Although as of late last month Boeing continued to cite a fourth-quarter target for authorization from the U.S. Federal Aviation Administration (FAA) for the Boeing 737 Max to return to service, the company also harbored contingencies­—namely, further production-rate reductions or a complete shutdown of the narrowbody assembly line in Renton, Washington—in the event the approval process extended beyond the end of the year. Furthermore, Boeing CEO Dennis Muilenburg acknowledged during the company’s third-quarter earnings call the likelihood of a “phased” certification process in which approval from aviation authorities outside the U.S. would come after the FAA gives its blessing. The timing of the Max’s return, he said, would depend solely on those authorities’ determinations.

Muilenburg also called plans by U.S. airlines to delay Max service entry until early in 2020 “consistent” with the manufacturer’s timing appraisal. At the time, United Airlines’ schedules called for the Max’s return on January 6, American Airlines’ timetables showed January 16, and Southwest Airlines' schedules reflected a February 8 return. However, early this month American and Southwest delayed their planned EIS targets again, to March 5 and March 6, respectively,

Outside the U.S., Norwegian Air Shuttle last month said it didn’t expect the airplanes to resume operations until late March “at the earliest,” and Ryanair reported that it didn’t expect a return until March or April. While the Gulf region’s most prominent Max operator and the second-largest in the world next to Southwest Airlines—FlyDubai—has resisted setting a return date for the first of its 14 grounded airplanes, it has said it expects a significant effect on its finances if the grounding extended into next year.

The prospects for the Max’s re-certification by year-end hinged on Boeing’s ability to finish validating a software update that added three more “layers of protection” to the airplane’s maneuvering characteristics augmentation system (MCAS), flaws in which investigators believe set in motion the series of events that led to the 737 Max crashes in Indonesia and Ethiopia. Meanwhile, a second software update unrelated to the MCAS also needed to pass muster with U.S. authorities.

Boeing insists that its changes address all the concerns highlighted by the final report from Indonesia’s National Transportation Safety Committee (KNKT) into the October 2018 crash of a Lion Air Max in which 189 people lost their lives. Investigators have concluded that accident stemmed from an incorrect reading by an angle of attack (AOA) sensor that triggered the MCAS to force the airplane into a dive from which the pilots could not recover. A similar scenario appears to have taken place in the March 2019 crash of an Ethiopian Airlines 737 Max in which all 157 occupants died.

For the Indonesia crash, off the coast of Java, investigators blamed design flaws in the MCAS combined with insufficient oversight by U.S. regulators, failures in crew resource management (CRM), and maintenance lapses. A final report into the accident released by the KNKT on October 25 gives nine main contributing factors that led to the crash, including the fact that a faulty reading in only one AOA sensor caused the airplane’s MCAS to pitch the nose of the airplane downward.

Other factors included a lack of documentation about the existence and function of the MCAS in flight manuals or training programs and the FAA's over-reliance on Boeing’s own employees to oversee the system’s certification. The report also named the absence of an AOA disagree alert—a system that came standard on the 737NG.

Still, the KNKT did not place all blame on the airplane design, noting that maintenance crewmembers failed to detect a miscalibration of an AOA sensor they replaced on the Max before its fateful flight. However, investigators could not determine whether or not the mechanics properly performed the installation test of the AOA sensor, only that the miscalibration “was not detected.”

Meanwhile, the KNKT attributed a lack of detection of the miscalibration by maintenance and the flight crew’s failure to recognize the reasons for the uncontrolled dive at least partly to a lack of documentation related to MCAS.

Finally, the flight crew did not “effectively manage” the multiple alerts, MCAS activations, and distractions resulting from numerous ATC communications due to a CRM failure, said the report.

For its part, Boeing said it made several changes to the Max that address all the shortcomings cited by the KNKT, including a redesign in the way AOA sensors work with the MCAS. First, the MCAS will compare information from both AOA sensors before activating, meaning the system will now turn on only if both sensors agree. Moreover, the new design will ensure that the MCAS will activate only once in response to erroneous AOA, and will always allow pilots to override its activation with the control column.