An aircraft system includes an aircraft fuselage, an auxiliary power unit and an air inlet assembly. The auxiliary power unit is within the aircraft fuselage. The auxiliary power unit may be configured as or otherwise include an engine. The air inlet assembly includes an inlet orifice, an inlet duct and a door. The inlet duct fluidly couples the inlet orifice with an airflow inlet of the engine. The door is configured to pivot about a generally horizontal pivot axis between an open position and a closed position. The door opens the inlet orifice in the open position and substantially closes the inlet orifice in the closed position.