Nov 18 (Reuters) - Spirit Airlines ( SAVE ) has filed

for bankruptcy protection, it disclosed on Monday, after the

pioneer of no-frills air travel in the U.S. struggled with a

long run of quarterly losses and significant debt.

The airline's woes deepened after the collapse of its $3.8

billion planned merger with JetBlue Airways ( JBLU ) in January

and the impact of RTX's Pratt & Whitney Geared Turbofan

(GTF) engines snag that grounded many of its aircraft.

Spirit, recognized for its bright yellow livery, had been

losing money despite strong travel demand, as it struggled with

bloated costs.

The airline listed its estimated assets and liabilities in

the range of $1 billion to $10 billion each, according to a

court filing on Monday.

Spirit has entered into an agreement with its bondholders

that is expected to reduce total debt and provide increased

financial flexibility.

The airline, as part of the prearranged Chapter 11

bankruptcy protection, has received commitment for a $350

million equity investment from existing bondholders.

Existing bondholders will also provide $300 million in

debtor-in-possession (DIP) financing, which, together with

available cash, is expected to support the airline through the

Chapter 11 process.

Spirit expects to be delisted from the New York Stock

Exchange in the near term.

The company started out as a long-haul trucking company in

1964 before shifting to aviation around 1983. It offered leisure

packages to popular destinations under the name Charter One

Airlines and rebranded to Spirit in 1992.

The discount carrier became popular with budget-conscious

customers willing to forgo amenities like checked bags and seat

assignments.

Ultra-low-cost carriers, which excelled at keeping their

expenses low and offering affordable, no-frills travel, have

struggled since the pandemic as travelers prefer to pay extra

for a more comfortable journey as they pursue experiences.

Spirit's troubles, along with those at some of its rival

budget carriers, have spurred talks of a flawed business model

among some Wall Street analysts.