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Suppose, someone buys 300 Pratt & Whitney engines from the USA and 150 Mig-35 air frames from Russia. Then, they use their in-house expertise to fit those engines to those air-frames.

What challenges would have to be faced by the user?

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    $\begingroup$ A really weird one, but... perhaps not off-topic enough to close? $\endgroup$ – Therac May 2 '20 at 13:13
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    $\begingroup$ It sounds like you are saying "if I have the expertise to do this thing, then can I do this thing?" Naturally your expertise would include knowing how to cut any holes in the structure needed to make the engines physically fit, fairing over those holes (I'm thinking of the numerous "blisters" that appeared on the Bf-109), etc etc etc... $\endgroup$ – quiet flyer May 2 '20 at 13:35
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    $\begingroup$ Rather than "legal" you might consider rephrasing to ask about the feasibility of getting them certified airworthy. Assuming this is what you meant... $\endgroup$ – Michael Hall May 2 '20 at 20:14
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    $\begingroup$ With enough time, effort, trial & error, and above all, MONEY, then all sorts of amazing & difficult (and even, ill-advised) things are possible. Is it going to be easy? Nope. Is it going to be cheap? No, no, and absolutely NO! Is it going to be worth doing? Highly doubtful! But with enough billions of $, you could probably get it to work & get it certified as airworthy. But really, what's the point of going to all that time, effort, and expense? Just to say that you did it? $\endgroup$ – Ralph J May 3 '20 at 3:07
  • $\begingroup$ @RalphJ one reason I can think of is spares availability for the engines. Soviet engines are notoriously unreliable, needing far more maintenance than their western counterparts. This was a big reason for Germany and others to retire their almost new MiG-29s when joining NATO. If you can get the jets and engines on the black market, but no aircraft and correct engines for them, maybe it's your best option to make them fit. $\endgroup$ – jwenting May 4 '20 at 11:30
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Re-engine an existing airframe is such a complex undertaking, in a way that so much R&D is required that the entire aircraft ends up being reverse engineered to make it work. Or on some occasions, the airframe is first licensed or reverse engineered, after which, experience and expertise is gained to enable the re-engine effort.

China is notable in this area due their Russian lineage of airframes and a long lasting desire for western engines, and a mixed relationship with both. India is in a similar position but I'm not aware of any such attempts.

Notable examples:

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  1. We're not on law.stackexchange.com, but from what I can tell, the legal consequences would be limited to losing some benefits from warranty/support agreements.

  2. There is no reason to do this. The engines on modern Russian fighters are quite good and among their selling points. You'd be concerned with upgrading their avionics - which are, of course, one of the most tightly-controlled export items - not their already excellent flying qualities.

  3. This is not possible. For airliners or other pylon-engine configuration aircraft, re-engining is easy. For fighters with internal engines, it's not. The airframe is built around the engines, with special ring frames that allow room for the engine while carrying through the wing loads.
    The engines are not geometrically compatible. None of P&W turbofans that could suffice to power a MiG-35 would fit in it.

TL;DR version: To paraphrase the old joke, you want a fighter that flies like a Flanker, sneaks like a Raptor, shoots like a Rafale, and maintains like a Gripen... not the other way around.

The MiG-35 is Russia's "low" component, similar to the F-16 in the US arsenal - lower cost, lower maintenance, high but not best achievable performance, and in its case also good improvised airfield capability. As part of that, it's a mid-size airframe, with smaller than average engines, optimized for efficiency and durability. The only US-built engine that could both fit and power the MiG-35 is the GE-414, after some modifications.

However, it's not the best possible engine for the MiG-35. The latest version of its own engine, the VK-10M, offers more thrust than the GE-414, has vectored thrust versions, and is a perfect fit for the frame. If you want the best, you'd want a custom engine, like an enlarged higher-bypass version of the EJ-200 (that's doable).

But, then, if you want the very best, you'd probably start with the Su-35 or the F-35. The MiG-35 is a workhorse.

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If the question is not limited to specific airframes, the Irkut MC21 airliner is planned to have two engine options : Pratt & Whitney PW1400G (American) and Aviadvigatel PD-14 (Russian).

So the answer is Yes, if you include airliners; you can even order them from the factory that way. First deliveries were scheduled for this year : I suppose that may be subject to change.

(there were also plans to give the Sukhoi Superjet a P&W engine option; these seem to have fallen by the wayside at the moment)

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    $\begingroup$ Designing an airframe to use multiple engine types had been done for decades (the 747 was launched with three different engine options from the major manufacturers) but that is a very different issue from retrofitting an engine that was never designed to be there. And it doesn't imply that you can buy the aircraft configured for engine X, and then change your mind and fit engine Y instead - not without some very extensive (and expensive) rework. $\endgroup$ – alephzero May 3 '20 at 1:34
  • $\begingroup$ @alephzero all that is very true - and pod mounting makes the substitution vastly easier than in a fighter jet anyway. But unless someone has a line on 150 surplus MIGs (eBay?) I doubt the answers will have any practical relevance anyway. $\endgroup$ – user_1818839 May 3 '20 at 14:03
  • $\begingroup$ You could then mention IL-96M (powered by PW2337). But this was still made by the OEM, so doesn't quite count in the spirit of the question. $\endgroup$ – Zeus May 4 '20 at 0:49

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