Pressurization

Pressurization exists to protect occupants from hypoxia while the airplane operates in air too thin to breathe. A typical system maintains a cabin pressure altitude of about 8,000 ft at the airplane's maximum designed cruising altitude — at or below 8,000 ft cabin altitude no oxygen equipment is required, and the system also exchanges cabin air fast enough to clear odors and stale air (PHAK ch. 7).

Holding 8,000 ft also prevents uncomfortable or injurious rapid changes in cabin altitude. Because decompression is always possible, a pressurized airplane still carries supplemental oxygen (AC 61-107B para 2-9).

• Aircraft altitude — actual height above sea level at which the airplane is flying
• Ambient pressure — pressure in the area immediately surrounding the airplane
• Cabin altitude — cabin pressure expressed as an equivalent altitude above sea level
• Differential pressure — the difference between cabin pressure and atmospheric pressure, the load the fuselage actually carries (PHAK ch. 7)

Worked from the PHAK standard-pressure chart: at 28,000 ft ambient standard pressure is 4.8 psi; add a cabin differential of 6.1 psid and you get 10.9 psi in the cabin, which is the standard pressure found at 8,000 ft (PHAK ch. 7, fig 7-41).

• Turbine engines — bleed air from the compressor section
• Older turbine aircraft — superchargers pumping into the sealed fuselage
• Piston aircraft — air from each engine's turbocharger through a sonic venturi (flow limiter), or an engine-driven pneumatic pump (PHAK ch. 7, AC 61-107B para 2-9)

Air leaves through the outflow valve. By regulating the exit, the outflow valve permits a constant inflow while holding cabin pressure — pressurization is controlled on the way out, not on the way in. Note the coupling on a piston twin: lose the turbocharger and you can lose pressurization with it (AC 61-107B para 2-11).

• Cabin pressure regulator — controls cabin pressure to the selected value in the isobaric range and limits it to the preset differential in the differential range
• Outflow valve — meters air out of the cabin to hold that schedule
• Cabin air pressure safety valve — a combination unit: pressure relief (stops cabin pressure from exceeding the design differential above ambient), vacuum relief (lets outside air in if ambient exceeds cabin pressure), and dump (flight deck switch to ram, dumping cabin air overboard) (PHAK ch. 7)

The regulator can no longer hold the selected cabin altitude. Once the difference between inside and outside equals the highest differential pressure the fuselage is designed for, any further increase in airplane altitude produces a corresponding increase in cabin altitude. Differential control exists specifically to keep the design maximum from being exceeded (PHAK ch. 7).

That maximum is set by the structural strength of the cabin and the relationship of cabin size to probable rupture areas — windows and doors. PHAK puts it plainly: the degree of pressurization and the operating altitude of the airplane are limited by several critical design factors, primarily the maximum cabin differential the fuselage is built to withstand (PHAK ch. 7).

Keep the two ceilings straight, because the examiner may push on it: maximum operating altitude is the structural/pressurization limit described here, while service ceiling is a performance number — the altitude at which the airplane's max rate of climb falls off to a specified minimum. 61.31(g) triggers off whichever of the two is lower.

• Cabin differential pressure gauge — reads the difference between inside and outside pressure in psid; monitor so the cabin does not exceed the maximum allowable differential, which is marked as a limit on the gauge
• Cabin altimeter — a check on system performance in thousands of feet (often combined into one instrument with the differential gauge)
• Cabin rate-of-climb indicator — cabin vertical speed (PHAK ch. 7, fig 7-42)

Cabin rates of descent should generally not exceed 500 to 600 ft/min, and cabin pressure should equal ambient before landing or ear injury can result (AC 61-107B para 2-6).

• Pressurized aircraft meeting the requirements of 14 CFR part 23 or 25 have a cabin altitude warning system that activates at 10,000 ft cabin altitude
• Aircraft meeting the more stringent part 25 requirements have automatic passenger oxygen mask-dispensing devices that activate before the cabin exceeds 15,000 ft
• Many airplanes carry automatic visual and aural warnings of an unintentional loss of pressure
• Some have a negative pressure relief valve to equalize pressure in a sudden decompression or rapid descent, keeping cabin pressure from going below ambient (AC 61-107B para 2-9)

Some aircraft require the crew to disable automatic mask deployment before landing at airports above 10,000 ft MSL, then re-arm it after departure (AC 61-107B para 2-9).

