs, and you never know when you or a passenger may have to put on the mask.

To learn your response to hypoxia, take a high-altitude physiology course and a ride in an altitude chamber. Your local Flight Standards District Office (FSDO) can help direct you to the nearest chamber.

The type of system you select is up to you, of course, but remember that conservation of oxygen can be a false economy with dreadful consequences.

As a final arbiter of your own or your passengers' oxygen status, consider making your own measurements. If you have $425 to spare, then contact Aerox Aviation Oxygen Systems, Inc. and buy a Nonin Onyx portable finger oximeter. Then you can see for yourself how well your oxygen equipment performs for you and your passengers at altitude. It may be that flow metering devices or cannulas aren't for you. To reiterate, you want blood saturation levels in the high 90s. If you're not there, then something's wrong.

O 2 gear: Care and feeding

  • Store oxygen tanks securely so they can't fall over. This could damage the regulator and set the stage for cracks.
  • Don't keep portable oxygen tanks in hot, enclosed areas, such as the inside of an airplane on a hot day, or in the trunk of a car. Compressed gases can expand, causing dangerous pressure rises and the chance of a tank explosion.
  • Keep oxygen equipment clean. Dirt particles can contaminate regulators and valves, and create sparks at altitude, where the ambient air is dry.
  • Store masks and cannulas in their containers, and out of the sun.
  • Have your tank inspected every five years, as per FAA or manufacturer rules. This includes a hydrostatic test to check the tank's strength and integrity, just like the tests administered to scuba tanks.
  • Allow no smoking around oxygen equipment (Duh!). Oxygen burns readily.
  • Use no lip balms, lipstick, sun block, or other petroleum-based products (such as makeup) when using oxygen. In the presence of oxygen, these products can burn.
  • Make sure your mask and regulator connectors are of a compatible design. In order to have leak-free connections, all components must be compatible, and a mask connector that works with one regulator may not properly fit another.
  • When having your tank filled, make sure it's filled slowly. Yes, you can fill a tank in a few minutes, but the heat generated by the compression of the oxygen will cause high pressure readings and potentially dangerous internal stress. You'll have to stop filling so that you don't exceed the tank's pressure redline (very often, this is 1,800 psi, but check your gauge for your tank's limits). Later, when the tank cools off, the pressure drops and you're left with a less-than-full tank. Also, be absolutely sure that your tank's filling port and associated hardware is compatible with your oxygen supplier's fill fittings.
  • Have your gear inspected regularly to make sure O-rings, fittings, and masks are working properly.
  • Yes, you can legally fill your aviation oxygen tank with medical oxygen. It used to be that pilots were warned against using medical oxygen. The rationale was that medical oxygen has a high moisture content, and that this moisture might freeze at altitude and block the oxygen flow. It turns out that aviation and medical oxygen have the same properties. Moisture is indeed added to medical oxygen (to prevent drying of the airways), but it's added at the treatment site by an independent moisturizing unit.


Effective Performance Time
15,000 to 18,000 feet — 30 minutes plus
22,000 feet — 5 to 10 minutes
25,000 feet — 3 to 5 minutes
28,000 feet — 2.5 to 3 minutes
30,000 feet — 1 to 2 minutes
35,000 feet — 30 to 60 seconds
40,000 feet — 15 to 20 seconds
45,000 feet — 9 to 15 seconds

Time of Useful Consciousness
15,000 feet — Indefinite
20,000 feet — 10 minutes
22,000 feet — 6 minutes
24,000 feet — 3 minutes
26,000 feet — 2 minutes
28,000 feet — 1 minute
30,000 feet — 30 seconds
35,000 feet — 20 seconds
40,000 feet — 15 seconds

Links to all Web sites referenced in this issue can be found on AOPA Online ( E-mail the author at [email protected].

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