Embraer (NYSE:ERJ) suffered a disappointing customer loss earlier this month, as JetBlue Airways (NASDAQ:JBLU) ordered 60 A220-300s from Airbus (NASDAQOTH:EADSY) to replace its fleet of Embraer E190s.

Embraer's management continues to express confidence in the outlook for its next-generation E2-series jets. Nevertheless, the Brazilian aerospace company is still looking to land its first marquee customer for that product family. As of the end of June, Embraer had just 127 firm orders for E2-series jets in its backlog, excluding a 100-aircraft order from SkyWest that is contingent on changes to the major U.S. airlines' pilot contracts.

However, Embraer has a legitimate chance to win a big order from ultra-low-cost carrier Spirit Airlines (NYSE:SAVE), which is currently an all-Airbus airline. Here's why.

Spirit Airlines needs to order more planes

Way back in 2012, Spirit Airlines placed a big order for Airbus planes, including 45 A320neos. The A320neos from that deal will be delivered between 2019 and 2021. However, Spirit hasn't placed a major aircraft order in the intervening years.

A yellow Spirit Airlines jet parked at an airport gate

Spirit Airlines hasn't placed a major aircraft order since 2012. Image source: Spirit Airlines.

As a result, the fast-growing ultra-low-cost carrier's order book is thinning out. The airline aims to grow 13% to 15% annually for the next few years, but as of the beginning of this year, it didn't even have enough aircraft on order to enable 10% capacity growth in 2019.

Since then, Spirit has agreed to lease seven more A320neos, with deliveries in late 2018 and early 2019, allowing it to grow 14% next year. However, this seems like a suboptimal solution, given that management has previously stated that Spirit can save about $1 million annually per aircraft by buying rather than leasing.

Spirit Airlines will face the same problem for 2020 and 2021. Its current order book is only enough for annual growth of around 11% in those years. And the carrier has no orders at all for 2022 and beyond.

The near-term options are limited

In the long run, the A320neo family will almost certainly be the backbone of Spirit Airlines' fleet. However, Airbus has a backlog of nearly 6,000 firm orders for A320neo family planes, whereas it is building fewer than 700 per year. Consequently, A320neo production is likely sold out through 2021.

Furthermore, to maintain a low-teens growth rate beyond 2021, Spirit would need to expand its fleet by an average of about 25 aircraft a year. Unless Airbus is able to ramp up production beyond its current plans within the next few years -- which could be challenging due to supply constraints -- it may not have enough delivery slots left to meet Spirit's needs in 2022.

An Embraer E195-E2 aircraft in flight

Spirit Airlines may add a smaller aircraft type to its fleet. Image source: Embraer.

Spirit Airlines has been interested in adding smaller planes to its fleet, to help it serve small and midsize markets. It already operates 31 145-seat A319s, but the A319 has higher unit costs than state-of-the-art alternatives like the A220-300 and the Embraer E195-E2.

However, from Spirit's perspective, the A220-300 may share a flaw with the A320neo family: a lack of near-term delivery slots. Airbus plans to make all A220-300s for the U.S. market at its new production facility in Mobile, Alabama. However, thanks to recent orders from JetBlue and Moxy Airways -- a planned start-up created by JetBlue founder David Neeleman -- most of the Mobile A220 line's capacity is booked through 2024.

Why the E195-E2 could be a good bet

By contrast, Embraer's sales slowdown of the past few years means that it should be able to deliver E195-E2s in significant numbers starting as early as 2020. Embraer also claims that the E195-E2 is cheaper to fly than the A220-300, both on a per-seat basis and on a per-trip basis.

Some of those claims may be exaggerated. Nevertheless, it is clear that the E195-E2 is quite competitive with the A220-300 in terms of unit costs. That could be especially true for Spirit Airlines. If it wanted to utilize almost all of the A220-300's maximum seating capacity of 160, it would need to have a fourth flight attendant, whereas the E195-E2 has a maximum seating capacity of 146 and only requires three flight attendants.

One drawback of the E195-E2 is that it doesn't have full transcontinental range, although it's pretty close. That appears to have been a big issue for JetBlue, which has a large presence in the transcontinental market. However, Spirit's average stage length is around 1,000 miles, and nearly all of its routes are less than 2,000 miles, well within the E195-E2's capabilities.

Finally, Embraer should be highly motivated to snag a big order for E2-series jets at this point. Based on Spirit Airlines' growth plans -- and the need to eventually replace its A319s -- Spirit could potentially be in the market for around 100 small jets during the next decade. But whether it can reach a mutually advantageous deal with Embraer still remains to be seen.