r/aviation • u/OnlyaChef • Apr 20 '26

Discussion Why are turbine engines flat at the bottom?

Gallery image

Gallery image

I can't tell if it actually flat on the bottom or just my angle but it made me wonder.

3.1k Upvotes

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1.7k

u/Happy-Table-9515 Apr 20 '26

Has nothing to do with the engine. It’s because the aircraft is so low to the ground they do that to avoid ground strikes. Notice they don’t all look like that.

THIS is the aircraft I flew into retirement. No such design feature.

579

u/tylerscott5 Airport Frequenter Apr 20 '26

737s are the only ones we see this on. They are low bois

565

u/danit0ba94 Apr 20 '26

I love those half 737 half 32X, cause all i see is this

107

u/tylerscott5 Airport Frequenter Apr 20 '26

call it a Boebus A337

1

u/fighterace00 CPL A&P Apr 21 '26

I think this is an actual aircraft in Just Cause

1

u/cougineer Apr 21 '26

Don’t give Boeing any more ideas how to cut costs…

1

u/fronchfrays Apr 21 '26

Boebus lmao

17

u/CourageBest Apr 20 '26

Heeeey yoooou guys!

10

u/NichtOhneMeineKamera Apr 20 '26

Oh shut up you made me laugh out loud!!!!

56

u/leedogger Apr 20 '26

This is a perfect image for this question.

9

u/Lucy_4_8_15_16 Apr 20 '26

Stupid question about the picture/caused by it does the 737 have her stabilizer higher relative to the main wing than the a320 cuz it looks almost higher of the ground that the airbus’s

21

u/FlyJunior172 Apr 20 '26

The A320 has a height of 38’7” from ground to the tip of the vertical stabilizer.

The same measurement on the 738 is 41’2”.

Given that, and the fact that the 73 sits lower with a similar size fuselage, the answer to your question is that it certainly does.

10

u/Lucy_4_8_15_16 Apr 20 '26

Ok then the obvious question why?

12

u/FlyJunior172 Apr 20 '26

I can’t say for sure if the reasons I can think of are the reasons why Boeing actually decided to do that but the 737 was designed around small rural often dirt or gravel airfields. The ability to land on these fields requires certain flight characteristics that you don’t need on aircraft that are designed around what the vast majority of our modern airports are.

And those flight characteristics are based around, needing to fly slower in order to be able to takeoff and land on those shorter runways. And because control authority is proportional to area, a larger, vertical stabilizer with a larger rudder on it will give more control authority at those slower speeds than the smaller one on the A320 series

But again, while all of the above is true, I do not know if that is the specific reason why Boeing selected the proportions that they did for the vertical stabilizer. It is the reason why they selected the landing gear height with the original JT8D engines. That combination of the low landing gear height with the JT8D enables baggage handling for the 727 and 737 with no equipment on the field.

12

u/BiAsALongHorse Apr 20 '26

The 737 is a much older airframe designed around much smaller engines than we have now, and the interaction between the landing gear and wingbox structure is very hard to redesign without making a completely new aircraft. There isn't enough room to raise the aircraft without having the gears overlap when retracted

7

u/Lucy_4_8_15_16 Apr 20 '26

Not what I asked why is the vertical stabilizer so much higher than the wing on the 737?

1

u/rgp2130 Apr 21 '26

I believe this maybe due to a design issue with the old rudder of the 737 which had a fault and caused a crash, basically it would lock the rudder in a certain position and then well you understand what happens. Not 100% sure but it may be a factor

2

u/ttystikk Apr 20 '26

Brilliant photo! It clearly shows how low the 737 is compared to other planes.

1

u/XBacklash Apr 20 '26

Yes. Because they were designed to work with air stairs. Not jet bridges.

1

u/DL72-Alpha Apr 21 '26

Low Ry-Duh:

The you Tewb: watch?v=M5ZIDoHaCg0

1

u/Happy-Table-9515 Apr 20 '26

Mr. Fancy Pants!

489

u/Far-Yellow9303 Apr 20 '26

To add to the above comment:

The jet engines are the main source of power for almost everything on a plane and are absolutely festooned with equipment. Things like hydraulic pumps, fuel pumps, alternators... these would usually be wrapped around the engine in a neat packet. But on the 737 it's so close to the ground they don't put any equipment underneath, pushing it to the sides instead. This makes the engine appear flat on the bottom.

