No. 45 Squadron IAF based at Sulur Air Force Station, Coimbatore was the first to have their MiG-21s replaced by Tejas aircraft at the base. The FOC campaign began in December 2014. Two critical parameters set by IAF for FOC clearance was expansion of angle of attack from 24 degree in IOC-II to 28 degree in FOC and inflight refueling capability. In February 2018, as part of the FOC campaign, the Tejas carried out a "hot refuelling" - refuelling with engine running, which shortens the turnaround time by 30% and doubles the sortie rate. In September 2018, the Tejas successfully completed its mid-air refuelling trials required for the aircraft to obtain its FOC. In January 2019, HAL received permission from CEMILAC to start production of the FOC standard Tejas. On 20 February 2019, during the Aero India 2019 show, FOC was formally awarded to the Tejas. The second Tejas squadron – No.18 Flying Bullets was formed at Sulur Air Force Station on 27 May 2020 with the first four serial production FOC aircraft. A full mission simulator (FMS) phase-1 was commissioned at Sulur Air Force Station on 23 October 2021. The Phase 1 of the FMS features training in aircraft handling and full envelope flying. Phase 2 will further augment the training with focus on weapons system and advance sensors onboard Tejas. Upgrades and further development In May 2015, the Comptroller and Auditor General of India (CAG) noted some shortcomings in the then-delivered Tejas Mark 1 IOC standard aircraft, which according to the CAG would limit the survivability and operational deployability of the aircraft in actual combat. A few of these shortcomings, including lack of combat endurance, were addressed in the Tejas Mark 1 FOC configuration aircraft. Tejas Mark 1 FOC resolved the issue of onboard fuel monitoring with an integrated Environmental Control and Fuel Management (ECFM) system. It can now perform aerial refueling from Ilyushin Il-78 and buddy refueling with Sukhoi Su-30MKI. Tejas is also undergoing butt firing trials and air-to-air firing at HAL, Nashik facility with Gryazev-Shipunov GSh-23. The shortcomings, such as the increased weight and reduced speed would be addressed in the upcoming Tejas Mark 1A aircraft by increasing the use of composites in manufacturing and reducing the supersonic drag by using more aerodynamic pylons. The MK1A will also have AESA radar, a self-protection jammer, updated avionics and electronic warfare capabilities, among other improvements. The shortcomings identified