Delta Air Lines will be borrowing $6.5 billion.

That’s not super surprising, given the state of the big air carriers in this pandemic economy. A Delta executive recently said the airline’s losing $27 million a day. But the interesting part of this deal is the asset that Delta’s putting up as collateral: its frequent flyer program.

Frequent flyer miles aren’t only meant to build customer loyalty. They’re also a product that airlines sell.

Samuel Engel, senior vice president with the consulting firm ICF, said the buyers are credit card companies.

“And the credit card companies in turn give those miles to consumers as a reward for using their credit cards,” he said.

Engel says miles are a money-making machine for airlines.

“At the beginning of this year, the programs of the four largest U.S. airlines together had been valued at $77 billion, if they were standalone businesses,” he said. 

Airlines can tap these programs to raise cash. One common method is to presell miles to banks to get cash upfront.

Henry Harteveldt, president of Atmosphere Research Group, said an airline can also spin off the entire program, as Air Canada did in 2005.

“No airline, though, until recently has leveraged its frequent flyer program as collateral against a loan,” Harteveldt said.

Earlier this year, United put up its loyalty program as collateral for a government loan. Harteveldt said it’s a creative way to raise cash “at a time when air travel demand, especially by their more profitable business flyers, has fallen off the cliff.”

Airlines have other creative ways to raise money. Scott Hamilton, managing director at the aviation consultancy Leeham Co., said it’s common for airlines to pledge their planes, and even their headquarters and airport gates, as collateral for loans.

“But the frequent flyer program is to me the most creative and the most visible sign of the desperation that the airlines are in,” Hamilton said.

The risk for airlines, Hamilton said, is taking on too much debt. That could make it harder for them to recover from the current slump.