China’s long-touted Chengdu J-20, claimed to be the nation’s first fully functional fifth generation fighter platform, is neither fully functional nor is it technically a fifth-generation fighter — despite already being entered into service in China’s People’s Liberation Army.

Fighter generations are not determined by a formal committee; generational labels are more like industry shorthand to help quickly assess the type of capabilities a platform possesses. The line between fourth and fifth generation fighters, as a result, tends to move depending on who you ask. For the most part, however, what separates a fifth generation platform from its predecessors can be boiled down to three things: an airframe designed from the ground up to maximize stealth capabilities, the ability to serve as a data hub the melds information from multiple sources, and “super-cruise” capabilities — which means maintaining supersonic speeds without having to engage the jet’s afterburner.

It’s in that third category that the J-20 continues to fall short. While China did develop the aircraft using stolen Lockheed Martin plans for the F-22, their method of developing new technologies through theft often comes back to haunt them. In the case of the J-20, it is widely believed that, despite having the plans for the aircraft, China lacks the ability to manufacture jet engines that are comparable to America’s or to produce the stealth absorbent coating seen as integral to the F-22’s small radar signature.

Questions remain regarding China’s capacity to produce and utilize that classified stealth coating material — but last week’s massive Chinese air show made it clear that they continue to struggle with the engine issue. The existing J-20s in service all utilize older Russian sourced powerplants that require the continued use of afterburners to maintain supersonic speed. This significantly limits the operational range of the fighter if it intends to close with a threat quickly because of the afterburner’s massive fuel requirements. As a result, China’s current J-20s can either operate for a normal amount of time or go supersonic. They can’t do both.

China’s home-built WS-15 engine was supposed to fix all that… but J-20s with operational WS-15 engines were nowhere to be found in China’s massive display of aviation technology this week. Now, the reason why has reached the media.

“The performance of the engine is still very unstable, and engineers have failed to find the key reason for the problems, even though its vector power is good enough now,” an unnamed Chinese official told the South China Morning Post when asked why the WS-15 had yet to make an appearance. “Now it’s very embarrassing because China now may need to ask for help from the Russians.”

Russia’s own fifth-generation fighter, the Su-57, also holds a “questionable” status within its generational category, but Russia recently announced no plans to put the jet into full-scale production.