PHOENIX — Mesa Air Group Inc., the parent company of interisland carrier go!, filed a federal court lawsuit yesterday against Delta Air Lines Inc. to keep its contract-flying agreement intact on the Mainland, a lawyer for Mesa said.

Mesa General Counsel Brian Gillman said the lawsuit filed in Atlanta was in response to Delta's announcement last week that it plans to end the contract.

"We're asking the court ... to require Delta to honor the contract as long as Mesa carries out its end of the bargain," Gillman said.

Losing Delta's contract would be a major blow to the Phoenix-based carrier. It was worth $20 million in revenue per month last year and comprised 21 percent of the carrier's total passenger revenue for the three-month period that ended Dec. 31.

Delta noted last week that Mesa's performance was lacking. Specifically, Mesa said, the Atlanta-based carrier criticized the number of ERJ-145 Delta Connection flights Mesa's subsidiary, Freedom Airlines, completed from September 2007 through February.

But Mesa said there's nothing wrong with the regional carrier's performance. It said Delta asked Mesa to cut flights and now is blaming the carrier for the low completion rate.

Mesa contends that Delta is simply trying to end its contract with Mesa in an effort to cut flights and boost profits.

"Now that Delta wants out of the contract, it has contrived a new way of calculating completion rate that counts those cancelations against Mesa," Gillman said.

"This new interpretation is obviously unreasonable because it would give Delta the ability to declare default whenever it wanted just by canceling enough of Freedom's flights," he said.

Delta spokeswoman Chris Kelly wouldn't comment about the lawsuit except to say that Delta would "vigorously defend" itself.

"Delta has notified Freedom that we will be canceling the contract because operational performance has fallen below minimum levels required under the contract," Kelly said. "We expect the pull-down to be orderly and have minimal customer impact."

Mesa operates 182 aircraft to 157 cities and employs about 5,000 people. It operates as Delta Connection, US Airways Express and United Express under contractual agreements with Delta Air Lines, US Airways and United Airlines, respectively. In June 2006, Mesa launched go!, an inter-island service in Hawai'i.

Bob McAdoo, an analyst with Avondale Partners, said it would be tough on Mesa if it lost the Delta contract. But the vacuum could be filled somewhat as go! responds to increasing demand in Hawai'i following the shutdowns of ATA Airlines and Aloha Airlines.