The C-17 Globemaster III is the United States Air Force’s second-largest airlifter after the C-5M Super Galaxy. The Globemaster’s production line shut down in 2015, and there is no clear replacement plan, meaning that the United States Air Force is set to continue flying them for many years to come. As aircraft age, parts become obsolete, or they break down and require replacement (sometimes the original contractor goes out of business, meaning a substitute part needs to be found).