7 US domestic routes all served by the Big 3

The world’s largest hub operation is operated by American Airlines , where the air carrier operates 925 daily departures from Dallas-Fort Worth International Airport (DFW). Delta Air Lines aims to retake the lead in 2025 with a planned 968 daily departures from Hartsfield Jackson Atlanta International Airport (ATL) in the summer of 2025.

Significantly trailing these two, United Airlines ' largest hub operation is currently at Chicago O’Hare International Airport (ORD), with the airline operating over 500 daily departures from the Windy City in January 2025.

While each of America’s 'Big Three' legacy airlines (American, Delta, and United) operates major hubs around the United States, they rarely share a hub. In this way, each airline carves out its own niche in the market, and it is actually not very common for all three to compete on the exact same route between a city pair. Indeed, there are many markets where two of them may directly compete, but it is less common for all three to fly the exact same domestic route.

With that fact explained, let's take a look at seven US domestic routes that are served by all three of America’s Big Three airlines. This list is not exhaustive and is based on planned schedules for January 2025.

Nonetheless, it highlights the point that there are still several city pairs that see direct competition between American Airlines, Delta Air Lines, and United Airlines. Some may immediately notice that Los Angeles International Airport (LAX) appears on this list multiple times as it serves as an important airport for all three of the United States' largest airlines.

1 San Francisco International Airport (SFO) to Los Angeles International Airport (LAX)

565 flights scheduled by the Big Three

San Francisco has been an important gateway for flights to Asia for a very long time. Pan American Airways launched the first regularly scheduled trans-Pacific flights from San Francisco in 1935, utilizing a Martin M-130 named China Clipper. San Francisco International Airport continues this tradition of linking Asia with North America and is an important hub for United Airlines.

It is also interesting to note how important this airport is in linking Northern California with Southern California, as the route from San Francisco International Airport (SFO) to Los Angeles International Airport (LAX) has been vital to intrastate travel for many decades. Once served by the likes of Pacific Southwest Airlines (PSA) and Western Airlines, SFO to LAX remains one of the busiest routes that is served by all the Big Three US airlines.

In January 2025, American, Delta, and United have a combined total of 565 flights planned on this route, with United Airlines clearly leading with a total of 281 scheduled flights. If the return flights were counted, that number would almost double. According to data from Cirium, an aviation analytics company, this is what the San Francisco to Los Angeles route looks like for these airlines in January 2025:

2 Denver International Airport (DEN) to Los Angeles International Airport (LAX)

United plans 272 flights

Next on this list is another United Airlines hub-to-hub route. Denver International Airport (DEN) to Los Angeles International Airport (LAX) remains busy year-round. Since Denver International opened in 1995, it has quickly become one of the world’s busiest airports, and it has, at times, rivaled Chicago O’Hare (ORD) as United’s busiest hub.

As such, you can imagine that the DEN to LAX route is important to United. Los Angeles is another very important connection point for transpacific flights and all the US majors have a strong presence there. With that in mind, it is interesting to note how American Airlines and Delta Air Lines also compete with United on the Denver to Los Angeles route. Cirium shows the three airlines fly a combined total of 420 flights from DEN to LAX. Here is what January looks like for this route:

3 Phoenix Sky Harbor International Airport (PHX) to Los Angeles International Airport (LAX)

167 flights scheduled by American Airlines

Los Angeles remains a consistent destination as we look at the next route on this list. Phoenix Sky Harbor International Airport (PHX) to Los Angeles International Airport (LAX) is an important route for the Big Three. Still, the data suggests it is especially important to American Airlines. Once the home of America West Airlines, Phoenix Sky Harbor remains a vital hub in the southwest of the United States.

The Big Three plan to link the Copper State to the Golden State with no less than 399 nonstop flights between PHX and LAX. As can be expected, American Airlines leads this route with 167 flights scheduled to LAX in January.

4 Chicago O'Hare International Airport (ORD) to Hartsfield Jackson Atlanta International Airport (ATL)

388 flights connect two of the largest hubs in the U.S.

The next route on this list takes a step away from Los Angeles to address a relatively short but quite busy route for both Delta Air Lines and United Airlines. Chicago O'Hare International Airport (ORD) is currently United’s busiest hub and is also a key Midwest base for American Airlines. On the other hand, Delta Air Lines' home base and main hub is Hartsfield Jackson Atlanta International Airport (ATL).

That said, it is not difficult to understand that one of the busiest domestic routes served by all three airlines is Chicago O'Hare International Airport (ORD) to Hartsfield Jackson Atlanta International Airport (ATL). Delta Air Lines leads with the most flights between these two cities, followed by United Airlines. The total number of nonstop flights American, Delta, and United have planned on the ORD to ATL route in January is set at 388 flights. Cirium reports the following:

5 George Bush Intercontinental Houston Airport (IAH) to Los Angeles International Airport (LAX)

277 flights scheduled by United Airlines alone

George Bush Intercontinental Houston Airport (IAH) to Los Angeles International Airport (LAX) is another route served by all three of America’s largest air carriers. Interestingly, it is also one of the longer US domestic routes flown by an Embraer E175. AA 4916 is an Embraer E175 operated by SkyWest Airlines for American between George Bush Intercontinental and Los Angeles International.

The flight is scheduled to depart Houston (IAH) at 7:35 p.m. and arrive at Los Angeles (LAX), 3 hours and 54 minutes later at 9:29 p.m. George Bush Intercontinental is another hub for United Airlines. As can be expected, United has the most significant presence on this route. However, all three of these US majors still serve the two airports. The total number of nonstop flights scheduled on this route by American, Delta, and United in January is 364, with United operating 277 of these flights.

6 Los Angeles International Airport (LAX) to Seattle Tacoma International Airport (SEA)

Delta connects its West Coast hubs with 157 flights

Another route served from Los Angeles International Airport (LAX) by the Big Three connects Southern California with the Northwest. The number of international flights serving Seattle Tacoma International Airport has increased over the last few years, making Seattle an important connection point for Alaska-bound flights and flights to Europe and Asia. Delta Air Lines expanded its operations at the airport, making it another hub in the airlines' vast network.

Not surprisingly, of the Big Three, Delta has the most flights scheduled between Los Angeles International Airport and SeaTac. The airline has planned 157 flights on this route, making up a sizable chunk of the combined total of 275 flights. Cirium noted the following numbers for the month of January 2025:

7 Los Angeles International Airport (LAX) to Daniel K Inouye International Airport (HNL)

266 flights connect the mainland with Hawaii

Number seven on the list is a popular destination for many Americans. Daniel K Inouye International Airport (HNL) in Honolulu, Hawaii, is the largest airport serving Hawaii and the international gateway to Aloha State. Therefore, it is an important destination for American, Delta, and United.

The airport advertises that there are non-stop departures to a number of cities, such as, but not limited to, Dallas-Fort Worth, Kansai, Manila, Pago Pago, and Sydney. That said, Los Angeles International Airport still serves as an important starting point for flights headed to Hawaii. American Airlines, Delta Air Lines, and United Airlines have 266 nonstop flights scheduled from Los Angeles (LAX) to Honolulu (HNL) in January.

When all is said and done, this is not an extensive list of every domestic route served by America's three largest airlines. Rather, it is a quick analysis of some of the busier routes that they all serve. Though American, Delta, and United generally do not have the same hub cities, there are still a number of nonstop routes between city pairs where they all compete - perhaps more than you might think!