The ACS asks you to operate the system, if equipment is installed, so fly the schedule, not just the switches — the specific controller and checklist come from your AFM (CA.VIII.B.S1):

1. Preflight/before takeoff — system armed per the AFM, dump switch normal, oxygen quantity checked and masks stowed within reach
2. Set the controller — cruise cabin altitude (or planned cruise flight level) plus destination field elevation, and the cabin rate on rate-controlled systems
3. Climb — verify the cabin is actually climbing on schedule and that the differential gauge stays below the marked maximum allowable differential (PHAK ch. 7, fig 7-42)
4. Cruise — cabin altimeter, cabin VSI, and differential gauge stay in the scan; a drifting cabin is a slow decompression until proven otherwise
5. Descent — reset the controller to destination field elevation early, hold cabin descent to 500 to 600 ft/min, and have cabin pressure equal to ambient before landing (AC 61-107B para 2-6)

Decompression is the inability of the pressurization system to maintain its designed pressure differential — from a system malfunction or from structural damage (PHAK ch. 7, AC 61-107B para 2-11).

• Explosive — a change in cabin pressure faster than the lungs can decompress, risking lung damage. Unrestricted lungs vent in about 0.2 seconds, so most authorities call anything faster than 0.5 seconds explosive and potentially dangerous
• Rapid — the lungs decompress faster than the cabin; much lower risk of lung damage
• Gradual or slow — dangerous precisely because it may not be detected; you rely on the warning systems (AC 61-107B para 2-11)

AC 61-107B says actual decompression times are difficult to calculate because of the many variables involved — it names the type of failure, the differential pressure, and the cabin volume — and it does not give you a formula (AC 61-107B para 2-11a(2)). Be careful how confidently you answer this.

The one relationship the AC does commit to is cabin volume. Given the same size hole and the same conditions, decompression of a small-volume airplane is more critical than a large one:

• A typical small pressurized airplane can decompress on the order of 10 to 200 times faster than a large one
• B-747 versus Learjet is a 223:1 cabin volume ratio — an extreme example, and the point of it is that human response, TUC, and the protective equipment required are identical (AC 61-107B table 2-8)

So the crew of the small airplane simply has less time to take lifesaving actions. Same physiology, smaller clock.

• Noise — a leaky seal, a departing window, structural breach, or the alarm system
• Fog — from the rapid drop in temperature and change in relative humidity; on a small airframe it can fill the cabin
• Flying debris, dust, and dirt toward the opening
• Wind blast and a sharp temperature drop
• Gas expansion — most noticeable in the ears and the GI tract; ears normally clear automatically
• Air rushing out of the mouth and nose as the lungs vent (PHAK ch. 7, AC 61-107B para 2-11)

Loose items become projectiles — secure baggage and oxygen cylinders before flight.

Same sequence regardless of type:

1. Oxygen mask on, 100 percent, breathe slowly — the primary danger of decompression is hypoxia, and the mask goes on before anything else (PHAK ch. 7, AC 61-107B para 2-11)
2. Emergency descent to a safe altitude — below 10,000 ft MSL if oxygen is unavailable or symptoms persist (see Task IX.A for the maneuver)
3. Advise ATC and get the airspace
4. Check on passengers once you're established
5. Land as soon as possible if symptoms persist or the airframe is damaged

Recognition is what makes recovery survivable — cabin altitude warnings, the differential gauge, and hypoxia symptoms are all triggers (AC 61-107B para 2-11).

Because it doesn't announce itself. The fog, noise, wind blast, and debris that make a rapid decompression unmistakable may not be evident at all in a gradual one. Meanwhile the insidious onset of hypoxia and the accompanying depression of mental function decrease your ability to recognize the emergency and carry out the recovery — the failure and the impairment feed each other (AC 61-107B para 2-7).

By its nature a rapid decompression commands attention; a slow one goes unnoticed until the hypoxia is already in charge. Your defenses are mechanical, not perceptual: the cabin altitude warning at 10,000 ft, a cabin altimeter in the scan, and a pulse oximeter.

If the pressurization air supply depends on the turbocharger compressor, a turbocharger malfunction or failure can cause loss of cabin pressurization. Indications of turbocharger trouble are increased oil temperature, decreased oil pressure, and a drop in manifold pressure, and the airplane may be unable to sustain altitude on the remaining power.

AC 61-107B para 2-11 is explicit about the trap: monitor pressurization carefully during the resulting descent so a slow decompression doesn't produce hypoxia while you're busy managing the engine. Notify ATC of the descent.

The ACS lists impairment and symptoms of hypoxia under this task too (CA.VIII.B.K2a, K2b), so have the short version ready without reaching for Task VIII.A:

• Impairment starts almost immediately — TUC is the time to loss of proper corrective and protective action, not the onset of unconsciousness, and the potential for impairment begins right away (AC 61-107B para 2-7)
• Symptoms — cyanosis, headache, increased reaction time, impaired judgment, euphoria, visual impairment, drowsiness, dizziness, tingling, numbness (PHAK ch. 17)
• Why that's the trap — euphoria and impaired judgment give you a false sense of security while performance is already degrading, so you will not reliably self-diagnose

That is exactly why the response is mechanical and immediate: mask on, 100 percent, breathe slowly, descend. Full treatment is under Task VIII.A.

