A flowpath assembly for a gas turbine engine , the assembly comprising: a plurality of flowpath insert ducts arranged circumferentially in a cascade configuration, each flowpath duct including: a radially inward wall (32-3, 32-4); a radially outward wall (32-5, 32-6); a first side wall (32-1); and a second side wall (32-2), wherein a flowpath volume is defined between the inward, outward, first side and second side walls, wherein the first side wall of a given one of the plurality of flowpath insert ducts is positioned circumferentially adjacent to the second sidewall of an adjacent one of the plurality of flowpath insert ducts in the cascade configuration, characterised in that the flowpath assembly further comprises a plurality of outward-facing retention hooks located on each of the first and second side walls of each of the flowpath ducts; and a plurality of housing cartridges configured to engage the opposed retention hooks of adjacent flowpath ducts to provide engagement therebetween, wherein each housing cartridge comprises: a body ; a plurality of cutout notches defined in the body configured to accept the retention hooks of flowpath ducts ; a threaded member ; a movable member threadably engaged with the threaded member ; a retention plate engaged with the movable member and positioned proximate a row of the plurality of cutout notches , wherein rotation of the threaded member produces translational movement of the movable member and the retention plate such that retention hooks of at least one of the flowpath ducts can be removably retained in the cutout notches by the retention plate . a plurality of flowpath insert ducts arranged circumferentially in a cascade configuration, each flowpath duct including: a radially inward wall (32-3, 32-4); a radially outward wall (32-5, 32-6); a first side wall (32-1); and a second side wall (32-2), wherein a flowpath volume is defined between the inward, outward, first side and second side walls, a radially inward wall (32-3, 32-4); a radially outward wall (32-5, 32-6); a first side wall (32-1); and a second side wall (32-2), wherein a flowpath volume is defined between the inward, outward, first side and second side walls, wherein the first side wall of a given one of the plurality of flowpath insert ducts is positioned circumferentially adjacent to the second sidewall of an adjacent one of the plurality of flowpath insert ducts in the cascade configuration, characterised in that the flowpath assembly further comprises a plurality of outward-facing retention hooks located on each of the first and second side walls of each of the flowpath ducts; and a plurality of housing cartridges configured to engage the opposed retention hooks of adjacent flowpath ducts to provide engagement therebetween, wherein each housing cartridge comprises: a body ; a plurality of cutout notches defined in the body configured to accept the retention hooks of flowpath ducts ; a threaded member ; a movable member threadably engaged with the threaded member ; a retention plate engaged with the movable member and positioned proximate a row of the plurality of cutout notches , wherein rotation of the threaded member produces translational movement of the movable member and the retention plate such that retention hooks of at least one of the flowpath ducts can be removably retained in the cutout notches by the retention plate . <EOS>