Southwest Airlines (WN, Dallas Love Field) has filed a petition with a US federal court over a US Department of Transportation (DOT) letter that effectively allows Delta Air Lines (DL, Atlanta Hartsfield Jackson) to remain at Dallas Love Field past July 6.

In January of this year, Delta announced it had reached an agreement with United Airlines (UA, Chicago O'Hare) allowing the former to maintain its Dallas Love Field operations through to July 6. Delta was to have lost its access to the central Dallas airport in October last year when American Airlines (AA, Dallas/Fort Worth) transferred its gates at the airport, one of which was leased to Delta, to Virgin America (San Francisco) as part of its merger settlement with the US Department of Justice.

Then, earlier this month, United announced it would be withdrawing from Dallas Love Field in favour of Dallas/Fort Worth with its two Love Field gates to be subleased to Southwest. While the Texan LCC said it would continue to honour United's agreement with Delta, it said nothing about Delta's future at Love Field post July 6.

However, Southwest's ambitious plans for the gates have now been thrown into turmoil after it emerged that the DOT's General Counsel, Kathryn Thomson, informed Dallas City council lawyers in December last year that any airline currently operating out of Dallas Love Field could maintain its operations out of the airport “as long as the carrier continues to operate the accommodated flights.”

According to the Dallas Morning News, Southwest has since filed a petition in a federal appeals court in Washington, D.C. to review the letter, seen as a directive, which it has labelled "arbitrary and capricious, an abuse of discretion, and otherwise not in accordance with law."

It has since emerged that while the US Department of Justice Antitrust Division and the owner and operator of Love Field, the City of Dallas, reportedly approved United's sublease contract with Southwest, their consent warned Southwest that terms of the sublease could be subject to Thomson’s letter. Southwest and United in turn registered their strong disagreement that the DOT has "the authority to impose the terms of accommodation" as stated in Thomson's letter.

As such, Dallas City lawyers have called into question the nature of Thomson's letter with debate over whether it should be interpreted as a directive or as simple guidance.

Southwest, which currently holds 16 out of Love Field's 20 available gates, has already announced that beginning in April of this year, it will use the two additional gates to offer daily nonstop flights to nine new cities including Memphis International, Milwaukee General Mitchell, and Seattle Tacoma International while boosting coverage to existing destinations.

In related news, Virgin America has annnounced that starting April 28, it will start competing with Southwest on a first intra-state route linking Love Field with Austin-Bergstrom International five times daily using A319-100s. Southwest currently operates up to ten daily services on the route.