Pilots from U.S. airlines Delta and Northwest now occupy a unified seniority list under an arbitrator's formula, an airline spokesman said.

The two airlines merged in October, but smoothly integrating two seniority lists have plagued airlines in the past, The Atlanta Journal-Constitution reported Tuesday.

The new formula, using a "ratioed status and category" method of determining seniority, "marks another important step in bringing our work groups together as quickly as possible," Delta spokesman Kent Landers said.

Lee Moak, chairman of The Air Line Pilots Association, which represents Delta and Northwest pilots, called the agreement "a historic labor first."

The agreement stands "in stark contrast to traditional airline merger time lines where labor issues can take years to resolve, often at the expense of both labor and the merged corporation," Moak said.

The combined carriers include 12,000 pilots, about 5,000 from Northwest and 7,000 from Delta, the report said.