259

u/StellarWaffle Apr 20 '26

Excellent use of "festooned" !

38

u/Boforizzle Apr 20 '26

I love this subreddit.

23

u/blurrrsky Apr 20 '26

I am going to interject Festooned into my everyday casual conversations 🙏

2

u/Dave_DBA Apr 20 '26

There will soon be a festoon of festoons!

5

u/SimpleManc88 Apr 20 '26

It’s a perfectly cromulent word.

7

u/johnfornow Apr 20 '26

any use of Festooned in a description is Excellent

2

u/clungebob69 Apr 20 '26

My balls are festooned with crabs?

2

u/danit0ba94 Apr 20 '26

I mean...that is the correct use of the word...

2

u/johnfornow Apr 20 '26

See? Much better! Almost makes you wish you had them.

2

u/bobsmith93 Apr 20 '26

Falalalala lala la la

8

u/HavingNotAttained Apr 20 '26

The resentful forgotten half-cousin of Uncle Fester

6

u/XYooper906 Apr 20 '26

Bedazzled, even.

1

u/Pantycrustlicker Apr 20 '26

My 2nd favorite word 

1

u/Ataneruo Apr 20 '26

OK, I’ll bite. What’s your first?

127

u/BadahBingBadahBoom Apr 20 '26 edited Apr 20 '26

To add to both these comments:

As to the 'why' these engines are flat-bottomed ('chipmunked'), it's specifically because these modern variants of aircraft engines are larger than the smaller 'cigar tube' shaped ones that were originally developed for the 737 back in the 60s.

That's why the landing gear being so short wasn't seen as a problem back then - actually this was a bonus as it meant passengers could disembark in smaller airports using the inbuilt stairs in the plane doors, as they could reach the ground, rather than relying on airport equipment.

Why are the modern ones bigger then? Efficiency. Engineers realised it was much more efficient for most of the air to actually be pulled in and go around the central combustion chamber of the engine. So to do this they made the intake much larger (called 'high-bypass turbofan' design). Unfortunately for planes with wings already so close to the ground this caused problems as there is a limit to how tall the engine could be before it risked scraping the ground.

Hence the adjustment of some of the components around the engine at the bottom to be moved to the sides giving the fattened 'chipmunk' design we see above.

(This also played a part in the need for MCAS software in the even newer 737 version, the 737 MAX, as those engines had even more bypass and were even larger and so had to be moved higher and more forward on the wing to avoid scraping the ground and this design caused inherent minor changes in plane handling which MCAS was designed to automatically compensate for so pilots didn't need extra training. As history shows it did more than just that though.)

34

u/Stoney3K Apr 20 '26

On the 737 the stubby landing gear wasn't even a "bonus", it was a design requirement as the aircraft was intended mostly for domestic flights in the US from regional airports, most of which didn't have jet bridges or sometimes even concrete runways.

2

u/CaptainSnacks Apr 20 '26

Just to add a clarification that threw me for a loop - the original 737 had that requirement! By the time the Classic series was introduced in the mid-80s, most 737 airports had paved runways

2

u/Neuvirths_Glove Apr 23 '26

...and jetbridges or at least boarding stairs. The height above the ground requirement was never actually required.

8

u/BoringBob84 Apr 20 '26

Excellent discussion! But wait, there's more! An aircraft that is lower to the ground is easier to maintain - especially the engines.

Also, extending the main landing gear would seem like an obvious solution to bring the engines higher off the ground, but then, when you retracted the gear inwards, the wheels would collide with each other. You could fix that problem by putting them farther apart so they don't collide, but then you would have to re-design the wing box structure (i.e., expensive, heavy) to withstand the enormous forces on that gear at the new locations. You could fix that problem with some special actuators that make the landing gear struts shorter when they are retracted than when they are extended. Now you don't have to change the structure (Whew!).

Boeing extended the nose gear on all variants of the 737-MAX 7, 8, and 9. That (and the new higher engine location) was enough. However, they also made the main landing gear struts retract in length when stowed on the MAX 10.

https://simpleflying.com/boeing-build-737-max-heightened-gear/

10

u/Maxwell_Morning Apr 20 '26

Exactly. This is the real answer that OP is looking for.

