This report examines several issues associated with the cost-per-flying-hour (CPFH) metric used in the Department of Defense (DoD). CPFH is calculated as the ratio of an aircraft fleet's operating-and-support (O&S) costs divided by its flying hours. Subsets of an aircraft fleet's total annual O&S cost are budgeted in DoD for the flying-hour program used to achieve and maintain flight-crew proficiency and used to calculate hourly rates when DoD aircraft are flown on a reimbursable basis. In addition, other aggregations of costs are used to calculate CPFH and compare O&S costs of different aircraft for various other reasons, including informing decisions on aircraft acquisition and force structure.