– Delta Air Lines, working to keep peace with pilots ahead of union contract talks, agreed to retain its own international flights and not outsource trips to partner Virgin Atlantic Airways.

One of the deal’s terms is the first of its kind in the industry, an assurance that Delta’s overseas operations always will stay larger than Virgin’s, said Rick Dominguez, executive administrator of the U.S. carrier’s chapter of the Air Line Pilots Association. The accord also ensures that Delta will use its own pilots and planes on trips to London, rather than Virgin pilots.

Delta paid $360 million last year for 49 percent of Virgin, the England-based airline founded by billionaire Richard Branson. The purchase helped Delta expand service across the North Atlantic, the busiest international travel market — and stirred union concern that jobs would be threatened if the Atlanta-based carrier let Virgin jets do more flying.

“With Delta’s large equity position, it concerns us that Delta could start to grow Virgin and not to grow Delta,” Dominguez said in a telephone interview.

Delta enjoys a relatively good relationship with its pilots compared with other airlines, Dominguez said. Keeping them happy could be crucial as the two sides start contract discussions in April.

A Delta spokeswoman, Kate Modolo, said the airline had no comment about the pilot agreement. Delta’s 12,000 pilots are the only major organized work group at the airline, the least-unionized carrier in the U.S. industry.

Under the Dec. 12 accord, Delta will have to retain about 49,000 hours of flying time a year between the U.S. and London’s Heathrow Airport on its wide-body jets, Dominguez said. Delta also will have to continue flying about three times as much globally as Virgin, no matter how much the U.K. airline grows, Dominguez said.

The union sought to protect the bulk of Delta’s U.K.-U.S. flying and ensure that Virgin doesn’t erode operations on other routes around the world. The pilot deal would let Virgin add eight more wide-bodies to its current 38, according to its website, after which Delta would have to maintain about three times as much international capacity as the U.K. airline, Dominguez said.