Reviving a Forgotten Concept: Can HAL's HJT-39 Combat Air Trainer Revive the IAF's Stage-III Trainer Requirement?

The Indian Air Force's (IAF) recent Request For Information (RFI) for 150 Stage-III jet trainers has sparked a renewed debate about the suitability of Hindustan Aeronautics Limited's (HAL) HJT-39 Combat Air Trainer concept for the IAF's Stage-III trainer requirement. The HJT-39 concept, initially developed in 2005, was designed as a twin-engine transonic jet trainer with secondary combat capabilities. This concept was more compact than the current HLFT-42 concept, with a maximum take-off weight estimated to be between 7-9.5 tonnes, depending on configuration.

The HJT-39 concept called for a digital fly-by-wire flight control system, a modern glass cockpit HOTAS arrangement, a HUD, and a mission computer, as well as avionics and weapon systems compatibility. A modernised HJT-39 concept would be even more intriguing, given HAL's access to advanced design tools and technologies not available a decade-and-a-half ago. Most crucially, the development of the HTFE-25 turbofan engine gives HAL an opportunity to create an indigenous powerplant for a trainer aircraft.

The HTFE-25 engine is designed to power basic trainers as well as advanced trainers, as a single engine solution for an aircraft weighing up to 9 tonnes. Notably, HAL has already produced two core engines for the HTFE-25, which are currently undergoing development testing. This suggests that a completely redesigned HJT-39-like trainer aircraft, which employs two HTFE-25 engines, can possibly be configured to deliver a combined dry thrust of 50 kN in a 9 tonne airframe.

While the larger HLFT-42 clearly has a significant edge if the IAF wants a trainer that has the flight characteristics of a front-line fighter and can carry meaningful combat payloads, there is a potential economic concern. The closer the design of the trainer is to that of a front-line combat aircraft, the more expensive it is likely to be to acquire and operate. This is a crucial consideration given that the IAF has requested around 150 aircraft, which will serve as training aircraft for many years, accumulating thousands of flight hours.

Unless there is a specific combat requirement for the trainer aircraft, it may not be economically viable to equip a 16 tonne aircraft with fighter-grade avionics, sensors, armaments, and engines. If the IAF prefers an option that has a substantial commonality with its potential front-line aircraft, the LCA-based LIFT option would be much more attractive than the HLFT-42. On the other hand, the LCA-based trainer would also carry a disproportionately high cost if it is intended only as a trainer.

The sweet spot in the requirements for a trainer may well lie somewhere in between the HLFT-42 and the LCA-based LIFT. In this context, the HJT-39 concept is worth exploring as a viable option. This way, HAL can develop a trainer aircraft that is optimised for the requirements of a Stage-III training aircraft while also retaining some combat capabilities as a secondary option. A possible configuration would see a basic training aircraft have a relatively simple avionics suite, while more sophisticated aircraft within the same family carry upgraded radar, EW suites, targeting systems, or even additional weapon stations as and when required.

It is not essential for every 150 aircraft to be a multi-role combat aircraft – it would be much more economical to have a subset of them configured as such. The original HJT-39 concept also envisaged a secondary ground-attack capability as well as multiple weapon stations. This suggests that the original CAT concept was also based on the two-role philosophy.

The main challenge in this scenario would be the time required to develop the aircraft. India cannot afford to have another 'decade trainer'. Ideally, HAL should take advantage of the capabilities afforded by the existing HTFE-25 engines, employ a mature avionics architecture, and start the development process as soon as possible.

Given the IAF's requirements for a fleet of around 150 Stage-III trainers, HAL has three options: the larger HLFT-42, the LCA-based LIFT, and a light twin-engine CAT concept based on the HJT-39 concept. The third option should be seriously considered by HAL as a viable path ahead. A roughly 9 tonne twin-engine aircraft could potentially offer a much-needed economical alternative to the other two options while also offering the potential to serve as a supplementary combat aircraft in times of conflict.

In conclusion, the HJT-39 Combat Air Trainer concept, initially developed in 2005, is worth revisiting as a viable option for the IAF's Stage-III trainer requirement. With the HTFE-25 turbofan engine in development, HAL has a solid foundation to build upon, offering a cost-effective solution for the IAF's Stage-III requirement. Reviving the HJT-39 concept could showcase HAL's capabilities in indigenous engine development, making it a viable path ahead.