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?

full_context: Document 1: Aérospatiale SA 321 Super Frelon: The Aérospatiale (formerly Sud Aviation) SA 321 Super Frelon ("Super Hornet") is a three-engined heavy transport helicopter produced by aerospace manufacturer Sud Aviation (later Aérospatiale) of France. It held the distinction of being the most powerful helicopter to be built in Europe at one point, as well as being the world's fastest helicopter.
Document 2: Airbus: 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.
Document 3: Delta Air Lines: Delta Air Lines, Inc. ("Delta"; ) is a major American airline, with its headquarters and largest hub at Hartsfield–Jackson Atlanta International Airport in Atlanta, Georgia. The airline along with its subsidiaries and regional affiliates operate over 5,400 flights daily and serve an extensive domestic and international network that includes 319 destinations in 54 countries on six continents, as of October 2016. Delta is one of the four founding members of the SkyTeam airline alliance, and operates joint ventures with Air France-KLM, Alitalia, Korean Air, China Eastern Airlines, Virgin Atlantic, and Virgin Australia. Regional service is operated under the brand name Delta Connection.
Document 4: Delta Air Lines–Northwest Airlines merger: 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."
Document 5: Aerospace industry in the United Kingdom: The aerospace industry of the United Kingdom is the fourth-largest national aerospace industry in the world and the third largest in Europe, with a global market share of 6.4% in 2016. In 2013, the industry employed 84,000 people. Domestic companies with a large presence in the British aerospace industry include BAE Systems (the world's third-largest defence contractor), Britten-Norman, Cobham, GKN, Hybrid Air Vehicles, Meggitt, QinetiQ, Rolls Royce (the world's second-largest maker of defence aero engines) and Ultra Electronics. Foreign companies with a major presence include Boeing, Bombardier, Airbus (including its Astrium, Cassidian and Surrey Satellite Technology subsidiaries), Leonardo (including its AgustaWestland and Selex ES subsidiaries), General Electric (including its GE Aviation Systems subsidiary), Lockheed Martin, MBDA (37.5% owned by BAE Systems), Safran (including its Messier-Dowty and Turbomeca subsidiaries) and Thales Group (including its UK-based Thales Air Defence, Thales Avionics and Thales Optronics subsidiaries). Current manned aircraft in which the British aerospace industry has a major role include the AgustaWestland AW101, AgustaWestland AW159, Airbus A320 family, Airbus A330, Airbus A340, Airbus A380, Airbus A400M, BAE Hawk, Boeing 767, Boeing 777, Boeing 787, Bombardier CRJ700, Bombardier CSeries, Bombardier Learjet 85, Britten-Norman Defender, Britten-Norman Islander, Eurofighter Typhoon, Hawker 800, Lockheed Martin C-130J Super Hercules and Lockheed Martin F-35 Lightning II. Current unmanned aerial vehicles in which the British aerospace industry has a major role include BAE Taranis, HAV 304 Airlander 10, QinetiQ Zephyr and Watchkeeper WK450.
Document 6: American Airlines Group: American Airlines Group, Inc. is an American publicly traded airline holding company headquartered in Fort Worth, Texas. It was formed December 9, 2013, in the merger of AMR Corporation, the parent company of American Airlines, and US Airways Group, the parent company of US Airways. The airline groups together form the largest airline in the world, with more than 6,700 daily flights to 336 locations in 56 countries worldwide, about $40 billion in operating revenue, over 100,000 employees, and plans to take delivery of 607 new aircraft, including 517 narrowbody aircraft and 90 widebody international aircraft. The integration of American Airlines and US Airways was completed when the Federal Aviation Administration granted a single operating certificate for both carriers on April 8, 2015.
Document 7: Northwest Airlines Flight 253: 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.
Document 8: Delta TechOps: 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.
