Shares of Delta Air Lines Inc. (DAL) tumbled to their lowest level in more than 20 years Friday amid yet another ratings downgrade and growing fears that the struggling carrier will file for bankruptcy.

The nation's third-largest airline is seeking $1 billion in concessions from its pilots that it says it needs to survive. Pilots, however, have criticized the proposal as too high and have not scheduled any new negotiation sessions, raising uncertainty about the outcome.

Goldman Sachs analyst Glenn Engel cited the deteriorating tone of pilot negotiations in his downgrade of Delta's stock Friday to underperform from in-line. He said in a research note that "the probability of a Delta bankruptcy filing is rising."

That assessment, coupled with a Fitch Ratings' downgrade Thursday of Delta's credit rating on some of its debt, sent the Atlanta-based company's stock sharply lower.

Delta shares closed down 39 cents, or 8.7 percent, at $4.11 in trading Friday on the New York Stock Exchange (search). That's the lowest level since at least 1980, spokeswoman Meghan Glynn said. She did not know if the company's shares ever traded lower since Delta first went public in April 1957.

Fitch said in its downgrade decision that there is a "heightened risk that a stalemate in negotiations over pilot cost restructuring may ultimately force the carrier into a Chapter 11 filing."

Glynn, however, said the airline has not given up hope of reaching an agreement with the Air Line Pilots Association (search).

"The way Delta sees it, we are still engaged with ALPA," Glynn said.

Union spokesman Chris Renkel reiterated in a memo to pilots that management must outline a comprehensive restructuring plan that includes all other Delta stakeholders and address other union concerns before the union will respond further to the airline's latest proposal.

Pilots have proposed up to $705 million in concessions, including a 23 percent pay cut. The company said it needs a minimum of $1 billion in concessions. Its proposal includes a 35 percent pay cut.

Delta, hit hard by high fuel costs and competition from low-fare carriers, has lost more than $5 billion in the last three years. It has laid off 16,000 employees during that time.