Why Are The Bottom Of Boeing 737 Engines Flat?

If you have ever looked at a Boeing 737 engine you might have noticed something odd. That the bottom circle of the engine casing flattens out instead of continuing the same circumference. There is a very special reason for this and we will explain why.

Boeing 737 engine
The Boeing 737 engine. Photo: Davidilit via Wikipedia

The history of the Boeing 737

To answer this question, we first need to examine the initial design principles of the Boeing 737. Originally the aircraft was built for airports in a different age, one in which only a few airports had stair cars and even less had jet bridges. An aircraft back then had to have doors (especially cargo doors) that an operator or cargo handler could climb into without any ladder or truck.

With that in mind, the Boeing 737 series was made to operate as low as possible, with a belly that almost scraped along the surface of the runway (overdramatic but you get the idea).

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Lufthansa 737
A look at Lufthansa’s 737-100 on the ground. The first Boeing 737s didn’t need big engines or lots of room. Photo: Lars Söderström via Wikimedia Commons

However, as engine technology improved, engineers discovered the more air that you can put into an engine, the more efficient its fuel will be. This is called the bypass ratio.

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To get more air into the engine you need a bigger turbine which is powered by a bigger fan. Bigger fans require a bigger intake and an engine casing to fit. Problem is, unlike the Airbus A320 (the Boeing 737 rival) that was built with bigger engines in mind, the Boeing 737 was actually too low to the ground to have any bigger engines.

Boeing could have moved the engine to a different area of the plane (such as a trijet model or overwing) but this would have significantly changed the aerodynamic profile, leading to pilots having to retrain and be recertified (this story might seem familiar for long time readers, but don’t worry we will explain at the end).

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Thus Boeing was out of luck and faced an expensive redesign of the 737.

Southwest 737 MAX
You can see the flat design principles still on the Boeing 737 MAX. Photo: Southwest Airlines

Why is the engine bottom flat?

However, engineers looking at the aircraft realized that the casing of the engine was the problem. Could they simply have a shape that wasn’t round and thus would not hit the ground as the plane rolled along the runway?

Thus the new Boeing 737 engine was flattened along the bottom, despite still containing a larger fan and turbine. This did have bad aerodynamic properties and caused more drag than a round engine, but the increased amount of air entering the engine caused the bypass ratio to expand so much that is made any problems negligible.

The Boeing 737 classic series (-300 -400 -500) was the first to feature the CFM56 engine, well known for its ‘hamster pouch’ non-round design.

Because the aircraft still flew the same and had the same aerodynamics, pilots did not have to retrain nor be recertified. In retrospect, perhaps not improving the Boeing 737 design to take bigger engines back in the day resulted in the situation we have with Boeing 737 MAX, however that is beyond the scope of this question and best left for the comment section.

What do you think? Do you like the look of the Boeing 737 flat engines? Let us know in the comments.

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William

The engine position Boeing used on the B737-100 enabled easy service by technicians standing at ground level not possible in rear engined aircraft. The low height of the aircraft meant baggage loading and unloading could be done straight from the baggage cart into the hold without waiting for a conveyor. There was no major issue on the positioning of the LEAP 1C engines. It just wasn’t engineered properly.

atlas

And after this upgrade should Boeing have started to develop a 737 successor, Because they did know that newer engines will be bigger.
But money is money, and now they have lost more money and time than it will cost them a clean sheet design.

Phil

Boeing wanted to but Airbus came out with the NEO which began the process of forcing Boeing’s hand. Then American went ahead and ordered the plane before Boeing even announced it. Southwest said they need a new plane with very few changes so pilots don’t have to be retrained. While this doesn’t excuse Boeing for taking short cut, it does tell the MAX came to be.

Erwin

Not to mention how many lives were altered and lost? The hell with money do it right or don’t do it at all!

DAVID

Why Are The Bottom Of Boeing 737 Engines Flat? It was the first step toward disaster !

FIRST OFFICER

SPARE ME YOUR SARCASM

Leonardo

I think the flat looking in the engines of the Boeing 737 is fantastic design ,bat in my opinions the flat portion on the botton of the engines help with the lifting or sustentation of the airplaing in the air.

TonytTDK

It might make it prettier.? Beauty IS in the eye of the beholder.!
Arerodynamically, the non-uniform shape actually CREATES drag.
That unbalanced shape might possibly create a tiny bit of lift, but anything it creates is more than offset by the amount of additional drag it generates.!

Raghunathan Ampalaya.

This shape of the casing have structural advantage apart from assisting to reduce height,but the drag has to be negated.

Bill Blais

The flattened engine housing is a step sibling to the MAX problem– instead of building a new plane, jury-rig an old one.

Moaz Abid

Why didn’t they raise the clearance for the Max 8, just like they did for Max 10? Or was it to continue the famous flat bottom design?

