Northwest immediately said it would appeal.
Judge Allan Gropper in New York wrote that he does not have the authority to block a strike as Northwest had requested.
Flight attendants have said they may begin random, unannounced strikes after 9:01 p.m. CDT Aug. 25 unless Northwest negotiates a new contract with them. Northwest imposed pay cuts and work rules on flight attendants last month after they voted down a negotiated settlement.
"Now, Northwest management has one more chance. They have the choice to either set greed aside for once and agree to a fair and equitable contract, or they will face CHAOS," the union's acronym for 'create havoc around our system,' said Mollie Reiley, who runs the Northwest branch of the Association of Flight Attendants. "Management needs to reduce their demands and do it in a hurry."
The union has not said what kind of strike it plans; it could range from occasional, unannounced strikes at specific gates or flights to a full-scale walkout. The idea is that even small-scale disruptions can cause passengers to book away from an airline, giving the union leverage.
Northwest sought to downplay the risk of disruptions.
"While we are disappointed with Judge Gropper's ruling and will appeal it, we remain committed to continuing to serve our customers professionally and transporting them to their destinations safely and reliably. Our customers can continue to book Northwest with confidence," Northwest Chief Executive Doug Steenland said.
Gropper wrote that federal labor law generally bars federal courts from blocking strikes. And nothing in bankruptcy law overrides that, wrote Gropper, who is overseeing Northwest's reorganization under Chapter 11.
In the same ruling, Gropper denied the union's request to force Northwest to impose a different contract that was more favorable to workers.
Northwest has negotiated settlements with its other large unions, but flight attendants have twice voted down negotiated deals that would have saved the airline $195 million a year. Flight attendants have said Northwest has asked for cuts that were too large.