the only Tu204 in China has been modified to a inflight testbed for the new radar and avionics of J20 project.
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BTW ,there is quite some such testbeds in china's test fight units...
1.the early IL14PB testbed in 1980-90s...most of 147X serial PD radars were mounted on this platform for test in 1990s. this plane has retired..
2.the famous testbed plane--Y8( #079 ) ...the J10B's AESA and J11B's PD Radars were tested on this platform...
3.one of the J11A Prototypes was chosen as the J16 attacter's AESA radar testbed.
4.the Y7 (#713) testbed for the newest H6 bomber varient-H6K's radar(bombing radar).
5.another Y8 platform--#728 for some Special Purpose Airplanes' avionics and radar tests