A gas turbine engine comprising: a rotor including a row of rotor blades rotatable about an axis; a blade outer air seal radially outwards of the row of rotor blades, the blade outer air seal including a plurality of seal arc segments (66; 166; 266), each of the seal arc segments (66; 166; 266) having radially inner and outer sides (68a, 68b), first and second circumferential ends (70a, 70b), and first and second axial sides (72a, 72b); a plurality of primary seal supports; and a plurality of secondary seal supports (80a, 82a); wherein each of the seal arc segments (66; 166; 266) includes first and second primary radial supports (74a, 74b) upon which respective ones of the plurality of primary seal supports carry the seal arc segment (66; 166; 266) in a primary radial position which is the initial assembled position and the position in which the seal arc segment (66; 166; 266) is in during operation of the engine, and first and second secondary radial supports (76a, 76b; 176a, 176b) upon which respective ones of the plurality of secondary seal supports (80a, 82a) carry the seal arc segment (66; 166; 266) in a secondary radial position upon release of the seal arc segment (66; 166; 266) from the first and second primary radial supports (74a, 74b), the secondary radial position is radially offset from the primary radial position, the first and second primary radial supports (74a, 74b) are first hook supports, and the first and second secondary radial supports (76a, 76b; 176a, 176b) are second hook supports, wherein the first and second circumferential ends (70a, 70b) include the first and second primary radial supports (74a, 74b), and the first and second axial sides (72a, 72b) include the first and second secondary radial supports (76a, 76b; 176a, 176b), wherein the first and second secondary radial supports (76a, 76b; 176a, 176b) protrude, respectively, axially forward and aft of the radially inner side, wherein the seal arc segment (66; 166; 266) has a U-shape cross-section, with the radially inner side forming the bottom of the U-shape cross-section and the first and second axial sides (72a, 72b) forming the upright legs of the U-shape cross-section, and the first and second secondary radial supports (76a, 76b; 176a, 176b) are at the top of the upright legs, and wherein the plurality of secondary seal supports (80a, 82a) include forward secondary seal supports on a forward vane structure and aft secondary seal supports on an aft vane structure. a rotor including a row of rotor blades rotatable about an axis; a blade outer air seal radially outwards of the row of rotor blades, the blade outer air seal including a plurality of seal arc segments (66; 166; 266), each of the seal arc segments (66; 166; 266) having radially inner and outer sides (68a, 68b), first and second circumferential ends (70a, 70b), and first and second axial sides (72a, 72b); a plurality of primary seal supports; and a plurality of secondary seal supports (80a, 82a); wherein each of the seal arc segments (66; 166; 266) includes first and second primary radial supports (74a, 74b) upon which respective ones of the plurality of primary seal supports carry the seal arc segment (66; 166; 266) in a primary radial position which is the initial assembled position and the position in which the seal arc segment (66; 166; 266) is in during operation of the engine, and first and second secondary radial supports (76a, 76b; 176a, 176b) upon which respective ones of the plurality of secondary seal supports (80a, 82a) carry the seal arc segment (66; 166; 266) in a secondary radial position upon release of the seal arc segment (66; 166; 266) from the first and second primary radial supports (74a, 74b), the secondary radial position is radially offset from the primary radial position, the first and second primary radial supports (74a, 74b) are first hook supports, and the first and second secondary radial supports (76a, 76b; 176a, 176b) are second hook supports, wherein the first and second circumferential ends (70a, 70b) include the first and second primary radial supports (74a, 74b), and the first and second axial sides (72a, 72b) include the first and second secondary radial supports (76a, 76b; 176a, 176b), wherein the first and second secondary radial supports (76a, 76b; 176a, 176b) protrude, respectively, axially forward and aft of the radially inner side, wherein the seal arc segment (66; 166; 266) has a U-shape cross-section, with the radially inner side forming the bottom of the U-shape cross-section and the first and second axial sides (72a, 72b) forming the upright legs of the U-shape cross-section, and the first and second secondary radial supports (76a, 76b; 176a, 176b) are at the top of the upright legs, and wherein the plurality of secondary seal supports (80a, 82a) include forward secondary seal supports on a forward vane structure and aft secondary seal supports on an aft vane structure.