Why do some B747-400 engines have this back cover?

Hi guys,

So I wanted to ask why do some B747-400 engines have something like a cover on the back of the engines?? (See the 2 images below) Is it because of different engine manufactures or aerodynamics etc? Thanks!

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They are different engines hence the different look.

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The second is made by Rolls Royce and I think the first one is made by CFM or GE, it’s because Airlines can choose which manufacturers they want to equip their planes’ engines.

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The engine types on the 747-400 are General Electric (GE) CF6, Rolls Royce (RR) RB 211 and Pratt and Whitney (PW) PW4000
Fun fact: CFM international is a collaboration between GE and Safran aircraft engines

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I believe as everyone stated above that there different engines, but part of it is also that some of them have a form of a hush kit on them. Correct me if I’m wrong. 🤗

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The RR engines are the quietest hence the extended engine cover as well it also has an air mixer reducing turbulence inside and the back of the engine I’m pretty sure or at least on the earlier engines varients

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The RB211 is a triple spool gas turbine where as the other two engines are 2 spool engines hence the RB211 is ‘shorter’ and fully covered by the engine casing.

The extended centre portion of the GE and the PW covers the back of the power turbine assembly.

HTH

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