As responsibility for each WBS element is allocated to members of the team, it is important that the boundaries and connections between each element are unambiguously defined. The wing-to-fuselage joint, for example, is a typical interface where different team members might be responsible for the wing and the fuselage design. A good interface definition will ensure that both team members understand the geometry as well as the structural and functional interface requirements. In this example, there may also be a third team member who might be responsible for electrical systems, and the wing-to-fuselage interface definition will clearly need to include electrical connections. Examples of diagrams for the SPOTTER aircraft of the sort that can help define clear interface definitions have been given earlier in Figures 6.1 and 6.2.
It is often worth ensuring the each interface definition is owned by a specific team member. There is often a logical allocation of this ownership. For example, in the wing-to-fuselage interface mentioned above, it might be more logical for the wing designer to “own” the interface definition, as it is intimately connected with wing design decisions such as spar diameter and root chord.