Question: The first airline before Empires Airlines was absorbed into Delta Air Lines and was the largest airline in the world until what year?

Context articles: Northwest Airlines
Delta Air Lines–Northwest Airlines merger
Delta TechOps
Empires Airlines
SkyMiles
Endeavor Air
DAL Global Services
Delta Air Lines fleet
Pinnacle Airlines Corp.
United Airlines

Northwest Airlines Corp. (often abbreviated as NWA) was a major United States airline founded in 1926 and absorbed into Delta Air Lines, Inc. by a merger.
 Approved on October 29, 2008, the merger made Delta the largest airline in the world until the American Airlines-US Airways merger on December 9, 2013.
 Northwest continued to operate under its own name and brand until the integration of the carriers was completed on January 31, 2010.
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."
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.
Empires Airlines was an airline that merged with Zimmerly Airlines in 1946 to form West Coast Airlines.
 West Coast Airlines later merged with two other airlines to form Hughes Airwest in 1968.
 In 1980 Hughes Airwest was acquired by Republic Airlines, which was acquired in 1986 by Northwest Airlines.
 Northwest Airlines was approved to merge with Delta Air Lines on October 29, 2008.
SkyMiles is the frequent-flyer program of Delta Air Lines that offers points (or "miles") to passengers traveling on most fare types, as well as to consumers who utilize Delta co-branded credit cards, which accumulate towards free awards such as airline tickets, business and first class upgrades, and luxury products.
 Created in 1981 as the "Delta Air Lines Frequent Flyer Program"; its name was changed to SkyMiles in 1995.
 Delta claims that it is the only major U.S. airline without mileage expiration, so travelers can redeem awards at their leisure.
 The airline also has a separate SkyBonus program that provides small to mid-sized business owners with a way to earn points for trips taken by employees, good towards free flights, upgrades, Silver Medallion status and other travel awards.
Endeavor Air is an American regional airline that operates as Delta Connection for Delta Air Lines.
 The airline was founded as Express Airlines I in 1985 and changed names to Pinnacle Airlines in 2002.
 In 2012, Pinnacle's parent company filed for chapter 11 reorganization, then emerged as a wholly owned subsidiary of Delta Air Lines.
 The name of the airline was changed to Endeavor Air on August 1, 2013.
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.
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.
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.
United Airlines, Inc., commonly referred to as United, is a major U.S. airline headquartered in Chicago, Illinois.
 It is the world's third-largest airline when measured by revenue, after American Airlines and Delta Air Lines.
 United operates a large domestic and international route network, with an extensive presence in the Asia-Pacific region.
 United is a founding member of Star Alliance, the world's largest airline alliance.
 Regional service is operated by independent carriers under the brand name United Express.
 Its main competitors are American Airlines, Delta Air Lines, and Southwest Airlines.