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Supplemental Oxygen

Hypoxia does not feel like suffocating. It feels fine, and the part of you that would notice otherwise is the first part to fail — which is why the rules are written in altitudes rather than in symptoms.

Technical Subject Areas — FI.II.N

Helios 522

On August 14, 2005, a Boeing 737-300 climbed out of Larnaca for Athens with its pressurization system still set to manual, where maintenance had left it. The cabin never pressurized. A warning horn sounded and the crew read it as a takeoff configuration problem. By the time the airplane leveled off in the flight levels, both pilots were hypoxic. The autopilot flew on until the fuel ran out and the airplane crashed near Grammatiko, Greece, killing all 121 people aboard — the deadliest accident in Greek aviation history.

Two trained airline pilots sat with oxygen masks a few inches above their heads and never reached for them. Nothing was hidden from that flight deck. The warning was audible, the passenger masks had dropped in the cabin behind them, and both pilots had the training to assemble those pieces into a diagnosis. What they no longer had was the capacity to do it. Hypoxia takes away the faculty you would use to recognize hypoxia, and it takes it early.

What thin air does to you

Air stays about 21 percent oxygen all the way up. What changes is pressure, and as pressure falls the partial pressure of oxygen in your lungs gets too low to push oxygen across into your blood. That is hypoxic hypoxia, the one high-altitude flight hands you.

The brain notices first, and losing mental function at the moment you need it is what makes hypoxia lethal rather than merely unpleasant. Judgment goes early and goes quietly — you become euphoric, slow to respond, content with decisions you would normally reject. Around that sit the physical signs: tingling fingers and toes, headache, light-headedness, drowsiness, degraded vision, and cyanosis, the blue tinge in your fingernails and lips that a passenger may see before you do.

Hyperventilation shadows all of this. Stress drives your breathing rate up without you deciding to, you blow off too much carbon dioxide, and the result feels a great deal like hypoxia. The two are hard to tell apart in the airplane, so treat it as hypoxia first: get on oxygen, then slow your breathing deliberately.

The legal thresholds are not where the effects start. Night vision degrades at cabin altitudes well below any of them, because the rods in your retina are unusually sensitive to a shortage of oxygen. If your vision starts to go, treat that as the symptom it is — get on oxygen, and descend.

Time of useful consciousness

Time of useful consciousness is the maximum time you have, after losing your oxygen at a given altitude, to make rational life-saving decisions and carry them out. It is not the time until you pass out; it is the shorter window before you stop being useful to yourself:

Those are for a gradual loss. A rapid decompression is worse, because air is driven out of your lungs. At the bottom of that list there is no time to diagnose anything. There is time to put a mask on, and that is all.

What the regulation requires

Supplemental oxygen lives in 14 CFR 91.211, written in cabin pressure altitude. In an unpressurized airplane that is the airplane's own pressure altitude, not what the altimeter reads unless the setting is 29.92.

For unpressurized flight the thresholds sit in 91.211(a), and the wording carries more than a summary of it can. The first one applies “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.” Note where the band begins: above 12,500 feet, not at it. Note also that the 30 minutes and the oxygen both attach to the same thing, that part of the flight at those altitudes — read the paragraph itself before you plan a flight that lives near the edge of it. Then 91.211(a)(2) takes the allowance away above 14,000 feet, where the required minimum flight crew uses oxygen during the entire flight time at those altitudes, and 91.211(a)(3) adds that above 15,000 feet each occupant must be provided with supplemental oxygen. Provided, not required to use it. Only the crew has a duty to breathe it.

Pressurized airplanes get 91.211(b). Above FL250 the rule calls for at least a 10-minute supply of supplemental oxygen available for each occupant in case a descent is forced by loss of pressurization — and that supply is required “in addition to any oxygen required to satisfy 91.211(a),” so it is a reserve stacked on top of the crew requirement rather than a replacement for it. Above FL350, 91.211(b)(2) puts one pilot at the controls in a mask that is secured and sealed, with a stated exception at or below FL410 when there are two pilots at the controls and each has a quick-donning type of oxygen mask. If one pilot leaves the controls above FL350, the remaining pilot puts on and uses a mask until the other returns.

