The U.S. domestic market continues to be shaped by hub dynamics and route specific supply decisions. Unsurprisingly, the airports that dominate the list are serving some of the most populated catchment areas. Drawing on supply data [1] of seats for three consecutive summers (June to August of 2023, 2024 and 2025, referred to as SYY from here on), the analysis identifies the dominant carriers by airport pair and highlights the trajectory of their dominance.

In S25, the largest legacy carriers; United Airlines, Delta Airlines, American Airlines – carved out a market share of 66% across the top 10 domestic airport pairs with United Airlines far ahead of the pack with 28% market share of the top 10 US domestic airport pairs.

The pecking order for the top 10 domestic pairs tell an interesting story. Largely, the pairs have held their ranking in S25 as compared to S24 with at most a drop or gain in ranking by one position for some airport pairs. A three-year comparison of summer schedule across top airport pairs reveals how carriers have fortified their dominance, how low-cost and ultra low-cost carriers (LCCs and ULCCs) are tactically adjusting, and route-specific supply decisions are tightly aligned with broader network planning objectives.

The JFK>LAX corridor is the busiest U.S. domestic airport pair in S25 with a 10.3% YoY growth from S24, and represents a strategic battleground for transcontinental traffic. The entry of Frontier with a 4.9% share in S25 signals possible ULCC experimentation, though the pair remains highly concentrated among legacy and hybrid carriers – Delta ~ 44% and JetBlue 33% and American 19% market share.

LGA>ORD has dropped 1 position to #2 ranking with a 4.4% YoY growth in S25 as compared to S24. The move up in the ranking from 7^th position in S23 reinforces its status as a high-frequency business trunk pair. United Airlines has increased its market share (by 6 percentage points) to 48% as the carrier continues leveraging its stronghold at Chicago O’Hare. The casualty of the battle has been American and Delta – losing market share by 5.0 and 3.0 percentage points respectively. Spirit Airlines coming out of bankruptcy has filed for second bankruptcy within a year due to its inability to fix its cost structure. Operating expenses have continued to exceed revenue, forcing the airline to cut several loss-leading routes. Its market share has dropped from 9.8% in S24 to 3.0% in S25, and the company plans to further reduce its presence in the near future as part of additional cost-saving measures.

On the West Coast, LAX>SFO route posted a 3.8% increase in seat supply in S25 as compared to S24, and retained its #3 rank in seat supply volumes. United and Delta are dominant with 33% and 25% market share respectively. Notably, American Airlines has ramped up service and increased market share from 2.6% to 6.6%, while Frontier and Horizon have emerged with ~5% shares respectively, reflecting diversification of supply. In contrast, JetBlue and Alaska have fully exited as part of the larger network restructuring to cut unprofitable routes.

ORD>LAX, one of the fastest growing pair, increased seat supply by 12% and moved up one spot to #4 position in S25 – United (47.8%) and American (40.0%) dominating the pair. Frontier is the new kid on the block with a 3.4% market share.

At #5 position, the seat supply for the East Coast corridor LGA>ATL fell sharply (-13.9% drop) with the exit of JetBlue and the reduced service from Spirit, illustrating the challenges of contesting in Delta’s fortress hubs. Delta increased its dominance to nearly 70% market share. Again, Frontier was the only other airline to grow seat volumes by 1.5%. Other competitors, including JetBlue and Spirit, exited.

In the #6 position DEN>PHX corridor, not surprisingly dominated by Southwest, who remains the market leader (48.0%) but has ceded ground to United (23.6% market share, 2% seat supply growth in S25). While Frontier’s growth has plateaued, American has rebounded modestly. The market contracted by 7.0% in S25 compared to S24.

At the #7 position EWR>LAX market contracted by 1.2% in S25. United was the dominant carrier (69.2%), with declining seat volumes for JetBlue, Alaska, and Spirit, reflecting incumbents’ advantage in long-haul markets. This dominance is expected to increase with United’s moving their transcontinental premium “PS” service from JFK to EWR from October.

At the #8 position, JFK>SFO route is dominated by Delta and JetBlue controlling ~69% combined share, Alaska Airlines continued to retreat, cutting seat volumes by 5.5% in S25 and 8 percentage points compared to S23. This route grew 5.4% in seat supply in S25.

Leisure markets – #9 EWR>MCO is stagnant with S25 volumes similar to S24 while #10 PHL>MCO volumes in S25 are down compared to the S24 by 4.0%.

Overall, S25 trends illustrate a re-concentration of supply around high-demand transcontinental and business-focused corridors, led by legacy carriers strengthening their share amid selective ULCC entry and exit. The overall seat supply in S25 for the top 10 domestic markets increased by 1.2%. United, Delta, and American have garnered nearly two-thirds of the market share of the top 10 domestic markets, cementing their network advantages, while ULCC entries remain limited.