Friction Stir Welding (FSW) can achieve high quality welds in aluminum alloys that are of interest to the aerospace industry (e.g. alloys 2014, 2219, 7050 and numerous aluminum-lithium alloys). The low distortion solid-phase welds exhibit metallurgical and mechanical properties, including fatigue, which are superior to conventional fusion welds achieved by arc processes. FSW, although a relatively new welding technique, has been systematically developed and proved by The Welding Institute (TWI) under contract to an international group of sponsors, one of which is The Boeing Company.To further validate the process, The Boeing Company conducted separate development activities including detailed mechanical testing of welds made from the FSW process. Weld property data showed a dramatic improvement over conventional arc processes and additional studies concluded that the FSW process could provide significantly improved weld quality and lower production costs on critical aerospace components. Further validation studies have been conducted, much of which has been published within the aerospace industry (Ref.1,2,3), all of which concluded similar results. However, one critical study needed to be done with respect to actual operational testing of the process with aerospace hardware. This paper summarizes the results from a study conducted by The Boeing Company to determine mechanical behavior and operational service testing of FSW. TWI was placed under contract by Boeing to fabricate several subscale experimental cylindrical tanks, each fabricated entirely by the FSW process.