Cabin Pressurisation: Helping us breathe at 35,000 feet since 1921

Why We Need It

At 35,000 feet, the air is so thin that you’d lose consciousness in seconds. The oxygen partial pressure is too low to sustain brain function, and your “time of useful consciousness” at that altitude is roughly half a minute.

So, modern aircraft carry their own atmosphere. A pressurised cabin keeps everyone breathing normally while the airplane cruises above weather, turbulence, and traffic.

For pilots, understanding how this system works isn’t optional. It’s one of the most common interview and checkride topics because it directly connects engineering, physiology, and operational safety.

Why Aircraft Are Flown So High

There are two main reasons airplanes cruise at such great heights :

Fuel efficiency. At high altitude, air is thinner, reducing drag and improving engine performance. The same speed can be maintained with less fuel.

Smooth air. Climbing above storms and convective activity gives a smoother, more efficient ride.

But to live in that thin atmosphere, the aircraft must be either pressurised or equipped with supplemental oxygen for everyone onboard.

The Basics of Pressurisation

A pressurised aircraft essentially seals its fuselage into an airtight shell that can hold air at a higher pressure than the surrounding atmosphere. The cabin, flight compartment, and baggage compartment are combined into a sealed pressure vessel.

Pressurised air is tapped from the engine compressor section or from turbochargers, this is known as bleed air. In modern airliners, this bleed air is conditioned through the air conditioning packs before entering the cabin.

Turbine aircraft use bleed air from the engine’s compressor section to pressurise the cabin. Image source: AOPA

Piston aircraft use air supplied by engine-driven turbochargers or superchargers routed through flow limiters called sonic venturis. Image source: Boldmethod

Modern airliners like the Boeing 787 have gone “bleedless,” using electrically powered compressors instead.

Inside, the cabin is maintained at a “cabin altitude”, usually around 6,000-8,000 feet when the aircraft is cruising at altitudes like 37,000 feet. That’s a comfortable, safe pressure level where oxygen equipment isn’t required for passengers or crew.

How the System Works

Pressurisation is all about controlling airflow: how much air goes in and how much is let out.

Cabin pressure results from the differential between the internal pressure created by the system and the ambient pressure outside. Image source- Boldmethod

Air Supply - Conditioned bleed air or compressor air enters the cabin through the air conditioning packs, which cool, filter, and regulate it.

Cooling - Compressed air flows through venturi ducts and heat exchangers (or intercoolers) to cool it before entering the cabin

Airflow Regulation - Outflow valves, located at the rear of the cabin, regulate cabin pressure by controlling the rate of air leaving the fuselage. The primary valve maintains the desired cabin altitude, while a secondary safety valve acts as a backup relief mechanism. Think of the outflow valve as the system’s exhaust valve.

Control - A cabin pressure controller (manual or automatic) maintains target cabin altitude and rate of change.

Safety Valves - Relief and dump valves prevent the cabin from over or under-pressurising. The cabin safety valve opens automatically if the cabin pressure exceeds the maximum allowable differential, preventing structural damage..

The result: constant fresh air flow, stable pressure, and no buildup of stale or odorous air.

Cabin Pressure Profiles

As the aircraft climbs, the pressurisation system lets the cabin altitude rise gradually, typically 300–500 feet per minute, so passengers’ ears can adjust comfortably.

At cruise, the pressure differential (difference between cabin and outside pressure) stabilises around 8–9 psi depending on aircraft design. This keeps the cabin feeling like 8,000 ft while the airplane may be flying at 41,000 ft.

During descent, the system allows the cabin altitude to lower (and cabin pressure to increase) smoothly so that by touchdown, internal and external pressures match. This prevents discomfort and structural stress on doors and panels.

Understanding the Terms

Aircraft altitude: the height above mean sea level.
Cabin altitude: the cabin pressure expressed as an equivalent altitude in Standard atmosphere.
Differential pressure: the difference between cabin and outside air pressure.
Ambient pressure: the pressure surrounding the aircraft.
Ambient temperature: the outside air temperature.

These are basic but important, you’ll often get asked to define them in interviews or oral exams.

The Controls and Instruments

A typical pressurisation panel includes:

Cabin Pressure Controller: sets target altitude and rate.
Cabin Differential Pressure Gauge: shows pressure difference between inside and outside.
Cabin Altimeter: displays the current cabin altitude.
Rate-of-Climb Indicator: shows how quickly the cabin altitude is changing.

Cabin Altitude and Differential Pressure Indicator; Image Source: Learn to Fly Blog.

Cabin Rate of climb indicator; Image source: Learn to Fly Blog.

Most modern systems blend the controller and valves into an automatic loop, with manual control available as backup.

Decompression and Emergencies

Decompression means the system can no longer maintain pressure. It can happen from system failure or structural damage.

There are two main types:

Explosive decompression: pressure drops faster than the lungs can equalise (under 0.5 seconds). Can cause lung damage or disorientation.
Rapid decompression: pressure drops quickly but slower than lung equalisation time. It’s less violent but equally dangerous due to hypoxia.

In both cases, you’ll see fog or dust form briefly inside the cabin, that’s moisture condensing as temperature and humidity change (temperature reaches dew point and below)

Immediate pilot actions:

Oxygen masks on, establish crew communication.

Initiate emergency descent to around 10,000 ft.

Verify outflow valves and system status.

If decompression is caused by structural damage, those near openings risk being tossed or exposed to extreme cold and wind blast, hence the emphasis on seatbelts whenever seated.

The Real Dangers

The primary physiological risk is hypoxia, lack of oxygen to the brain. It impairs judgment before you even realise it. At FL350, you may have only 30 seconds of useful consciousness.

Time of Useful Consciousness decreases with increasing altitude. Image Source: Code 7700.

Another is decompression sickness, where dissolved nitrogen forms bubbles in tissues when pressure drops suddenly. Crew at high altitudes may also experience this if oxygen use or descent is delayed.

That’s why regulatory guidance and airline SOPs require masks to be donned immediately and 100% oxygen selected at high altitudes.

Interview & Checkride Focus Points

Expect questions like:

“Explain the pressurisation system in your aircraft.”
“What’s the typical cabin altitude at cruise?”
“At what point does the cabin altitude warning trigger?”
“What is the maximum differential pressure limit?”
“What are your immediate actions during pressurisation loss?”

Checks and exams often involve scenario-based questioning. In DGCA or airline interviews, they test whether you understand not just what happens, but why it happens and how you’d respond operationally.

Key Takeaways

Aircraft cabins are pressurised to maintain a breathable environment at high altitudes, typically around 8,000 ft cabin altitude.
Bleed air or electric compressors supply conditioned air to the cabin.
Outflow and safety valves regulate and protect against over-pressurisation.
Cabin pressure differential is what actually keeps the cabin air dense enough to breathe.
Regulations require supplemental oxygen above FL250 and continuous use by one pilot if operating above FL350.
A failure at cruise altitude can reduce time of useful consciousness to seconds, making this system one of aviation’s quiet lifesavers.

Big Picture

Pressurisation is one of those systems most passengers never notice, yet it’s what makes modern air travel possible.

For pilots, it’s a reminder of why systems knowledge matters. When it fails, you don’t get much time to think, only to act. Understanding how it works is essential for survival.