Over the past few years, Spirit Airlines (NYSE:SAVE) has dramatically changed its route network strategy. Five years ago, the budget carrier's approach was as simple as entering big hubs with lots of traffic and undercutting the incumbents on price to fill its planes. However, once the legacy carriers began to match Spirit's fares, this strategy became untenable.

Spirit Airlines hasn't completely abandoned the top hubs. But it has focused more of its recent growth on large leisure markets like Florida and Las Vegas -- where low-fare giant Southwest Airlines (NYSE:LUV) tends to be its main competitor.

This strategy shift has already started to pay dividends. As a result, Spirit appears to be doubling down, based on several new routes that it announced last week.

Spirit finally gets back on track

In the first half of 2018, Spirit faced severe margin pressure due to a jump in fuel prices combined with continued unit revenue declines. However, the carrier culled underperforming routes after the summer peak season while ramping up its growth in Florida, a market that benefits from strong demand during the fall and winter (unlike most other U.S. airline routes).

The result was a huge improvement in its performance in the second half of the year, driven by a return to strong unit revenue growth. That enabled Spirit to post impressive earnings growth in the third and fourth quarters, including an 89% surge in adjusted earnings per share in Q4.

The airline's momentum is on pace to continue in 2019. Indeed, analysts expect adjusted EPS to roughly double in the first half of the year. The return to consistent unit revenue growth indicates that Spirit's new route strategy is succeeding. Not surprisingly, management plans to stick with what's working.

A yellow Spirit Airlines jet parked at an airport gate

Spirit Airlines' earnings are set to soar this year. Image source: Spirit Airlines.

New routes coming

Last week, Spirit Airlines made three separate new route announcements. On Tuesday, it revealed plans to add Burbank Airport in Southern California to its route network. The carrier will fly three times a day between Burbank and Las Vegas beginning on June 20.

One day later, Spirit made Sacramento, California, its next new city. As with Burbank, the carrier will offer three daily round-trips to Las Vegas starting on June 20. Spirit Airlines noted that customers traveling to and from Burbank and Sacramento will be able to connect to dozens of other cities across its route network via Las Vegas.

On Thursday, Spirit Airlines began service in Indianapolis, with daily flights to Orlando, Florida, and Las Vegas. In conjunction with starting flights there, the carrier announced two new routes to Florida that will take off on Nov. 14: daily service to Tampa and Fort Myers.

There are two interesting things to note about these four new routes. First, Spirit continues to target markets where Southwest Airlines is the dominant carrier. Southwest is the only airline with nonstop service on the Burbank-Las Vegas and Indianapolis-Tampa routes today. It is also the only carrier offering daily year-round service on the other two routes: Frontier Airlines flies between Sacramento and Las Vegas, but just three times a week, while Frontier and Delta Air Lines offer seasonal, less-than-daily flights from Indianapolis to Fort Myers.

Second, Spirit will fly three times a day on the two new Las Vegas routes. As of last fall, only about a quarter of its routes had more than one flight a day. This follows a recent trend of Spirit operating multiple daily flights on some routes in an effort to gain market share by offering more schedule flexibility. This summer, Southwest will operate 11 daily flights to Las Vegas from Burbank and eight from Sacramento. That highlights the depth of demand -- and the importance of having multiple daily flights to be remotely competitive.

Check out the latest earnings call transcripts for Spirit Airlines and Southwest Airlines.

Spirit stock is absurdly cheap

Wall Street analysts currently expect adjusted EPS to soar nearly 50% at Spirit Airlines this year, reaching $6.55. That would imply a pre-tax margin of roughly 15%.

There's no reason to think that level of profitability is unsustainable. Spirit routinely posted even higher margins between 2013 and 2016. Moreover, the carrier's rock-bottom cost structure remains a big competitive advantage. Spirit Airlines can also rely on its growing scale and much-improved customer service to bolster its profitability. And its new route strategy of focusing on large leisure markets seems just as promising as the previous focus on big hub cities like Chicago, Dallas, and Houston.

Nevertheless, Spirit shares currently trade for just eight times the company's projected 2019 earnings. With the budget carrier likely to grow its revenue at a double-digit annual rate for the foreseeable future, Spirit Airlines stock looks like a tremendous bargain.