Simply because it takes time maturing new engines. Testing alone can take 1-2 years. WS13 does not make sense simply because PAF has built up rebuild and overhaul facilities.The most important is why not kilomov already developing and maturing RD93MA series engine when its probable parameters already out in news and on PDF threads 10 years ago. This should be a confirm news at least at least in 2018 and thus as a matter of good planning, Block-III first flight should be with RD93MA Why only on this July-20 Kilomov move forwards? after first Block-III experimented in Dec-19. Why?