It slashes it. Oxygen in the lungs is exhaled rapidly, reducing pressure on the body, which drops the partial pressure of oxygen in the blood and reduces effective performance time by one-third to one-fourth its normal time (PHAK ch. 7). AC 61-107B is more aggressive: assume TUC following decompression to altitudes between 25,000 ft and 43,000 ft is reduced by 50 percent (para 2-7(a)), and that rapid decompression cuts TUC by at least 50 percent generally (para 2-7(b)).

Above 43,000 ft it collapses to the circulation time from lung to brain plus the brain's reserve — about 9 to 12 seconds from the start of the decompression to loss of functional capability (AC 61-107B para 2-7). That is the entire justification for the 91.211(b) mask-wearing rules above FL350 and for practicing the don from time to time.

• Evolved gas decompression sickness — with pressure off the body, nitrogen comes out of solution and forms bubbles in the tissues, with adverse effects; risk rises with exposure duration at altitude and with a high rate of ascent (PHAK ch. 7, AC 61-107B para 2-7 and table 2-7). After an in-flight rapid decompression, do not fly for at least 24 hours, and stay alert for delayed symptoms (AC 61-107B para 2-7)
• Being blown or tossed out if a structural opening is nearby — anyone seated near openings should keep the harness or seatbelt fastened whenever the cabin is pressurized
• Wind blast and extreme cold through the breach
• Lung damage in an explosive decompression (PHAK ch. 7)

Structural damage is also why "descend and continue" may not be the answer — you may have an airframe that needs to be on the ground.

It applies to a pressurized aircraft with a service ceiling or maximum operating altitude, whichever is lower, above 25,000 ft MSL.

Ground training endorsement covers:

• High-altitude aerodynamics and meteorology
• Respiration
• Effects, symptoms, and causes of hypoxia and other high-altitude sickness
• Duration of consciousness without supplemental oxygen
• Effects of prolonged oxygen use
• Causes and effects of gas expansion and gas bubble formation
• Preventive measures
• Physical phenomena and incidents of decompression

Flight training endorsement covers:

• Normal cruise operations above 25,000 ft MSL
• Simulated rapid decompression without actually depressurizing
• Emergency descent procedures (61.31(g))

Exceptions exist for pre-April 15, 1991 experience and part 121/125/135 PIC proficiency checks (61.31(g)(3)).

On pressurized airplanes the pressurization air system removes smoke from the cabin. If the smoke is intense, it may become necessary to either depressurize at altitude — only if oxygen is available for all occupants — or execute an emergency descent (AFH ch. 18).

That is the whole task in one decision: the pressurization system is a tool you can use, an oxygen requirement you must satisfy, and an emergency descent trigger when neither works. Identify and shut down the faulty system first, use the extinguisher before opening air vents, and get on the ground.

Do it on the ground, in plain language:

• What they'll notice — a loud bang or hiss, fog filling the cabin, sudden cold, ears popping. The fog is normal and is not smoke
• Masks — where they are or that they'll drop automatically, don your own first, then help children or seatmates
• Breathe normally and slowly; the bag may not visibly inflate
• Seatbelt on and stay seated — the descent will be steep and deliberate, and that's the plan working
• No smoking, ever, with oxygen flowing
• Expect an unscheduled landing and that I may be too busy on the radio to talk to you for a few minutes (CA.VIII.B.S3)

Then, in flight, back it up with your own actions: mask first, autopilot or hand-fly the descent, ATC, then passengers.

The ACS wants single-pilot resource management or crew resource management, as appropriate (CA.VIII.B.S4). The defining feature of this emergency is that the failure attacks the decision-maker, so the resources have to be lined up before it happens:

• Aeronautical decision-making — set the trigger on the ground: cabin altitude warning or an unexplained cabin climb means mask on first, diagnose second. Never troubleshoot before you're on oxygen
• Automation — let the autopilot hold the airplane while you don the mask, then use it to fly the emergency descent (Task IX.A)
• Task management — mask, descend, ATC, passengers, land. In a crew airplane, split it explicitly: pilot flying flies and descends, pilot monitoring works ATC and the system
• External resources — declare with ATC early; "unable to maintain, emergency descent" gets you the block and the vectors
• Situational awareness — a pulse oximeter and the cabin altimeter give you objective numbers when your own judgment is the thing that's degraded