The Boeing 737 is an immensely popular family of narrowbody airliners. We are less than a month away from the 53rd anniversary of the type's first commercial flight. German flag carrier Lufthansa had the honor of operating the launch service in February 1968. Just over 50 years later, the 10,000th 737 rolled off the production line. But which airlines have operated the most examples of airliners from this seemingly ubiquitous family of aircraft?

Considerable commercial success

As our recent article on the most produced commercial aircraft in history touched upon, Boeing has sold an enormous number of 737s. It has delivered almost 10,500 examples of the type to customers around the world. Its recent difficulties with the 737 MAX have admittedly seen demand slow somewhat, and it has a backlog of around 450 MAX aircraft waiting to be delivered.

Nonetheless, before coronavirus brought commercial aviation screeching to a halt last year, one could find 737s in every corner of the globe. At the peak of pre-COVID air travel, Boeing was producing 47 737s every month, and around 1,200 examples would be in the air worldwide at any given time. A further 4,000 737s are currently on order, so it seems that the 737 will continue to be a staple of contemporary commercial aviation for many more years to come.

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Southwest Airlines - the largest operator

Dallas-based Southwest Airlines is the world's largest low-cost carrier. Correspondingly, it also operates the world's largest fleet of Boeing 737 aircraft. According to Planespotters.net, it presently consists of 728 examples of aircraft from the popular narrowbody family, with an average age of 12.5 years. This includes 135 grounded aircraft, due to coronavirus and the aforementioned 737 MAX issues. The present fleet is made up of the following variants.

• -700, 477 aircraft, launch customer. Average age - 16.4 years.
• -800, 207 aircraft. Average age - 5.5 years.
• MAX 8, 44 aircraft. Average age - 2.5 years.

Southwest also has the smaller MAX 7 on order, of which it intends to be the launch customer. However, as well as considering its fleet's future, it is also useful to examine past variants. In addition to its existing 728-aircraft fleet of Boeing 737s, Southwest has also previously operated a further 319 planes from this family. This brings its all-time total 737 fleet size into four figures, up to a staggering 1,047 examples! Its historic 737s consist of the following variants.

• -200, 62 aircraft, 1971-2005.
• -300, 195 aircraft, 1984-2017.
• -500, 25 aircraft, 1990-2016, launch customer.
• -700, 37 aircraft, first delivered in December 1997.

United Airlines - a healthy inheritance

US legacy carrier United Airlines inherited a significant number of Continental's 737s when the two companies merged in 2012. Today, with 364 examples, it almost has a 737 for every day of the year! Of these, 80 remain parked due to the current circumstances. United's present 737 fleet consists of the following variants.

• -700, 50 aircraft. Average age - 20.7 years.
• -800, 141 aircraft. Average age - 17 years.
• -900, 148 aircraft. Average age - 9 years.
• MAX 9, 25 aircraft. Average age - 2 years.

Moving forward, United has two other MAX variants on order. Specifically, these are the MAX 8 (11 orders) and the MAX 10 (100 aircraft). Historically speaking, United has operated a further 295 aircraft from the 737 family, bringing its total all-time fleet size to 659 aircraft. United's historical 737s were made up of the following variants.

• -200, 100 aircraft, 1968-2001, launch customer.
• -300, 101 aircraft, 1986-2009.
• -500, 94 aircraft, 1990-2009 & 2011-2013.

Ryanair - a strong 737-800 focus

Irish airline Ryanair is Europe's largest low-cost airline by scheduled passengers flown. Much like Southwest, it is well-known for operating an all-737 fleet. Presently, all but one of these are the -800 variant, of which Ryanair is the world's largest operator. It flies a total of 435 737-800s across the Ryanair group, including its partners Malta Air and Buzz. Additionally, it operates a single corporate 737-700, which groups such as sports teams often charter.

Historically, Ryanair has also operated three other 737 variants. Specifically, these were the -200 (21 aircraft), -300 (seven aircraft), and -400 (one aircraft). Looking to the future, Ryanair is planning to modernize its fleet with the high-capacity 737 MAX 200. With 210 examples reportedly ordered, this would take the carrier's all-time 737 fleet size to 675 aircraft.

American Airlines - increasingly uniform

American Airlines also operates a large 737 fleet on its domestic and short-haul international networks. Presently, this consists primarily of the -800 variant, of which American operates 304 aircraft, with an average age of 11.2 years.

The remainder of its current 737 fleet are examples of the MAX 8 variant, with 35 having been delivered to date, with an average age of 2.2 years. This gives a total of 339, although 124 of these aircraft (99 -800s and 25 MAX 8s) remain grounded.

In years gone by, American also operated several older and smaller varieties of the 737. Totalling 46 aircraft, this historical fleet consists of the following variants.

• -100, 2 aircraft, 1987-1990.
• -200, 21 aircraft, 1987-1991.
• -300, 8 aircraft, 1987-1992.
• -400, 14 aircraft, 2013-2015.
• -800, 1 aircraft written off after a runway overshoot at Kingston, Jamaica in December 2009.

Honorable mentions

Several other carriers across the US and China have significant 737 fleets that are also well worth mentioning. For example, US legacy carrier Delta Air Lines presently operates an impressive fleet of 207 aircraft, with an average age of 10 years. Historically speaking, Delta also operated a further 116 737s, bringing its all-time fleet size for the type up to an impressive 323!

The 737 is also hugely popular in China. Indeed, three of the country's carriers have operated over 200 examples of the type. These consist of the following airlines.

• Air China - 242 aircraft (134 current + 108 historical).
• China Eastern Airlines - 215 aircraft (152 current + 63 historical).
• China Southern Airlines - 295 aircraft (213 current + 82 historical).

One final honorable mention should go to Argentine flag carrier and largest airline, Aerolíneas Argentinas. While its all-time 737 fleet size is not comparable to the other airlines mentioned (44 current + 84 historical), it does have a particular quirk. Reportedly, it was the first and only carrier to have operated the 737 Original, Classic, and Next-Generation series simultaneously.

What are your thoughts on the Boeing 737 family? How many of these airlines have you flown with on their 737s? Let us know your thoughts and experiences in the comments!