The T-10 was spotted by Western observers and assigned the NATO reporting name 'Flanker-A'. The development of the T-10 was marked by considerable problems, leading to a fatal crash of the second prototype, the T-10-2 on 7 July 1978, due to shortcomings in the FBW control system. Extensive redesigns followed (T-10-3 through T-10-15) and a revised version of the T-10-7, now designated the "T-10S", made its first flight on 20 April 1981. It also crashed due to control problems and was replaced by T-10-12 which became "T-10S-2". This one also crashed on 23 December 1981 during a high-speed test, killing the pilot. Eventually the T-10-15 demonstrator, "T-10S-3", evolved into the definitive Su-27 configuration.