The repair design is primarily driven by non-engineering requirements such as: the repair facility capability, the type of damage found, whether or not the repair scheme is to be installed on or off the aircraft and the accessibility of the damaged area. The capability of suitable repair facilities has the strongest influence on the repair design. Flight line repairs are generally restricted to simple plug/patch repair types, whereas a depot-level repair should cover all repair types. In aircraft structures where light-weight structures are an essential design requirement, stiffness is often more critical than strength. Aerodynamic/hydrodynamic smoothness is an important consideration when maximum speed or fuel efficiency is required. The operating temperature influences the selection of repair materials, particularly adhesives and composite resins. The combination of extreme temperatures with environmental exposure, the hot/wet condition, is often the critical condition for which the repair must be designed.