The U.S. Air Force is grappling with the challenge of aging fleets and when it might be optimal to replace those fleets. This monograph examines commercial aviation data with the goal of drawing inferences and lessons about aging aircraft that may be relevant to the Air Force. It focuses on "aging effects" - i.e., how commercial aircraft maintenance costs change as aircraft grow older. Although commercial aircraft clearly differ from military aircraft, commercial aviation aging-effect estimates might help the Air Force to project how its maintenance costs will change over time and how those costs might evolve for new commercially analogous aircraft not yet in its inventory. This study found that commercial-airline inflation-adjusted total aircraft maintenance costs, per flight hour, rise substantially as aircraft come off the manufacturer's warranty after a few years of operation, and then rise at about a 3.5 percent annual rate for aircraft six to 12 years old, but are nearly unchanged for aircraft 12 to 25 years old. Read Less