Even as they expand their operations, many major airports around the world are operating at or beyond capacity. Jakarta’s Soekarno–Hatta International Airport, which was designed to accommodate 22 million passengers a year, handled more than 52 million in 2013; in 2014, Guangzhou Baiyun International Airport in Guangzhou, China, operated at nearly 20 percent above capacity. Other major airports—in cities such as São Paolo and Hong Kong—come close to saturation during peak travel periods and show no signs of slowing down.

Most airline executives know that congestion is a growing trend and have a general sense of what to expect when a hub or home airport begins approaching maximum capacity. For instance, they might rightly suspect that on-time performance drops as airports become constrained. And they may have a hunch that aircraft become harder to schedule and utilize. But beyond executives’ intuition and anecdotal experience, there is little information that can help airlines understand the impact of an airport becoming constrained, or how they should respond. Most important, there is little consensus around an essential question: Is a congested hub airport good or bad for home airlines?

To begin to answer these questions, we looked closely at trends from some of the world’s most constrained airports: Beijing Capital International Airport, Haneda Airport in Tokyo, and London Heathrow Airport. We reviewed both runway and terminal capacity, where data were available. We then analyzed the ways in which passenger numbers, aircraft types, ticket prices, and flight routes have shifted at those airports over time, and compared those shifts with the patterns seen at similarly large but unconstrained airports throughout the world. We identified five trends—some of which are straightforward and intuitive, and others that are more complex—that can help airlines better understand what happens as airports begin to approach maximum capacity and in turn plan their response.