Chicago-based United also has issued a request for proposals to other regional carriers to cover flying in the event that Mesa's situation worsens, said a person familiar with its plans. United, the nation's second-largest carrier, used Mesa for about 18 percent of its regional operations during 2007, according to Securities and Exchange Commission filings.

Chicago-based restructuring expert Bill Brandt said United and US Airways likely will find other airlines to pick up the slack if Mesa collapses because many have seen their business dealings with major airlines shrink, leaving them with excess