Document 9: Daimler AG: Daimler AG (] ) is a German multinational automotive corporation. Daimler AG is headquartered in Stuttgart, Baden-Württemberg, Germany. As of 2014, Daimler owns or has shares in a number of car, bus, truck and motorcycle brands including Mercedes-Benz, Mercedes-AMG, Smart Automobile, Detroit Diesel, Freightliner, Western Star, Thomas Built Buses, Setra, BharatBenz, Mitsubishi Fuso, MV Agusta as well as shares in Denza, KAMAZ, Beijing Automotive Group. The luxury Maybach brand was terminated at the end of 2012, but revived in April 2015 as "Mercedes-Maybach" versions of the Mercedes-Benz S-Class and G-Class. In 2016 Daimler sold 3.0 million vehicles. By unit sales, Daimler is the thirteenth-largest car manufacturer and is the largest truck manufacturer in the world. In addition to automobiles, Daimler manufactures buses and provides financial services through its Daimler Financial Services arm. The company is a component of the Euro Stoxx 50 stock market index.
Document 10: Delta Air Lines fleet: 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.
Document 11: Leonardo S.p.A.: Leonardo S.p.A., formerly Leonardo-Finmeccanica and Finmeccanica, is an Italian global high-tech company and one of the key players in aerospace, defence and security. Headquartered in Rome, the company has 180 sites worldwide. It is the ninth largest defence contractor in the world based on 2014 revenues. The company is partially owned by the Italian government through the Ministry of Economy and Finance, which holds 30.2% of the company's shares and is its largest shareholder.
Document 12: US Airways: US Airways (formerly known as USAir) was a major American airline that ceased to operate independently when the Federal Aviation Administration granted a single operating certificate (SOC) for US Airways and American Airlines on April 8, 2015. Publicly, the two carriers appeared to merge when their reservations systems and booking processes were merged on October 17, 2015; however, other systems were still separate at that time. The airline had an extensive international and domestic network, with 193 destinations in 24 countries in North America, South America, Europe and the Middle East. The airline was a member of the Star Alliance, before becoming an affiliate member of Oneworld in March 2014. US Airways utilized a fleet of 343 mainline jet aircraft, as well as 278 regional jet and turbo-prop aircraft operated by contract and subsidiary airlines under the name US Airways Express via code sharing agreements.
Document 13: Airbus A350 XWB: The Airbus A350 XWB is a family of long-range, twin-engine wide-body jet airliners developed by European aircraft manufacturer Airbus. The A350 is the first Airbus aircraft with both fuselage and wing structures made primarily of carbon-fibre-reinforced polymer. Its variants seat 280 to 366 passengers in typical three-class seating layouts. The A350 is positioned to succeed the A340, and compete with the Boeing 787 and 777.
Document 14: Eurocopter EC135: The Eurocopter EC135 (now Airbus Helicopters H135) is a twin-engine civil light utility helicopter produced by Airbus Helicopters (formerly known as Eurocopter). It is capable of flight under instrument flight rules (IFR) and is outfitted with digital flight controls. It entered service in 1996; over a thousand aircraft have been produced to date. It is widely used by police and ambulance services and for executive transport; by 2013, more than 500 EC135s were providing helicopter emergency medical services across the world. While the EC135 is primarily used by civil operators, a combat-capable military-orientated variant of the EC135, designated as the Eurocopter EC635, has also been produced.
Document 15: Air Partner: Air Partner Plc is a global aviation services group. The Group is structured into four reporting divisions: Commercial Jet air charter, Private Jet Charter, air freight and Baines Simmons. The Commercial Jet division charters large airliners to move groups of any size. They also provide remarketing programmes for all types of commercial and corporate aircraft. Air Partner is headquartered alongside Gatwick airport in the United Kingdom. Air Partner operates 24/7 year-round and has 20 offices across Europe, North America, the Middle East and Asia. The company is publicly listed on the London Stock Exchange as LSE: [ AIR] , and is also ISO 9001:2008 compliant for commercial airline and private jet solutions worldwide.
ground_truth: Northwest