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Home > Airplanes > Why do the engines of Boeing 737 have flat bottom?
Why do the engines of Boeing 737 have flat bottom?
Aviation - Airplanes
Saturday, 19 November 2005 00:36
Dear Capt Lim,

I don*t know if you*d know the answer to my question, but it*s worth a try.

Why do the engines of the Boeing 737 have flat bottom? It seems kind of odd but there must be a reason behind it.


Matt Hill

Hi Matt,

Of course, there is a reason why the newer Boeing 737 engines are flat at the bottom.

On the earlier Boeing 737-200s (Classic), they had rounded engines. In 1985, when Boeing decided to stretch the Boeing 737-300 and creating the Boeing 737-400, they extended the fuselage by a further 3 meters (10 feet). They also fitted them with more powerful engines, strengthened the wings and landing gears components. The newer engines were larger than the previous ones on the Classic where they were built into the wings rather than slung under the wings with pylons.

As a result of the pylons and having the engines slung below the wings plus the extra 3 meters, the center of gravity (CG) of the plane also moved forward and the ground clearance was reduced. (The forward CG is a good thing in a total engine failure scenario as it tends to recover from any inadvertent stall by naturally pitching down). How to solve this problem? Simple. Have a flat bottom engine. But with this, new problems arose. Well, they had to relocate the engine accessories that used to be fixed at the bottom to the side.

The solution of flattening the bottom gave an extra of about 18 inches clearance from the ground. To a pilot, this is added clearance and safety, especially in a crosswind landing where there is a fear of scraping an engine on the downward wing with a wrong pitch attitude!


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What problem?
Hi, I was researching this very question and came across this thread. Capt Lim, in your explanation to Matt's question you said "How to solve this problem? Simple. Have a flat bottom engine". However, after carefully rereading your response I fail to see exactly what "problem" needed to be solved. You discuss the CG moved forward as a result of bigger engines and lengthening the plane and reducing ground clearance. Is specifically "reduced ground clearance" the problem you are referring to? As I am writing this I am beginning to assume that is the problem beause you address ground clearance again in your last paragraph. But the clarification would help.

Thanks. Also glad to know I am not the only who notices these oddities and gets to researching an answer. :-)
Russel , 08 Sep, 2013
The problem Capt Lim was ground clearance caused by the addition of the Engine pylon.

Joe Sands , 03 Oct, 2015

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