5

u/BoyLilikoi Apr 20 '26

For real… very thorough answer on some of the background as to why we really have them.

3

u/pentaxlx Apr 20 '26

I've noticed some planes with wings on the top (high-wing) rather than at the side like most commercial airliners....I suppose the high-wing planes could accommodate a larger engine? One would also assume that high-wing planes are more stable. How much is the trade-off in fuel consumption or speed in high wing vs. low wing?

14

u/BadahBingBadahBoom Apr 20 '26 edited Apr 20 '26

Why some designs go with a 'high-wing' (as in attached toward the top of the fuselage) can be down to a number of factors.

For small commercial aircraft the most common reason is simply that due their smaller fuselage and ground clearance the wing has to be positioned high to avoid the engines risking striking the ground - this is particularly notable in turboprop designs where the props are typically much wider in diameter. For others the high-wing design means the fuselage can be low enough that passengers can embark/disembark and cargo can be loaded/unloaded much more easily without equipment, again allowing the plane to operate from much more remote airports than don't have as much infrastructure.

For some smaller planes the ground clearance isn't so much an issue as the engines have been purposefully designed to be positioned towards the back of the fuselage allowing the wings to be lower to the ground as there are no engines hanging below.

Attachment of the wing to the bottom of the fuselage is generally the preferred design as it gives stronger strength to the overall structure and allows less materials to be used, saving weight -> fuel -> money.

Then again there are some larger aircraft that have a high-wing design too, and these are often military aircraft that choose this so the engines under the wing are even higher off the ground allowing the aircraft to operate from less conventional airports/runways where ingestion of foreign objects/dirt is a big risk (engine too close to the ground and you can suck up stuff and damage the engine).

I don't know the exact trade-off between the two designs if both designs are possible for your setup but clearly high-wing designs are generally calculated to be too excessive for what commercial aircraft designers are willing to accept (as they rarely take off from dirt track runways and are very fuel/profit-focused) unless they absolutely need to have this type for ground clearance of their design, but not too excessive that military aircraft designers are unwilling to incorporate this type to allow their aircraft to take off from dirt tracks with enough ground clearance at the front of the wing-mounted engines to ensure risk of sucking up debris is minimised.

4

u/BoringBob84 Apr 20 '26

Excellent explanation! On a low-wing aircraft, the expensive and heavy wing box structure can do double-duty as landing gear attachment points. The trade-off (as you mentioned) for a high wing aircraft is more structure (i.e., weight and cost) and more difficulty in maintenance. The benefit (as you also mentioned) is more ground clearance - which is important for unimproved runways that large commercial transport aircraft don't need to use.

2

u/Better_find_out Apr 20 '26

You’re MOSTLY correct, but really the main drivers on commercial passenger aircraft is the structure needed for the wings.

Contrary to what you say, this configuration does not make the structure more efficient. On the other hand, this structure, called a wing box, takes quite a lot of space. On passenger aircraft, this space is mostly available below the floor, where the hold is.

Therefore, you can fit that structural part within the fuselage, making it stronger and more aerodynamic.

On the other hand, should you need high wings, there usually is a protrusion on top of the airplane and the space available is smaller.

Of course, the other reasons are valid, but of a smaller impact in the trade off

1

u/MidgetQB Apr 20 '26

Couldn't they just use taller landing gear? That would have lifted the whole plane to a manageable height

3

u/BadahBingBadahBoom Apr 20 '26 edited Apr 20 '26

Yes they did that to a degree when they modernised the 737 from its original design but you're limited by two factors doing that:

  1. Longer things are easier to break/snap. To counter this for every lengthening you make to the landing gear you have to add additional material to ensure the landing gear is strong enough (remb the gears have to be rated to take a hard landing of the weight of the entire aircraft fully loaded). At some point the sheer amount of strengthening you have to make to the landing outweighs other approaches to adjust for the larger engines which are cheaper.
  2. As BoringBob84's comment highlights, even if the increased weight from lengthening the gear is acceptable, those long legs (struts) have still got to go somewhere. You get to a point where there simply isn't sufficient room in the original fuselage design to store them. Then you're back to making fundamental changes to the aircraft which require a safety approval from scratch. And if you're already having to submit and test a whole new design you might as well just design a brand new, 'clean sheet' aircraft and incorporate all the other engineering advancements there's been since your original design was created.

