The Hydraulic Piping System of the DC-10 Airplane represents a break with the Military Standard tubes and fittings used for the previous quarter century. This paper summarizes the reasons for the selection of ARMCO 21-6-9 stainless steel tubing over several other candidate materials. Factors of weight, cost, availability and reliability were important, as well as compatibility with the various joining processes contemplated. Perhaps even more important were ease of fabrication of tube segments and assemblies, installation, and field repair. The McDonnell Douglas material specification is summarized, with emphasis on tube inspection requirements. The tubing is giving excellent service, and was introduced into production with relatively few problems.