Semantron 23 Summer 2023

The Boeing 737 Max crisis

The design: AoA & MCAS

The CFM LEAP-1B engines on B737 MAX aircraft are 15% more fuel efficient, thanks to their high bypass ratio of 9:1, compared to 5.1:1 for the CFM56-7B engines on B737NG aircraft. 37 Generally, the higher the bypass ratio, the more fuel efficient it is, as more air will pass through the bypass duct (the cold duct) for every kg of air that passes through the core (the exhaust hot duct). In order to increase thrust, passenger jets put more air into the engine, and therefore, a larger engine is needed to increase airflow. Meanwhile, the cool air in the bypass duct can cool down the inner core, thus reducing noise emission and increasing efficiency. 38 Boeing has a great engine, but how were they going to fit the much

Figure 7: The structure of a turbofan engine. The air which does not pass through the engine core is the ‘bypass air’.

larger engine onto a previous generation B737NG fuselage? In order to decrease the development cost and shorten the design stages to keep up with Airbus’ competition, Boeing had decided to use the same fuselage as the B737NG. They came up with an idea – to

Figure 8: Comparison of a B737NG aircraft (left) and a B737 MAX aircraft (right)

place the engine even more forward, and to push the engine higher than the wing itself. Given that the engine-to-ground distance on a B737NG aircraft is 627.38mm maximum or 480.1mm minimum, 39 and that the CFM56-7B engines are 1,829mm in height, 40 and given that the CFM LEAP-1B engines are 2,256mm tall, 41 one can easily imagine how high the engine would be over the wings in order to maintain the same ground clearance, alongside adding eight inches to the nose gear.

However, lifting the engines caused a huge technical problem. When the engines were in full thrust, the aeroplane will point too far upward. 42 This would potentially lead to stall due to the high Angle of Attack (AoA), and decreasing airspeed. The Angle of Attack, or AoA, is defined as ‘ the angle between the mean chord of the wing (a line drawn between the leading

Figure 9: Angle of Attack

37 Brady 2021, The Boeing 737 Technical Guide, 83 rd ed : 225. 38 Captain Joe 2021, HOW DO JET ENGINES work and WHY do they get BIGGER? : 12:20 39 SUNNY 2010, Technical Specifications NG . 40 Wikipedia 2022, CFM International CFM 56 . 41 Federal Aviation Administration 2018, Type Certificate Data Sheet E00088EN : 4. 42 VOX 2019, The real reason Boeing’s new plane crashed twice : 5:59.


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