929th GLITs received its first T-50 at Akhtubinsk for further testing and GSI state trials. However, severe issues were discovered during PI-1; in addition to the structural issues, the aircraft suffered from engine problems, including the AL-41F1 compressor stall during the MAKS-2011 airshow. In June 2014, the fifth prototype was severely damaged by an in-flight fire and written off, and an incomplete "first stage" airframe was finished using parts salvaged from the fifth prototype. Both preliminary and state trials were delayed by the fire and the structural redesign; the second phase of preliminary trials, PI-2, ran from 2014 to 2019 and mainly used structurally reinforced "first stage" aircraft while GSI-1 was halted until 2016 in order to wait for the updated "second stage" airframes. Armament trials were also delayed, with external weapon trials starting in May 2014 and internal trials starting in March 2016. GSI-1 was finished on 8 February 2018 with formal signing in May 2018. After more than 3,500 flights, GSI-2 was planned to be completed by 2019, but this was pushed to 2020, partly because of the crash of the first production aircraft in December 2019. As the Su-57 is gradually being inducted into Russian military service, Sukhoi is testing upgrades for the improved Su-57M variant. First flight of the izdeliye 30 engine occurred on 5 December 2017 with the second prototype (T-50-2, bort no. 052). Prototypes were also used for unmanned aerial vehicle (UAV) teaming tests with the Okhotnik UCAV, with a video of flight tests released by the Ministry of Defence on 27 September 2019. On 28 June 2020, TASS, with reference to anonymous sources within the military-industrial complex, reported that a 'swarm' teaming experiment had been conducted with a group of Su-35s and an Su-57 acting as a command and control aircraft. The networked information exchange significantly increases the efficiency of combat missions. Reportedly, the experiment was conducted in "real combat conditions." Syrian combat evaluation On 21 February 2018, two Su-57s performed their first international flight as they were spotted landing at the Russian Khmeimim air base in Syria. The aircraft were deployed along with four Sukhoi Su-35 fighters, four Sukhoi Su-25s, and one Beriev A-50 AEW&C aircraft. Three days later two more Su-57s were reported to have arrived in Syria. The deployment was criticised by some experts as overly risky, especially after reports of drone attacks at Khmeimim air base, as well as