I have a question about that fat part at the back of some 737-200's engines. What is it for?

You've got to be way more specific as to what you are exactly referring to. Perhaps you mean this: (?)http://www.airliners.net/open.file/383612/M/If this is what you are referring to, this is a modification made to make the engine quieter. This needs to be done because of the noise regulations (Stages I, II, and III) that are implemented by the JAA. Without the modification, the engines on this aircraft would be louder than the permitted amount...Does this answer your question?

Yes this answers my question. Thanks:-beerchug

Glad to hear it! :-)If you want more information on noise regulations, here are a couple of sites:Boeing's site on the laws and regulationshttp://www.boeing.com/commercial/noise/flash.htmlA nice diagram and explaination of how these huskits actually workhttp://search.fedex.com/us/hushkit/configuration/?link=2

Is that one of GE's aft fan turbofans?Colin

Nope- GE would only do maintainence on one of their own engines. The JT8D is a Pratt and Whitney manufacture, and most Stage III conversion work is done by third-party companies.

Could you equip a DC-9 or 727 with this device?

Yes, they do these conversions for those types. It's very easy to see an equipped 727, because of the bulge in the second engine intake:With modification:http://www.airliners.net/open.file/358996/M/Without modification:http://www.airliners.net/open.file/100380/M/The DC-9 hushkitting is far less visible, even to the point where it's impossible to tell if it has the mod or not:http://www.airliners.net/open.file/182190/M/These conversions can be (and have been) done on any aircraft type, including the 707, DC-10, MD-80s, etc... Pretty much any old aircraft that doesn't meet the regulations.

That 727 in the first picture doesn't have JT-8Ds. There is a coversion avaiable that changes the JT-8Ds out with Rolls Royce Tays. The reason for the bulge is because a Tay engine is larger than a JT-8D is.

That's right- any conversion to the engine or aircraft, including complete replacement of the engine unit is considered hushkitting. I never said that the converted 727's have JT8Ds.The bulge isn't because of an increased engine size, though, since you have to consider that the second engine is inline with the other two. There is an "S" duct that takes air to the engine below (the actual engine placement can be seen at the "tailpipe" of the second engine. The duct size was increased to allow for greater airflow to the new engine- the Tay's need that higher airflow rate to be efficient.Take a look at these two other 727 conversions:727 with MD-80 engines (JT8D-200) (a.k.a. the "Valsan Engine Conversion")http://www.airliners.net/open.file/279888/M/727 with JT8D 200 engines in #'s 1 and 3 position, with #2 remaining the original engine with thrust reversers removed.http://www.airliners.net/open.file/186705/M/

I heard somewhere that the tay engine was right at the front of the intake and the S duct was just used for exaust.