Abstract: A method and apparatus is provided for air conditioning two passenger decks in an aircraft, especially a high capacity aircraft. The engine tap air stream is provided for air conditioning the upper deck passenger cabin and the lower deck passenger cabin. The air stream is cooled in at least one air conditioning plant, mixed with recirculated air from the lower cabin in at least one premix chamber, and then mixed with recirculated air from the upper cabin in local mixing chambers. The finished air streams are directed to the upper deck and the lower deck passenger cabins. The upper deck exhaust air is essentially completely used as recirculation air being mixed with the conditioning air in the local mixing chambers. The lower deck exhaust air is partially used as recirculation air being mixed with the conditioning air in the premix chambers, and is partially exhausted overboard.