The 747 working utility supply

The 747 "bleeds" high-pressure, high-temperature air from its engine compressors and uses it as a workhorse utility supply: air-conditioning packs, cabin pressurisation, engine and APU starting, and wing and engine anti-ice all depend on it. It is why the 747 belongs to the "conventional" pneumatic generation of airliners.

Key facts
Source	Engine compressors, plus the APU when engines are off
Users	Air conditioning packs, pressurisation, engine/APU starting, wing and engine anti-ice, some pneumatic actuation
Ground need	A ground pneumatic (bleed-air) supply heats or cools the cabin while engines are shut down
Exception	The Boeing 787 is bleedless; the 747 was not

Modern turbofans such as the JT9D, CF6, RB211 and PW4000 supplied plenty of hot, high-pressure bleed air, but it carries a small fuel-efficiency penalty - the air is "stolen" from the compressor path.

The 747-400 and 747-8 use the same philosophy: bleed air from the engine fan/core stages is conditioned and distributed to the packs. Boeing documents ground pneumatic power requirements for heating and cooling in the 747-8 airport-planning manual.

The trade-off of conventional bleed

Tapping compressed air for utilities works, but it is not free. Every unit of bleed air is air the engine would otherwise use to produce thrust, and the energy spent heating and pressurising it comes out of overall efficiency. This is the fundamental reason the industry eventually moved toward more-electric architectures.

The 747 sat on the cusp of that transition. It kept bleed because the pneumatic architecture was deeply integrated and well understood, and because the aircraft's roles - long-haul passenger, freighter, and Combi - valued the reliability of a simple, mechanically robust utility source. The 747-8, the last of the line, still bled.

What runs on bleed air

Bleed air feeds several critical consumers, and the routing is why a single engine's bleed failure can cascade into a dispatch problem:

• Pneumatic engine starting - high-pressure starter air rotates the engine through cranking.
• Air conditioning packs - bleed air is chilled and then reheated to condition cabin and cockpit air.
• Pressurisation - conditioned packs supply the cabin and the outflow control system manages cabin altitude.
• Wing and engine anti-ice - hot bleed air runs through the leading-edge piccolo tubes to shed ice.
• APU start and some pneumatic actuation - minor consumers, but they close the start chain.

Because these consumers are largely independent, the minimum-equipment lists for the 747 (see the Transport Canada MMEL in the sources) allow the aircraft to dispatch with bleed-related items inoperative provided enough redundancy remains for the intended operation.

Related

• Fuel system
• Hydraulic systems
• Electrical system
• 747-8 systems differences