Pressurization keeps the cabin at a lower effective altitude
The fuselage does not maintain sea-level pressure at cruise; it keeps cabin altitude within an acceptable range. That reduces structural stress while providing enough oxygen for normal passenger activity.
Passenger masks provide temporary oxygen
When cabin altitude rises beyond a preset threshold, masks can deploy automatically or be released by the crew. Their purpose is to cover the period while the aircraft descends.
Pilots use dedicated oxygen supplies
Flight crews have quick-donning masks connected to a separate oxygen system so they can remain functional while handling the emergency and communicating with air traffic control.
The descent is planned around terrain and traffic
Over ocean or flat terrain, crews may descend promptly toward a standard safe altitude. Over high mountains, they may need a published escape route that preserves terrain clearance until lower ground is reached.
The source of the pressure loss may not be obvious immediately
A leak can come from a door seal, valve, structural damage or pressurization-system fault. Crews generally stabilize oxygen and altitude first, then diagnose what happened.
The flight normally diverts after significant depressurization
Even if pressure is restored, inspection is often required. The aircraft may land at the nearest suitable airport rather than continue to destination.
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