Double box composite seal assembly with insert for gas turbine engine

A method of fabricating a seal for a gas turbine engine, comprising:

• forming a hollow core including a plurality of triaxially braided core plies along a pair of mandrels arranged in an opposed relationship;
• positioning a platform insert along the core;
• forming an overwrap including a plurality of biaxially braided overwrap plies that establish a passageway;
• inserting the core plies and the platform insert at least partially into the passageway such that the core, the platform insert and the overwrap cooperate to establish a sealing portion extending circumferentially between opposed mate faces, wherein the core and the overwrap cooperate to establish an engagement portion having an opening along each of the mate faces, the opening is dimensioned to receive a respective mounting block;
• and
• removing the pair of mandrels from the core to establish an internal cavity in the core, wherein the internal cavity spans between the openings along the mate faces, and the internal cavity is bounded between a first wall of the sealing portion establishing a seal face and a second wall of the engagement portion establishing a backside face.