What did Boeing change to get 737 MAX re-certified to fly?

It has been almost two years since Ethiopian Flight 302 crashed on the outskirts of Addis Ababa in a similar fashion to the Lion Air Flight 610 which crashed in the Java Sea five months earlier. These two incidents caused the grounding of the 737 MAX fleet with the flight certification being stripped from the aircraft.

With the 737 MAX being recertified in the United States and Europe after a two year absence from our skies, what did Boeing change to make sure that the 737 MAX is safe for air travel?
Boeing 737 MAX airplanes sit, grounded, in Seattle.(Stephen Brashear / Getty Images)

MCAS - Maneuvering Characteristics Augmentation System

The main cause for the crashes in Indonesia and Ethiopia was the failure of the MCAS system. MCAS stands for Maneuvering Characteristics Augmentation System which is an automated flight control developed by Boeing engineers. This system was designed to correct the angle of attack of the aircraft while climbing, in case of an unusually high angle of attack.
Impact of Angle of Attack on Lift
What is Angle of Attack?
In aerodynamics, the angle of attack simply refers to the angle between the wing and the oncoming airflow.

The system corrects a high angle of attack by pitching the plane downwards until it reaches an acceptable angle.

Originally, the data that was gathered for this metric was from a single sensor; however, in both of the crashes, the sensor fed false information to the aircraft computer causing the system to believe that the plane was in a high angle of attack even after the pitch had been corrected. This caused the plane to go into a nose-dive. Pilots unable to override the system; the aircraft plunged into an uncontrollable dive which is seen in the data gathered from the black box.

You can download the raw data that I used in my video to show the vertical speed and altitude of JT610 during its last one minute of flight from Flightradar24 below:
Two main fixes
Angle of Attack (AOA) Sensors
First, measurements from two Angle of Attack (AOA) sensors will now be compared which is a change from the only one sensor used prior. Each sensor will submit its own data to the airplane’s flight control computer. MCAS will only be activated if both sensors agree. The addition of data from the second sensor allows the flight computer to process two sets of data, reducing the likelihood of failure in the system. Both sensors would have to be faulty in order for any potential problem to occur.
No Pilot Override
Second, MCAS will only be activated once and will never override the pilot’s ability to control the airplane using the control column alone. Even if everything went wrong, both sensors were faulty and fed the flight computer faulty information causing the plane to pitch down, this system would be activated only once, preventing it from continuously forcing the plane to try to decrease its pitch. The system would never override the pilot’s ability to control the plane as it did in the two crashes prior to the grounding.

Boeing also addressed certain additional issues not related to the MCAS system:

One of the things that they fixed includes the development of software updates to address a theoretical horizontal stabilizer issue. During testing, Boeing found that there was a theoretical combination of faults that could lead to a runaway stabilizer condition. Boeing tells us that this failure never occurred during the 200 million hours of flight operations prior to the grounding of the aircraft family and that they addressed the issue to ensure that it never occurs.

Boeing also modified some wiring to meet regulatory requirements and now requires the inspection of stored aircraft for Foreign Object Debris (or FOD). This is because some FOD was found in the fuel tanks during routine maintenance of aircraft in storage. Boeing recommends that aircraft be checked and any FOD removed. In addition, the company updated software to address a remote possibility of autopilot disengagement, increased Angle of Attack integrity, and developed additional crew alerting testing.

Boeing also published a new pilot training program, collaborating with pilots, engineers, and safety experts to create a proposal that calls for 737-8 and 737-9 pilots to complete additional training, review of technical documents, and demonstration of their knowledge in a flight simulator. Boeing took input from 80 airlines from around the world in order to enhance its flight simulator to provide pilots with the best possible training. These measures will make sure that pilots are familiar with the new 737 MAX generation of aircraft.
FEBRUARY 28, 2021
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