The Unexplained
Boeing’s ill-fated 737 MAX series planes have larger engines than the earlier 737NG series. You may have wondered (as I did) why those larger engines caused MAX planes to pitch upwards during takeoff, necessitating the disastrous MCAS software. Basic logic suggests that the new, heavier engines would tend to make the planes pitch downwards.
Being a curious engineer, I started researching and found enlightenment on the subject, which I would like to share with you.
Why Larger Engines?
The main feature that differentiates the MAX series from earlier 737s is its larger engines. Boeing needed to put in larger engines to make the 737s more fuel-efficient. 1 They had to do this to stop American Airlines from ordering a huge batch of A320neo planes (which were more efficient than the contemporary 737NG) from Airbus and order on them instead. 2
The Surprising Explanation
As is evident from the image below, the new engines are mounted higher than in the previous 737 series (737NG). They are now in line with the wing, and in fact, some parts of the engine housing are actually higher than the wing (not visible in the image).
At normal speeds and normal thrust, the larger, heavier engines do not create any additional aerodynamic phenomena.
However, at an angle of about 14 degrees coupled with higher thrust, the engine casing begins to act as a wing and starts generating additional lift, pushing the nose of the plane upward, as shown in the below diagram:
And of Course…
Boeing installed software (called MCAS) to automatically push the nose down in this situation, which, coupled with a defective sensor, resulted in the deaths of more than 300 people. But that’s another blog post.
I hope this explanation was of value to you. Please feel free to put down any thoughts you want to convey in the comment box below.
As Sir Francis Bacon is reputed to have said, “Knowledge is Power.”
Further reading:
Boeing’s automatic trim for the 737 MAX was not disclosed to the Pilots
Bibliography
- “Why Jet Engines Keep Getting Bigger”,
- “Boeing didn’t want to re-engine the 737–but had design standing by”, Leeham News, https://leehamnews.com/2019/03/20/boeing-didnt-want-to-re-engine-the-737-but-had-design-standing-by/
