The invention relates to a plate heat exchanger forming between its plates a first flat flow tube in which a first fluid circulates between a first inlet orifice and a first outlet orifice, and a second flat flow tube in which a second fluid circulates between a second inlet orifice and a second outlet orifice. The two flat tubes form parallel arms which extend in a longitudinal direction (L) perpendicular to a transverse air flow traversing the heat exchanger, the first tube being upstream in the air flow and the second tube being downstream.