Faced with maintaining just the right cabin atmosphere for a space that could accommodate up to 840 passengers, Airbus scoured the industry for the most powerful environmental control system (ECS) ever conceived. The chosen system, which Hamilton Sundstrand eventually selected for development (see Chapter 7), was a double-spool, air-generating system that promised better thermodynamic efficiency, occupied less volume, and offered more redundancy. Instead of opting for the conventional air conditioning system approach, in which high-temperature, low-pressure bleed air from the engine compressor stages is converted to pressurized cabin air at room temperatures, the A3XX was to have two innovative double-packs. Each pack contained four air cycle machines, each of which had four stages, and were 85 percent more powerful than any previous ECS developed by the company. Each unit performed separate functions within the overall cycle.