This paper reports experimental results for the performance of a freely hinged door between two incompressible air flows of different total pressures and different velocities. The apparatus used is a laboratory idealisation of the tertiary door system for an aircraft propulsion nozzle. The governing parameters are identified and the performance is presented in terms of the equilibrium door angle and the relative magnitude of the mass flow admitted. Three doors of different shapes were used and results are also presented for a plain opening. The main aerodynamic parameters are shown to be the velocity ratio and a non-dimensional pressure difference across the door. The shape of the downstream door jamb is found to have a considerable influence on the operation of the door at low angles. Removal of the door results in a lower entry mass flow over most of the operating range. It is shown that the mass flow ratio can be predicted quite well from the measured door angles by means of a simple theory.