Philadelphia International Airport handles 30.7 million travelers and nearly 420,000 airplane takeoffs and landings a year.
By another measure the airport is among the best at connecting passengers around the world, according to a recent study, and it's dominated by one major airline.
Among major airline hubs, or so-called megahubs, Philadelphia is the 14th-most-connected airport in the world, and No. 11 in the United States.
The aviation data firm OAG calculated that on a single peak summer travel day - Aug. 7, 2015 - fliers passing through or beginning trips in Philadelphia had 87,483 possible flight connections within an eight-hour period.
A "connection" was defined as a nonstop flight, or one or two stops or plane changes, and up to three legs of a trip to reach a final destination.
The Top 50 hub airports, as defined in the study, had the highest ratio of number of possible flight connections to the number of destinations served by each airport. Philadelphia airport is a hub for American Airlines and merger partner US Airways.
Are big hub airports good or bad for travelers?
Brett Snyder, author of CrankyFlier.com, an airline-industry blog, said they were "fantastic" for passengers in terms of "the breadth of opportunities and the frequency of flights."
However, ticket prices can be more expensive on some routes. "If one airline holds a dominant position, and has a significant advantage in terms of number of nonstop flights vs. anyone else, they are going to be able to charge more for that," Snyder said.
"Where it gets much tougher are the midsize hubs that we've seen slowly disappearing - Memphis, Cleveland, and Cincinnati. For them, the fares get really high," Snyder said.
Airports with the most connections are not necessarily the largest, nor do they have the most international flights. Instead, they serve more regional markets with the highest frequency of flights. They ranked ahead of some bigger airports such as John F. Kennedy in New York and Newark Liberty International, known for their long-haul international flying, according to the hubs report.
"Los Angeles, New York, and Newark serve quite a few destinations and international destinations, but with extremely low frequency," said John Grant, senior analyst with the British-based OAG. "Philadelphia has a pretty high frequency to a number of destinations, and more domestic connectivity than New York or Los Angeles."
The wave of consolidation and mergers over the last decade has left four big carriers - Delta, American, United, and Southwest - controlling more than 80 percent of the U.S. airline market.
Major carriers concentrate their flying in hubs, which increases efficiency and profit by creating more critical mass. Megahubs are economic engines for communities. "These hubs create more employment and more wealth in the region," Grant said.
The study, which analyzed airlines' flight schedules, also looked at flight connections on a peak travel day five years ago - July 9, 2010. On that basis, nine of the 20 largest hub airports, including Philadelphia, have lost connections. Airlines are flying larger aircraft; packing planes fuller; and have reduced in some cases the number, or frequency, of flights.
Philadelphia International saw an 11 percent decline, from 98,262 connections on a single day in July 2010 to 87,483 on Aug. 7, 2015.
The main reason was the retreat here by Southwest Airlines, which arrived to great fanfare in 2004 when US Airways was in bankruptcy.
As US Airways became financially stronger, with 66 percent of the flights in Philadelphia, Southwest scaled back. In 2011, Southwest acquired AirTran Airways and absorbed the AirTran operations.
Southwest at its peak flew 71 daily nonstops to 20 cities from Philadelphia. Today the combined Southwest-AirTran flies to 10 cities, averaging about 25 daily departures. "If you do the math, that's about a 60 percent decline," Philadelphia airport CEO Mark Gale said.
Philadelphia has lost "flight activity" largely because of mergers and consolidation, "with an emphasis on the Southwest and AirTran reduction," Gale said.
OAG's Grant said the country has had four big airline consolidations and "all of them have shaved capacity and reduced frequency across the market."
United and Continental merged in 2010 and today operate 50 percent fewer flights in Philadelphia than they did as competitors, Gale said.
"We advocated for the merger of American Airlines and US Airways, but we knew there were certain cities that they both served, and it was very possible we would see consolidation of the flight schedule," Gale said.
The good news for Philadelphia was that American and US Airways competed on only three nonstop routes: Dallas/Fort Worth, Chicago, and Miami. The result has been "a very small" pruning of flights, Gale said.
"What US Airways and American were doing as individual airlines back in 2010, compared to this year as one airline, is very close - 3 percent to 4 percent to the negative," Gale said, regarding less flight activity at Philadelphia airport.
At the same time, the low-cost carriers Frontier, Spirit, and JetBlue, Alaska Airlines, and Qatar Airways have all added nonstop flights in Philadelphia.
Jeffrey Erlbaum, president of ETA Travel in Conshohocken, said "prices tend to be lower" between large airline hubs, such as Philadelphia and Dallas, if there's competition on the route.
Erlbaum said that fares in Philadelphia were "on average lower" than they used to be, but the planes were fuller. "So even if there is a low fare in the market, you are not necessarily going to get it unless you book pretty far in advance - at least a month, probably more," he said.
"The biggest issue that I see now, because of the lack of competition in smaller markets, the fares are higher," Erlbaum said. "If you are going from Philadelphia to a small market, or from a small market to Philadelphia, the fares are going to be high. Before the mergers, there were more airlines. You had more options."