smaller cross sectional area heat exchanger leads to enabling its use in a duct of a smaller cross sectional area to thereby improve the compactness of the resulting cooling subsystem achieving the same cooling of a working fluid such as a lubricant passing through such a smaller cross sectional area heat exchanger generally requires that a larger volume of air pass through that heat exchanger per unit time about the passageways thereof to which the working fluid is delivered and through which the previously heated working fluid is flowing to be cooled and thereafter subsequently be introduced to the remainder of the system making use of that fluid such an increase in volume rate of air passing through the heat exchanger in a smaller cross sectional area duct generally requires that the pressure gradient from the duct entrance to the duct exit be sufficiently greater to result in such a flow although the present invention is useable with any working fluid provided in connection with a gas turbine engine that needs cooling the remainder of the specification will describe an embodiment in which the working fluid is a lubricant a relatively large pressure gradient exists in a turbofan engine between the beginning of the fan stream in the turbofan engine and the ambient atmosphere thus locating the duct in the engine nacelle lower bifurcation or in some other engine nacelle bifurcation to extend from an upstream side near the engine fan to a downstream side.