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Commercial pilot · ACS CA.VIII.B · lesson pack

Pressurization

Why this one matters

In a pressurized airplane at high altitude, a decompression gives the pilot seconds to a few minutes of useful consciousness. Pilots have been lost to slow leaks they never noticed. The response has to be automatic: mask on first, then descend, then think.

1 · Know these cold

  • •Pressurization pumps air into a sealed cabin (usually engine bleed or turbocharger air) and controls the cabin altitude by letting it out through an outflow valve.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
  • •Differential pressure is the difference between cabin and outside pressure; the structure has a maximum, protected by a safety (relief) valve. A dump valve releases it all.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
  • •Decompression types: explosive (under about half a second), rapid (a few seconds), gradual (slow, the most insidious because you may not notice).(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization); AC 61-107, Aircraft Operations at Altitudes Above 25,000 Feet MSL and/or Mach Numbers Greater Than .75)
  • •Explosive decompression (under about half a second) is faster than the lungs can vent, so lung damage is possible; in a rapid one (about 1 to 10 seconds) the lungs vent faster than the cabin and hypoxia is the main danger; a gradual one may show only on the instruments. Rapid decompression can cut the time of useful consciousness roughly in half: masks on first, immediately.(AC 61-107, Aircraft Operations at Altitudes Above 25,000 Feet MSL and/or Mach Numbers Greater Than .75)
  • •After a decompression: oxygen on, then an emergency descent to a safe altitude (generally 10,000 feet or the minimum safe altitude), then deal with the cause.(Airplane Flying Handbook (FAA-H-8083-3C), Emergency Procedures)
  • •The warning is about cabin altitude, not the airplane's: it comes on when the cabin climbs above about 10,000 feet. At FL350 with a normal 6,000-foot cabin, no warning; at FL200 with an 11,000-foot cabin, an emergency.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
  • •Brief passengers on masks: on themselves first, then help others.(Commercial Pilot ACS (FAA-S-ACS-7B), CA.VIII.B.S3)

2 · The lesson

How it works
  • Compressed air (from engine bleed, a turbocharger or a compressor) is sent into the cabin; the outflow valve, set by the controller, holds the cabin altitude you select (the isobaric range) and then limits the pressure to a maximum differential (the differential range). A typical system holds about an 8,000-foot cabin at the airplane's maximum cruise altitude.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
  • Operating it, per the POH: before takeoff, set the controller for the planned cruise cabin altitude and the landing field elevation, and a comfortable rate; in the climb, verify it's pressurizing and the gauges behave; in cruise, monitor cabin altitude, rate and differential; depressurize on or after landing as directed.(Your airplane's POH/AFM)
  • The Part 91 rules (91.211(b)): above FL250, at least a 10-minute oxygen supply for each occupant for a descent; above FL350, one pilot at the controls wears a sealed mask, with the FL410 two-pilot quick-donning exception.(14 CFR 91.211)
  • Gauges: cabin altitude, cabin rate of climb, differential pressure. A safety valve limits differential pressure; a negative-pressure valve prevents outside pressure from exceeding cabin pressure; a dump valve releases cabin pressure.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
  • Failure modes: a leak (a door seal, a window), a failed outflow valve or controller, a loss of the air source (an engine failure in some designs). The warning comes on as cabin altitude climbs, typically around 10,000 feet.(PHAK (FAA-H-8083-25C), Aircraft Systems (oxygen and pressurization))
Physiology
  • Hypoxia is too little oxygen reaching the body. Hypoxic hypoxia comes from altitude; hypemic from carbon monoxide or blood loss; stagnant from poor circulation (including G-loads); histotoxic from alcohol or drugs.(PHAK (FAA-H-8083-25C), Aeromedical Factors)
  • Symptoms vary by person and come on insidiously: euphoria, poor judgment, headache, tunnel vision, tingling, drowsiness, slowed reactions, cyanosis. Learn your own (an altitude chamber course helps).(AIM, Chapter 8, Medical Facts for Pilots)
  • Time of useful consciousness shrinks fast with altitude: about 20-30 minutes at 18,000 feet, 3-5 minutes at 25,000, 1-2 minutes at 30,000, 30-60 seconds at 35,000. A rapid decompression can cut it roughly in half.(AIM, Chapter 8, Medical Facts for Pilots; AC 61-107, Aircraft Operations at Altitudes Above 25,000 Feet MSL and/or Mach Numbers Greater Than .75)
  • Night vision degrades from as low as 5,000 feet; smoking, alcohol, fatigue and some medications lower your tolerance. A pulse oximeter shows trends but doesn't replace the rules.(AIM, Chapter 8, Medical Facts for Pilots)
  • A rapid decompression brings a loud noise, fog in the cabin, flying debris, cold, and pain in ears and sinuses; trapped gas expands, and decompression sickness is possible at high cabin altitudes.(AIM, Chapter 8, Medical Facts for Pilots)
The response
  • Masks on and oxygen flowing, crew first so you can help passengers; then an emergency descent per the POH; declare an emergency and squawk 7700.(Airplane Flying Handbook (FAA-H-8083-3C), Emergency Procedures; AIM, Chapter 6, Emergency Procedures)
  • Level at a safe altitude (generally 10,000 feet or the minimum safe altitude if higher), assess the passengers, and land at a suitable airport. Use SRM: declare, use ATC, hand passenger briefing to a second pilot or capable passenger if you have one, and flying alone, put the airplane and the descent ahead of troubleshooting and passenger management until you're at a safe altitude.(Airplane Flying Handbook (FAA-H-8083-3C), Emergency Procedures; Risk Management Handbook (FAA-H-8083-2A))
  • Be alert for a gradual decompression: a cabin altitude climbing slowly while you're busy. Scan the pressurization gauges like any other system.(AC 61-107, Aircraft Operations at Altitudes Above 25,000 Feet MSL and/or Mach Numbers Greater Than .75)

