My confusion with the WS-19 not being offered for export is that this engine is primarily for the J35, and the J35 was made with export in mind.
The Chinese airforce is now taking in large quantities of WS19 so they need sufficient engines and will take priority - that makes sense. If a foreign customer wants the J35 in the meantime, then they need to take the WS21 for now - that makes sense too. But longer term once China meets its WS19 needs, I dont understand why the engine would be blocked to sell, especially to a close ally like Pakistan.
Also I thought the WS21 has a limited production line and its been an interim engine until the WS19 is ready. So it seems strange even if its now more mature but less powerful than WS19.
If you look at the US its shared its latest and sensitive engine on the F35 with its allies.
If you are fully aware that the demands for military aviation engine are the most stringent and serious from any fighter jets operators, you will understand if PAF wants to field J35AE as fast as possible, WS19 unlikely will be the best choice.
Currently there’s no too much publicly available information on WS19, but the most optimistic estimate is WS19 in series production starting probably early this year. Do note like any brand new fighter jet engine, it needs time including periods of field operations to mature. At initial production stage, engine makers need to monitor closely that the engine produced from actual production lines matches the prototype engine certified upon completion of development stages. As such, initial batches of WS19 engines from production lines will be used by PLAAF so that the engine makers can work closely with PLAAF to rectify potential problems of a new engine design under real field and actual production conditions. WS10 was design certified as early as in 2005, but once in real production, WS10 engines experienced many issues which only until 2010s had been solved. Given the current expertise of China in manufacturing fighter jet engine, WS19 unlikely will experience the same issues of WS10 in production, but it still need undergo production stage to ramp up production volume and prove ultimate reliability.
People generally overlook the reality of fighter engine development, in another thread that I participated in the debate, people against my view are so optimistic that they believe their nation without any prior high thrust low bypass turbofan engine development experience, can complete the circle of such engine development in 6 years, from core engine paper design finalised in 2026, to fully power their 5th gen heavy fighter in 2032.
But PAF surely is experienced in this area so I won’t be surprised if they opt to WS21, likely a more field proven engine for quicker deployment of a true 5th G fighter.