FORT WORTH, Texas — American Airlines on Wednesday pushed out its scheduled return to service of the Boeing 737 MAX another six weeks. The airline said it now expects to begin commercial service with the MAX on January 16 and will then "slowly phase the MAX in our operation over the course of a month."

"Additional refinements to our schedule may occur through Feb. 12," American said. "Affected customers will be contacted directly."

The move reflects continued delays in Boeing submitting its final software fixes for the MAX to the Federal Aviation Administration (FAA). The manufacturer is engaged in a back-and-forth with both U.S. and foreign regulators about what analysis and testing they require to clear the aircraft to re-enter passenger service.

Boeing was expected to submit its final software changes last month, but still hasn't done so. After it does, initial clearance to fly could come about a month later, once the FAA and other regulators complete an analysis of the software upgrades and conduct test flights.

Afterward, airlines will vary in the time each needs to get the software installed, take engines out of mothballs, and run and test the airplane systems for each aircraft, while also getting pilots trained on the changes to the systems.

It now looks like the earliest any MAX could realistically return to passenger service is December. However, airlines with more airplanes — American has 24 MAXs currently grounded — and a large number of pilots to be trained will take more time to put the plane back in their schedules.

Southwest Airlines, which has the largest fleet of MAXs and more than 9,000 pilots, had already pushed out the return of the MAX to January 5. United Airlines hasn't updated its plans since it delayed the return of the MAX to mid-December.

Boeing's software upgrade includes an update to the MAX's new flight-control system — Maneuvering Characteristics Augmentation System (MCAS) — that activated erroneously on two crashed flights in Indonesia and Ethiopia, resulting in the grounding of the aircraft. It also includes a separate upgrade to the software architecture, so that the MAX's automated flight-control systems take input from both flight-control computers and compare data inputs instead of, as in the original design, using only one computer on each flight.

All American Airlines travelers booked previously on MAX on flights through Jan. 6 will be automatically accommodated on the same flights but flying on an older 737-800. The airline is still working out the details for travelers booked on MAX flights between Jan. 7 and Jan. 15.

The majority of those passengers will be accommodated on the same flight using either a 737-800 or an Airbus A320. Beginning Oct. 13, American's reservations team or a travel agent will contact passengers affected by potential flight cancellations. Passengers will see their online reservation updated that day.

American said cancellations could extend to flights that were not scheduled to be flown with a MAX, since flight crews may have to be reassigned to cover a MAX route with a different aircraft, in order to affect the smallest number of travelers.

In total, American said it is canceling approximately 140 flights per day through Jan. 15.