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Cabin Pressurization

A pressurized cabin is what lets an airplane cruise where the air is too thin to keep you conscious. It is also a system whose most dangerous failure gives your body nothing to feel.

Technical Subject Areas — FI.II.O

The airplane can go higher than you can breathe

At sea level the atmosphere presses on you at about 14.7 psi, and oxygen makes up roughly 21 percent of it. Climb, and that percentage does not change — the mixture is the same all the way up. What changes is how much of it there is. Lower pressure means fewer molecules in every breath, so the partial pressure of oxygen in your lungs falls and less of it reaches your blood.

That is the problem a pressurized cabin solves. Rather than feed everyone bottled oxygen for the whole flight, you seal the cabin, flight deck and baggage compartment into one vessel and pump air in faster than it leaks out. The airplane flies in thin air; the people inside it do not.

Helios 522

On August 14, 2005 a Boeing 737-300 took off from Larnaca as Helios Airways Flight 522. The cabin failed to pressurize during the climb, the crew did not correctly identify what the warnings were telling them, and hypoxia took them before they acted. The autopilot flew on toward Athens and held there until the fuel ran out, and the airplane crashed near Grammatiko, Greece, killing all 121 on board — the deadliest accident in Greek aviation history.

The airplane was not silent about it. A cabin altitude warning horn sounded, which is precisely what that horn is for. What the failure gave nobody was a sensation — no bang, no fog, no rush of air, nothing the body could feel and react to. That is the reason to learn this one properly: the warning arrives as information you have to interpret, at the exact point in the flight where your ability to interpret anything is quietly draining away.

Hypoxia, and the clock it starts

Hypoxia is oxygen deficiency in the body, and what matters in flight is the brain. Judgment goes early, which is what makes it dangerous: the first thing you lose is the faculty you need to notice you are losing anything. Euphoria and a mild sense that everything is fine are symptoms, not the absence of them. What to look for, in yourself and in the person next to you:

• Impaired judgment and slower reactions
• Euphoria, or a strange lack of concern
• Headache, drowsiness, lightheadedness or dizziness
• Visual impairment
• Tingling in the fingers and toes
• Cyanosis — blue fingernails and lips

Time of useful consciousness is the time you have left to make and carry out rational, life-saving decisions at a given altitude without supplemental oxygen — not the time until you pass out, but the shorter window before you stop being useful. The PHAK's figures run from twenty to thirty minutes at 18,000 feet, to three to five minutes at FL250, to thirty seconds to a minute at 35,000 feet, and a rapid decompression cuts those to about a third or a quarter of what they were. Which is why the response is a reflex, not a diagnosis.

The other things thin air does

Hyperventilation is breathing faster and deeper than the body needs, blowing off too much carbon dioxide in the process. The PHAK puts the usual in-flight cause down to emotional stress, anxiety or fear; hypoxia is one trigger among several, not the definition. The symptoms overlap closely enough with hypoxia that treating for hypoxia first is the right call. Vision goes before you feel anything: the rods lose sensitivity first, so night vision is the earliest casualty, and it starts well below any altitude where a regulation requires you to act.

Gas trapped in body cavities expands as pressure falls, which is where ear and sinus pain come from — usually from a head cold you flew with anyway. Reduced pressure can also bring nitrogen out of solution as bubbles, which is decompression sickness and the reason flying after scuba diving needs a waiting period.

How the cabin is pressurized

Ambient air is compressed, cooled, and pushed into the sealed cabin. In a turbine airplane the source is bleed air tapped from the engine's compressor section; in a piston airplane it is usually the turbocharger's compressor or an engine-driven pneumatic pump. Compressing air heats it, so it goes through a heat exchanger before it reaches you.

Air arrives faster than it is let out, so controlling cabin pressure is really about controlling the leak. The cabin pressure regulator holds the cabin altitude you selected. The outflow valve does the work, metering how fast air escapes. The safety valve is the backstop, three functions in one housing:

• Pressure relief — stops cabin pressure exceeding the airplane's maximum differential above ambient
• Vacuum relief — lets outside air in if ambient pressure exceeds cabin pressure, as it can in a fast descent
• Dump valve — dumps cabin air overboard, on a flight deck switch

Differential pressure is the limit that matters

Differential pressure is the difference between cabin pressure and the atmospheric pressure outside, expressed in psi. The higher you fly, the more differential it takes to hold the same cabin altitude, and every airframe has a maximum differential pressure it is built to contain — an airplane-specific number that lives in your POH. Once you are at that limit the cabin cannot hold its altitude any longer: climb higher and the cabin altitude climbs with you.

