Parts Manufacturer Approval (PMA) parts and designated engineering representative (DER) repairs are parts and repairs that are provided by third-party companies and certified by the Federal Aviation Administration to be airworthy and interchangeable with original equipment manufacturer (OEM) parts or repairs. Currently, all major U.S. carriers use PMA parts and DER repairs in their own fleets so they can introduce competition, save costs, and maintain a more robust supply chain of parts and repairs. The Department of Defense (DoD) is interested in increasing its use of similar kinds of alternate parts and repairs. This report assesses the feasibility and extent to which DoD might decrease its aircraft operating and support costs without a loss of safety or reliability through the increase in the use of non-OEM parts and repairs. The authors performed case studies on two engines used by the Air Force and the Navy; conducted a literature review; interviewed subject matter experts, engineers, and contracting personnel; and mined PMA and DoD data to analyze parts commonality among DoD engines. The authors find substantial evidence of cost savings where the Air Force and Navy have used PMA parts and DER repairs; however, they also find large differences in how, and how aggressively, the military services pursue the use of these practices in comparison with commercial airlines. The authors offer recommendations on how DoD and the services can revise their processes to realize greater benefits from PMA parts and DER repairs.