Over the past decade, Spirit Airlines (SAVE +0.00%) has grown faster than nearly any other airline in North America. The carrier operated just 37 aircraft at the end of 2011. By the end of 2021, Spirit will have 173 jets in its fleet.

The first few years of Spirit's rapid growth paid off handsomely for shareholders. Since then, though, the airline has struggled to protect its margins and operational reliability while growing rapidly. Nevertheless, Spirit Airlines recently doubled down on its plans to expand capacity at a double-digit annual rate by signing lease deals for additional Airbus (EADSY +0.47%) aircraft.

A gap in aircraft orders

Several years ago, Spirit Airlines began a thorough fleet review to decide whether to continue operating exclusively Airbus A320-family aircraft or add a second fleet type. In late 2019, the budget airline finally decided to stick with Airbus, signing a deal to buy 100 A320neo-family jets.

However, due to the A320neo's immense popularity, Airbus had very few near-term delivery positions by then. Deliveries under the new order will run from 2022 to 2027, but the delivery schedule is back-loaded within that window.

Meanwhile, Spirit was due to receive the last aircraft under its previous Airbus order in 2021. This left a gap in the carrier's order book, which management planned to fill by sourcing additional A320neos from aircraft-leasing companies.

Image source: Airbus.

The COVID-19 pandemic gave Spirit Airlines more options to smooth out its delivery schedule. First, Airbus agreed to defer some of Spirit's 2020 and 2021 aircraft deliveries to 2022 and beyond as the airline slashed its schedule in response to the pandemic.

Second, many other airlines ran into even greater financial problems and canceled or deferred orders originally scheduled for delivery over the next couple of years. That opened up more near-term delivery slots for Airbus, allowing Spirit Airlines to accelerate the timing of some deliveries.

Even so, Spirit Airlines ended the second quarter with just 17 aircraft deliveries scheduled for 2022. By contrast, its official fleet plan called for adding 24 new jets next year. At that time, Spirit also needed 16 additional jets in 2023 (beyond those already under contract) to meet its growth target.

Firming up lease deals

Last month, Spirit Airlines took a step toward filling the gaps in its fleet plan, signing a deal to lease 10 Airbus A321neos from Air Lease, with deliveries in 2023 and 2024. Spirit also arranged sale-leaseback financing from Air Lease for five A320neos in its existing order book due for delivery in 2021 and 2022.

Of course, this didn't fully address the shortfall in Spirit's orders for 2023 -- and it didn't add any additional aircraft for delivery next year. However, Spirit Airlines wasn't done. Last week, AerCap announced a deal to lease 20 A320neo-family jets to the carrier, with deliveries running from 2022 to 2024. That should enable Spirit to meet its fleet growth targets.

Image source: Spirit Airlines.

Can Spirit Airlines handle this growth?

At the beginning of 2020, Spirit Airlines had 145 jets in its fleet. Its current fleet plan calls for growing that figure to 197 next year and 230 by the end of 2023, with growth continuing at a high rate thereafter.

Spirit Airlines clearly has plenty of room to expand as it taps new sources of demand for cheap air travel. Yet the current U.S. labor shortage will make it hard to digest this rapid expansion. In late July and early August, Spirit suffered a massive operational meltdown, as its staff was stretched too thin to recover from a series of severe weather events. This led the company to reduce its schedule for the rest of the summer.

Other airlines with aggressive capacity plans are encountering similar problems. Just in the past week, Southwest Airlines suffered its second major operational crisis of 2021, leading to thousands of flight cancellations.

Spirit Airlines stock has massive long-term potential if the company can hire staff fast enough to grow at a double-digit pace over the next decade. However, investors have ample reason to be skeptical of Spirit's aggressive growth plan until the airline proves it can pull it off without compromising its operational reliability.