a schematic cross sectional view of the arrangement of a bypass channel 20 alongside a ram air channel 30 the inlet opening 34 is again illustrated in the top left hand corner of the figure the ram air inlet channel 32 which is part of the ram air channel 30 is connected to the inlet opening 34 by contrast to the conventional construction from fig1 in this arrangement according to the invention a bypass channel 20 branches off from a common inlet 25 of the ram air channel 30 and of the bypass channel 20 bypasses the air conditioning system 10 and merges into the ram air outlet channel 40 the bypass channel comprises a non return valve 22 the bypass channel 20 is fed by the cool air from the ram air inlet channel 32 the air entering the ram air outlet channel 40 from the air conditioning system 10 is sometimes very hot and must be cooled in this instance this is achieved by the bypass channel 20 merging into the ram air outlet channel 40 the cool air from the bypass channel 20 is thus fed into the ram air outlet channel 40 and mixes with the hot air from the air conditioning system 10 the air in the ram air outlet channel which is released.