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Engine Pylon Construction

Mon Jan 24, 2005 8:39 am

Everytime I fly or look at an airliner, I can't help but notice how puny the engine pylons look in comparison to how much force is exerted on them. Does anyone have a webpage or more info about what these look like inside and what materials are used to construct them and how they are attached to the airplane and engines? I would assume that all the fuel, heater, and other lines run through there too. They obviously are more beefy than they appear, but I'm just curious...thanks!!
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Joined: Mon May 24, 2004 9:32 am

RE: Engine Pylon Construction

Mon Jan 24, 2005 9:19 am

Structurally, pylons are very complicated. I'm not an expert on them but here is a general description of how a pylon works. There are forged attach fittings that attach to the front spar and underside of the wing. A complicated box built up from sheet metal and more forgings is fastened to the fittings. There are also strut braces which are tubes that attach to the wing and the front portion of the pylon and serve to carry loads from the front of the pylon to the wing in a cantilever fashion. This entire structure is then covered with an external skin. You don't externally see any of the load carrying structure of the pylon. Pylons are inspected rather frequently and if cracking is found you will usually find it on all pylons of the same configuration eventually. When we have encountered this at DL pylon mods were designed incorporating titanium doublers, triplers, and quadruplers to reinforce the cracked areas. The mods were then incorporated on the entire affected fleet.

I didn't find any good photos of a pylon in the database but I did find this picture of Dl ship 252 having it's pylon repaired in DEN.

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Photo © Hisham Atallah

The #1 engine was run into by ground equipment damaging the engine and pylon. The pylon was removed and sent to the DL TOC in ATL for repair and then sent back to DEN along with a new engine to be refitted to the aircraft. Cant remember how long it took but I will try to find out and get back.

757 Most beautiful airliner in the sky!
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RE: Engine Pylon Construction

Mon Jan 24, 2005 9:41 am

Yeah, that is good skinny. Few people realize that the sheet metal structure you can see from your seat is not what is supporting the engine, its thrust and other forces.

The pylon is more of a fairing for the actual attach fittings and various plumbing and wiring. Some contained the fire bottles for the engine, but not so much anymore.

Happiness is not seeing another trite Ste. Maarten photo all week long.
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RE: Engine Pylon Construction

Mon Jan 24, 2005 10:10 pm

This is the best picture I could find:

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Photo © Alastair T. Gardiner

If you look at the pylon where it transitions from the forward horizontal firewall to the aft vertical firewall (about where the 2nd upper access door is) you'll see a complex shaped doubler. You'll notice it is on both visible pylons. Just about every classic you see will have this double, sometiems you'll see a tripler. This was an area that is highly prone to cracking. The interior structure has been strengthened with stronger materials (usually some titanium alloys). The doubler you see is the only visible sign of this modification.
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RE: Engine Pylon Construction

Wed Jan 26, 2005 9:26 am

Just to add a little bit more:

The nacelle struts or pylons are basically frame and skin structures.
This structures are riveted and bonded together to form a torque box.
Outside, the skin form the fairings to provide smooth air flow. Inside the structure hold frame; bulkheads; stringers and spars. Mostly, aluminum alloys are used here.
Now in the case of the P&W JT9D, the engine is attached to the nacelle struts in three places, one at the forward end and two at the aft end.
These mounts are designed to absorb engine thrust, vertical and side loads, and to allow axial and radial growth due to thermal expansion.
Regarding materials, steel, stainless steel and titanium are used due to high strength and heat resistance required.
As stated above, that area is frequently inspected by different methods and several AD's and SB's has been issued for modification. (NW and El Al engine separation accidents)


"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
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RE: Engine Pylon Construction

Wed Jan 26, 2005 11:02 pm

There is so much to consider when thinking about the pylon. First of all I am no engineer, only a mech who has performed many engine changes on C-5, C-17, KC-10 etc.

The C-5 engine for example is held onto the pylon by only 3 thats right 3 bolts!

The pylon has to withstand the loads that drag, thrust, gravity, turbulance, etc put on it.

It has to give a little, some call it "engine nodding" basically it means the pylon can't be so stiff that it doesn't allow some flex or give, because otherwise the pylon would crack.

On the C-17, when engines are running you will see the engine wiggleing or moving up and down and side to side. That is the pylon allows this to prevent the cracking I mentioned in the last paragraph.

As stated in other post the pyon that most people see is one covered by panels for aerodynamic efficiency, but underneath is very structurally sound, and complicated piece of hardware.

Hope my rambling helps,
At Pope, where not happy, until you're not happy!
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RE: Engine Pylon Construction

Thu Jan 27, 2005 6:23 am

If it helps, the NTSB report on AA191 (ORD, May, 1979) has some diagrams of the pylons...
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.

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