Oxygen Use in Aviation

Table of Contents

Overview of Federal Aviation Regulations

• The Issue
• FAR 91.211 Supplemental oxygen
• Military Specifications

Oxygen and the manufacturing process

Physiology and hypoxia

• Effective Performance Time (EPT)
• Time of Useful Consciousness (TUC)

Oxygen Systems

• Constant flow
• Altitude adjustable
• Altitude compensating
• Demand
• Pressure Demand

Ascertaining the flow

Masks

Cannuals

O2 gear: Care and feeding 

Finding FBOs that provide oxygen – AOPA’s online Airport Directory

Overview of Federal Aviation Regulations

The Issue

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 something 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.

FAR § 91.211 Supplemental oxygen

(a) General. No person may operate a civil aircraft of U.S. registry—

(1) At cabin pressure altitudes above 12,500 feet (MSL) up to and including 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen for that part of the flight at those altitudes that is of more than 30 minutes duration;

(2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required minimum flight crew is provided with and uses supplemental oxygen during the entire flight time at those altitudes; and

(3) At cabin pressure altitudes above 15,000 feet (MSL) unless each occupant of the aircraft is provided with supplemental oxygen.

(b) Pressurized cabin aircraft. (1) No person may operate a civil aircraft of U.S. registry with a pressurized cabin—

(i) At flight altitudes above flight level 250 unless at least a 10-minute supply of supplemental oxygen, in addition to any oxygen required to satisfy paragraph (a) of this section, is available for each occupant of the aircraft for use in the event that a descent is necessitated by loss of cabin pressurization; and

(ii) At flight altitudes above flight level 350 unless one pilot at the controls of the airplane is wearing and using an oxygen mask that is secured and sealed and that either supplies oxygen at all times or automatically supplies oxygen whenever the cabin pressure altitude of the airplane exceeds 14,000 feet (MSL), except that the one pilot need not wear and use an oxygen mask while at or below flight level 410 if there are two pilots at the controls and each pilot has a quick-donning type of oxygen mask that can be placed on the face with one hand from the ready position within 5 seconds, supplying oxygen and properly secured and sealed.

(2) Notwithstanding paragraph (b)(1)(ii) of this section, if for any reason at any time it is necessary for one pilot to leave the controls of the aircraft when operating at flight altitudes above flight level 350, the remaining pilot at the controls shall put on and use an oxygen mask until the other pilot has returned to that crewmember's station.

Military Specifications

The Department of Defense document MIL-PRF-27210G describes the performance specifications for oxygen, aviator’s breathing, liquid, and gas.

Oxygen and the Manufacturing Process

In the United States, the last plant producing oxygen by a process called hydrolysis closed in 1972. Since then, all oxygen in the United States has been made by an industrial process known as liquefaction. The process places air under very high pressure. As the pressure increases, the temperature of the air also increases, eventually converting the gas to a liquid that boils off, leaving a pure gas, oxygen, as a result. 

Oxygen is oxygen, and the same gas is used for aviation, medical, and industrial purposes. All oxygen comes from the vendor in a dry state. Medical oxygen has water vapor (bubbling oxygen through water) added at the patient’s bedside .

All oxygen supplies come from a very small number of vendors and is normally delivered in 25,000 gallon refrigerated tanks. The manufacturing process is so thorough and clean that the finished product meets all usage specifications right from the tank. However, just to be sure, any lot of oxygen destined to be medical oxygen is batch tested for aromatics (oils, Benzene, and other impurities that appear in the manufacturing equipment). ABO (aviation breathing oxygen) is also tested for moisture content, while welding oxygen comes straight from the vendor with no additional additives or testing. 

FAR Part 135 requires that the operator must provide ABO or an “acceptable replacement.” But under Part 91, it is at the pilot (operator’s) discretion. Although hundreds of FBOs around the country have aviation oxygen available, industrial oxygen (remember, it’s all the same) can also be used for Part 91 operations.

Physiology

Hypoxia

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 about 70,000 feet. 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 partial pressure, those molecules are spaced farther apart. Consequently, the partial pressure of oxygen in the bloodstream is significantly reduced; so 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 retinal 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, and they can vary from individual to individual. That's why a ride in an altitude chamber is essential for recognizing your particular response to hypoxia. There are altitude chambers all over the country that provide one day classroom training and a ride in the chamber.

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.

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 usually, at this point the pilot doesn't care.

People are 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 a smoker, under a great deal of stress, or don’t exercise regularly to increase your heart rate, your EPT and TUC will be considerably shorter than the published guidelines. A pack-a-day cigarette smoker is physiologically 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, so at sea level, the smoker's blood-oxygen concentrations are already at the 7,000-foot level.

For this reason, smokers and those with more sedentary lifestyles conk out faster at altitude than the smoke-free and fit, and they should use begin using oxygen at altitudes lower than required by the regulations.  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 seen in high pollution metropolitan areas like Los Angeles, Mexico City, Tokyo. Doctors and hospital staff want to see your blood oxygen saturation level at 96 to 98 percent. That's considered normal. You can measure O2  saturation with a relatively inexpensive pulse oximeter that clips over your finger tip. 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.

Effective Performance Time (EPT)

Time of Useful Consciousness (TUC)

Oxygen Systems

Many pilots have just 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.

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 use of a flow meter.

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 better assurance of a proper flow rate.

Altitude compensating . This type of system is typically used in permanently installed 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 system . 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 high 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

Portable oxygen systems are not addressed 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. If you have a red/green color vision deficiency, have someone else check the color of the flow indicator!

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

Masks

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

The partial rebreather is the most common. With these, there's an external plastic bag that inflates each 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. A set of check valves and ports allows a mixture of oxygen and outside air into the mask, and allows 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.

Cannulas

Nasal cannulas have become increasingly popular alternatives to face masks. Cannulas free you to talk, eat, or drink without the hassle of a conventional facemask. 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.

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 (scheduled airlines).

O2 Gear: Care and feeding

• Store portable oxygen tanks securely so they can't fall over or become a projectile. 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 Oxygen burns robustly!
• Use no petroleum-based lip balms, lipstick, sun block, or 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 the 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.

Finding FBOs that provide oxygen – AOPA’s online Airport Directory

AOPA’s online Airport Directory provides a searchable database that will identify United States airports that provide oxygen. More than 800 FBOs across the country have oxygen available. To view a comprehensive list of all airports that provide oxygen: Under “Airport Directory Search,” click on “Advanced Search” and check “Oxygen.” The results list the hundreds of airports with FBOs that have oxygen. Click on “FBO/Facility/Fuel Information” to find which specific FBOs on the airports supply oxygen.

For a more specific airport search, for instance to find out which FBO at your intended landing facility provides oxygen, just input the airport identifier into the airport database and click “Search.” When the airport information is displayed, click on “FBO/Facility/ Fuel Information” and read which FBOs on that airport supply oxygen. And be sure to call the FBO before departure to confirm the current availability of oxygen.