U.S. airlines' moves to raise cash this week may be followed by more creative ways to boost liquidity this winter if business travel fails to recover, industry observers said this week.

Carriers have mortgaged everything from aircraft and spare parts to frequent-flier miles and airport slots and gates to replenish balance sheets still burdened with too much debt in an environment of weak traffic and low fares.

The latest efforts to raise cash came late Tuesday and early Wednesday. U.S. Airways Group Inc. (LCC) said it is selling 26.3 million new shares, Delta Air Lines Inc. (DAL) said it will offer $500 million in senior second lien bonds and American Airlines said it will sell $450 million in three-year notes through a private offering.

American's parent, AMR Corp. (AMR), disclosed on Monday it plans to sell 30 million common shares and $250 million in convertible senior notes, just days after announcing a deal for $2.9 billion in fresh funding.

AMR's plan to sell convertibles is viewed by some analysts as a sign of what others may try to do, while some see aircraft and engine manufacturers and other vendors as potential sources of cash.

Airline cash reserves dwindled in a year in which industry analysts expect North American carriers to lose $2.6 billion. A year ago, AMR ended the third quarter with $5.1 billion in cash and short-term investments.

"I personally think the carriers have pretty much exhausted all their asset-backed options," said CreditSights analyst Roger King, saying that US Airways has traditionally been the best at "squeezing juice out of the lemon" of vendor financing.

A round of industry fundraising and recent signs that the drop in average fares has at least stabilized has removed any threat of bankruptcy. Eyeing liquidity needs, several airlines have sold common stock this year, including Continental Airlines Inc. (CAL), US Airways and JetBlue Airways Corp. (JBLU). But any supply-side shocks, including the H1N1 virus, could still tip some carriers into trouble.

Meanwhile, airline stocks have surged this year. The NYSE Arca Airline Index (XAL) has more than doubled from its March lows.

The shift by carriers to convertibles, or to other methods such as vendor financing at the hands of order-starved manufacturers, would hardly be unprecedented. When airline stocks rise, carriers typically use the opportunity to seek fresh sources of funding. And a drought in orders presents airlines with opportunities to negotiate.

But this highlights the near-exhaustion of the industry's traditional cash till - liens against aircraft fleets - and raises questions about cash burn if business travel doesn't recover during the airlines' seasonal winter cash trough.

So-called unencumbered assets aren't the solution, King said. The legacy carriers could raise at most a few hundred million dollars each on what some of them say are billions of dollars in unencumbered assets. But many are not liquid, he said, or are of uncertain use outside the carrier.

"When Delta's management was asked about it on the first-quarter earnings call, they said that there were basically none," said Standard & Poor's Philip Baggaley.

Last week, when AMR secured the $2.9 billion in funding, which included $1.3 billion from selling frequent-flier miles, AMR Chairman and Chief Executive Gerard Arpey said the financing had cut unencumbered assets to $2 billion from around $3.9 billion. However, he acknowledged that what remained would be more difficult to use as collateral.

That cash raise included aircraft. "American benefited from the fact that the 737-800 is probably the best collateral from the perspective of potential lenders," Baggaley said.

There will be some borrowing against collateral, he said, as the airlines will borrow again on existing assets once they become free. But, barring upstarts like Southwest Airlines Co. (LUV) and a few of the legacy carriers who tout their unencumbered assets, "the other airlines are pretty much tapped out."

There are others who are more sanguine about the prospects of borrowing against aircraft.

AMR will have at least $700 million in assets freed up for refinancing in October, said Helane Becker of Jesup & Lamont Securities.

"There is absolutely more to be done" in the bond markets, she said. "As debt is paid down, it frees up assets to be sold and mortgaged again."

But the cases where re-mortgaging can make a real difference appear few to most observers.

"The good news is, the airlines are able to raise cash, and they're doing so in advance of the winter," Baggaley said. "The bad news is that they're steadily working their way down to the bottom of the barrel as far as collateral is concerned."

-By Brendan Conway, Dow Jones Newswires; 212-416-2670; brendan.conway@dowjones.com