Some aeroplanes are offered with a choice of engines; for example, both the Boeing 777-200 and the Airbus A330-300 could be ordered with Pratt and Whitney, Rolls Royce or General Electric engines. Does each type of engine require considerably different mountings and other fittings thus preventing them from being interchanged, or are the engines interchangeable perhaps with some minor adjustments?

They have considerably different mountings as far as I know. They did try to have a common mounting for the 787 engines, so they could be changed, but I don't think it got anywhere.

For most engines it requires a pylon change to change engine manufacturer. That also includes a change of the thrust reversers It is pretty big and expensive job so so you just don't see many mods of this type anymore. The last I can really think of are the UPS 727 mod that swapped out the JT8 for the Rolls-Royce Tay. Before that were the DC-8 reengine programs.

On the military side there was the KC-135 and soon to be the B-52. It helps to have a military budget.

Thanks for the information. I thought that it might be quite a complex job.

I've just read that the 777X and the A-350 don't have any choice; the former has GE, the latter RR only. Similarly, the A-330-200 and -300 had a choice of P+W, RR and GE, but the -800 and -900 have RR only. I wonder if there might be a trend towards having fewer or no choices in engines.

DrPaul wrote:

Thanks for the information. I thought that it might be quite a complex job.

I've just read that the 777X and the A-350 don't have any choice; the former has GE, the latter RR only. Similarly, the A-330-200 and -300 had a choice of P+W, RR and GE, but the -800 and -900 have RR only. I wonder if there might be a trend towards having fewer or no choices in engines.

As the next generation of engines become more expensive to develop it is unlikely (at least for widebodies) that more than one supplier will commit to the programs.

StTim wrote:

DrPaul wrote:

Thanks for the information. I thought that it might be quite a complex job.

I've just read that the 777X and the A-350 don't have any choice; the former has GE, the latter RR only. Similarly, the A-330-200 and -300 had a choice of P+W, RR and GE, but the -800 and -900 have RR only. I wonder if there might be a trend towards having fewer or no choices in engines.

As the next generation of engines become more expensive to develop it is unlikely (at least for widebodies) that more than one supplier will commit to the programs.

Yes this is the reason. It has nothing to do with a swap like the OP was asking about. With the exception of the 787 I don't think having multiple engine choices was used for swap opportunities. The multiple choices were driven by the engine companies that wanted market share on what ever new aircraft was offered. The development cost have just gotten too large. They all pretty much require to be the single source to recoup the investment. The narrow body market is really the only opening for multiple engines with the GTF and the LEAP both offered on the A320neo.

Dalmd88 wrote:

StTim wrote:

DrPaul wrote:

Thanks for the information. I thought that it might be quite a complex job.

I've just read that the 777X and the A-350 don't have any choice; the former has GE, the latter RR only. Similarly, the A-330-200 and -300 had a choice of P+W, RR and GE, but the -800 and -900 have RR only. I wonder if there might be a trend towards having fewer or no choices in engines.

As the next generation of engines become more expensive to develop it is unlikely (at least for widebodies) that more than one supplier will commit to the programs.

Yes this is the reason. It has nothing to do with a swap like the OP was asking about. With the exception of the 787 I don't think having multiple engine choices was used for swap opportunities. The multiple choices were driven by the engine companies that wanted market share on what ever new aircraft was offered. The development cost have just gotten too large. They all pretty much require to be the single source to recoup the investment. The narrow body market is really the only opening for multiple engines with the GTF and the LEAP both offered on the A320neo.

That is a recent development though, for many years the narrow-bodied market was almost completely single-option (JT8D, and then CFM56 untill IAE came along.)