Loss of cabin pressure is a potentially serious emergency that can arise when the sealed setting falls below the safe equivalent of 6,000-8,000 ft cabin altitude. Causes include pressurisation system malfunction or structural failure; either unit failing allows the internal air mass to escape. Crew mistakes - like an inadvertent switch movement - or deliberate intervention can open the outflow path. Failures range from slow leaks that let pressure drift downward to sudden events where a hull breach creates a rapid decompression event capable of ejecting people and items.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
Why does a plane lose cabin pressure?
A plane can lose cabin pressure due to different reasons. Aircraft cabins are pressurized to an equivalent altitude of 6,000-8,000 ft (1,829-2,438 m) because high altitudes reduce the ability of humans to take in enough oxygen. Conditioned air, compressed bled from the engines and cooled by an air-cycle machine, flows continuously in. An automatic controller keeps cabin altitude low by sending excess air out through a spring-loaded outflow valve. That valve also acts as a safety-relief valve, so the fuselage is never stressed beyond its pressure limit. Any breach of this sealed system or any upset of the inflow-outflow balance will therefore make cabin pressure drop.
Pilots have access to mode controls of the cabin-pressure control system, and an incorrect setting of the air-pressurisation unit leads to loss of cabin pressure just as surely as a deliberate crew intervention. Machine faults are equally effective: loss of the conditioned-air source - be it failure of all engines or failure of all air conditioning units - leads to loss of cabin pressure, because the engine actively pumping air into the cabin ceases to supply flow. The automatic controller cannot compensate when the source itself is gone.
Structural failure is another cause. Any breach to a door or window or any rupture of the fuselage leads to an explosive or rapid decompression in which the airtight seal keeping pressure inside the aircraft is broken. Failures range from sudden catastrophic loss of airframe integrity (explosive decompression) to slow leaks or equipment malfunctions that allow cabin pressure to drop-every one a path by which the pressurised vessel fails to hold the air that keeps passengers safe.
What happens when a plane cabin depressurizes?
When a plane cabin depressurizes, the air pressure inside suddenly drops. Higher-pressure air gushes out and molecules get further apart. This drop in pressure makes it harder for lungs to supply adequate oxygen to the blood, so oxygen pressure falls even though the percentage of oxygen in the air stays about the same.
The immediate signs of cabin depressurization are a loud bang, a massive wind blast roaring out of any hole, and sometimes heavy fog filling the aircraft cabin as the remaining air cools and moisture condenses. Anything not tightly secured will fly out, and violent, immediate suction throws people and items around the aircraft cabin or even ejects them out of the plane.
Within seconds passengers and crew feel dizziness and a rapid deterioration of cognitive abilities. If the exposure continues they drift into unconsciousness, and eventually lack of oxygen leads to death. To prevent this, oxygen masks drop down once pressure dips below acceptable levels; the masks give pure oxygen in a concentration and duration long enough to keep occupants alive during the emergency descent.
The flight crew will don their own masks and begin a rapid descent to about 10,000 feet (3,048 meters), an altitude at which passengers can breathe without supplementary oxygen. Throughout the event the pressurisation control system is no longer able to maintain the normal cabin altitude of about 8,000 ft (2,438.4 m), and the aircraft must reach thicker air before hypoxia incapacitates those on board.
How often does a plane lose cabin pressure?
An FAA study determined the odds of experiencing cabin depressurization are one in 54,300 flight hours. Loss of cabin pressure is normally classified as gradual, yet the same carrier records multiple rapid events: one airline's Boeing 737-300 experienced three rapid depressurizations in a 30-day period in 2018, and on 9 March that year the same type suffered a gradual loss while cruising at 39,000 ft (11,887 m) when a 60-inch-long (152.4 cm) gash appeared where a joint failed.
