SEATTLE, WASHINGTONAmerican ultra-low-cost carrier Allegiant Air reportedly orders 50 Boeing 737 MAX jets worth $5 billion at list prices, according to sources familiar with the matter.

With this deal, Allegiant seems to change its policy of picking-up second-hand Airbus A320 jets at bargain prices.

The switch of suppliers is the third such airline defection in as many weeks, this time working in Boeing's favor after the U.S. planemaker lost hard-fought medium-haul contests to Airbus at Dutch KLM and Australia's Qantas.

Such "flips" are rare due to the cost of retraining pilots but reflect fierce competition for new business as the aerospace industry seeks to recover from its worst-ever recession.

The deal is the latest sign of accelerated growth among "ultra-low-cost" carriers that combine rock-bottom fares with optional charges. Carriers like these are expected to emerge in a position of relative strength from the COVID-19 pandemic.