Abstract: Apparatus for sealing the side edges of flaps of a propulsion nozzle of an engine and the walls of a casing of the nozzle by sealing elements carried by the flaps and extending across a clearance between the side edges of the flaps and the walls. The sealing elements are carried in slots in the side edges of the flaps with clearance for capability of movement towards the walls. The sealing elements contact the walls to form a primary seal therewith. The flaps are formed with chambers carrying a pressure medium and plungers are supported in the flaps for being subjected to the pressure of the pressure medium and bearing against the sealing elements to urge the sealing elements against the surfaces of the walls. A differential pressure exists on both sides of the flaps and acts on the sealing elements in the clearance to press the sealing elements against respective side surfaces of the slots to form a secondary seal thereat.