Want it all on one page, with scenarios? Read the full lesson.

3 · Say it out loud

Answer each one out loud, then check yourself and mark how it went.

  1. Remember Recall it

    “What does the outflow valve do?”

  2. Understand Explain why

    “Why is a gradual decompression more dangerous than it sounds?”

  3. Apply Use it on a flight

    “At FL250 there's a bang, the cabin fogs, and the warning sounds. Walk me through it.”

4 · Common mistakes

  • Troubleshooting before donning oxygen.

    Useful consciousness may be seconds.

    Instead: Mask first, always.

  • Ignoring the cabin-altitude gauge.

    A slow leak goes unnoticed.

    Instead: Include it in the scan.

  • Descending too slowly after a decompression.

    Passengers without oxygen are at risk.

    Instead: Emergency descent per the POH.

5 · Quick check

Answer, then reveal. Flag anything you want to see again.

  1. 1. What is differential pressure?

  2. 2. What protects the structure from too much differential pressure?

  3. 3. Name the three types of decompression.

  4. 4. What is the first action after a rapid decompression?

  5. Scenario5. After decompression and descent, to what altitude do you generally descend?

Trained it? Now prove it: practice pressurization out loud with our examiner.

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Connect it

One situation that runs across this lesson and its neighbors in the same ACS area. Decide first, then check.

Connect it · High Altitude Operations

A pressurized single at FL 230 with a cabin at 10,000 feet. A loud bang, fog in the cabin and the cabin altitude climbing fast.

What happens to you, and in what order do you act? Decide, then see the worked answer

Rapid decompression: the cabin altitude climbs to the airplane's altitude, about 23,000 feet. The FAA's figure for time of useful consciousness there is roughly 5 to 10 minutes at rest, a rapid decompression can cut it by as much as half, and impairment can start at once. Don't hold your breath. Your own mask first: on, 100% oxygen, and flow verified. Then make sure the passengers' masks are on and flowing. Start the emergency descent immediately under your 91.3(b) emergency authority, without waiting for ATC, and squawk 7700 and declare as soon as you can. Fly the POH checklist down to 10,000 feet MSL or the lowest safe altitude, whichever is higher. 12,500 feet is only the 91.211(a)(1) threshold for crew oxygen, not a descent target. Then land as soon as practicable. Hypoxia can cloud your judgment without warning, so act on the checklist rather than on how you feel, and brief the passengers on their masks before every high-altitude flight.

PHAK (FAA-H-8083-25C), Aeromedical Factors; AFH (FAA-H-8083-3C), High Altitude Operations; 14 CFR 91.3, 91.211

Brings together: Supplemental Oxygen · Pressurization

Teaching this? The instructor lesson plan →

Study aid, not FAA guidance. Sources: Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C); Aeronautical Information Manual (AIM); 14 CFR 91.211; AC 61-107; Commercial Pilot ACS (FAA-S-ACS-7B); Your airplane's POH/AFM. Checked against current sources 2026-11-13.