Stall and spin behaviour is not a niche concern in a trainer; it is the heart of the job. Students are taught to stall, to spin and to recover, deliberately and repeatedly, so that they will not be killed by an inadvertent departure later in their careers. An aircraft that drops a wing unpredictably, or that resists recovery, cannot be cleared for that syllabus however good its other qualities.

By February 2014 the HJT-36 had flown more than 800 test sorties, yet the Indian Ministry of Defence still reported that its stall characteristics needed refinement before final clearance. Six limited-series aircraft had been built but not delivered. Reports in 2017 described the aircraft as unfit for service and its spin problems as hard to solve. HAL tried nose strakes, ventral fins and wing fences without curing the wing drop.

The solution, verified in wind tunnels including France’s ONERA facility, was more radical: the vertical tail was moved back by about a metre, the rudder was extended down to the rear fuselage, increasing its area by about 30 per cent, and leading-edge strakes were added at the wing roots. An anti-spin parachute was fitted for the trials in 2020. In January 2022 the modified aircraft completed six-turn spins in both directions, the milestone the programme had been chasing for well over a decade, and the way was open for the relaunch as the Yashas.

The original HJT-36 avionics suite was developed with Smiths Aerospace, later part of GE Aviation, and included a head-up display, mission computer and attitude and heading reference system, with the head-up display and repeater supplied by Elbit. The prototypes used Russian Zvezda K-26LT zero-zero ejection seats. HAL designed the aircraft with light alloys and composites, a tricycle undercarriage with a twin nosewheel, and an 18-degree swept wing, and it has said that about a quarter of its line-replaceable units are common with the Tejas trainer.

For the Yashas relaunch in 2025, HAL described a weight-reduction effort, a new glass cockpit with voice command, and the replacement of obsolete imported units with Indian equipment. The FADEC-controlled AL-55I gives the pilot carefree handling of the engine, an important point for students learning to manage power at jet speeds for the first time. HAL also used the design as the basis for its HLFT-42 lead-in fighter trainer project.

• PT-1: first prototype, first flight 7 March 2003 with the Larzac; first to fly with the AL-55I in May 2009.
• PT-2: second prototype, first flight March 2004; landing gear failure in February 2009.
• Prototype lost 28 April 2011: crashed near Hosur, Tamil Nadu; Group Captain Anant Mathur and Wing Commander Patra ejected safely.
• LSP aircraft: limited-series production airframes, six built by 2014 but not delivered.
• LSP-4 (S3854): the modified spin-test airframe, first flight with the new tail on 17 April 2019.
• HJT-36 Yashas: the relaunched production standard presented at Aero India 2025.

No operating cost has been published for the HJT-36, which has never entered squadron service. Single-engine jet trainers of its size are commonly estimated at roughly US$3,000 to 6,000 per flight hour, depending on whether fuel alone or full maintenance and support is counted; that is several times the cost of a turboprop basic trainer, and one reason air forces now push more of the syllabus onto propeller aircraft and simulators.

The heavier cost has been time. The project was sanctioned in 1999 at Rs 180 crore; by 2013 research and development spending was reported at about Rs 600 crore, and total commitments, including production advances, at more than Rs 4,000 crore. Early AL-55I engines needed overhaul after about 150 hours, far short of what the air force wanted, a gap later improvements were meant to close.