(Updated on September 28)

The fourth Boeing 737 Max 8 returned to Seattle Tuesday following six days of service ready operational validation (SROV) with launch customer Southwest Airlines that saw the fully equipped narrowbody fly routes out of Dallas Love Field to Long Beach, Albuquerque, Denver, Chicago and Phoenix.

Pioneered on the 787 program in Japan with the launch customer for that airplane, All Nippon Airways, Boeing's SROV gave hands-on experience to Southwest maintenance and ground crew at each airport. There, they carrid out all the tasks typically associated with revenue service operations, ranging from towing and fueling the airplane to conducting fit checks of ground support equipment and performing maintenance.

Boeing flew the airplane—one of ten it has so far rolled out of its Renton factory--to Dallas on September 22 for a “Meet the Max” event for Southwest Airlines employees and local media ahead of the trials. The first flight of the validation took place on September 23 and the trials ended with the airplane's return to Seattle on September 27.

Although Southwest serves as the launch customer for the 737 Max program by virtue of placing the first order, Boeing still hasn’t announced the first operator. It expects to deliver the first airplane some time during the first half of next year.

While speculation that a dispute between Southwest Airlines pilots and management over whether or not the airline could fly the Max before the sides agreed to a new labor contract led to the ambiguity over first delivery, Boeing continues to insist that uncertainty over the identity of the first operator has nothing to do with the contract talks.

That issue could become moot by November, however, when the pilots vote on a tentative deal reached on September 22 that would include pay and work rule gains as well as a clause that would allow Southwest to enter formal interline agreements with foreign carriers for the first time.