The making of Delta’s A350-900

As of October 2017, the Delta Air Lines mainline fleet comprises 853 aircraft in service, making it one of the largest airline fleets in the world. Before merging with Northwest Airlines in 2009, Delta Air Lines operated an all-Boeing and McDonnell Douglas fleet; it incorporated Northwest's Airbus aircraft into its fleet following the merger and subsequently ordered further Airbus types. Delta is known for its policy of generally buying older generation or used aircraft and for continuing to fly aircraft for 20-30 years, much longer than most other major airlines. As such, it has one of the oldest fleets of any American airline, with an average fleet age of 17 years as of 2017. Fellow U.S. legacy carriers American Airlines and United Airlines have much younger fleets; as of December 29, 2016, American had an average fleet age of 10.3 years, while United had an average fleet age of 14.1 years.