American Airlines says it its Boeing 737 MAX airliners again on Dec. 29, marking the return of that troubled plane to commercial service after two disastrous crashes led to a 20-month grounding. The Federal Aviation Administration has now , which made me wonder what changes Boeing made to earn recertification. We have the details here.

The FAA published a of its review of the Boeing 737 MAX. Buried about nine pages in, the summary describes the changes made to MAX aircraft.

Before we go over what Boeing had to change, we should explain how Boeing got here in the first place. As noted in the , the 737 MAX features larger and more powerful engines compared with its 737 NG predecessor. Because these engines are larger, they had to be installed higher and farther forward than the previous engines. As a result, the aerodynamic balance of the plane changed.

Through use of what is called the Maneuvering Characteristics Augmentation System (MCAS), the MAX was supposed to fly like the 737 NG and thus not require pilots to take additional training to fly the MAX. The system works by automatically adjusting the trim of the horizontal stabilizer at the rear of the plane to push the nose down. It’s supposed to activate when the plane is at a high angle of attack to prevent stalls.

Many pilots of the system’s existence. The changes to the MAX without using the MCAS system . However, Boeing did not want that, as it may have affected sales to the airlines. It designed the MAX to be flown by 737 NG pilots .

In practice, the MCAS system accepted readings from . In the event of a bad sensor reading, the MCAS initiated repeated nose-down inputs. The cockpit alarm for AOA disagreement was also an expensive upcharge.

So Boeing made some changes to the MAX and the MCAS system. The MCAS system now has a maximum limit of one nose-down input during a single event of high angle of attack. The limit doesn’t reset if the pilots activate the electric trim switches. Further, an AOA sensor monitor was added to make sure MCAS doesn’t use AOA input if sensors disagree with each other by more than 5.5 degrees. The Flight Control Computer itself also no longer relies on a single sensor. Another important change is with the AOA DISAGREE alert. Previously, this alert was part of an optional AOA Gauge offered by Boeing. Now the AOA DISAGREE alert is always enabled, regardless of whether the airline has the option or not. All these changes are in the .

In addition to the above changes, Boeing also improved training procedures and added safeguards to the Flight Control Computer to detect and stop erronous stabilizer commands. It also added a requirement for operators to test AOA sensors after repairs have been made.

In essence, the changes Boeing made are honestly what should have been done in the first place. Modern aviation is known for its redundancy. MCAS should have never relied on a single sensor, and the system should have never been allowed to continuously enter nose-down inputs.

Pilots will be extensively trained on the MAX before it returns to the skies for commercial duty. When it does, Boeing and the FAA are confident the MAX’s woes are over.

(Updated 3/3/22 with new details)