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This paper explains, via specific examples, that no one-to-one correlation exists between design for assembly and parts-count reduction. It explains how overall cost reductions cannot be maximized if the cost of each and every step in a process is minimized separately, as it occurs. A distinction is drawn between goals and the metrics used to measure satisfaction of the goals, and concern is expressed at how frequently one now sees the metric usurping the function of the original goal - to the extent that satisfying the metric can result in directly violating what once was the goal. Illustrations are provided of how, and when, parts consolidation can be of overall benefit. Other examples reveal how parts-count reduction has been counterproductive. A better metric, interface control, is recommended as an alternative.

The process of eliminating, or at least minimizing, critical interfaces in the structural assembly process is shown to be very powerful in reducing overall costs. Generic, but specific, examples are used to illustrate the technique. This approach is compared to the more traditional technique of parts-count reduction, and shown to be the more powerful of the two. The procedure for implementing interface control is known as dimensional management. The location of interfaces is defined by the dimensions, and tolerances, of the many parts in assemblies and subassemblies and by those of any fixtures used. Maximizing the permissible tolerances at any one stage of assembly is shown to be achieved by reducing both the number of assembly steps and the number of assembly fixtures involved. Part-to-part assembly is justified on the basis of the number of critical interfaces it eliminates.

With the DC-10 carbon-epoxy rudder, the Douglas Aircraft Company achieved one of the greatest percentage weight savings associated with composite structures. Apart from minor damage from lightning strikes, the 15 rudders put into service have experienced virtually trouble-free operation for about a decade. This paper explains why a multirib, postbuckled skin design was used for the DC-10 composite rudder, how it was justified, and how it would have compared with more conventional sandwich design concepts. Special attention is devoted to the reasons why, for such postbuckled designs, it is better to allow the skin to wrinkle and unload itself than to reinforce it and make it resist buckling until some higher load level is attained. With minor changes in the manufacturing technique, this design concept is ideal for the control surfaces on many aircraft. The paper includes suggestions on how to make even better composite control surfaces in the future.

This paper addresses the repair of advanced composite structures by mechanical fasteners or by adhesive bonding. It is shown that many of today's composite designs are unreasonably difficult to repair. Conversely, the knowledge to design repairable structures is already available, if only it is applied during the initial design stage. Bolted or riveted repairs require only the avoidance of extremely orthotropic composite fiber patterns; those near the quasi-isotropic layup are the most suitable. Mildly orthotropic fiber patterns are appropriate for structures in which there is a dominant load direction. Thick composite structures are shown to require bolted or riveted repairs while thin structures favor adhesively bonded permanent repairs, although provisions can be easily made for temporary mechanical repairs. The reasons why integrally stiffened cocured composite designs are usually impractical to repair are explained and alternative repairable design concepts are presented.

The Primary Adhesively Bonded Structures Technology (PABST) program conducted by McDonnell Douglas for the USAF Plight Dynamics Laboratory has broadened the horizons for future applications of adhesive bonding to aerospace structures. A major portion of this program was concerned with the use of new environmentally-resistant adhesives and primers, and an improved surface preparation involving the use of phosphoric acid anodize. This portion has been well publicized and explains past service failures and how to avoid their recurrence. This paper emphasises structural considerations.