US aerospace giant  Boeing says deliveries of its beleaguered 737 MAX aircraft could resume in December but it now does not expect a return to commercial service until January. The manufacturer until recently had stuck by its assumption there would be a return to service in the fourth quarter but that seemed increasingly unrealistic after major US airlines Southwest and American took the aircraft out of their schedule until March. READ: Boeing 737 MAX safe says panel of experts. Boeing continues to work closely with the US Federal Aviation Administration on getting the aircraft back in the air and on Monday outlined the progress it had made so far "While the FAA and other regulatory authorities will determine the timing of certification and return to commercial service, Boeing continues to target FAA certification of the MAX flight control software updates during this quarter,'' Boeing said in a statement. "Based on this schedule, it is possible that the resumption of MAX deliveries to airline customers could begin in December, after certification, when the FAA issues an Airworthiness Directive rescinding the grounding order. "In parallel, we are working towards final validation of the updated training requirements, which must occur before the MAX returns to commercial service, and which we now expect to begin in January." The manufacturer, which says it is confident the MAX will emerge from the process as "one of the safest airplanes ever to fly", outlined five key milestones it needs to complete before a return to service. It said the first of these -- an eCAB simulator certification session -- was completed in the past week and it was working with the FAA towards a line pilots test crew workload evaluation and an FAA certification flight test. The multi-day eCab simulator evaluation with the FAA ensured the overall software system performs its intended function, both normally and in the presence of system failures. The FAA line pilots crew workload evaluation is a separate, multi-day simulator session with airline pilots to assess human factors and crew workload under various test conditions. The crucial FAA certification flight test will see administration pilots conduct a certification flight or flights of the final updated software. After the flight test, the final product will be submitted to the FAA for certification and a multi-regulatory body, the Joint Operational Evaluation board, will conduct simulator sessions over several days using global regulatory pilots to validate training requirements. This will see the Flight Standardization Board release a report for a public comment period, followed by final approval of the training. "At each step of this process Boeing has worked closely with the FAA and other regulators,'' Boeing said. "We’re providing detailed documentation, had them fly in the simulators, and helped them understand our logic and the design for the new procedures, software and proposed training material to ensure that they are completely satisfied as to the airplane’s safety. "The FAA and other regulatory authorities will ultimately determine return to service in each relevant jurisdiction. This may include a phased approach and timing may vary by jurisdiction." The European regulator, EASA, has said it does not expect to give the aircraft the green light until some weeks after the FAA.