Why pressurise?
At 39 000 ft the outside air pressure is about a fifth of that at sea level and the temperature around −56 °C. Without pressurisation the useful consciousness time of an unprotected person is well under a minute. A pressurised cabin keeps the occupants at a comfortable, safe cabin altitude while the aircraft flies high where it is efficient.
| Term | Meaning |
|---|---|
| Cabin altitude | The altitude in the standard atmosphere whose pressure equals the cabin pressure |
| Differential pressure (ΔP) | Cabin pressure minus outside (ambient) pressure, in psi (psid) or hPa — the load on the structure |
| Cabin rate | The rate of change of cabin altitude (ft/min); typically 300–500 ft/min climbing and about 300 ft/min descending for comfort |
| Negative ΔP | Outside pressure greater than cabin pressure — possible in a fast descent; the structure tolerates only a small value |
Cabin altitude not above 8 000 ft at the maximum operating altitude in normal operation, and not above 15 000 ft after any probable failure (CS 25.841). A warning must tell the crew when cabin altitude exceeds 10 000 ft. Fresh air at least 0.25 kg (0.55 lb) per minute per occupant; carbon monoxide no more than 1 part in 20 000 and carbon dioxide no more than 0.5 % (CS 25.831). Typical maximum ΔP is 7.5–9 psi (about 9.4 psi on the B787, which keeps a 6 000 ft cabin).
Example: holding an 8 000 ft cabin (10.92 psi) at 40 000 ft (2.72 psi ambient) needs a ΔP of 8.2 psi; at 24 000 ft (5.70 psi) the same cabin needs only 5.2 psi.
Air supply
- Engine bleed — air is taken from the engine compressor: normally from an intermediate (low-pressure) stage, switching to a high-pressure stage through the HP valve when the engine is at low power. A pressure-regulating and shut-off valve (PRSOV) limits the pressure (typically around 40–45 psi), and a precooler — an air-to-air heat exchanger cooled by fan air through a fan-air valve — limits the temperature (around 200 °C) before the air enters the wing and fuselage ducts. The bleed system itself is covered in Chapter 18.
- APU bleed — the APU's load compressor supplies air on the ground and, within limits, in flight.
- Ground supply — a high-pressure ground cart connects to the pneumatic system (and can feed the packs or start engines), while a low-pressure pre-conditioned air (PCA) unit connects directly to the mix manifold, bypassing the packs.
- Bleedless designs (B787) — electrically driven cabin air compressors draw in ram air instead of using engine bleed.
- Ozone converters — catalytic converters in the supply ducts break down atmospheric ozone, which can be high above about FL300.
The air-cycle air-conditioning pack
Bleed air at around 200 °C must become air at a few degrees above freezing, at a controlled flow and with the water removed. The pack (usually two, mounted in unpressurised bays under the centre wing box) does it with an air cycle machine (ACM) — the "bootstrap" system.
- Pack (flow-control) valve — opens and shuts the pack and regulates the mass flow (often selectable low/normal/high) using a venturi or flow sensor.
- Primary heat exchanger — the first cooling stage, using ram air.
- ACM compressor — raises the air's pressure (and temperature again). The higher temperature lets the next heat exchanger remove more heat, and the higher pressure gives a bigger temperature drop in the turbine.
- Main (secondary) heat exchanger — the second ram-air cooling stage.
- High-pressure water separation — a reheater and a condenser (cooled by the cold turbine outlet air) make the moisture condense before the turbine; a water extractor swirls the droplets out. The water is sprayed into the ram-air inlet of the heat exchangers to improve their cooling.
- ACM turbine — the air expands, doing work that drives the compressor and fan on the same shaft, and its temperature drops sharply — often below 0 °C.
- Temperature control (bypass) valve — mixes hot air around the ACM into the outlet to set the pack outlet temperature; an anti-ice function stops ice forming at the turbine outlet or water separator.
- Ram air enters through a modulating ram-air inlet door and leaves through an outlet door or louvres. On the ground there is no ram effect, so the ACM fan (or an electric fan on some types) draws cooling air through the heat exchangers. Inlet doors are often closed during take-off and landing to keep out debris.
- Older and smaller systems use a low-pressure water separator downstream of the turbine — a coalescer bag in which droplets grow, then swirl vanes and a collector — with an anti-ice valve to keep the turbine outlet just above freezing. Its bag must be changed periodically.
- ACMs turn at tens of thousands of rpm on air or oil-lubricated bearings; loss of cooling air or blocked heat exchangers make the pack run hot and trip.
Vapour-cycle cooling
A vapour-cycle machine (VCM) works like a domestic refrigerator: the refrigerant absorbs heat when it evaporates at low pressure in the evaporator and gives it up when it condenses at high pressure in the condenser. It is used for cabin cooling on turboprops, business jets and smaller aircraft, for galley chillers, and for equipment cooling. The receiver-drier stores liquid refrigerant and removes moisture with a desiccant; a sight glass shows bubbles if the charge is low; the expansion valve (thermostatic, controlled by evaporator outlet temperature) meters the flow. Servicing uses service valves on the high and low sides, recovery equipment (refrigerant must never be vented) and nitrogen for leak checks.
