Dawn Gilbertson

The Republic | azcentral.com

Flights to Atlanta and Detroit begin in April.

Flights to Seattle and Portland begin in late June.

Introductory one-way fares range from $49 to $79 depending on the route.

New flights are part of an aggressive expansion by Frontier, with 42 new routes announced today.

Frontier Airlines will add service to four cities from Phoenix Sky Harbor International Airport this year.

The low-cost carrier, which is owned by Phoenix-based Indigo Partners, said Thursday that it will add non-stop flights to Atlanta and Detroit on April 15 and flights to Seattle and Portland on June 29 and June 30, respectively. It also is resuming its seasonal non-stop service to Cincinnati and Salt Lake City.

The Portland flights will be offered on Tuesdays, Thursdays and Sundays, while flights to the other cities will operate on Mondays, Wednesdays, Fridays and Saturdays.

Frontier has changed its business model in the past couple years to be similar to discounters Spirit Airlines and Allegiant Air and, like those carriers, it is growing rapidly. The airline charges a low base fare and then extra fees for almost everything else.

The airline said introductory one-way fares are $49 to Seattle, $59 to Portland and $79 to Atlanta and Detroit, but a quick search turned up availability on just a handful of days, if any, depending on the route. Regular one-way fares will be $79 to Seattle and Portland and $119 to Atlanta and Detroit. Use the "month view'' feature after entering your preferred dates and destination to find the lowest prices.

Frontier will have plenty of competition on the routes. Non-stop flights to each city are already available on other carriers out of Sky Harbor. Check those airlines to see if they are matching Frontier's prices.

Frontier's schedule is open for bookings through Aug. 15.

Spirit Airlines starting service between Phoenix and Los Angeles in April