Pressurisation and air conditioning

Bleed air, air conditioning packs and outflow valves keep the cabin habitable — and every pressurisation cycle is a fatigue cycle on the airframe.

At a 747’s cruising altitude the outside air is around −55 °C and far too thin to breathe. The cabin is kept habitable by taking air from the engines, conditioning it, and letting it out at a controlled rate.

Where cabin air comes from

On every 747 up to and including the 747-8, cabin air is bleed air: air tapped from the compressor stages of the engines, before combustion.

It arrives hot and at high pressure, and passes through air conditioning packs — heat exchangers and air cycle machines that cool it and control its moisture — before being distributed to the cabin.

This is worth contrasting with the 787, which is a more-electric aircraft: it draws cabin air through dedicated electric compressors instead of engine bleed. The 747-8’s GEnx-2B had to be adapted back from the 787’s GEnx specifically to provide bleed air again, because the 747 uses conventional pneumatic systems.

Keeping the pressure

Air is fed in continuously and let out through outflow valves. Cabin pressure is set by how fast air is allowed to leave, not by how fast it goes in.

The cabin is held at an equivalent altitude of roughly 8,000 ft at cruise — comfortable enough for almost everyone, while keeping the pressure difference across the fuselage within what the structure can take. The 747-8 improved on this, as did the 787, by lowering cabin altitude and raising humidity.

The fuselage is a pressure vessel

This is the structural consequence, and it drives more of the aircraft’s design and maintenance than anything else.

A pressurised fuselage is a balloon with people in it. Every flight inflates it and deflates it once, and each of those cycles puts a fatigue load into the structure. Metal fatigue depends on the number of load cycles far more than on hours flown.

That is why the 747’s most unusual variants exist. The 747-100SR, -300SR and 747-400D were built for Japanese domestic routes flying many short sectors a day, and were structurally reinforced specifically to absorb far more pressurisation cycles than a long-haul 747 would ever see. One ANA aircraft retired with more than 33,000 cycles.

It is also why the aft pressure bulkhead is such a critical component — and why a faulty repair to one destroyed Japan Air Lines 123 seven years and 12,318 flights after it was made.