Delta Air Lines and Northwest Airlines have both declared bankruptcy as the industry struggles with soaring oil prices and low-cost competition.
With the filings, both made in US bankruptcy court in the Southern District of New York, four of the seven largest airlines in the United States are now operating under Chapter 11 federal bankruptcy protection from their creditors.
Both airlines would likely use bankruptcy to slash labour and pension costs, following in the footsteps of United Airlines, and US Airways.
With respective assets of $21.6 billion and $14.4 billion, Delta and Northwest became the second- and third-largest US airline bankruptcies ever.
The filings could put added pressure on other carriers including American Airlines, by putting them at a disadvantage as their bankrupt rivals shed costs and ditch pensions, analysts said.
But other airlines could benefit if Delta and others cut back on domestic routes, getting rid of overcapacity that has made the US airline sector the most troubled worldwide.
US airlines are expected to post some $10 billion in losses this year.