The Boeing 737 Max family inched closer to return to service following the U.S. FAA’s release on Monday of a proposed airworthiness directive calling for four key design changes surrounding the maneuvering characteristics augmentation system (MCAS) and the angle of attack (AoA) sensors.

The notice of proposed rulemaking (NPRM), which will be open for comment for 45 days, stems from the ongoing review of the Max family in the aftermath of the Ethiopian Airlines Flight 302 and the Lion Air Flight 610 fatal accidents and was accompanied by the release of a Preliminary Summary of the FAA’s Review of the Boeing 737 Max. That summary might continue to be updated as it continues to work with Boeing on the certification review of the Max family, which has been grounded now for well more than a year, the FAA said.

While the agency called the release of the NPRM a key step, the FAA cautioned a number of key steps are still ahead before the aircraft can return to service, including multiple operations and flight training reviews. In addition, the FAA must still review and sign off on Boeing’s final design documentation, issue a continued airworthiness notification to the international community, rescind the grounding order, issue airworthiness certificates, and approve operator training programs.

Boeing welcomed the proposal, saying, "We're continuing to make steady progress towards the safe return to service, working closely with the FAA and other global regulators. While we still have a lot of work in front of us, this is an important milestone in the certification process."

The NPRM’s proposed changes, meanwhile, are designed to address issues surrounding aircraft controllability. The agency pointed to flight data recording indications that “if a single erroneously high AoA sensor input is received by the flight control system, the…MCAS can command repeated airplane nose-down trim of the horizontal stabilizer.” This condition could impede aircraft control and lead to “excessive airplane nose-down attitude, significant altitude loss, and impact with terrain, the FAA said.

Specifically, the NPRM would require updating flight control software with new control laws to prevent erroneous MCAS activation; incorporating new and revised flight crew procedures in the airplane flight manual to ensure the flight crew can recognize and respond to erroneous stabilizer movement; installing new Max display system software to ensure the pilots are alerted if the aircraft’s AoA sensors disagree by a certain amount and indicate a potential failure; and changing the horizontal stabilizer trim wire routing installations to bring them into compliance with the agency’s latest standards.

In addition, it would require operators to complete an AoA sensor system test and perform an operational readiness flight before returning each airplane to service. The NPRM further directs operators to incorporate more restrictive provisions in the FAA-approved minimum equipment list surrounding the dispatch of the aircraft with certain inoperative equipment.