2.3 Pressurisation & Air Conditioning Systems
Key Takeaways
- Conditioned air is normally taken as bleed air from the engine compressors, cooled in air conditioning packs, mixed with filtered recirculated air and distributed to the cabin.
- Cabin altitude is controlled by the outflow valve, which modulates how fast air leaves the fuselage; typical maximum cabin altitude in cruise is around 6,000 to 8,000 feet.
- Differential pressure is the difference between cabin and outside pressure and is limited by structural design, which is why cabin altitude rises as the aircraft climbs.
- A cabin altitude warning is normally triggered at about 10,000 feet cabin altitude, and passenger oxygen masks deploy automatically at approximately 14,000 feet cabin altitude.
- Recirculated cabin air on modern aircraft passes through HEPA filters, and total cabin air is typically exchanged every two to three minutes.
Why the cabin must be pressurised
At the cruising altitudes flown by jet transports the outside air pressure is roughly a quarter of sea-level pressure and the temperature is around minus 50 degrees Celsius. Unprotected human beings lose useful consciousness in well under a minute at those levels. The pressurisation system therefore does two jobs at once: it keeps the cabin at a pressure the body can work at, and it supplies conditioned, ventilated air at a comfortable temperature.
Where the air comes from
| Stage | What happens |
|---|---|
| Bleed air | High-pressure, high-temperature air is tapped from the engine compressor stages. On the ground the APU or a ground air cart supplies it |
| Pre-cooler | Bleed air temperature is reduced before it enters the air conditioning system |
| Packs | The air conditioning packs — normally two — cool and dry the air using an air cycle machine, producing conditioned air at a controlled temperature |
| Mix manifold | Conditioned pack air is blended with filtered recirculated cabin air drawn through recirculation fans |
| Distribution | The mixture is distributed through ducts to the cabin, flight deck and cargo compartments, with zone temperature control |
| Outflow valve | Air leaves the fuselage through a modulating outflow valve, usually at the rear, which is what actually sets cabin pressure |
A minority of modern types, notably the Boeing 787, use electrically driven compressors instead of engine bleed air, but the downstream principle is unchanged.
Air quality. Roughly half the air supplied to a modern cabin is recirculated, and that fraction passes through HEPA filters that capture the great majority of airborne particles including bacteria and viruses. The complete cabin air volume is typically exchanged every two to three minutes — far faster than in most buildings. Cabin crew should be able to explain this to anxious passengers, and should report any unusual smell immediately because odour is the primary detection method for a bleed air contamination event.
Cabin altitude and differential pressure
Two numbers describe the state of the system.
- Cabin altitude is the pressure altitude corresponding to the pressure inside the cabin. It is not the aircraft's altitude. A cabin altitude of 7,000 feet means the occupants are breathing air at the pressure found at 7,000 feet, whatever the aeroplane is doing.
- Differential pressure is cabin pressure minus outside pressure. The fuselage is a pressure vessel with a certificated maximum differential, typically in the region of 8 to 9 pounds per square inch on narrow-body jets.
Because differential pressure is structurally limited, cabin altitude must rise as the aeroplane climbs. The system schedules a gentle cabin climb so that the maximum cabin altitude in cruise is normally about 6,000 to 8,000 feet. Descent is scheduled to bring the cabin back to field elevation at landing.
Rate of change matters more than the value. Cabin rate of climb is normally kept to a few hundred feet per minute, and cabin descent rates are held lower still, because the middle ear equalises outward far more easily than inward. This is why passengers with colds suffer on descent rather than climb.
Warnings and automatic protections
| Cabin altitude | What happens |
|---|---|
| Normal cruise, about 6,000–8,000 ft | No indication; comfortable operation |
| Approximately 10,000 ft | Cabin altitude warning — aural warning and master caution on the flight deck. The flight crew will don oxygen masks and begin diagnosis or descent |
| Approximately 14,000 ft | Passenger oxygen masks deploy automatically from the passenger service units, with a manual override available on the flight deck |
| Above 14,000 ft | Supplemental oxygen essential; time of useful consciousness shortens rapidly with altitude |
Nig. CARs adds an operational rule in Part 8: the pilot-in-command shall require all passengers to use oxygen continuously at cabin pressure altitudes above 4,000 m (13,000 ft), and shall ensure that breathing oxygen and masks are available in sufficient quantity for flights at altitudes where lack of oxygen might harm passengers.
Part 8 also states the safeguarding principle behind the design: cabin crew shall be safeguarded so as to ensure a reasonable probability of retaining consciousness during any emergency descent following loss of pressurisation, and shall have such means of protection as will enable them to administer first aid to passengers during stabilised flight afterwards. That is the regulatory reason portable oxygen bottles exist in the cabin.
Recognising a pressurisation problem
| Symptom | Likely condition | Immediate cabin crew action |
|---|---|---|
| Loud bang, rush of air, fog or mist in the cabin, dust and loose articles moving, sudden cold, ears popping violently | Rapid or explosive decompression | Grab the nearest oxygen mask and don it, sit down and hold on, secure yourself; assist others once the aircraft is below a safe altitude |
| Gradual onset of unexplained fatigue, euphoria, headache, tingling, poor coordination, cyanosis; masks may drop with no bang | Insidious or slow decompression | Suspect hypoxia in yourself first; take oxygen, notify the flight deck immediately |
| Ear and sinus pain, whistling noise near a door or window, condensation at a seal | Possible seal or door leak | Report location and description to the flight deck without delay |
| Unusual smell — oily, musty, like wet dog or dirty socks | Possible bleed air contamination / fume event | Notify the flight deck immediately, identify the area, record who is affected, do not attempt to ventilate by opening anything |
The cardinal rule in any decompression is oxygen first, for yourself, before anything else. A crew member who tries to help a passenger before donning a mask usually becomes a second casualty and removes the cabin's only trained responder.
At approximately what cabin altitude do passenger oxygen masks deploy automatically on a typical transport aeroplane?
What does the term cabin altitude describe?
Which component directly controls cabin pressure in flight?
A cabin crew member notices a persistent oily, musty smell shortly after take-off, and two passengers complain of headache. What is the correct immediate action?