The thermal mixed convection boundary-layer flow over a flat horizontal cooled plate is revisited. It is shown that this flow is very similar to the one taking place in a free convection hypersonic boundary layer (with a shock in x(3/4)): the observed singular solutions which branch out may then be reinterpreted in the framework of "triple deck" theory. Two salient structures emerge, one in double deck, if the buoyancy is very small, and the other one in single deck, if the buoyancy is O(1). These two structures are a reinterpretation of Steinruck's [J. Fluid. Mech. 278 (1994) 251-265] results. A numerical simulation of the unsteady boundary layer in the case of impulsively started and cooled plate is carried out. It leads to the separation of the boundary layer as predicted by the triple deck theory. A region of reverse flow is obtained which depends on the outflow boundary condition.