What happens during cabin decompression, and how do pilots respond?
Cabin decompression explained: symptoms, oxygen-mask use, emergency descent, ATC calls and the procedures pilots follow to land safely.
In real-world aviation, cabin decompression at jet cruise altitude lowers cabin pressure and oxygen partial pressure, causing hypoxia unless oxygen is used. Pilots immediately don oxygen masks, establish communication, control the aircraft and, if pressurisation cannot be maintained, make an emergency descent to 10,000 feet or the minimum safe altitude, whichever is higher.
What actually happens when cabin pressure is lost?
Cabin decompression is a loss of the controlled pressure difference between the cabin and the outside atmosphere. An airliner cruising high above the ground does not maintain sea-level pressure inside; our explanation of how cabin pressure normally behaves at FL370 provides useful context for the difference between aircraft altitude and cabin altitude.
The pressurisation system supplies conditioned air and controls how quickly it leaves through one or more outflow valves. A failed air source, incorrectly positioned outflow valve, leaking seal, damaged window or structural opening can make the cabin altitude climb. Airbus simmers can see the control logic in our guide to how the A320 outflow valve is represented and monitored.
| Type | Development | Likely indications |
|---|---|---|
| Slow decompression | Pressure is lost gradually over several minutes or longer. | Rising cabin altitude, abnormal cabin rate, ear discomfort, a warning message or an unusually noisy seal. The crew may receive an alert before anyone notices physical symptoms. |
| Rapid decompression | Cabin and outside pressure equalise within seconds. | A bang or roar, strong airflow, mist, falling temperature, oxygen-mask deployment and immediate pressurisation warnings. |
| Explosive decompression | Pressure equalises almost instantaneously after a large opening or structural failure. | Violent airflow, flying loose objects, possible debris and greater risk of injury or structural damage. |
The white mist sometimes seen after rapid decompression is condensation caused by the sudden pressure and temperature change; it is not necessarily smoke. Air moves towards a breach while pressure equalises, but there is no continuing vacuum afterwards. A sufficiently large opening can still eject an unrestrained person or loose equipment located nearby.
The immediate medical danger is hypoxia. Early effects include poor judgement, slowed reactions, tunnel vision, confusion, euphoria and loss of coordination, so affected people may not recognise that anything is wrong. At high cruise levels, useful consciousness can be measured in seconds rather than minutes; our discussion of high-altitude cruise, oxygen and decompression risk explains why altitude changes the urgency so sharply.
Rapid pressure changes can also cause severe ear or sinus pain, expansion of trapped gases and, after high or prolonged exposure, decompression sickness. These effects vary with altitude, health and the speed of pressure loss.
What do pilots do first during cabin decompression?
Pilots protect their ability to think and fly before attempting to diagnose the pressurisation fault. Exact memory items, oxygen-regulator settings and switch names differ by aircraft, but the operational priorities are consistent:
- Put on crew oxygen masks. Both pilots don their quick-donning masks, set the regulators as the aircraft checklist requires and verify that oxygen is flowing. Troubleshooting before using oxygen is a dangerous reversal of priorities.
- Control the aircraft and establish crew communication. The pilot flying maintains the flight path, normally using a functioning autopilot to reduce workload, while the pilots select their mask microphones or interphone and confirm that they can hear each other.
- Begin an emergency descent when required. If cabin altitude is excessive or pressurisation cannot be recovered at cruise altitude, the crew descends immediately towards breathable air while respecting terrain and aircraft limitations.
- Declare the emergency and protect the cabin. The crew transmits Mayday, reports its descent and intended altitude, and may select transponder code 7700. Seat-belt signs, passenger-mask deployment and a cabin announcement are completed when time and workload permit.
- Run the aircraft checklist. Once oxygen, control and descent are secured, the crew completes the QRH, ECAM or EICAS actions, checks cabin altitude and differential pressure, and identifies whether the cause is an air-source failure, valve problem or structural leak.
Pilots do not delay life-saving actions while waiting for ATC clearance. ATC should be told as soon as practical so it can clear traffic below, provide terrain information and coordinate the diversion, but an aircraft in distress has emergency authority to leave its assigned altitude.
How fast and how low is an emergency descent?
An emergency descent is a rapid but controlled descent within the aircraft's approved limits, not an uncontrolled dive. Crews normally reduce thrust, use speed brakes as permitted and fly the speed specified by the type-specific checklist while monitoring VMO, MMO and structural indications.
The usual target is 10,000 feet or the minimum safe altitude, whichever is higher. Ten thousand feet is normally low enough for passengers to breathe without continuous supplemental oxygen, but it may be below surrounding terrain. Over mountains, the crew follows an escape route or remains at a calculated terrain-safe altitude until it can move towards lower ground.
Suspected structural damage changes the decision. The checklist may require a lower speed, limited manoeuvring or cautious speed-brake use rather than the aircraft's fastest normal emergency descent profile. Passenger oxygen is finite, however, so the crew must balance structural limits, terrain and the time required to reach breathable air.
What happens to passengers and the aircraft afterwards?
Passenger masks provide temporary supplemental oxygen; they do not repressurise the cabin. On many transport aircraft they deploy automatically at a cabin altitude around 14,000 feet, although the exact threshold and oxygen system vary by type. Pilots can also deploy them manually.
Passengers should pull a mask towards them if instructed, place it over the nose and mouth, secure the elastic band and breathe normally. On many systems, pulling the mask starts the oxygen supply. The reservoir bag may not inflate visibly even when oxygen is flowing, so an apparently flat bag is not proof of failure.
Cabin crew use the nearest available oxygen, secure themselves and avoid moving through the cabin during the initial descent. Passenger oxygen is intended to bridge the descent to a safe altitude, not make prolonged flight at the original cruise level safe.
After levelling at a safe altitude, the pilots reassess the cabin, aircraft handling, fuel, weather and possible structural damage. They normally divert to the nearest suitable airport, where passengers and crew can receive medical assessment for hypoxia, decompression illness, ear or sinus injury and trauma. The aircraft remains out of service until maintenance or engineering personnel identify the fault and complete the required inspection.
The mistakes most likely to worsen the emergency are diagnosing the system before donning oxygen, waiting for ATC permission to descend, aiming blindly for 10,000 feet despite high terrain, exceeding aircraft speed limits and assuming that a passenger mask has failed merely because its bag is not fully inflated.