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“However, Boeing’s timely or earlier communication with the operators would have helped to reduce or eliminate possible confusion,” the F.A.A. said.

The anti-stall system, created to compensate for the Max’s large new engines, will push down the nose of the plane if the angle of attack sensors indicate the plane is dangerously close to stalling.

But the system relied on only one of the two angle of attack sensors, introducing a potential single point of failure into a critical flight system. And the anti-stall system was also changed late in the design process to make it much more powerful.

Airlines and pilots were not informed about the system until the Lion Air crash.

When Boeing explained to pilots in one meeting how systems on the Max worked, the company said that the disagree alert would function on the ground. In the late November meeting, Boeing told pilots for American Airlines (which had bought the add-on) that their disagree alert would have notified them of problems before takeoff.

“We were told that if the A.O.A. vane, like on Lion Air, was in a massive difference, we would receive an alert on the ground and therefore not even take off,” said Dennis Tajer, a spokesman for the union representing American Airlines pilots. “That gave us additional confidence in continuing to fly that aircraft.”

But in the last several weeks, Boeing has been saying something different. Mr. Tajer said the company recently told American pilots that the system would not alert pilots about any sensor disagreement until the aircraft is 400 feet above the ground.