Although China declared that its J-20 stealth fighter had officially entered service in September 2017 production of the J-20 appears stalled. The manufacturer had planned to build three a month initially but since mid-2017 production appears to have been zero. There are several potential problems with the J-20 but the main ones have to do with stealth (the delicate materials on the airframe that make radar detection less effective) and engines. The most obvious problem is the engines. The WS-10s currently installed are a stopgap and not efficient enough to support supercruise (go supersonic without using the afterburner and becoming easier to spit). China has had persistent problems developing high-performance jet engines. China has been developing the more powerful (and supercruise ready) WS-15 engine since the 1990s for a larger aircraft like the J-20 but has not been able to get the engine to work. Officials also confirmed rumors that a WS-15 exploded during a 2015 static (on the ground) test. That failure had been a secret but when an engine this big fails by blowing up the incident is difficult to hide.
No date was given as to when the WS-15 would be available for use or whether it would have the same vectoring (ability to move the hot jet exhaust in different directions in order to make the fighter more maneuverable) the F-22 uses. At first a more powerful and reliable version of the WS-15 for J20 was believed possible by 2020 but changes in the shape and weight of the WS-15 would require modifications to the shape of the J-20 and that would require a lot of testing to ensure that stealth was not compromised. The factory would have to install new or modified manufacturing equipment and suppliers would have to do the same to produce the new airframe components. Meanwhile the WS-15 reliability problems are still not completely resolved. Another potential J-20 problem is its ability to operate effectively in a wide variety of climates. For example development and pre-production J-20s were flown frequently in a wide variety of climates during 2017 and that may have revealed unanticipated problems requiring fixes that are still in the works.
In early 2017 Chinese officials revealed that at least a dozen J-20s had been delivered to the Chinese air force as part of a new fighter squadron. This information was apparently released to help with export sales because at the time Chinese media was reporting that Pakistan had agreed to buy J-20s. Numbers were not announced and there are still questions about how effective the J-20 actually is. Pakistani interest in the J-20 may have more to do with the fact that China is the only major-power ally they have, their main supplier of weapons and, best of all, a neighbor.
While China began offering its 18 ton J-31 stealth fighter to export customers in 2014 (as the FC-31), it was not until 2017 that efforts were made to export the more advanced J20. The manufacturer of the J-20 (CAC. Chengdu Aircraft Company) also produces the JF-17 and J-10. The JF-17 is a joint effort with Pakistan and Pakistan is the main customer.
The J-20 made its first flight in 2011, and many more since then. Before scheduled mass production could begin in late 2015 eight prototypes were built. There were at least two original J-20 prototypes, and in 2013 a new prototype appeared that had several modifications and is estimated to have a max weight of 36 tons. Since then several more prototypes have been built along with at least twenty production models. All of these were built with the understanding that some major (and time-consuming) refurbishing might be needed once the WS-15 was ready for service.
Work on the J-20 began in the late 1990s, and the Chinese went forward on the assumption that it could be 25 years or more before they had a competitive stealth fighter-bomber in service. The twin engine J-20 first appeared to be about the same weight class as the non-stealthy 30 ton F-15C but the production model was closer in weight to the F-22. By comparison the U.S. F-35A stealth fighter-bomber is a 31 ton, single engine fighter, while the twin-engine F-22 is slightly larger at 38 tons. The Russian Su-57 (T-50) weighed in at 37 tons and its production is also stalled over technical issues. The Russians can make more powerful (and reliable) engines than the Chinese but are still having problems with their other techs (stealth coatings and electronics).
