A center wing box assembly for an aircraft, characterised in that it comprises: a composite lower wing skin; a composite upper wing skin; a composite spar which extends between the composite lower wing skin and the composite upper wing skin; and a composite stringer which extends along a web of the composite spar in a wing span direction, wherein: the composite spar has a co-cure securement with the composite lower wing skin and with the composite upper wing skin; and the composite stringer has a co-cure securement with the web of the composite spar, wherein the composite spar comprises: a first composite trough configuration which comprises a first web, wherein: a first curved transition wall portion extends from a first end portion of the first web and to a first flange of the first composite trough configuration; the first flange of the first composite trough configuration extends along the composite lower wing skin; a second curved transition wall portion extends from a second end portion of the first web and to a second flange of the first composite trough configuration; and the second flange of the first composite trough configuration extends along the composite upper wing skin; and a second composite trough configuration which has a second web, wherein: a first curved transition wall portion extends from a first end portion of the second web and to a first flange of the second composite trough configuration, the first flange of the second composite trough configuration extends along the composite lower wing skin; a second curved transition wall portion extends from a second end portion of the second web and to a second flange of the second composite trough configuration; the second flange of the second composite trough configuration extends along the composite upper wing skin; and the web of the composite spar is formed by the first web of the first composite trough configuration and the second web of the second composite trough configuration abutting each other, the first web having a co-cure securement with the second web. a composite lower wing skin; a composite upper wing skin; a composite spar which extends between the composite lower wing skin and the composite upper wing skin; and a composite stringer which extends along a web of the composite spar in a wing span direction, wherein: the composite spar has a co-cure securement with the composite lower wing skin and with the composite upper wing skin; and the composite stringer has a co-cure securement with the web of the composite spar, the composite spar has a co-cure securement with the composite lower wing skin and with the composite upper wing skin; and the composite stringer has a co-cure securement with the web of the composite spar, a first composite trough configuration which comprises a first web, wherein: a first curved transition wall portion extends from a first end portion of the first web and to a first flange of the first composite trough configuration; the first flange of the first composite trough configuration extends along the composite lower wing skin; a second curved transition wall portion extends from a second end portion of the first web and to a second flange of the first composite trough configuration; and the second flange of the first composite trough configuration extends along the composite upper wing skin; and a first curved transition wall portion extends from a first end portion of the first web and to a first flange of the first composite trough configuration; the first flange of the first composite trough configuration extends along the composite lower wing skin; a second curved transition wall portion extends from a second end portion of the first web and to a second flange of the first composite trough configuration; and the second flange of the first composite trough configuration extends along the composite upper wing skin; and a second composite trough configuration which has a second web, wherein: a first curved transition wall portion extends from a first end portion of the second web and to a first flange of the second composite trough configuration, the first flange of the second composite trough configuration extends along the composite lower wing skin; a second curved transition wall portion extends from a second end portion of the second web and to a second flange of the second composite trough configuration; the second flange of the second composite trough configuration extends along the composite upper wing skin; and the web of the composite spar is formed by the first web of the first composite trough configuration and the second web of the second composite trough configuration abutting each other, the first web having a co-cure securement with the second web. a first curved transition wall portion extends from a first end portion of the second web and to a first flange of the second composite trough configuration, the first flange of the second composite trough configuration extends along the composite lower wing skin; a second curved transition wall portion extends from a second end portion of the second web and to a second flange of the second composite trough configuration; the second flange of the second composite trough configuration extends along the composite upper wing skin; and the web of the composite spar is formed by the first web of the first composite trough configuration and the second web of the second composite trough configuration abutting each other, the first web having a co-cure securement with the second web.