A gas turbine engine, comprising: an annular gaspath extending around a central axis, the annular gaspath defined between a first casing (26, 126) and a second casing; and a variable guide vane assembly having: variable guide vanes circumferentially distributed about the central axis, the variable guide vanes having airfoils extending between first and second stems at respective first and second ends of the airfoils, the variable guide vanes rotatable about respective spanwise axes; a unison ring (44, 144) rotatable about the central axis, the unison ring (44, 144) operatively connected to the variable guide vanes for rotating the variable guide vanes about the respective spanwise axes, and a segmented bushing (46, 146) having bushing segments circumferentially distributed around the central axis, the bushing segments (46a, 146a) radially supported and axially constrained by the first casing (26, 126), the unison ring (44, 144) rollingly engaged to the first casing (26, 126) via the bushing segments (46a, 146a), the unison ring (44, 144) axially and radially constrained to the first casing (26, 126) via the bushing segments (46a, 146a); characterised in that the segmented bushing (46, 146) is secured to the first casing (26, 126) and the unison ring (44,144) slides on the bushing segments (46a, 146a) when the unison ring (44, 144) rotates about the central axis; or the bushing segments (46a, 146a) are secured to the unison ring (44, 144) such that the bushing segments and the unison ring rotate together about the central axis, wherein the first casing defines a casing groove defined between two rims (126b, 126c) of the first casing, the two rims extending circumferentially around the central axis and protruding from a wall of the first casing, the bushing segments axially constrained by the two rims within the casing groove, a distance between the two rims corresponding to an axial depth of the bushing segments, the bushing segments slidingly engaged to the first casing within the casing groove. an annular gaspath extending around a central axis, the annular gaspath defined between a first casing (26, 126) and a second casing; and a variable guide vane assembly having: variable guide vanes circumferentially distributed about the central axis, the variable guide vanes having airfoils extending between first and second stems at respective first and second ends of the airfoils, the variable guide vanes rotatable about respective spanwise axes; a unison ring (44, 144) rotatable about the central axis, the unison ring (44, 144) operatively connected to the variable guide vanes for rotating the variable guide vanes about the respective spanwise axes, and a segmented bushing (46, 146) having bushing segments circumferentially distributed around the central axis, the bushing segments (46a, 146a) radially supported and axially constrained by the first casing (26, 126), the unison ring (44, 144) rollingly engaged to the first casing (26, 126) via the bushing segments (46a, 146a), the unison ring (44, 144) axially and radially constrained to the first casing (26, 126) via the bushing segments (46a, 146a); characterised in that variable guide vanes circumferentially distributed about the central axis, the variable guide vanes having airfoils extending between first and second stems at respective first and second ends of the airfoils, the variable guide vanes rotatable about respective spanwise axes; a unison ring (44, 144) rotatable about the central axis, the unison ring (44, 144) operatively connected to the variable guide vanes for rotating the variable guide vanes about the respective spanwise axes, and a segmented bushing (46, 146) having bushing segments circumferentially distributed around the central axis, the bushing segments (46a, 146a) radially supported and axially constrained by the first casing (26, 126), the unison ring (44, 144) rollingly engaged to the first casing (26, 126) via the bushing segments (46a, 146a), the unison ring (44, 144) axially and radially constrained to the first casing (26, 126) via the bushing segments (46a, 146a); characterised in that the segmented bushing (46, 146) is secured to the first casing (26, 126) and the unison ring (44,144) slides on the bushing segments (46a, 146a) when the unison ring (44, 144) rotates about the central axis; or the bushing segments (46a, 146a) are secured to the unison ring (44, 144) such that the bushing segments and the unison ring rotate together about the central axis, wherein the first casing defines a casing groove defined between two rims (126b, 126c) of the first casing, the two rims extending circumferentially around the central axis and protruding from a wall of the first casing, the bushing segments axially constrained by the two rims within the casing groove, a distance between the two rims corresponding to an axial depth of the bushing segments, the bushing segments slidingly engaged to the first casing within the casing groove.