I'm curious about the frames & cases at the front of an engine. I've seen OEMs call them the centerbody, fan hub frame, and intermediate case. Are these all the same thing or are these each a separate part? It seems that each OEM has their own vocab.

The fan case is the one I can figure out. I'm more talking about the area right behind it before the HPC

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    $\begingroup$ You mean "cowls" don't you? Case refers to the engine itself. $\endgroup$
    – John K
    Nov 16, 2019 at 20:24
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    $\begingroup$ I'm referring to the engine, not the nacelle. Everyone seems to use the same vocab to describe that so it's pretty clear. $\endgroup$
    – Jim12
    Nov 16, 2019 at 20:46
  • $\begingroup$ Welcome to aviation.SE! This seems like a case where a picture would be very helpful to clarify what you're asking. Can you upload or link to one that shows the components you're asking about? You might also like to check out the tour to see how this site works. $\endgroup$
    – Pondlife
    Nov 17, 2019 at 3:14
  • $\begingroup$ If your question is "are centerbody, fan hub frame and intermediate cases the same thing?" you may edit the title to reflect it. It make it easier to crawl in a list of questions about the same topic. Moreover, you should transform the title in a question as this is a Q&A website. $\endgroup$
    – Manu H
    Nov 18, 2019 at 9:57

1 Answer 1


On Boeing aircraft, the "centerbody" is part of the exhaust system. It is the inner is the inner part of the exhaust as shown in the picture below (the outer part being called the primary nozzle). The centerbody is also referred to informally as the "plug". Since your question seems to be asking about fan and compressor cases, this doesn't seem to fit. I'm not sure what Airbus or other airframers call this part, so maybe the word "centerbody" is used differently in different contexts. But for a Boeing airframe, that's the centerbody.

For turbofans which use a "quick engine change" concept, the fan frame actually splits into two pieces. The "fan hub frame" referred to the inner part. Hence the word "hub" in the name. I've marked it up in the image below. This photo also illustrates the concept pretty well. This shows an engine being assembled. The outer part of the fan frame (with fan case attached) is on the right, and the fan hub frame (with the rest of the engine core attached) is on the left. The "fan hub frame" proper is the part just to the right of the yellow tape.

The term "intermediate case" is ambiguous to me. I could think of two or three different things that that might refer to. On a two spool engine, that probably refers to the booster case, which is usually immediately forward of the fan frame. But on a three spool engine, it probably refers to the intermediate compressor case, which is usually just aft of the fan frame. Do you have more context?

enter image description here

  • $\begingroup$ Thanks! For the intermediate case, I'm referring to the diagramn on page 3: gknaerospace.com/globalassets/downloads/sweden/…. Hard to tell, but it looks like there is a second major frame or case behind the fan hub frame. Am I crazy or is this a thing? $\endgroup$
    – Jim12
    Nov 17, 2019 at 23:05
  • $\begingroup$ Can you mark up the drawing and point out the specific area that you have questions about? Still not sure I understand what you are asking... "behind" to me typically means "aft", but the thing labeled "intermediate compressor case" is forward of the fan hub frame, so not sure that I understand what part you're talking about $\endgroup$
    – Daniel K
    Nov 18, 2019 at 1:20

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