An aircraft assembly comprising: a pair of covers; a spar web extending between the covers in a thickness direction, the spar web having a length which extends in a span-wise direction; and a container which extends from the spar web and houses at least part of a system component. The container comprises first and second side walls which are spaced apart from each other across the spar web in the thickness direction, and inboard and outboard walls which are spaced apart from each other along the spar web in the span-wise direction. The spar web and at least part of the container are integrally formed as a single piece. Typically the assembly is a wing assembly with a fuel tank; a track; a high-lift device carried by the track; an actuation mechanism for moving the track and high-lift device between an extended high-lift position and a retracted low-lift position; and a track container which extends from the spar web into the fuel tank and houses at least part of the track when it is in its retracted low-lift position. The spar web and at least part of the track container are integrally formed as a single piece, typically from a laminar composite material.