Southwest Airlines has been Lambert's largest carrier for over a decade and is almost solely responsible for the passenger gains that have brought the airport's overall passenger counts back to levels not reached since the early 2000's. While other airlines might add a flight or two, they are contributing passenger gains in the tens of thousands or low hundreds of thousands. But these are all dwarfed by Southwest's additions. Southwest flies more than half of all passengers at Lambert and shows no signs (MAX airplane issues aside) from losing that dominance anytime soon.

Southwest began at the airport in the late 80's serving as a low cost alternative to the dominant TWA. During the 80's and 90's, St. Louis gained some Southwest flights as a good midway point for the airline's all 737 fleet. The airline was steadily expanding before American (post-TWA) reduced service at the airport and then Southwest began to fill in many of the gaps appearing in St. Louis' route map along with getting a boost of being allowed to have service direct to Dallas Love-Field beginning in 2006. Over the past decade, Southwest has continued its expansion, increasing passenger numbers 67% between 2010 and 2018 to 9.6M, adding new gates from the long shuttered Terminal D, and establishing new routes across the country. Southwest has served both the local passengers and been increasingly using St. Louis as a connecting airport in it's network.

In the last 5 years, St. Louis has played a prominent role in Southwest's overall expansion plans. Only 3 airports (ATL, DAL, and FLL) flying at least 1M departing Southwest passengers have seen a higher percentage growth than STL's 43.8% over that period. The expansion has occurred in many different ways, some established routes like Washington DC, that saw 770 flights in 2014, expanded to serve 1,327 flights in 2018. Some existing routes like BWI were upgauged where 13,000 additional seats were added in the 5-year period even though total flights decreased by 9 in the period. And many new routes were started, both short feeder routes like Wichita, Little Rock, and Des Moines (all flying over 150k passengers in 2018) and long coastal routes like Portland and Oakland. All of these changes added together have made a big difference in the airport's overall numbers and given St. Louisians lots of flight options.

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When looking at other cities served by Southwest, St. Louis flew the 11th most domestic passengers of any Southwest city and all the cities in front of it were operating base for Southwest. St. Louis is a focus city for Southwest and has been the largest domestic non-operating base for Southwest since 2017. Hopefully with a few more routes, it'll make sense to open a full fledged base in St. Louis.

There are so many interesting routes to pick apart here, so I'll try to limit myself to some of the more interesting spots. Two routes that saw the most reduction in flights over the last 5 years were DAL and MDW, but part of that reduction happened in conjunction with more direct flights being added to new destinations. Both of those airports handle lots of connecting traffic, but with direct flights available, the need to connect through other big Southwest hubs has been reduced. And St. Louis has seen its connecting passenger traffic grow which can be seen in the growth of short routes that don't serve as much O&D to St. Louis. Kansas City (while this route has been around a long time) is a good example of these short routes. While 295k people flew on the STL-MCI route, only 81k people flew between St. Louis and Kansas City without continuing on for some connection. Kansas City's O&D is actually very interesting because no airlines expect Southwest show up in their O&D numbers. Southwest has the only direct route here, so other airlines would have to take a much longer route, but with over 80k people traveling between the cities, you would expect at least some other airline to show up. Des Moines is another example of a shorter route that has been added and is beefing up connecting numbers. Des Moines, which is just under a six hour drive from St. Louis, was added as a route in 2016. The demand for a direct route between these cities isn't very high with just over 30k people traveling between the cities via plane in 2018, but Southwest still averaged good numbers on the DSM-STL route in 2018 with 158k passengers and an 81.6% Load Factor. Over 80% of the Des Moines passengers are connecting in some capacity compared longer routes like STL-SEA or SFO-STL which only have 33% and 26% of their passengers connecting.

Not surprisingly, Southwest is the dominant O&D carrier at most to most of it's destinations. It also has generally low fares across the board and has the highest percentage of passengers flying on a free fare. When looking at individual fares on a route, any ticket that cost less than $15 is considered a free fare and these fares are removed from our average fare calculations for a route. All of Southwest's top 20 O&D destinations have a free fare percent over ten percent with some much higher. Anecdotally, I can say that this squares with my feeling that I reach a free flight with Southwest's Rapid Rewards quicker than other carriers. Although, I'm a little skeptical that I may need to adjust this calculation when seeing a route like RSW (Fort Meyers) show 40% of tickets being less than $15. Maybe people just loved to using points to travel to South Florida?

Southwest flies only Mainline,so there are no regional breakouts in the data below and they have an all 737 fleet, so there are too many interesting points to be had about the aircraft types flown from St. Louis. I won't clutter this post with too many other insights, but please take a look at the data below to undercover some interesting facts on your own. There is a lot to dig into. And with so much data, the graphs are a little cluttered in their current state, so you can click here to check out some much larger versions to dig through.