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Chandigarh: The outcome of India’s Atmanirbhar or indigenous military aviation ambitions is determined less by design maturity and technological competence, but largely by a persistent dynamic rarely articulated openly, yet operationally critical: delivery schedules of imported engines around which all these aerial platforms are structured.

Nowhere is this constraint more evident than in the twin delivery bottlenecks affecting two US-origin engines powering Hindustan Aeronautics Limited’s Tejas Mk1/Mk1A fighters and the Hindustan Turbo Trainer-40 (HTT-40) – platforms central to both frontline combat readiness and foundational (Stage I) pilot training. Persistent delays in the supply of General

Electric’s F404-IN20 afterburning turbofan for the Tejas and Honeywell’s TPE331-12B turboprop for the HTT-40 expose a critical vulnerability that India’s defence industrial base has yet to resolve.

For decades, this propulsion constraint has been structurally embedded in India’s defence aerospace ecosystem, where platform development and integration are largely domestic, but engine supplies remain externally controlled. This, in turn, has effectively translated into foreign influence over the Indian Air Force’s (IAF’s) operational timelines, shaping both platform induction rates and fleet availability.

Against this backdrop of overseas reliance, a clear divergence in public, media, and official attention has emerged in recent months between Hindustan Aeronautics Limited’s Tejas fighter and HTT-40 trainer programmes, particularly due to delayed engine deliveries from the two US suppliers.

In the case of Tejas Mk1A, deferred deliveries of F404 engines have attracted sustained scrutiny across mainstream and digital media, as well as in institutional discourse, due to their adverse downstream impact on the IAF’s combat squadron strength, which has fallen from a sanctioned 42 to around 29. These delays have also had serious knock-on consequences for HAL’s Nashik facility, where Mk1A fighters are being series-produced.

In response, HAL and GE have recently proposed a dedicated depot-level support arrangement with the IAF for the F404 ecosystem. However, despite official optimism, these measures remain under negotiation and have yet to translate into tangible operational relief.

By comparison, a clear asymmetry in attention persists between fighter and trainer programmes. Fighter aircraft, by their very nature, command visibility, urgency and sustained scrutiny; basic trainers do not. Lacking the appeal of frontline platforms, they are often treated as less urgent, despite forming the foundation on which future generations of pilots – and, by extension, combat capability – are built. Reflecting this imbalance, the HTT-40 programme remains largely under-scrutinised in official and media circles, even though it is subject to propulsion constraints similar to those affecting the Tejas Mk1A, with its contractual framework and ensuing delays underscoring this divergence.

The HTT-40 programme’s constrained progress is reflected in the limited activity surrounding its initial series-production aircraft. The first two such platforms – TH-4001/02-undertook maiden flights in Bengaluru and Nashik in late 2024, marking a nominal transition from prototype to production configuration. But these sorties were conducted using refurbished Category-B engines drawn from earlier test inventories rather than newly supplied power plants. Such a workaround further underscored the continuing disconnect between airframe readiness and propulsion availability, with HAL effectively forced to recycle existing engine assets to sustain programme momentum.

Moreover, in 2023, the IAF had signed a Rs 6,600 crore contract with HAL for 70 HTT-40s, in which 12 aircraft were scheduled for delivery by March 2026, followed by an annual production rate of 20 units thereafter, with manufacturing split between HAL’s Nashik and Bengaluru facilities till the order was completed.

Timelines not materialising

But these timelines have not materialised, despite HAL’s July 2022 contract with Honeywell – valued at around US$100 million – for the supply of 88 TPE331-12B turboprop engines, equipped with  Electronic Engine Controller (EEC) for the HTT-40. Of these, 32 engines were to have been imported, and the remaining 56 licence-produced by HAL under a transfer of technology arrangement.

HAL sources in Bengaluru said engine deliveries had slipped well beyond their November 2025 deadline, reportedly due to Honeywell’s global order book prioritisation and competing higher-volume commercial and defence commitments. As a result, no HTT-40 trainers have yet entered Indian Air Force service, with induction timelines slipping beyond the original 2024-25 schedule and overall completion – once envisaged around 2030–now expected to drift into the next decade.

