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)
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