Flow, temperature and humidity control
- Flow — set by the pack valves, adjusted for the number of passengers, single-pack operation, APU supply or fuel-saving low flow.
- Temperature — each zone (flight deck, forward cabin, aft cabin…) has a temperature selector and zone and duct temperature sensors. The controller sets the packs to the temperature the coldest zone needs and opens each zone's trim-air valve to add hot air from a trim-air manifold (supplied through a pressure-regulating valve). Simpler systems control each pack's outlet temperature directly.
- Humidity — high-altitude air is extremely dry, so cabin humidity typically falls to 10–20 %. The water separators remove free water to prevent fogging, icing and corrosion; some newer aircraft add humidifiers, mainly for the flight deck and crew rests.
- Distribution — overhead and sidewall ducts with outlets along the cabin, individual gasper outlets, flight-deck outlets, and riser ducts from the mix manifold; the flight deck usually receives fresh pack air first. Air leaves through floor-level grilles into the lower lobe.
- Recirculation — fans draw part of the cabin air (typically around half) through HEPA filters that trap about 99.97 % of 0.3-micron particles, including bacteria and viruses, and return it to the mix manifold. This reduces bleed demand and keeps humidity up.
- Other ventilation — lavatory and galley extract fans; avionics cooling by blower and extract fans (overboard on the ground, through a skin heat exchanger or into the cargo hold in flight); cargo-hold heating and ventilation.
The system page on ECAM or EICAS shows pack valve positions, pack outlet and compressor outlet temperatures, duct and zone temperatures, trim-air valve positions and bleed pressures, with cautions for pack overheat, duct overheat and pack faults. Pack controllers run built-in tests and record faults for the central maintenance computer.
Pressurisation control
The packs put a nearly constant mass of air into the fuselage; the cabin pressure is set by how much is let out. The cabin pressure controller (CPC) drives the outflow valve — a motorised butterfly or flap valve, sometimes shaped to recover a little thrust — to hold the scheduled cabin altitude and rate.
- Modes of control: in the isobaric mode the controller holds a constant cabin altitude; if holding it would exceed the maximum differential, it changes to a constant-differential mode. Modern digital controllers follow a schedule computed from the aircraft's altitude and the landing-field elevation (from the FMS or set manually).
- Phases: on the ground the outflow valve is fully open; at take-off power many systems pre-pressurise slightly (about 0.1 psi) so there is no pressure bump at rotation; the cabin climbs in proportion to the aircraft, holds in cruise, and descends so that it reaches the destination elevation (just pressurised) at touchdown; after landing the valve opens fully to depressurise the cabin.
- Redundancy: two automatic channels (alternating flight by flight or on failure), each with its own motor on the outflow valve, plus a manual mode in which the crew drives the valve directly with a switch while watching cabin altitude, rate and ΔP.
- Older aircraft used pneumatic controllers: a reference chamber and diaphragms in the outflow valve, with the crew selecting cabin altitude, rate and barometric setting.
Safety and warning devices
| Device | Function |
|---|---|
| Safety (positive-pressure relief) valves | Independent of the controller, usually pneumatic; open at a set differential (typically 8–10 psi, just above the normal maximum) to protect the structure. CS-25 requires the relief capacity to cope with full inflow |
| Negative-pressure (inward) relief valves | Open inwards if outside pressure exceeds cabin pressure by a small amount, protecting the structure and allowing doors to be opened |
| Cabin altitude limiter | Closes the outflow valve if the cabin altitude climbs too high (around 13 000–15 000 ft), whatever the controller is doing |
| Dump function | Rapidly equalises cabin and outside pressure (opening the outflow valve fully), e.g. for smoke removal |
| Ditching | Closes the outflow valve and other valves below the waterline so that a ditched aircraft floats longer |
| Pressure equalisation / decompression panels | Open between cabin, cargo holds and other compartments in a rapid decompression so the floor and bulkheads are not overloaded |
| Cabin altitude warning | Horn or master warning when cabin altitude exceeds 10 000 ft; passenger oxygen masks drop automatically at about 14 000 ft (Chapter 17) |
| Excess ΔP and residual-pressure warnings | Alert the crew to overpressure, and warn against opening doors while pressure remains (Chapter 4) |
| Pack and duct overheat protection | Compressor-outlet or pack-outlet overheat closes the pack valve; duct overheat closes the trim-air valves; bleed-leak loops detect hot-air leaks |
Not the pack valve. The packs set the flow (and temperature); the outflow valve sets the pressure. Safety valves are a back-up that only open when the differential exceeds its limit.
Heating and ventilation of smaller aircraft
(Background — examined for B1.2 and B3.) Unpressurised light aircraft use ram-air vents for ventilation and heat the cabin with an exhaust shroud (muff) heat exchanger or a fuel-burning combustion heater. Both can leak carbon monoxide into the cabin through cracks, so they are pressure-tested and inspected at intervals, combustion heaters have overheat protection, and CO detectors are recommended. Some aircraft use electric heaters.