First for some basic pulmonary physiology. The atmosphere is composed of approximately 21 percent oxygen and about 78 percent nitrogen. At sea level, atmospheric pressure is 760 millimeters of mercury (mmHg), and the partial pressure of oxygen is 160 mmHg (21 percent of 760). This results in 95-percent to 100-percent blood oxygen saturation as measured with a fingertip oximeter.
As an aircraft climbs, atmospheric pressure and the partial pressure of oxygen decrease, but not the percentage of oxygen. For the lungs to transfer adequate oxygen to the bloodstream requires at least 100 mmHg (the FAA standard for certifying oxygen masks is 122mm Hg). In an International Standard Atmosphere, 122 mmHg oxygen is reached at 7,000 feet, and 100 mmHg at 12,500 feet. Above these altitudes, oxygen partial pressure is insufficient to force enough oxygen across the lungs’ small sacs—alveoli—into the bloodstream.
Most of us will be mildly hypoxic, with 90 percent or less blood oxygen saturation, by 10,000 feet. While we will not lose consciousness, our judgment will be dulled. By 18,000 feet almost all pilots will feel the effects of hypoxia.
At 45,000 feet and above, total atmospheric pressure is so low that even 100-percent oxygen is insufficient. Oxygen must be supplied to the lungs under pressure. This results in reverse or pressure breathing. Inhalation is effortless, but expiration requires you to force the air out of your lungs.
Oxygen delivery options
Supplemental oxygen can be delivered in several ways: continuous flow, demand flow, diluted, or 100 percent. Depending on the mask, one or more of these modes or combinations may be provided. The aircraft’s ceiling dictates the mask modes required.
The simplest oxygen delivery system provides continuous flow to a nasal cannula or mask. These are the most common options for nonpressurized aircraft. A mask delivers oxygen more effectively than a cannula, as it presents flow to both nose and mouth. Either can be equipped with a demand system that optimizes oxygen usage. Sensing each inhalation, the system provides a blast of oxygen. Depending upon the sophistication of the constant-flow system or demand system, flow rate in liters per minute may be pilot adjustable. Demand systems are not efficient if masks seal poorly to the face, or are structurally flabby.
Rebreather bags are offered as options on constant-flow masks. They conserve oxygen by collecting it between breaths.
But for all their features, most oxygen delivery systems for unpressurized aircraft simply are not robust. The plastic tubing is thin and easily kinked. Flow indicators may be inaccurate. Masks seal poorly to the face and are insecurely held by elastic ribbons. Nasal cannulas are easily dislodged.
Crews of pressurized aircraft are provided more sturdy and complex systems. Oxygen lines are more robust and less likely to kink or be compressed. Masks seal well to the face and are securely held in place by inflatable harnesses. This is important because the higher you fly, the more critical mask fit becomes. And here’s something to think about: Oxygen masks do not seal adequately over beards.
Constant-flow masks are available for passengers in pressurized aircraft. Masks may be dropped by crew action, or automatically by a cabin-pressure-sensing switch typically set to a cabin altitude of 13,000 to 14,000 feet. However, flow does not commence until passengers individually pull on a lanyard attached to each mask.
The oxygen system should be tested as part of the preflight check, although the checklist may not be at all detailed in some aircraft. All oxygen valves should be opened, and the mask test button or paddles pressed. This should result in a hissing sound, partial inflation of the harness, and the blinker indicating flow. Mask communication also should be tested. The mask microphone should be activated, and then—as each mask is tapped with a finger—the tapping should be heard in the headsets.
Five seconds takes practice
Regulations require that the crew be able to remove quick-don masks from their stowage boxes, secure them to their faces, and receive oxygen within five seconds—using just one hand. This may seem like an unnecessarily short time, but if you are already hypoxic, the faster you re-oxygenate, the more likely you are to survive.
But no way can you expect to meet the five-second standard without practice, and well-thought-out choreography. Prior to donning the mask, the headset must be removed, but not necessarily your glasses. (Have you checked that your glasses will not be disturbed during the process?) Even pulling the mask out of its mount poses issues. The mask must be grasped with one hand, properly oriented. If not, the mask will be presented to the face inverted or backward. This will then require using the other hand to reorient the mask. However, the other hand is occupied with the headset...and then your glasses slip. You can see the problems. Some pilots plan to park the headset over a clip, or over their knee if necessary.
With the mask now positioned and sealed, the headset should be put back on. If necessary, the mask mode can be adjusted, and if there is smoke, goggles should be donned (if not incorporated into the mask).
A blinker in the oxygen line or on the stowage box should turn from red to clear to confirm oxygen flow. However, the only confirmation you are adequately oxygenated is the reading on a fingertip pulse oximeter.
Now communications need to be reestablished. Depending on your airplane type, a mask-microphone switch may be tripped automatically when the mask is removed from its housing. Otherwise, the crew has to manually set a microphone-select switch to the Mask position.
Communications with a mask in place are complicated by breathing noises. Headphones provide a sidetone (you can hear yourself talk), but the mask microphone also picks up breathing noises. You may find it necessary to hold your breath to best hear ATC communications. It may also be necessary to turn off the intercom, as breathing noises from others may overlay communications.
Masks should be stowed in 100-percent (demand) mode so when first donned, the maximum amount of oxygen is suppled to remedy hypoxia as rapidly as possible. Once adequately oxygenated, the mask can be switched to diluter-demand (normal) mode. This preserves oxygen supply by regulating the percentage (flow) of oxygen depending on cabin altitude. Typically, above about 32,000 feet the two modes are identical, as there is no dilution—but at lower altitudes, oxygen savings may be considerable.
Masks may also provide an emergency mode providing 100-percent oxygen under constant pressure. This limits inhalation of any cockpit smoke or noxious fumes, and is typically used in conjunction with goggles, which may be a permanent part of the mask. To keep goggles free of contamination, a vent valve must be opened so oxygen enters the goggles and continuously flushes them clear of smoke or fumes.
Masks require factory overhauls every five (Eros masks) or six (B/E masks) years. None of the components is field replaceable. This might be the time to consider upgrades or replacement. Zodiac, for example, has optional microphones that suppress breathing noise. Also available are controls that allow the pilot to vary mask-sealing pressure; your current mask may be uncomfortably tight when worn for long periods. This is a consideration for Part 121 or 135 operations, because when only one pilot is in the cockpit, continuous use of a mask is required (even if it is quick donning). Ferrying an airplane with cabin altitude above 10,000 feet may also require long-term wear. And no, placing the mask on your lap does not satisfy regulations or safety guidelines, but it seems to be a common occurrence.
Oxygen masks may be rarely required, but they are lifesaving tools that deserve attention to detail during training; donning practice; and routine preflight testing. Ignore this, and you could pay a heavy price when a depressurization or other emergency occurs, and you’re suddenly faced with what could be chaos as you try to use your mask. That sounds simple, but you’d be surprised.
Dr. Ian Blair Fries is an orthopedic surgeon and TBM 900 owner.