 Throughout the Su @-@ 35 's flight test programme , active controls during manoeuvres such as the Pugachev 's Cobra and tailslide could not be maintained . The eleventh Su @-@ 27M ( T10M @-@ 11 ) was built by KnAAPO and delivered in 1995 for the installation of exclusive systems to give it thrust @-@ vectoring capabilities . The resultant Su @-@ 37 technology demonstrator made its first flight in April 1996 . A second Su @-@ 35 was modified into an Su @-@ 37 in the late 1990s . In 2001 , AL @-@ 31F engines with fixed nozzles , an upgraded fly @-@ by @-@ wire controls , and improved cockpit systems were fitted to a Su @-@ 37 for testing . 