While the J-20 looks like the American F-22 when viewed head on, it's overall shape, weight, and engine power is closer to the older, non-stealthy American F-15C. In other words, the J-20 is 20.4 meters (67 feet) long, with a wing span of 13.5 meters (44 feet). The J-20 has about the same wing area as the F-15C, which is about 25 percent less than the F-22 (which is a few percent larger than the F-15 in terms of length and wingspan). Worse for the J-20, is the fact that its engine power is about the same as the F-15C, while the F-22 has 65 percent more power. With the afterburner turned on, the J-20 has more power than the F-15C and nearly as much as the F-22. But because the afterburner consumes so much fuel you can't use more than a few minutes at a time. The afterburner also generates a lot more heat which makes an aircraft more visible to heat sensors. The new J-20 model appears to be able to supercruise with a powerful enough engine, joining the F-22, Eurofighter, and the Gripen as aircraft that can supercruise. The J-20 appears capable of doing lots of engine dependent things during tests or in theory that is has not been able to sustain in regular service.
The J-20 has some stealthiness when it's coming at you head on. But from any other aspect, the J-20 will light up the radar screen unless it has effective radar absorbing material on the exterior. For this reason the J-20 appeared to be a developmental aircraft, not the prototype of a new model headed for mass production. China soon made it clear that the J-20 was the basis for a new fighter and would go through as many design and shape changes needed to become combat ready. Based on recent Chinese warplane development projects (J-11 in particular) it was believed that the J-20 had a long development road ahead of it. There were some obvious changes between the first and the later prototypes, but nothing all that drastic. Thus it was surprising when the J-20 was declared ready for service in 2017 but not so surprising when production was quietly halted in 2017 because of unspecified problems. Yet the Chinese have been competent and relentless in developing complex technologies and there is no reason to believe they won’t get the J-20 working.
The J-20 is only the fifth stealth warplane to fly, the others being the American F-22 and F-35, plus the Russian Su-57. The older U.S. F-117 was actually a light bomber and the B-2 was obviously a heavy bomber. While the shape of the J-20 confers a degree of stealthiness (invisibility to radar), even more electronic invisibility comes from special materials covering the aircraft. It's not known how far along the Chinese are in creating, or stealing, these materials or the needed engines. China would most likely use the J-20 singly, or in small groups, to seek out and attack American carriers. To make this possible F-22 class engines are needed and that is still in development.
For the J-20 to be a superior fighter it would need electronics (including radars and defense systems) on a par with the F-35 and F-22 as well as powerful and reliable engines and effective radar absorbing materials. So far, the Chinese have not caught up with stuff used by current American fighters. But the gap is being closed, faster than it was during the Cold War when the Russians were creating, or stealing, their way to military tech equivalence with the West. The Russians never made it but the Chinese believe they can succeed. It may be that the J-20 is not meant to be a fighter but a stealthy strike aircraft, like the first American “stealth fighter” the F-117. This innovative aircraft was actually a stealthy light bomber that first flew in 1981 and entered service a few years later. It was very successful as a strike fighter and the Chinese may have noted that such an aircraft would also make an excellent interceptor. That would also explain Pakistani interest as they still use fighters armed with nuclear weapons and the main foe here is India, a country that has been improving its air defense systems a lot lately.
China is also developing other support technologies, like the AESA radar, highly efficient cockpit, stealth, and software to tie everything together. Developing, or even copying, this tech is not easy. But the Chinese already know that, having decades of experience adapting stolen technology to their needs. Thus, it appears that China was planning on having the J-20, in some form, ready for service by the end of the decade. The key factor is their ability to develop or steal the needed technology by then. The J-20 appears to be a fighter-bomber, as this kind of aircraft would be most useful dealing with the U.S. Navy and key targets in Taiwan or Japan. In any event, the J-20 is an attempt to develop some kind of 5th generation aircraft, complete with stealth.
The J-31 and J-20 are further evidence that China is determined to develop its own high tech military gear. While China is eager to develop advanced military technology locally, it recognizes that this takes time and more effort than nations new to this expect. Thus, China is trying to avoid the mistakes Russia made in this area. That means having competing designs and developing necessary supporting industries as part of that. All this takes a lot of time and involves lots of little (and some major) failures. The Chinese are doing it right and are willing to wait until they get military tech that is truly world class.