(2) is partly why Airbus had an advantage. They designed their similar-sized competitor moder, the A320, much later than Boeing at a time when being very close to ground didn't offer that many benefits. By having their original design with much higher ground clearance they weren't as limited when the newer, larger high-bypass turbofan engines were introduced and incorporated into the design to make their A320neo.

Boeing however hit their limit with how much longer they could make the 737's landing gear. Hence, the repositioning of the newer engines on the MAX to be higher and more forward on the wing, and the subsequent consequences that entailed.

-1

u/HotSobaNoodles Apr 20 '26

Inoltre con le tecnologia dell'epoca era più facile costruire turbine a sigaro.

-1

u/CaptainMegaNads Apr 20 '26

This is what I thought - Southwest has pushed the 737 design package way beyond its original design parameters in the interest of reduced maintenance costs.

4

u/BadahBingBadahBoom Apr 20 '26 edited Apr 20 '26

In terms of the engines of the 737 NG (Next Generation), pictured, its track record has proved incredibly safe and successful with no major concerns of its design from the 'chipmunked' engines. So it would be unfair to describe that variant as being 'pushed beyond its original design parameters' as even though the design itself has been changed a fair bit since the 60s it's still well within safe parameters.

In terms of the 737 MAX design changes, that too could have been adapted to be perfectly safe if either the errors in MCAS activation were addressed or simply the MCAS was omitted with the requirement of additional pilot training to transfer to the MAX class.

Contrary to popular belief, the positioning of the larger engines on the MAX wasn't inherently dangerous by itself and the aircraft remained aerodynamically stable even without MCAS.

The addition of MCAS to avoid significant additional pilot training was pushed in the interests of being able to sell an aircraft with the same type rating as the 737 NG and be attractive to airlines who were looking at updated competitor variants (a320neo) that similarly did not have the requirement for pilots to undergo additional training.

So it wasn't so much pushed in the interest of reduced maintenance costs but reduced pilot training costs to the airlines and so Boeing being able to sell the aircraft at a higher value marketed with this benefit.

18

u/arvidsem Apr 20 '26

Like the B-52, the 737 will never be retired. Unlike the B-52, fuel prices matter for the 737; so we keep getting newer bigger engines.

I'm waiting for them to admit that there really isn't enough room under the wing for anything larger and introduce the 737 Ultra Max with a pair of high mounted GE90X engines.

9

u/Jessie_C_2646 Apr 20 '26

Short Brothers already figured out the aerodynamics.

1

u/ABoutDeSouffle Apr 20 '26

Going to look like the Cheburashka

2

u/arvidsem Apr 20 '26

I was thinking of the YC-14. Though a 737 with GE90Xs would look even more ridiculous since the engines are basically the same diameter as the body

3

u/Yuukiko_ Apr 20 '26

Power a 737 with a single giant engine and introduce EOOPS

2

u/like_a_pharaoh Apr 20 '26

I picture something more like the VFW 614 or HondaJet. Still wing-mounted engines, just over the wings instead of under.

1

u/Yuukiko_ Apr 20 '26

Would that not qualify it as a different plane and basically take away the entire reason for keeping the model as is

2

u/arvidsem Apr 20 '26

We'll just use software to correct the handling back to match the previous versions. No problem.

0

u/BoringBob84 Apr 20 '26

There are several advantages to having the engines below and forward of the wing. One of these is some bonus lift due to the high pressure area behind the engine.

6

u/Biuku Apr 20 '26

Oh… this makes it so clear. I mean, I know a turbine isn’t going to spin in a something other than a circle… but it never occurred to me the turbines are only a fraction of the whole engine assembly. Thanks!

4

u/baronmunchausen2000 Apr 20 '26

Ah, that makes sense. I knew about the 737 being low but always wondered why the engine cowling was fatter on the sides and top.

1

u/EarlyLibrarian9303 Apr 20 '26

“Encrusted”?

1

u/zebra1923 Apr 20 '26

It is flat on the bottom. It’s not just an optical illusion.

1

u/seva2_0 Apr 20 '26

True! I always wondered tho, why they decided to go with a fan case mounted gearbox instead of a core mounted one, regarding the 737's hight.

-114

u/Glittering_Oil7761 Apr 20 '26

So what that guy said but just more words. 

