The board, authorized under the Railway Labor Act, prohibits a strike by workers or a lockout by management for up to 60 days. The action fulfills a promise the president made earlier this year and extends the 30-day cooling-off period that was set to expire Friday.
"The president is concerned about the economy, particularly after Sept. 11 and the effect that airline strikes would have on the economy, on the ability of the public to travel at this time," White House spokesman Ari Fleischer said.
This is not the first time Bush has moved to keep planes flying. Earlier this year, he prevented a strike by Northwest Airlines mechanics and helped push through settlements between Delta Airlines and its pilots and American Airlines and its flight attendants.
Spokesmen for United and the machinists pledged to resume talks to resolve the dispute but the machinists still expressed outrage over Bush's action.
"Today's decision by President Bush to block a negotiated settlement at United Airlines will go down in history as the defining moment in this administration's war against American workers' collective bargaining rights," Tom Buffenbarger, president of the IAM said in a statement.
"It's clear United Airlines and the Bush administration agreed in advance to ignore the intent of collective bargaining law and force the dispute into the hands of a management-friendly presidential emergency board."
The board will have 30 days to investigate the dispute and make recommendations.
"The president's directive appointing a presidential emergency board is part of the normal process of the Railway Labor Act and guarantees uninterrupted services for the next 60 days," United said, adding that it would "respect the process."
In issuing his order, Bush said the contract dispute between United and the IAM "threatens substantially to interrupt interstate commerce to a degree that would deprive sections of the country of essential transportation service."
United has been wracked by labor unrest for more than a year, even though the carrier is mostly employee-owned. In the summer of 2000, pilots staged a slowdown in a successful bid to force concessions at the bargaining table. Flight attendants also have staged job actions.
United said it is trying to be open with its unions, laying open its operating and financial problems "to give union leadership every opportunity to help in crafting turn-around plans."
"Our hope is that a common understanding of the facts will lead to some common measures to preserve good jobs and return the company to profitability," the company said.
The union said United's last contract offer did not provide for any wage increases. Machinists' salaries have been frozen since 1994. The union voted Dec. 14 to authorize a strike.
"Throughout negotiations, United behaved like a deadbeat dad," Buffenbarger said, "dodging its responsibility to employees with one bad excuse after another. ...
"Airline employees everywhere are bitter. The administration that wouldn't lift a finger to help their laid off co-workers is now leading the way to prevent the remaining workers from helping themselves. This abuse has gone far enough."
The union also charges the airline industry would like to see collective bargaining agreements canceled and arbitration imposed.
Airlines argue the aviation industry is at a crossroads as a result of the Sept. 11 terrorist attacks and a faltering economy, despite the airline bailout package enacted by Congress