Des Moines International Airport saw a 3.1 percent gain in passenger traffic in March over the previous March. The January-through-March passenger count was 3.3 percent higher than the same period in 2017. For the year so far, Southwest Airlines leads the gains with 21 percent more passengers getting on or off planes in Des Moines. The biggest carrier, American Airlines, is up 5.7 percent. United Airlines is off 14.5 percent.