An air seal assembly for a gas turbine engine comprising: a forward first rotor disk defined about an axis of rotation; an aft second rotor disk defined about said axis of rotation; a vane structure axially between said first rotor disk and said second rotor disk; a first cover plate with a first radially extending knife edge seal defined about said axis of rotation, said first cover plate mountable to an aft surface of said first rotor disk for rotation therewith, said first radially extending knife edge seal extending outward from a cylindrical extension that extends from said first cover plate and interfacing with said vane structure, said first cover plate further comprising a second radially extending knife edge seal which extends outward from said first cylindrical extension; and a second cover plate with a third radially extending knife edge seal defined about said axis of rotation, said second cover plate mountable to a forward surface of said second rotor disk for rotation therewith, said third radially extending knife edge seal interfacing with said vane structure; characterised in that: said first cover plate has only a single cylindrical extension having knife edge seals, and said vane structure comprises apertures for communicating purge air into an upstream rim cavity formed between said first rotor disk and said vane structure to prevent ingestion of hot gas core airflow and its contaminants into a rotating cavity between the first and second stage disks, a downstream rim cavity being formed between the second rotor disk and the vane structure, purge air flowable from the upstream rim cavity to the downstream rim cavity past the radially extending knife edge seals (88A, 88B, 90A) due to the lower pressure at the downstream rim cavity relative to the upstream rim cavity. a forward first rotor disk defined about an axis of rotation; an aft second rotor disk defined about said axis of rotation; a vane structure axially between said first rotor disk and said second rotor disk; a first cover plate with a first radially extending knife edge seal defined about said axis of rotation, said first cover plate mountable to an aft surface of said first rotor disk for rotation therewith, said first radially extending knife edge seal extending outward from a cylindrical extension that extends from said first cover plate and interfacing with said vane structure, said first cover plate further comprising a second radially extending knife edge seal which extends outward from said first cylindrical extension; and a second cover plate with a third radially extending knife edge seal defined about said axis of rotation, said second cover plate mountable to a forward surface of said second rotor disk for rotation therewith, said third radially extending knife edge seal interfacing with said vane structure; characterised in that: said first cover plate has only a single cylindrical extension having knife edge seals, and said vane structure comprises apertures for communicating purge air into an upstream rim cavity formed between said first rotor disk and said vane structure to prevent ingestion of hot gas core airflow and its contaminants into a rotating cavity between the first and second stage disks, a downstream rim cavity being formed between the second rotor disk and the vane structure, purge air flowable from the upstream rim cavity to the downstream rim cavity past the radially extending knife edge seals (88A, 88B, 90A) due to the lower pressure at the downstream rim cavity relative to the upstream rim cavity. said first cover plate has only a single cylindrical extension having knife edge seals, and said vane structure comprises apertures for communicating purge air into an upstream rim cavity formed between said first rotor disk and said vane structure to prevent ingestion of hot gas core airflow and its contaminants into a rotating cavity between the first and second stage disks, a downstream rim cavity being formed between the second rotor disk and the vane structure, purge air flowable from the upstream rim cavity to the downstream rim cavity past the radially extending knife edge seals (88A, 88B, 90A) due to the lower pressure at the downstream rim cavity relative to the upstream rim cavity.