The air distribution system on most aircraft mixes cold air from the air-conditioning packages (packs) and hot engine bleed air in the conditioned air manifold according to the temperature called for by the flight crew.
• Air from the engine bleed valve enters the air-conditioning unit through the engine bleed duct.
• The air is brought to the required cabin temperature and pressure by passing through a heat exchanger inside the air-conditioning which exchanges heat with cold air coming from outside the aircraft.
• The air is then passed into the cabin through the cabin air distribution system.
• Some of the stale air from the cabin is filtered and recirculated back to the cabin through the air mixing unit. Modern jetliners use "high efficiency particulate arresting" HEPA filters, which trap more than 99% of all bacteria and clustered viruses.
• Also, hot air from the hot air manifold is used to control the temperature inside the cabin.
