Which airline has the biggest or oldest fleet? Here's how Delta, United, American and Southwest compare.

Which airline has the biggest or oldest fleet? Here's how Delta, United, American and Southwest compare.

American Airlines, Southwest Airlines, and United Airlines planes at San Francisco International Airport.Tayfun Coskun/Anadolu via Getty ImagesUnited, Delta, American, and Southwest operate the world's largest airline fleets.Business Insider compared the age, size, and makeup of the Big Four's planes.United and Southwest prefer Boeing planes, while the other two have an even split with Airbus.US airlines dominate the world's rankings, operating …