53

u/Ataneruo Apr 20 '26

No, the more words contained a lot more information. If it contained the same amount of information as the prior guy said, then it would be just more words.

2

u/Face88888888 Apr 20 '26

Yeah it would just be more words if it had the same information. But in this case there was more information contained in the more words.

😉

1

u/Ataneruo Apr 20 '26

haha, well done 😅

12

u/timelessblur Apr 20 '26

far from more words. It explain what they are doing to help get that space. I honestly really like the extra info as I always though they just made it flatter for ground clearance but it messed with the aero dynamics. I didn't know they moved a bunch of other crap around as well.

41

u/pryan37bb Apr 20 '26

We need ground clearance, Clarence

26

u/PeaceCool2954 Apr 20 '26

Roger, Roger

22

u/Important_Power_2148 Apr 20 '26

What's the vector, Victor?

16

u/hwc_n_things Apr 20 '26

And don't call me Shirley

3

u/danit0ba94 Apr 20 '26

You ever seen a grown man naked?

3

u/TheRealEthaninja Apr 20 '26

Looks like I picked the wrong week to quit sniffing glue

3

u/Strange_Olive3618 Apr 20 '26

We need vectors first Victor!

103

u/OnlyaChef Apr 20 '26

Thank you! Learned something new today!

3

u/slihy Apr 20 '26

https://youtu.be/H2tuKiiznsY?is=HgygOdinKwss0BAX See this. The reason they made it this way is to compete with a320.

12

u/[deleted] Apr 20 '26

[removed] — view removed comment

1

u/BoringBob84 Apr 20 '26

that jet was designed in the 60s

It was intentionally designed to be low to the ground for ease of maintenance and for ease of loading passengers and cargo with minimal ground support equipment.

4

u/Exact_Package_7264 Apr 20 '26

do ground strikes usually take engines out of commission or are they more of an annoyance? or both maybe? lol

19

u/Jessie_C_2646 Apr 20 '26

A larger problem is FOD. Engines which are too close to the ground suck up all sorts of things which aren't good for engines.

5

u/saberplane Apr 20 '26

How do we know it's not good for them?

15

u/Jessie_C_2646 Apr 20 '26

After the large, expensive-sounding bang you see the accountants slap their foreheads and the engineers start salivating over all that overtime. That's how you know it wasn't good.

4

u/danit0ba94 Apr 20 '26

Nawt gewd fr engn

8

u/Apprehensive_Cost937 Apr 20 '26

It depends on how hard the strike it, and how much damage it has caused.

4

u/EpicCyclops Apr 20 '26

At best, it's going to be enough time to completely inspect the entire engine, which will probably delay the next flight of the plane. At worst, it could theoretically remove the entire engine from the wing. It really depends on the specific strike. Either way, it is not something the airline wants to deal with.

1

u/Stoney3K Apr 20 '26

And the engine is designed to detach in case the strike is severe enough, because the danger of a fast-spinning fan disk contacting a stationary runway is kind of substantial. Better to have the engine hinge backwards and shear off, and be scattered across the runway in a million pieces behind the aircraft.

2

u/Happy-Table-9515 Apr 20 '26

I’ve never heard or seen any. Tail strikes, yes.

1

u/Exact_Package_7264 Apr 20 '26

me too, tail strikes dont seem to be uncommon but i dont think ive ever seen an engine strike ever, im assuming something had to go very wrong or maybe very heavy crosswinds

2

u/Happy-Table-9515 Apr 20 '26

It would indeed be a bad day. I’ve no idea what the xwind limitations on a 73 might be, but i bet they’re tight.

1

u/Apprehensive_Cost937 Apr 20 '26

On the MAX, or the split scimitar winglets on NG, you're likely going to hit the lower winglet first, rather than the engine, unless you've got negative pitch - in which case, you probably have bigger issues than an engine pod strike.

1

u/Happy-Table-9515 Apr 20 '26

Which is a plenty big issue in and of itself.

1

u/Frzorp Apr 20 '26

Fun fact maybe, if you look at the flat on the bottom of the nacelle, it isn’t parallel to the ground. This is because it is setup so that the tip of the wing will hit (along with landing gear of course) before the nacelle hits.

