Delta and Northwest airlines cleared another hurdle in their effort to smoothly merge when an arbitrator late Monday issued a decision on integrating pilot seniority lists.

For Delta, which closed its acquisition of Northwest in October, integrating pilot groups has been one of the stickiest issues surrounding the deal.

The arbitrator’s decision, which creates a new, unified seniority list, “marks another important step in bringing our work groups together as quickly as possible so that our employees, customers and shareholders fully benefit from the merger,” Delta spokesman Kent Landers said.

Seniority affects pilots’ pay, schedules and aircraft assignments. Pilots from Delta on average had lower seniority than pilots from Northwest because many Delta pilots retired early to save pension benefits before Delta went through a recent bankruptcy reorganization.

Delta and Northwest pilot leaders had agreed to binding arbitration on how to merge the two groups, and the combined seniority list is effective immediately. The pilot groups already have a combined labor contract.

Lee Moak, chairman of the Air Line Pilots Association unit at Delta, said in a written statement that the single seniority list is “a historic labor first and stands in stark contrast to traditional airline merger timelines where labor issues can take years to resolve, often at the expense of both labor and the merged corporation.”

The Air Line Pilots Association union represents pilot groups from both Delta and Northwest. The merged airline has more than 12,000 pilots, including about 5,000 from Northwest and about 7,000 from Delta.

In a message to pilots, Moak termed both the process and outcome “fair and equitable.”

Combining pilot seniority lists can be complex.

According to the pilots union, the integration of Delta and Northwest pilot groups is based on a “ratioed status and category” methodology, which means pilots are “ratioed into the new list based on a staffing formula and the aircraft flown by each pilot group with a rational treatment for the minor attrition differences” between the two groups. It includes conditions and restrictions, known as fences.

Pilot seniority can be a particularly contentious issue in airline mergers. Pilots from US Airways and America West are still dealing with seniority integration issues, three years after the carriers merged. The two pilot groups went through arbitration on the matter, but US Airways pilots sought to vacate the arbitration award and this year voted in a new union.