China’s J-20, an advanced fighter built using stolen F-22 plans, is usually referred to as a “fifth-generation fighter.” But its capabilities question that designation. The problem with these sorts of classifications is that there is no universal or official set of characteristics an aircraft needs to exhibit in order to fit in a category—not unlike assessing whether you’re a millennial, part of Generation X or Generation Y, you’ll find that the standards for these titles aren’t universally accepted.

For instance, nearly everyone agrees that a fifth-generation fighter has to have stealth capabilities.  For most, that’s the primary delineation between an advanced fourth generation aircraft, like Russia’s highly maneuverable Su-35 or America’s forthcoming Block III Super Hornets, and the most advanced fighters on the planet, like the F-22, F-35, or indeed, China’s copy-cat J-20. However, that’s where things get complicated. A fifth-generation fighter must also be able to perform multi-role capabilities, such as relaying battle space data, and perhaps among the most important, being able to cruise at supersonic speeds without engaging afterburners.

The J-20 immediately meets the stealth criteria, thanks in large part to the F-22 it borrows so heavily from. And yet, issues with its engine have kept the fighter’s classification as a fifth-generation fighter honorary in the minds of many. The aircraft can’t maintain supersonic speeds without the use of afterburners, thanks to its dated, Russian sourced engines, meaning it can’t couple high speeds with large operational ranges due to the afterburners’ large fuel consumption.

Now, however, Chinese officials claim that they have almost all the kinks worked out of their purpose built WS-10B engines, and plan to begin fielding truly fifth generation J-20s by the end of the year. The only problem is that they don’t sound very convincing.

China may well be nearing production on their WS-10B engines, which are said to have a thrust rating that may even exceed that of the F-22 (though there is no evidence yet to support those claims), but even if they are ready to go full steam ahead on building these engines, there’s still a matter of mating them to the J-20s air-frame and testing the new combination. That means a limited run of J-20s will need to be built for the new engine platform, followed by a series of tests to ensure each component performs as it was intended, and just as integral, proves reliable under difficult circumstances. Equipment used in combat operations must be incredibly resilient to ensure the operator’s survival, and merely “finishing” an engine does not guarantee its usefulness.  There may still need to be some tweaks and modifications done once the engine is inside a functioning aircraft.

China will likely invest quite a bit into expediting this process, especially with another stealthy fighter currently in development, the J-31 (based on America’s F-35), but for now, their timelines seem optimistic at best.