The carrier cited the size of further cost cuts and the need to find bankruptcy-exit financing as reasons for such a drastic move.
The filing came a day before a federal bankruptcy court hearing on United's pension plans, which the company announced last month it would no longer contribute to while in Chapter 11.
That action has been assailed by United's unions and challenged by the government's pension-protection agency as violating federal pension law.
The Employee Retirement Income Security Act requires companies that promise defined-benefit pensions to employees to fund those plans unless the plan is terminated.
In its court filing, United cited the government's recent decision to reject its bid for a $1.6 billion loan guarantee along with sky-high jet fuel prices. The Elk Grove Village, Ill.-based airline said it "must have the cash flow and liquidity that the financial markets are willing to finance."
"We have taken every effort to restructure our business without affecting accrued pension benefits, and will continue to explore every other option," the company said in the 26-page filing. "However, given the magnitude of further cost reductions needed to create a viable business plan and attract exit financing, termination and replacement of all our defined benefit pension plans likely will be required."
The possibility of such a move surfaced last month when United, facing $725 million in 2004 payments to the funds alone and over $4 billion by 2008, defaulted on a payment and announced the cutoff in contributions.
When a pension plan terminates, the federal Pension Benefit Guaranty Corporation can step in to help employees.
But according to a statement by the PBGC, United Airlines' four pension plans — which serve 119,00 workers — are underfunded by $8.3 billion. Because of legal limits, the PBGC can pay only $6.4 billion. Employees would lose the rest.