Many airplanes hold a cabin somewhere near 8,000 feet at normal cruising levels, which is comfortable and keeps the cabin well below the altitudes at which 14 CFR 91.211(a) starts asking for oxygen. It does not put you outside the rule. The paragraphs that govern pressurized airplanes, 91.211(b)(1) and (b)(2), are written around flight altitude rather than cabin altitude, so they apply above FL250 and FL350 no matter how low the cabin is being held. What your particular airplane holds, and up to what altitude, is in its POH.

You watch three things: the cabin altimeter, showing the pressure altitude inside the airplane; the cabin differential pressure gauge, often combined into the same instrument; and the cabin rate-of-climb, showing how fast cabin pressure is changing. The rate needle is usually the first honest sign that something has changed, and cabin altitude is what the warnings and the drop-down masks are wired to.

Decompression, and what you do about it

Decompression is the system's inability to maintain its designated differential pressure, from a malfunction or from structural damage. The PHAK sorts it by what your lungs can keep up with. Explosive decompression is faster than the lungs can vent — the handbook puts it at less than half a second — and that is what makes lung damage possible. In a rapid decompression the lungs decompress faster than the cabin does, so lung damage is not the expected outcome. A slow loss of pressurization is the insidious case: it is not one of the handbook's two categories, but it is worth naming, because it can proceed to a dangerous cabin altitude without anyone noticing. Helios 522 is the extreme version of that problem — a cabin that never pressurized at all rather than one that lost a differential it had been holding — and it shows what an absence of physical sensation costs a crew.

A fast decompression announces itself: a loud bang, the cabin filling with fog as temperature and humidity change, dust and debris in flight, air rushing out of your mouth and nose as your lungs empty, wind blast and extreme cold, and a dazed few seconds while you work out what happened — seconds out of a time of useful consciousness that has already been cut. So the response is fixed and short:

1. Oxygen mask on, immediately — yours first, before anything else
2. Emergency descent to a safe altitude
3. Then, and only then, work out what happened and tell somebody

Getting down is the objective. Fly the emergency descent your POH specifies — in a piston airplane it accounts for the fact that hauling the power off for a fast descent can shock-cool the cylinders and crack them. The engine is a problem you can have later.

What the regulations require

Supplemental oxygen requirements are in 14 CFR 91.211, and the section itself is what you go by. Paragraph (a) is written around cabin pressure altitude. Above 12,500 feet MSL up to and including 14,000 feet MSL, the minimum required 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." Above 14,000 feet MSL the required crew uses it for the entire flight time at those altitudes. Above 15,000 feet MSL each occupant is provided with it.

Paragraph (b) covers pressurized cabins, and it keys off flight altitude. At flight altitudes above FL250, 91.211(b)(1) requires at least a 10-minute supply of supplemental oxygen for each occupant, in addition to anything paragraph (a) already requires, available for a loss of cabin pressurization. Above FL350, 91.211(b)(2) requires one pilot at the controls to wear and use an oxygen mask secured, sealed and supplying oxygen at all times, with a conditional exception at or below FL410 for two pilots at the controls each holding a quick-donning mask. Read (b)(2) in full before you rely on that exception — it carries its own conditions about what happens when one pilot leaves the controls.

Whatever you carry has to be aviator's breathing oxygen: medical oxygen holds water that can freeze in the lines, industrial oxygen may carry impurities, and oil, grease or a flame anywhere near pure oxygen is a fire. Most general aviation airplanes deliver it through a continuous-flow system — a cannula or a rebreather mask. A nasal cannula is generally limited to use below 18,000 feet, so above that the delivery is by mask; the approved limits for the system installed in your airplane are in its own documentation and the POH.

14 CFR 61.31(g) is where the pressurized-airplane training requirement lives: ground and flight training with an authorized instructor and a logbook endorsement before acting as pilot in command of a pressurized airplane that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 feet MSL. The section also sets out what that training has to cover and the circumstances under which it can be credited, so read it rather than a summary of it.

The CFR is the authority and it changes. Use these numbers to orient yourself, then read the current text of 91.211 and 61.31 before you fly or teach against them.

Where you meet it next

On a checkride this is a ground task, and what the examiner is testing depends on the certificate. On the Flight Instructor ACS the question is whether you can teach it; on the Commercial ACS you are asked to know it. Either way, three questions do most of the work — how high you can fly with no supplemental oxygen, how the most common general aviation oxygen system works, and what the procedure is for a rapid decompression.

In the airplane you meet it the first time you fly something with a cabin altimeter — an instrument that reports on you rather than on the airplane. Put it in the cruise scan and it will tell you about a slow loss of pressurization long before your body does. The pressure altitude tool is the same arithmetic that instrument is doing, run on the air outside. From here, the Supplemental Oxygen lesson takes the systems and the 91.211 requirements further, Human Factors covers hypoxia, hyperventilation and decompression sickness as aeromedical subjects in their own right, and Performance and Limitations deals with pressure and density altitude; all of them are in Learn.