Question:  As of September 2017, the Delta Air Lines mainline fleet comprises 852 aircraft in service, merging with Northwest Airlines in 2009, Delta Air Lines operated an all-Boeing and McDonnell Douglas fleet; it incorporated into which company's Airbus aircraft into its fleet following the merger?
Context: Northwest Airlines Flight 253 was an international passenger flight from Amsterdam Airport Schiphol in Haarlemmermeer, Netherlands, to Detroit Metropolitan Wayne County Airport in Romulus, Michigan, United States.  The flight was the target of a failed al-Qaeda bombing attempt on Christmas Day, December 25, 2009, in which a passenger tried to set off plastic explosives sewn to his underwear.  There were 290 people on board the aircraft—an Airbus A330-323E operated by Northwest Airlines, which had merged with Delta Air Lines the year before.  Had the attempt succeeded, it would have surpassed American Airlines Flight 191 as the deadliest aviation occurrence on U.S. soil and tied Iran Air Flight 655 as the eighth-deadliest of all time.  The incident was also the second in 2009 involving an Airbus A330, after the crash of Air France Flight 447 on June 1.  This was also the final accident/incident involving Northwest Airlines as it closed down a month later when it merged with Delta Air Lines. Delta Air Lines Flight 1086 (DL1086/DAL1086) was a scheduled Delta Air Lines domestic passenger flight between Atlanta and New York's LaGuardia Airport.  On March 5, 2015, the McDonnell Douglas MD-88 aircraft veered off the runway shortly after landing at LaGuardia Airport in New York City.  The plane ran up the seawall berm and struck the perimeter fence, sliding along it for approximately 940 ft before coming to rest with the nose of the aircraft hanging over the berm above Flushing Bay.  There were no fatalities, although 24 people suffered minor injuries.  The aircraft was seriously damaged. DAL Global Services, LLC (DGS), a Delta Air Lines company, is an aviation services provider.  DGS provides services such as aircraft ground handling, aircraft maintenance, cargo handling, and many other aviation-related services.  DGS services over 50 airports within the USA and the Bahamas.  They have contracts with multiple airlines and have not limited themselves to Delta Air Lines.  DGS has over 7,000 employees.  It is headquartered at the Delta Air Lines headquarters in Atlanta. On April 15, 2008, Delta Air Lines and Northwest Airlines announced a merger agreement.  The merger of the two carriers formed what was then the largest commercial airline in the world, with 786 aircraft.  The merged airline is called "Delta Air Lines." Northwest Airlink was the trade name of Northwest Airlines' regional airline service, which flew turboprop and regional jet aircraft from Northwest's domestic hubs in Minneapolis, Detroit, and Memphis.  Service was primarily to small-to-medium-sized cities and towns where larger aircraft might not be economical to operate and also to larger markets to either provide additional capacity or more frequent flights than could be justified using mainline aircraft.  Beginning in July 2009, the Northwest Airlink trade name was phased out, and replaced by the Delta Connection trade name for Delta Air Lines as part of the Delta/Northwest merger. As of September 2017, the Delta Air Lines mainline fleet comprises 852 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. Delta TechOps is the maintenance, repair and overhaul (MRO) division of Delta Air Lines, and is headquartered at Hartsfield-Jackson International Airport (ATL) in Atlanta, Georgia.  With more than 9,600 Technical Operations employees and 51 maintenance stations worldwide, Delta TechOps is a full-service maintenance provider for the more than 750 aircraft that make up the Delta Air Lines fleet.  In addition to maintaining the Delta Air Lines fleet, Delta TechOps also provides MRO solutions and support to more than 150 third-party operators around the world, making it the largest airline MRO provider in North America and the third largest worldwide. Mesaba Aviation, Inc. (operating as Mesaba Airlines) was an American regional airline based in Eagan, Minnesota The airline was a wholly owned subsidiary of Pinnacle Airlines Corporation.  Its flights operated as Delta Connection for Delta Air Lines and US Airways Express for US Airways.  Previously, the airline operated as Northwest Airlink and Northwest Jetlink on behalf of Northwest Airlines which subsequently merged with Delta.  Mesaba Airlines effectively ceased operations on January 4, 2012, when all aircraft and personnel were transitioned to the Pinnacle Airlines operating certificate.  Mesaba's operating certificate was surrendered on July 31, 2012. Pinnacle Airlines Corporation was an American holding company headquartered in Memphis, Tennessee, and was the parent company of Pinnacle Airlines and Colgan Air until Colgan was closed down in 2012.  The company contracted its airlines to fly in the aircraft livery of other airlines such as Delta Air Lines and United Airlines.  The company filed for bankruptcy protection under Chapter 11 in 2012 and emerged from Chapter 11 in 2013 as a wholly owned subsidiary of Delta Air Lines.  The company's only surviving airline, Pinnacle, became Endeavor Air. Airbus SE ( , ] , ] , ] ) is a European multinational corporation that designs, manufactures, and sells civil and military aeronautical products worldwide.  In addition to its primary civil aeroplane business, the company has two divisions for other products and services: "Defence and Space" and "Helicopters", the latter being the largest in its industry in terms of revenues and turbine helicopter deliveries.
Answer: Northwest