United Airlines, American Airlines and Hawaiian Airlines have warned employees that they might be furloughed.
United said that it has warned some 14,000 employees that they might be furloughed, and aviation unions made a new request to Congress and President Joe Biden for another $15 billion in government assistance to keep workers on the payroll through at least September 30.
Chicago-based United warned that once a second round of payroll support expires on April 1, airlines could be forced to make drastic new cuts as the coronavirus pandemic has slashed demand for air travel.
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United had recalled 13,000 employees from furlough when a $15 billion airline industry payroll package was passed in December to protect jobs through March.
"Despite ongoing efforts to distribute vaccines, customer demand has not changed much," United told employees, while saying that it is monitoring demand and advocating for continued government support.
The $15 billion in December helped bring back more than 32,000 airline employees and followed a $50 billion package in March for passenger airlines divided between payroll assistance and low-cost government loans.
Two union leaders representing 75,000 flight attendants wrote congressional leaders seeking quick action to extend the payroll support programme "with $15 billion to protect jobs" through September 30 or later.
Union leaders Sara Nelson and Julie Hedrick added, "The alternative is mass layoffs starting in April."
United's memo said, "We are all working hard toward the day when we can bring back our furloughed co-workers permanently."
American Airlines has said that some 13,000 employees are at risk of furlough when the US aid package for airline workers expires on April 1, blaming slow vaccine rollouts and new international travel restrictions for dampening demand.
"We are nearly five weeks into 2021, and unfortunately, we find ourselves in a situation similar to much of 2020," American Airlines chief executive Doug Parker and president Robert Isom said in a memo to employees that was also included in a regulatory filing.
Fort Worth, Texas-based American furloughed 19,000 workers when a previous round of government payroll support ended on October 1 but recalled them in December after the fresh $15 billion airline industry payroll package was passed to protect jobs through March.
"The vaccine is not being distributed as quickly as any of us believed, and new restrictions on international travel that require customers to have a negative COVID-19 test have dampened demand," American said, adding that the company will not fly all of its aircraft this summer as planned.
American said that it is launching a fresh round of exit packages in an effort to mitigate potential involuntary furloughs, similar to plans by United.
They are required by law to inform employees whose jobs are at risk, generally within 60 days.
American's potential furloughs include 1,850 pilots and 4,245 flight attendants. United's pilots approved a deal late last year to prevent furloughs until June.
Last month, American's wholly owned regional subsidiary, PSA Airlines, said that it plans to resume pilot hiring this year, as did privately owned Frontier Airlines.
The Allied Pilots Association, which represents American's pilots, said that actions by management and their treatment of the airline's balance sheet "have placed American in a more precarious situation than [its] competitors."
American is the most leveraged of the major US carriers. Last week it took advantage of a sharp rise in shares after a mention on Reddit's WallStreetBets forum to launch a fresh $1 billion stock sale to boost liquidity.
Hawaiian Airlines has also said that it had issues furlough warnings to employees.
Hawaiian's warnings have been issued to 900 employees.