TonytTDK

A good question
& one I have not yet heard a proper answer to.?
I would have thought that if they could fit ‘extending’ undercarriage legs to the MAX 10, to help with tail-strike issues, that they could fit it to all the MAX range, to lift the entire rear & mid-section up a few inches & allow them to reposition the engines, at least a bit.?
Every Little Helps.?

Bob T

Yet another band-aid implemented by Boeing to take a 1960’s era design and try to shoehorn it into modern-world realities. The 737 is basically a shorter version of the Boeing 707, which was introduced in 1959! Yet Boeing STILL tries to milk this cash cow — to the MAX, some might say… Putting dollars ahead of good engineering design. In the case of these flattened nacelles, no harm done to passengers. But in the case of MCAS (again, a way to cram yet BIGGER engines onto the ancient 737 airframe design), passengers and crew were killed. Boeing needs to accept… Read more »

TonytTDK

The MAX 10 will be longer than the original Boeing 707 with which it shares it’s fuselage
& will carry more passengers but only on 2 engines, when the B707 had 4 to power it.!
That’s how much jet-engine technology has changed, during the operational-life of this design of airframe.

Ravioliollie

Good grief, the Max 8 hasn’t been given a clean bill of health and you’re pimping the Max 10? Delusional, IMO.

Erwin

I’m sure that over the years they have put the modern technology into the 737 / 707 fuselage.
But looking at the top picture of the Lufthansa, you can see how much engines have changed for the better.

James

CFM56-3 engines on the 737 are shaped funny because of the accessory gearbox location being moved from the bottom to the side.

Raymond Chung

Would it have been possible to ‘jack up’ the plane with higher/taller landing gear?

wompusderg

total redesign. more material, more mass. retracted landing gear compartments would have to be enlarged. this would cause a domino affect on the entire nomenclature of the aircraft. taking away fuel space, decreasing range and adding weight.

Erwin

So then how is Airbus doing it so nicely? As I see it it is because of Airbus that Boeing scrambled and pushed for the MAX.

ssmith3104

That is what I was wondering! Even if only making the landing gear only, say, .6 of a meter (2 foot) taller, that would have improved the geometry of where/how the engine nacelle would bolt onto the wing; allowing it to achieve adequate ground clearance whilst being more conventionally “back” and not so disturbingly unconventionally forward as what has caused all the problems with the MAX.

dougcoleman

All other things being equal, how would that have prevented the 737 Max problems?

Ron

So use to it, never give it a second thought! But, as you stated, it the problem had been dealt with then, we would probably not have the MAX problem now.

janj

They should have averted this disaster on the Max when they redesigned the landing gear to accommodate the engine properly and take the advantage of the redesign.. At the end, everyone needs to do sim training, such lose of foresight.

If they had this in mind, they could have continually be slugging out the A320.

TonytTDK

The problem Boeing have, & have always had, with the B737 design, is that there is no space in the wing -root for any changes to the main undercarriage.
Boeing COULD have addressed this at any stage in the past, but it would have involved substantial changes to all the tooling for the wing to body attachment areas, which would have cost lots of money in both design & retooling. Presumably, this is why they DIDN’T do it a few years ago when they Did make lots of changes & ‘upgrades’ to the wing itself.?

David Knutson

Why didn’t they simply redesign the landing gear to raise the fuselage and allow more clearance for the larger engines in a better configuration under the wing?

B b salterike

As i understand it, some auxillary components were moved around the circumderence of the nacelle to provide addition clearence to the 5 and 7 oclock positions. Thus the hamster cheek appearance.

David

That sound like more an excuse than an explanation. If the engine change had made no difference to the aerodynamics there would not have been a need for secret software to correct it and so fool the pilots.
I don’t buy it.

Marek Rutkowski

Diameter of engine inlet is design required to supply required volume of air at any stage of engine operation. Fltening is required to keep CLEARENCE from surface to prevent suction foreign objects at take off when engine works at max power

Ashish Bharadwaj

Simply redesign the landing gear to telescopic design that fits into existing bay, yet extends deeper when deployed, more like an air suspension commonly seen in vehicles

Patrickk

If that could’ve been done as easily as you suggest it would’ve happened 20 years ago.

Curt

How’d you get a round fan in a housing that is flat on the bottom ?

Here2go

Gears are to the side. Normally they are at the bottom of the engine, and the housing designed to allow a more rounded appearance for efficiency

Craig Hanna

I thought that was the case.but a beautiful aeroplane.

Rudy Leyk

My experience with the 737 was in Hawaii from the late 60s to the early 80s. Aloha airlines flew them as opposed to Hawaiian Air that flew the dc-9. I liked its wider body seating 6 across. I thought it’s engine design was most interesting with the flat bottom because it did roll close to the ground. I always felt safe in that aircraft. That Faith proved correct when an accident that stripped the upper portion of the fuselage away exposing the passengers to the open-air, but the plane still flew and landed.