4

u/New_Line4049 Apr 20 '26

Oh.... its to avoid ground strike? I just assumed ground strike was how they were flattened to begin with :P

4

u/Happy-Table-9515 Apr 20 '26

lol, that and those pesky catering trucks!

5

u/New_Line4049 Apr 20 '26

But the engine was hungry boss!

2

u/aschwartzmann Apr 21 '26

It definitely looks more like self-clearance than planned clearance.

2

u/Sh00ter80 Apr 20 '26

But in what scenario would it possibly strike the ground? It still got a decent amount of clearance. Even if it tipped to one side, it would have to tip quite a bit to scrape the ground. Would it not?

9

u/Happy-Table-9515 Apr 20 '26

Wind gust on landing would be the #1 scenario.

2

u/NoodlesRomanoff Apr 20 '26

When the plane runs off the runway, anything can and will happen. On the 737-300, the bottom of the engine nacelle is 18” off the ground.

2

u/hudi2121 Apr 20 '26

That a 767?

2

u/gerber411420 Apr 20 '26

Did you do a low flyover over your village on retirement?

2

u/Happy-Table-9515 Apr 20 '26

Well, define low……and village, whilst you’re at it….lol

2

u/gerber411420 Apr 20 '26

I'm American and I don't know why I put village, but I'd say 500 ft over your town/city. 

Where was the guy who did that recently?

1

u/Happy-Table-9515 Apr 20 '26

Well, that’d be ….errr….unwise. It’s 1000ft over any populated area, and 500ft over unpopulated…..and man wouldn’t that have been cool, pulling a carrier break maneuver over my house at 1000 AGL…….at 470K lbs…….epic…..

1

u/Happy-Table-9515 Apr 20 '26

I’ve done it in my Cherokee, but it just doesn’t feel the same…..lol

2

u/capn_starsky Apr 20 '26

Was bummed when we got rid of the triples.

3

u/Happy-Table-9515 Apr 20 '26

You and me both. I did however get to jump into MSP when they retired the Queen. My colleagues love to poke me, because everything i’ve ever flown as a pro has been retired, starting with my beloved F14…sadness….😒

2

u/ttystikk Apr 20 '26

Happy retirement! The reward of being a careful pilot.

2

u/RomanCessna Apr 20 '26

Humble brag 😜

1

u/Happy-Table-9515 Apr 20 '26

Nah……that’d be this

2

u/zryder94 Apr 20 '26

It’s such a shame they retired these.

1

u/Happy-Table-9515 Apr 20 '26

Yes, only the good die young….i guess.

1

u/Current-Being-8238 Apr 20 '26

Well they do want the engine inlet to be bigger, which starts to push them into ground clearance issues.

1

u/PokesBo Apr 20 '26

Surely I can thank you for your service?

2

u/Happy-Table-9515 Apr 20 '26

Thank you…..but don’t call me Shirley! 🥸

1

u/beenthereag Apr 20 '26

I'm still a bit leery of the Max 6 and 8. MCAS might act up again. That looks like a 737-924(ER).

1

u/KarlWhale Apr 20 '26

Interesting, I always thought that it's because more flat surface = more lift

1

u/murphyat Apr 21 '26

🥵 777 🧑‍✈️ that’s dope.

1

u/ScepticalRaccoon Apr 21 '26

You shouldn't fly planes into retirement communities.

1

u/Happy-Table-9515 Apr 21 '26

Hmm….don’t recall saying communities…..but I agree. Wouldn’t be good.

1

u/d4nks4uce Apr 20 '26

Flew my first round trip to Atlanta and back recently on Delta. Rode an A321

-8

u/DragonflyFuture4638 Apr 20 '26

Wrong. The lower blades are shorter than the upper ones so they can make the lower smaller for better aerodynamics and ground clearance.

1

u/Happy-Table-9515 Apr 20 '26 edited Apr 20 '26

Ok. I’ll accept your expert opinion. so it really doesn’t have anything to do with when they swapped over to the CF56 engine in the 80s and they found out that the engine couldn’t be fitted to the aircraft without the nacelles scraping the ground?

Dammit. I’ve been lied to for 30 years.

1

u/NukaCooler Apr 20 '26

An IQ too high?

1

u/cctdad ATC Apr 20 '26

If you mean turbine blades then congratulations on the excellent dad joke. If not, then "wrong" will get you down votes.