As we have reported previously, it is no secret the U.S. has made and still make use of captured or otherwise acquired Russian aircraft for test, evaluation and training purposes including the development of U.S. radars, countermeasures and early warning systems (earlier this year we published some really rare images of a Russian Su-27 Flanker dogfighting with a U.S. F-16 inside Area 51…)

https://theaviationist.com/2017/06/14/russian-video-of-captured-u-s-f-5a-tiger-jet-dogfighting-against-mig-21-in-tests-raises-question-do-they-still-operate-american-jets/