Separately, 14 CFR 61.31(g) requires ground and flight training and a logbook endorsement before you act as pilot in command of a pressurized aircraft. The section defines that by performance rather than paperwork — an aircraft with a “service ceiling or maximum operating altitude, whichever is lower, above 25,000 feet MSL” — so the figures to check are the service ceiling and the maximum operating altitude, not the type certificate. Exceptions live in 61.31(g)(3), among them documented service as pilot in command of such an aircraft before April 15, 1991. Take no training, currency or endorsement requirement as settled from a lesson: the Code of Federal Regulations is the authority, it changes, and the current text of 91.211 and 61.31 is what governs your flight.

When the cabin lets go

Pressurization holds a cabin altitude lower than the flight altitude — commonly around 8,000 feet — so nobody wears a mask for hours. The gap between inside and outside is differential pressure, and every airframe has a maximum it is built to contain. A decompression is that system failing to hold it.

If the cabin changes pressure faster than your lungs can vent, in less than about 0.5 seconds, it is explosive decompression and lung injury is possible. Slower, and it is rapid decompression, where the lungs keep up. Either way the cabin usually fills with fog as temperature and humidity collapse, air rushes out of your mouth and nose, and the cold and wind blast are severe.

The response is short, and the order is not negotiable:

  1. Don your oxygen mask. Immediately, before diagnosing anything.
  2. Begin an emergency descent to a safe altitude.

Reaching a safe altitude is the top priority. A fast descent can cool a piston engine faster than it likes, so fly the emergency descent procedure and the airspeeds your POH gives for your airplane, cautions about power and cooling included — but do not trade consciousness for cylinder life.

Three kinds of oxygen system

Continuous flow is what you find in most general aviation airplanes. Oxygen flows all the time; a reservoir bag catches what you do not inhale, and once the bag empties you draw cabin air along with it. It wastes oxygen and it works. Cannulas belong to this family and are generally limited to 18,000 feet, above which you move to a mask.

Diluter-demand systems deliver oxygen only when you inhale, through a mask sealed to your face, and adjust the mix of cabin air and oxygen with altitude, up to 100 percent. They are usable to around 40,000 feet.

Pressure-demand systems force oxygen in under positive pressure, pressurizing your lungs, which is what it takes to move oxygen into the blood at the highest altitudes. Above roughly 40,000 feet nothing else works, and breathing against pressure is exhausting over time.

Aviator's breathing oxygen, and the bottle it lives in

Put only aviator's breathing oxygen into an aircraft system. It is 99.5 percent pure with almost no water content, and the water is the whole reason. Medical oxygen is comparably pure but carries enough moisture to collect in the plumbing and freeze at altitude, stopping the flow when you need it. Industrial oxygen is not intended for breathing at all and may carry impurities.

Bottles are typically stored at 1,800 to 2,200 psi, though the service pressure and duration figures that govern your flight belong to your equipment — take those from the POH and the equipment manufacturer's instructions. Whatever the placarded figure, cylinder pressure falls as temperature falls, so a lower indication on a cold morning is normal physics, not a leak. The real hazard is fire: materials that barely burn in ordinary air burn readily in pure oxygen, and oil or grease near a fitting can ignite. Keep fittings clean, keep petroleum products out of the system, and no smoking near oxygen in use. If the airplane has no installed system, the portable equipment has to be reachable in flight — a bottle in the baggage compartment is decoration.

Inspect it before every flight, on PRICE:

Where you meet it next

On a flight instructor practical test this is a technical subject area, examined orally. Three questions come up almost every time: how high you can fly without supplemental oxygen, what the most common oxygen mask in general aviation is and how it works, and what the procedure is for a rapid decompression. Answer the last one in the right order and you have understood the lesson.

It also shows up long before you see a flight level, on any cross-country over high terrain and on any night flight. Because 91.211 is written in cabin pressure altitude, the pressure altitude tool is the one that tells you which threshold you are actually flying against, and on a hot day at a high field the density altitude tool tells you what the airplane will manage getting there. The lessons on cabin pressurization and high-altitude operations pick up the same equipment from the airplane's side, and the rest of the ground school library covers those same trips.

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