"It's so embarrassing to change engines for such an important aircraft project several times," a military source told the South China Morning Post of the J-20.
Chinese state media announced on Friday that the Chengdu J-20 stealth jet had officially entered into service as a combat-ready platform — but inside sources say it's a long way from fighting fit and has an embarrassing flaw.
Citing military sources with knowledge of the J-20's development, the South China Morning Post reported that the jets that entered service didn't feature the engine China custom-built for the platform but used an older one instead.
The result is an underpowered, less stealthy jet that can't cruise at supersonic speeds and is therefore not a true fifth-generation fighter.
The Posts' sources pinned the jet's troubles on a test in 2015 in which the custom-built engine, the WS-15, exploded — something they attributed to China's inability to consistently build engines that can handle the extreme heat of jet propulsion.
"It's so embarrassing to change engines for such an important aircraft project several times … just because of the unreliability of the current WS-15 engines," one of the sources told the Post. "It is the long-standing core problem among home-grown aircraft."
The older engine, the WS-10B, is basically the same kind used in the J-11 and J-10 fighters in 1998 and 2002.
Without the new engine, the J-20 can't supercruise, or fly faster than the speed of sound without igniting its afterburners, like the US's F-22 and F-35 can.
"Afterburners do make any fighter much easier to detect, track, and target using Infrared and Electro-Optical systems at closer ranges when in use," Justin Bronk, a combat-aviation expert at the Royal United Services Institute, told Business Insider.
Experts have assessed that the goal of the J-20 platform is to launch long-range missiles at supersonic speeds, but they won't perform as well if they can't fire at such speeds, Bronk said.
"The major drawback from not having the ability to supercruise in this case would be having to choose between using a great deal of fuel to go supersonic or stay subsonic and accept shorter effective range from the fighter's missiles and an inferior energy position compared to a supercruising opponent," he said.
A senior scientist working on stealth aircraft who requested anonymity because of the sensitive nature of their work previously told Business Insider that the J-20's design had a decent stealth profile from the front angle but could be exposed from others.
According to Bronk, the older engine may exacerbate that problem.
A US Air Force affiliate researching the Chinese People's Liberation Army Air Force told Business Insider that an analysis of imagery suggested the service's 9th Brigade traded its Russian-made Su-30s for J-20s, but they disputed whether the jet was operational in the way Western militaries use the word.
"The aircraft and its pilots and maintenance group need to master the type before it can be sent on a 'real' mission, not a training mission," said the researcher, who spoke on condition of anonymity because their employer's report has not been released.
The researcher said that even for planes that aren't stealth and as radically different as the J-20, that could take up to a year, adding that the new WS-15 engines most likely won't be added until 2020.
So while China claims it has become the only nation other than the US to field a fifth-generation stealth jet, at the moment it looks as if it's hardly stealth, hardly fifth-generation, and a long way from the field.