PHOENIX — Frontier Airlines added four new non-stop flights out of Phoenix Sky Harbor International Airport this week.

Starting Thursday, the low-cost carrier will offer flights to Atlanta, Cincinnati, Cleveland and Detroit.

“We’re looking at the markets that are the most overpriced and underserved,” Frontier CEO Barry Biffle told financial news site The Street. “There are a lot of them.”

Frontier Airlines is owned by Phoenix-based Indigo Partners, and changed its business model two years ago to match budget airlines such as Allegiant Air and Spirit Airlines. Frontier charges a low base rate and then extra fees for most everything else, like carry-ons, checked bags and seat changes.

Biffle believes he coined the term ultra-low-cost-carrier, a business model that has seen healthy revenue gains in its relatively young age. Frontier’s capacity grew 20 percent in 2015 and is projected to do the same in 2016.

Frontier’s newest flights out of Phoenix are part of more than 40 new markets and seasonal service resumptions happening around airline’s network this week.

KTAR.com found round trip bare-bones tickets for later this month from Phoenix Sky Harbor to Atlanta International Airport for $138. Tickets to Cleveland cost $198.

Frontier will also add two new West Coast services in June 2016, offering flights to Seattle and Portland, Oregon.