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This question already has an answer here:

What is this big rectangular tab (?) thingy extending backwards just above the DC-10's #2 engine? What does it do?

photo
(wikimedia.org, picture modified to point out the tab thingy)

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marked as duplicate by kepler22b, Community May 25 '18 at 19:28

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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enter image description here
(airliners.net via szextant.blogspot.com)

It mostly houses the same stuff that's on an underwing engine pylon: bleed air ducts, wiring, fuel and hydraulic hoses and tubes that connect to the engine core and accessories. The aft part of the structure where it tapers is just there out of aerodynamic necessity.

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Also the stuff at the end of the fuselage below the engine is just a fairing. It will fold away and the engine is removed by installing a bracket on the pylon and using cables to lower the engine straight down. If it was deeper in the fin as Sean mentioned, it would need to be drawn backwards and then down, making it more complex to do an engine swap.

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DC10 rear engine pylon, same as underwing, for supporting the engine. The tube that looks like it could provide support is just a very long intake duct. enter image description here

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  • $\begingroup$ So why not put the engine further forward in that tube, which would allow the engine to be supported by the horizontal stabilizer directly and allow its ancillaries to be placed within the stabilizer itself rather than needing an external pylon? $\endgroup$ – Sean May 24 '18 at 22:28
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    $\begingroup$ @Sean - The long inlet is to straighten out the air after the fuselage disturbance, especially at high angles of attack. $\endgroup$ – ymb1 May 24 '18 at 23:42
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    $\begingroup$ @sean You don't want an engine explosion to blow all the control surfaces off the aircraft. UA232's #2 engine failure took out all three hydraulic systems and control of the tail, but were able to land. If the engine had been mounted farther forward they might have lost the control surfaces altogether and consequently the plane. $\endgroup$ – Pilothead May 25 '18 at 0:12
  • $\begingroup$ @Pilothead: Good point, although it wouldn't matter nowadays (since all jet engines are now required to contain any and all debris within the engine under any and all possible failure scenarios). $\endgroup$ – Sean May 25 '18 at 0:16
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    $\begingroup$ @sean As they always have, and as well demonstrated by SWA1380, SWA3272 in 2016, QF032 in 2010, AF066 in 2017... $\endgroup$ – Pilothead May 25 '18 at 0:22

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