An inlet door assembly for reducing air pressure loss and enhancing performance of an auxiliary power unit (APU) contained within an aircraft housing is provided. The inlet door assembly includes a duct and a diffuser. The duct, which is configured to be movably coupled to an aircraft and to move between open and closed positions, has a flow passage extending therethrough. Air external to the aircraft flows through the duct while it is in the open position, but not while it is in the closed position. The diffuser is coupled to the duct and configured to move therewith. The diffuser has a passage extending therethrough that is in fluid communication with the duct flow passage, and is configured to diffuse the air that flows therethrough.