America’s air safety agency announced the move on Wednesday, saying it was made after a comprehensive and methodical 20-month review process.
Regulators around the world grounded the Max in March 2019 after the crash of an Ethiopian Airlines jet, which happened less than five months after another Max flown by Indonesia’s Lion Air plunged into the Java Sea.
A total of 346 passengers and crew members on both planes were killed.
The planes will not return to the skies immediately as the Federal Aviation Administration (FAA) says it must approve pilot training changes for each US airline and airlines must perform required maintenance on the planes.
The FAA says the move was made in co-operation with air safety regulators worldwide.
“Those regulators have indicated that Boeing’s design changes, together with the changes to crew procedures and training enhancements, will give them the confidence to validate the aircraft as safe to fly in their respective countries and regions,” the FAA said in a statement.
The move came after numerous US congressional hearings on the crashes that led to criticism of the FAA for lax oversight and Boeing for rushing to implement a new software system that put profits over safety and ultimately led to the dismissal of its chief executive.
Investigators focused on anti-stall software that Boeing had devised to counter the plane’s tendency to tilt nose-up because of the size and placement of the engines. That software pushed the nose down repeatedly on both planes that crashed, overcoming the pilots’ struggles to regain control. In each case, a single faulty sensor triggered the nose-down pitch.
The FAA required Boeing to change the software so it does not repeatedly point the nose of the plane down to counteract possible aerodynamic stalling. Boeing says the software also does not over-ride the pilot’s controls like it did in the past. Boeing also must install new display systems for pilots and change the way wires are routed to a tail stabiliser bar.
“These events and the lessons we have learned as a result have reshaped our company and further focused our attention on our core values of safety, quality and integrity,” current Boeing chief executive David Calhoun said in a statement.
Boeing sales of new planes have plunged because of the Max crisis and the coronavirus pandemic. Orders for more than 1,000 Max jets have been cancelled or removed from Boeing’s backlog this year.
American Airlines is the only US airline to put the Max back in its schedule so far, starting with one round trip daily between New York and Miami beginning on December 29.
Nearly 400 Max jets were in service worldwide when they were grounded, and Boeing has built and stored about 450 more since then. All have to undergo maintenance and get some modifications before they can fly.
Pilots must also undergo simulator training, which was not required when the aircraft was introduced.
Boeing’s then-chief executive, Dennis Muilenburg, initially suggested that the pilots were to blame. However, congressional investigators discovered an FAA analysis — conducted after the first Max crash — that predicted there would be 15 more crashes during the plane’s life span if the flight-control software were not fixed.
After an 18-month investigation, the House Transportation Committee heaped blame on Boeing – which was under pressure to develop the Max to compete with a plane from European rival Airbus – and the FAA, which certified the Max and was the last agency in the world to ground it after the crashes.
The investigators said Boeing suffered from a “culture of concealment” and pressured engineers in a rush to get the plane on the market.
Boeing was repeatedly wrong about how quickly it could fix the plane and when those predictions continued to be wrong, and Boeing was perceived as putting undue pressure on the FAA, Mr Muilenburg was dismissed in December 2019.
Europe’s aviation regulator, the European Union Aviation Safety Agency, said it will take public comment on its plans to clear the Max for flight and expects to finalise a plan late this year or early in 2021. Some EU states will have to lift their own individual grounding notices as well.
Regulators in Canada and China are still conducting their own reviews.