Aircraft cabins are pressurized because air pressure and available oxygen drop rapidly with altitude. At typical cruising altitudes, a sudden loss of cabin pressure can make clear thinking and normal breathing difficult within seconds to minutes. Oxygen systems give passengers and crew enough breathable oxygen to stay conscious while pilots descend to a safer altitude.
They are a critical backup system, not a comfort feature.
Key Facts
- Atmospheric pressure decreases with altitude, so the partial pressure of oxygen also decreases.
- Partial pressure of oxygen is approximately PO2 = 0.21P, where P is total air pressure.
- Time of useful consciousness decreases as altitude increases, especially above about 25,000 ft.
- Passenger masks usually supply oxygen from chemical generators activated when the mask is pulled down.
- Crew masks usually use stored gaseous oxygen from high pressure cylinders for quick, reliable breathing support.
- Oxygen systems protect people while the aircraft descends to a breathable cabin altitude, often below 10,000 ft.
Vocabulary
- Cabin pressurization
- Cabin pressurization is the system that keeps air pressure inside an aircraft higher than the outside pressure at altitude.
- Hypoxia
- Hypoxia is a shortage of oxygen in body tissues that can cause confusion, poor coordination, unconsciousness, or death.
- Chemical oxygen generator
- A chemical oxygen generator is a device that produces oxygen through a heat releasing chemical reaction after a passenger mask is activated.
- Gaseous oxygen system
- A gaseous oxygen system stores oxygen as compressed gas in cylinders and delivers it through regulators and masks.
- Time of useful consciousness
- Time of useful consciousness is the approximate time a person can still think and act effectively after losing adequate oxygen at altitude.
Common Mistakes to Avoid
- Thinking the passenger mask bag must fully inflate. The bag may not inflate even when oxygen is flowing, because flow depends on breathing rate and system design.
- Assuming passenger masks use oxygen tanks above every seat. Most passenger masks on airliners are supplied by chemical oxygen generators, not individual compressed gas tanks.
- Waiting to help others before putting on your own mask. Hypoxia can impair judgment quickly, so securing your own oxygen first keeps you able to assist someone else.
- Believing oxygen masks are used to keep flying normally at cruise altitude after depressurization. They are mainly a short term emergency supply while pilots descend to a safer altitude.
Practice Questions
- 1 At sea level, pressure is about 101 kPa. If oxygen is 21 percent of air, what is the oxygen partial pressure at sea level?
- 2 A passenger oxygen generator supplies oxygen for 15 minutes. If the aircraft descends from 35,000 ft to 10,000 ft in 10 minutes, how many minutes of oxygen remain after reaching 10,000 ft?
- 3 Explain why pilots have dedicated crew oxygen masks with compressed gaseous oxygen, while passengers commonly use drop-down masks connected to chemical oxygen generators.