Erwin

That was a miracle the way that top was ripped off at high speed. But it is a strong plane for sure. Not knocking the plane, but I am when they want to make it a different plane. A redesign and new name would have been much better. You can not erase the lives lost because of greed! Now Boeing is paying for it anyway and still have no planes to show for it. I wouldn’t doubt that they will scrap the 737 Max.
The other 737’s are fine.

Donald Norman Scott

Interesting comment and in fact goes some way in the ultimate design of the 737 MAX which turned out to be a step to far.

Hans jansen

Dont care round, square or in between.so long we dont stay in the air for ever

Oma

It’s not the engine that is flat on the bottom, it’s the cowling that is flattened. If a turbine would have a flat side it wouldn’t go round that easy.

Paul

Lol

Erwin

I’m glad you said that. It’s true but the outside shell can even be altered and a round turbine inside works. Just a round cowling is more aerodynamic. That’s fine as long as the engines are below the wings.

LogoQRcode

Explains to you very well that this airplane is outdated and should not fly again specially with modern bigger engines! Just strange that so many airlines are not that smart and never notice that problem before committing to large purchases!

Leigh

And what about telescopic U/C legs to raise height of A/C on the ground?

Ghyndui

the -300, -400, and -500 are not “classic” 737s. The classic models were the -100, and -200.

Jacques. Descarrega

To me it’s a design flop.

Steve

The B737, when first introduced was a world beater aircraft. Nothing came close and airlines snapped them up. Over the years, though, Boeing has milked this design to the point now that the 737 is basically an unstable aircraft. In the last several iterations Boeing’s changes to the 737 have now manifested themselves into an airplane so unstable that software is required to mitigate the instability inherent in the hardware. How would you like to have car that a software failure would disable the steering?

Johnny H

why not above the wings giving more clerence fot the wings to be lowered and allowing for larger more eficient engine ,getting away from traditional designs

Andre Parent

Any change to a plane that will require compensation from software is a mistake from the start. A plane must be designed first with minimal software requirements, basic aerodynamic rules are the most important step into building a solid safe plane. Flat bottom engines was a mistake from the start as its first reason to exist was not a « flying », it was a loading cargo issue. Right there the possible birth of a major aerodynamic problem is possible. As a math graduate who has studied wing technologies I am oftentimes very unpleasantly surprised by the administrative bypasses that goes into… Read more »

Valmir

No sense at all. Boeing a long time try to mend their terrible design on 737 family. They made their plane without future vision aread. Looks like amateur building these planes not a professional engineer. I could do much better.

Denis

God bless Boeing!

John

One of the other design criteria for the Next Generation 737, was to eliminate the over-wing escape slide which would save weight and cost. This allowed the passengers to slide off the back of the wing without an escape slide.
The flat bottom engine cowl was also needed for ground clearance so it would be less likely to suck in runway debris.

Andre' Labuschagne

Yes , it’s made the 737 look different and i’ve wondered why, thanks for the info !

Scott

Nice explanation but just one observation; the fan does not drive the turbine. It’s the other way around…

John

It’s flat to maintain ground clearance and by virtue of the low clearance they were able to avoid having a over-wing escape slide. Emergency egress was allowed off the wing by sliding off the back of the wing.

Ken Hartwell

Yes I really like the look of a flat bottom engine. However I have flown on both the backseat 109 hundrednYes I really like the look of a flat bottom engine. However I have flown on both the Max 800 and 900 and the seat configuration is the smallest I’ve ever experienced. They need to rethink about the comfort of their customer!

tom bogucki

Just another example of what we call value engineering. Cost to redesign Vs the cost of lives. I think we know the answer. A great woman once said “Do your best until you know better. Then when you know better, do better !!! Hope Boeing can do better!!

Enock

No I don’t like the look, aircraft should’ve banned from the Aviation before it even fly

Chris Whiteley

The turbine powers the fan, not as is stated the other way round.

Shabbir Malik

Makes sense to a person outside the flying fraternity.
Please explain why the MCAS software should take more than a year to get rid of from the 737 Max. It was not installed in the rest of the 737s and to revert the Max to it’s older models by just re-setting the computer to the older model settings should have taken no longer than a month

Stuart Evans

Casing design is nice,innovative, even attractive, sleek. But knowing how this flattening has compromised !?
aerdynamics,obviously my answers is no I don’t like it.

Roman

Handled Boeing 37/400/500/700 etc., didn’t have any problems with engines shape. Looks cool. Sort of Boeing’s business card. Personally, I like it.

Nathaniel Iyanda

Redesigned engine would have been better. I see the flat part of the engine as cutting corners. All the same, the 737 has good safety record until the the 737 Max debacle. I think safety concerns should transcend fuel efficiency and aerodynamic considerations.

Erwin

Having the bottom flat is not the issue as much as having the engines above the wing as the Max. Boeing slept at the job when it comes to looking into the future of aircraft. Yes I say scrap the Max and redesign a new and proper aircraft. All lives matter!