The U.S. Air Force has about 5,600 aircraft, which range in age from just-delivered to 60 years old. Many of those aircraft are nearing the end of their service life. In this report, CBO estimates that replacing those aircraft would cost an average of $15 billion a year in the 2020s. That figure would rise to $23 billion in the 2030s and then fall back to about $15 billion in the 2040s. In comparison, appropriations for procuring new aircraft averaged about $12 billion per year between 1980 and 2017 (in 2018 dollars).