HAL officials were unavailable for comment on these slippages.

“The HTT-40  may be designed, assembled, and integrated domestically, but its operational scaling remains externally dependent on its power pack” said a two-star veteran IAF fighter pilot. In effect, this indicates that Honeywell controls the tempo at which trainee IAF pilots can transition from basic flying to advanced stages, because of their engine unavailability,  he added, requesting anonymity.

A cross-section of senior veterans concurred, noting that a basic trainer like the HTT-40 should be the most stable and predictable platform in any air force’s inventory. Yet, they said, in the IAF, even this foundational layer remains vulnerable to external supply variability. As a result, they warned, delays in engine deliveries do not merely affect fighter  availability; they also disrupt the training pipeline itself, slowing the progression of successive batches of pilots to operational platforms

That said, the HTT-40 programme has been marked by a prolonged and often contentious evolution.

Conceived as a replacement for the HAL-designed HPT-32 Deepak, which was grounded in 2009 following a series of fatal accidents, the HTT-40 initially faced strong resistance from the IAF. In  2012, the IAF rejected it, on the grounds that it was costlier than the 75 Swiss Pilatus PC-7 Mk II trainers it had acquired that year under an over $1 billion import deal, triggering a protracted disagreement with HAL over the indigenous aircraft’s viability and development trajectory.

Discord escalated in 2014, after the IAF declined to fund the programme, leaving HAL to finance the HTT-40s development independently. However, following the prototype’s maiden flight in early 2016 – and a second prototype a year later – IAF interest revived, further reinforced by the government’s Aatmanirbharta initiative, aimed at reducing materiel dependence on imports.

Even then, frictions persisted.

In mid-2019, HAL sought an IAF tender or request for proposal (RfP) to fast-track HTT-40s series production and unlock internal funding–reportedly around Rs 200 crore – for engine-related upgrades. Specifically, HAL maintained that this funding was aimed at replacing the EEC-enabled TPE331-12B engine with an upgraded Full Authority Digital Engine Control (FADEC)  to enhance performance ahead of approval for HTT-40’s series production.

The impasse that followed was driven in part by the IAF’s complex and evolving technical requirements, particularly relating to the HTT-40’s stall-and-spin performance envelope. HAL maintained that these demands had been addressed, or were in the process of being resolved incrementally through successive trials. This stall-and-spin training aspect is critical for fighter pilots, who must be familiarised with departures from controlled flight and precise recovery actions required in such situations.

HAL is also believed to have faulted the Indian Air Force for continually shifting its performance benchmarks, maintaining that the HTT-40 had already exceeded its initial staff qualitative requirements during flight testing. These included features like zero-zero ejection seats and multi-function displays, along with the capability for instrument, night and aerobatic training.

HAL further claimed that the trainer had a 450 km operational range, a 6 km service ceiling, load factors of +6G/−2G, a climb rate of 6 m/sec and a glide ratio of 12:1. Besides,  over 50% of the trainer – excluding the engine and ejection seats –had been indigenously sourced and that it could be armed with light munitions for employment in limited counter-terrorism missions.

Eventually, in early 2021, the IAF issued an RfP for 75 HTT-40s, including an option for 38 more. This was later rationalised during commercial negotiations, culminating in a Rs 6,800 crore contract in 2023 for 70 HTT-40s, with the additional option of 38 trainer platforms retained. Yet even this transition from requirement to contract has not translated into urgency on the ground, shaped by the realities of delayed deliveries of its imported engines.

Fundamentally, the HTT-40’s slow progress underscores a starker reality: India’s persistent inability to master aero-engine technology. Despite decades of futile effort, vast investment, and repeated programmes, it remains heavily dependent on foreign propulsion systems for even its most basic aviation requirements. And, until that gap is bridged, such delays will remain structural rather than episodic – where aircraft can be designed and built domestically, but not powered on sovereign terms.

In that sense, the HTT-40 is more than just a delayed trainer: it is a stark reminder that indigenously designed military aircraft will continue to fly with a critical foreign dependency embedded in their core. In practical operational terms, that is not merely inefficiency, but a strategic liability.

© The Wire