United Airlines, grounded 29 days by a bitter pilots' strike that canceled 85 percent of its flights, geared up yesterday to resume service to all 50 states in a back-to-work process expected to take about two weeks and to prompt continued court challenges and strong union protests.

United, the nation's largest air carrier, won important labor-cost concessions during the strike that ended midnight Friday with both sides claiming partial victories in the divisive walkout by 5,000 pilots and the 8,000 flight attendants who supported them.

Full service of 1,500 daily flights to 149 cities and 10 foreign destinations will resume gradually, United said, although pilots and flight attendants are to begin returning to work today.

United President James Hartigan said the airline will restore its schedule to 33 percent of capacity by June 20, 40 percent by June 25, and "virtually all of the schedule will be back in operation" by July 1.

A United spokesman said a promotional plan to be unveiled Monday probably will include discounts designed to lure passengers back into United's "friendly skies."

But officials of the Air Line Pilots Association (ALPA) and the Association of Flight Attendants (AFA) said yesterday that it will take considerable time for the friendliness to return to United.

"The company has bought itself a period of chaos," AFA spokesman Mark Bigelow said. "We tried to negotiate an orderly back-to-work agreement . . . , they chose chaos."

ALPA's United chairman, pilot Roger Hall, said the company used "punitive and vindictive" tactics in an attempt to "break the union."

United Airlines Chairman Richard J. Ferris called the settlement "a good solution to some complex and difficult issues."

In obtaining pay cuts of more than 50 percent for newly hired pilots, United will gain momentum for its international expansion plans and will influence wages throughout the industry, airline analysts said.

But United's victory could prove costly by further souring labor relations, spokesmen for both sides said. And United's plans could be changed by the U.S. District Court.

The unions have vowed to fight proposed seniority rules that would award higher-paying jobs and choice of home bases to unionists who crossed picket lines.

The back-to-work plan by United would give "super-seniority" to 2,000 AFA members and 260 ALPA pilots who worked during the strike, which began May 17. United would "rebid" jobs, giving nonstrikers first choice and possibly causing future layoffs or involuntary transfers of strikers. Those factors were so important to the unions that they prolonged the strike by three weeks after economic matters were settled May 24.

The 260 pilots who crossed union picket lines could receive raises of as much as $60,000 per year because they would be given first choice in flying larger planes at salaries of as much as $150,000, compared with the $90,000 average pay for United captains. Similarly, the 2,000 flight attendants who worked throughout the strike could get first choice in picking the longer, higher-paying routes and the more desirable home bases.

Ferris said the company insisted on back-to-work agreements that would "back its promises to employes. We said we would never forsake those who kept the airline flying." But U.S. District Judge Nicholas J. Bua is to begin hearing union complaints Monday that such seniority changes violate union contracts.

The unions' solidarity and management's pledge to reward employes who broke with the unions became the powerful issues of the nationwide strike, with both sides refusing to back down and, instead, deferring to the court. The court will also decide the fate of the 570 new pilots who sided with the union after completing their training. The company is refusing to hire them.

ALPA, once regarded as among the most successful unions, has seen its power greatly eroded because airline deregulation spawned dozens of lower-wage, nonunion airlines and prompted unionized firms to demand deep cuts in the name of remaining competitive.

In United's case, the three-year contract sets up a "two-tier" pay scale in which new pilots are hired at $21,600 and progress after five years to $27,792. Fifth-year pilots now earn $60,456.

Hall said the union is "alive and well," noting that ALPA was able to keep almost its entire membership on strike and to limit the two-tier system to five years -- after which negotiators and arbitrators are to decide how long to continue it. United sought a 20-year term.

Current pilots will get 9.5 percent raises over three years.

The two tiers are important to United because it expects to hire many new pilots as part of its plan to acquire Pan American's Pacific routes in a $750 million international expansion, said Lee Howard, executive vice president of Airline Economics Inc., a consulting firm.

"This was crucial to United" because of its nonunion competitors and unionized rivals with two-tier wage systems, including American, Republic and Piedmont, he said.