There’s a simple reason the Des Moines International Airport set a passenger record in June and followed it with a 12.3 percent year-over-year jump in traffic in July: more available seats. 

But that appears to be only the beginning. A computer analysis the airport performed at the Business Record’s request found that the airlines’ plans for this year, if they hold, will mean 23.7 percent more plane seats will be available this December than in the same month last year. 

Airport officials have said airport traffic needs to grow 3 percent annually to support a planned $434 million terminal and related construction. A move to bigger planes and added flights helped the airport log 249,944 passengers in July, just short of June’s all-time record of 251,265. August traffic was up 12.1 percent, at 250,046. For the year, traffic is up 6.3 percent.  

When the airlines offer more seats, we buy them. If the past few years have shown anything, it’s that if the airport managers can persuade airlines to offer a new flight, or an airline decides to on its own, Iowans will book seats in big enough numbers to make it profitable. 

That in turn stresses the need for a new terminal, said Kevin Foley, airport general manager and executive director. The airport has charted construction and reorganization that is expected to cost as much as $434 million even after some cost reductions. 

“Continued growth underscores the need for a new terminal and improvements to our existing infrastructure to ensure we have modern facilities with adequate capacity to maintain a positive experience for travelers to and from Iowa,” Foley said. 

The gain in seats crosses several airlines. The largest Des Moines carrier, American Airlines, should be offering 40.8 percent more seats after a gain of 30.8 percent in flights, Foley said. American added flights to Philadelphia in May and is moving to larger aircraft on a number of routes. 

Frontier Airlines, which has expanded its summer service to Orlando to year-round and added service to San Francisco three days a week, is expected to have added 33 percent more seats in that 12-month period. United Airlines should be up 29 percent; Southwest Airlines, 17.8 percent; and Delta Air Lines, 10.4 percent. 

Southwest has been trading 140-passenger planes for models carrying  156 to 160 people. Delta and United also are moving to larger planes.

The trend to larger planes can mean more seats, but also can mean fewer flights and less choice, Foley said. 

Foley said those data are based on his conversations with the airlines about their plans, which could change. “But this is as solid as it gets in this industry,” he said. “They are flights that are already loaded in the system.”

At times, airlines have traded routes to keep things fresh and profitable. Like the recent change in which Southwest Airlines took away flights to and from Chicago, but added St. Louis. United Airlines will stop its flights serving Newark, N.J., in October, but will use larger jets for service to Houston and may add more flights to the Texas city.

Southwest Airlines will fly daily to Phoenix from Nov. 4 to Dec. 15. 

Flights serving the Des Moines airport generally are more than 80 percent full — making them profitable for the airlines, Foley said. Saturday afternoon and Sunday can be slow, but on weekdays there are few empty seats. 

At the same time, fares are rising with fuel price increases, Foley said. 

It’s not just the airlines that are responding to strong demand in Des Moines. The TSA added overtime hours this summer after its own projections expected a bump of 11 percent in traffic at times this summer, Foley said.