Boeing 737 Max: Why Was It Grounded, What Has Been Fixed And Is It Enough?

John McDermid, University of York

The Boeing (BA) 737 Max began flying commercially in May 2017 but has been grounded for over a year and a half following two crashes within five months. On October 29 2018, Lion Air Flight 610 took off from Jakarta. It quickly experienced problems in maintaining altitude, entered into an uncontrollable dive and crashed into the Java Sea about 13 minutes after takeoff. Then on March 10 2019, Ethiopian Airlines Flight 302 from Nairobi suffered similar problems, crashing into the desert around six minutes after leaving the runway.

In total, 346 people lost their lives. After the second crash, US regulator the Federal Aviation Administration (FAA) decided to ground all 737 Max planes, of which around 350 had been delivered at the time, while they investigated the causes of the accidents.

Now, 20 months later, the FAA has announced that it is rescinding this order and has set out steps for the return of the aircraft to commercial service. Brazil has responded quickly, also approving the 737 Max. So, what went wrong – and can we be confident that it has been fixed?

The causes of the two accidents were complex, but link mainly to the 737’s manoeuvring characteristics augmentation system (MCAS), which was introduced to the 737 Max to manage changes in behaviour created by the plane having much larger engines than its predecessors.

There are some important points about the MCAS which we must consider when reviewing the “fixes”. The MCAS prevented stall (a sudden loss of lift due to the angle of the wing) by “pushing” the nose down. Stall is indicated through an angle of attack (AoA) sensor – the 737 Max is fitted with two, but MCAS only used one. If that AoA sensor failed, then the MCAS could activate when it shouldn’t, unnecessarily pushing the nose down. The design meant that there was no automatic switch to the other AoA sensor, and MCAS kept working with the erroneous sensor values. This is what happened in both crashes.

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This article is republished from The Conversation under a Creative Commons license.

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