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The Designs And Flaws Of The MCAS

Updated: Jun 16, 2023

Edison, NJ

Do you remember in 2018, when news networks talked about how the new Boeing 737 MAX was being grounded? Well, you might have heard that this was due to a mechanical fault in the plane. More specifically, this was due to flaws in the MCAS which was built into the 737 MAX. If you don't know what the MCAS is, or what it does, then this article brought to you by STEM-E will explain the system in its entirety, functions and flaws included.

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A 737 MAX taking off

First off, we need to know what the MCAS is and how it works. The MCAS is an acronym for the Maneuvering Characteristics Augmentation System, which was installed onto the 737 MAX. This system is commanded by the flight computer on the aircraft and is used to prevent an aircraft from stalling when the flaps are retracted. A stall is when an airplane loses speed. Due to this lack of speed, the airplane is not able to keep itself in the air, and it begins to descend rapidly. Meanwhile, flaps, when extended, increase drag and lift, which slows down the plane and reduces stall speed; for example, when a pilot slows down a plane for landing, they will have to extend the flaps, which prevents the plane from stalling due to a lack of speed.

The MCAS somewhat controls the elevators of the plane, which are surfaces at the back of the plane that make the nose of a plane point up and down, and rely on the Angle of Attack sensors, or the AoA sensors. These sensors are little black spikes that are built on the outside of the plane, which serve the purpose to detect the angle that the plane is climbing or descending. If the AoA sensors detect that a plane is close to stalling, the MCAS will point the nose of the plane downward, so that the plane can gain enough speed to not stall.

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A diagram from The Seattle Times describing the MCAS

Despite how it functions, the MCAS was not created to function as a stall protection system. The system was designed to counteract the forces from the engines of the 737 MAX. The LEAP engines are larger than the CFM56 engines on the previous 737NG because Boeing wanted the 737 MAX to have more fuel-efficient engines. These engines were placed higher and further forward on the wing to prevent the engine from colliding with objects on the ground, and thus, the turbulence created by the engine causes an effect where the nose of the plane will pitch upwards. This sudden effect could cause the pilots to accidentally pull the nose up further, which could stall the plane. So, the MCAS was built to pitch the nose down to prevent a stall.

Due to the new technology of the 737 MAX, it was the plane of choice for many airlines. However, this is where the flaws of the MCAS start to appear. A year after the introduction of the 737 MAX, on October 29th of 2018, Lion Air Flight 610 disappeared from the radar 13 minutes after takeoff from Soekarno-Hatta International Airport. Not long after, workers at a nearby oil rig reported what no one wanted to hear: the wreckage of the plane could be seen in the Java Sea. A year after the Lion Air accident, on March 10th of 2019, Ethiopian Airlines Flight 302 disappeared from radar screens 6 minutes after takeoff from Addis Ababa Airport, in similar circumstances to the Lion Air flight. The loss of two planes led to the worldwide grounding of the 737 MAX, which lasted 2 years. On October 25, 2019, the Indonesian NTSC ruled the cause of the Lion Air accident as a faulty AoA sensor, which caused the plane's MCAS to push the plane into a dive. Here, we find the main flaw with the MCAS: it only took data from one AoA sensor. This means that if the AoA sensor was faulty and transmitting false data to the MCAS, which was the case with Flight 610, the MCAS would push the plane into a dive even if nothing was wrong with the plane.

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A piece of wreckage from Flight 610

As has been shown, the MCAS is a system that was simply built to prevent a tragedy. However, the limitations of the system sadly caused two tragedies. Nowadays, the system is fixed and a situation like Flight 610 or Flight 302 will not happen again, but the MCAS, and the 737 MAX, have lost their shine.

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