Environmental control system (aircraft)

The environmental control system (ECS) of an aircraft provides air supply, thermal control and cabin pressurization for the crew and passengers. Avionics cooling, smoke detection, and fire suppression are also commonly considered part of an aircraft's environmental control system.

The systems described below are specific to current production Boeing airliners, although the details are essentially identical for passenger jets from Airbus and other companies. An exception was Concorde which had a supplementary air supply system fitted due to the higher altitudes at which it flew, and also the slightly higher cabin pressure it employed.

On jetliners, air is supplied to the ECS by being bled from a compressor stage of each gas turbine engine, upstream of the combustor. The temperature and pressure of this bleed air varies according to which compressor stage is used, and the power setting of the engine. A manifold pressure regulating shut-off valve (MPRSOV) restricts the flow as necessary to maintain the desired pressure for downstream systems.

A certain minimum supply pressure is needed to drive the air through the system, but it is desired to use as low a supply pressure as possible, because the energy the engine uses to compress the bleed air is not available for propulsion, and fuel consumption suffers. For this reason, air is commonly drawn from one of two (or in some cases such as the Boeing 777, three) bleed ports at different compressor stage locations. When the engine is at low pressure (low thrust or high altitude), the air is drawn from the highest pressure bleed port. As pressure is increased (more thrust or lower altitude) and reaches a predetermined crossover point, the high pressure shut-off valve (HPSOV) closes and air is selected from a lower pressure port to minimize the fuel performance loss. The reverse happens as engine pressure decreases.

...
Wikipedia