Tejas Light Combat Aircraft (LCA) - Design

Tejas has around 50% locally developed content, while the engines are American, radar, helmet display and laser pod of Israeli make and Russia has supplied the missiles. The LCA design has been configured to match the demands of modern combat scenario such as speed, acceleration, maneuverability and agility. Short takeoff and landing, excellent flight performance, safety, reliability and maintainability, are salient features of LCA design. The LCA integrates modern design concepts like static instability, digital fly-by-wire flight control system, integrated avionics, glass cockpit, primary composite structure, multi-mode radar, microprocessor based utility and brake management systems.

The avionics system enhances the role of Light Combat Aircraft as an effective weapon platform. The glass cockpit and hands on throttle and stick (HOTAS) controls reduce pilot workload. Accurate navigation and weapon aiming information on the head up display helps the pilot achieve his mission effectively. The multifunction displays provide information on engine, hydraulics, electrical, flight control and environmental control system on a need-to-know basis along with basic flight and tactical information. Dual redundant display processors (DP) generate computer-generated imagery on these displays. The pilot interacts with the complex avionics systems through a simple multifunction keyboard, and function and sensor selection panels.

A state-of-the-art multi-mode radar (MMR), laser designator pod (LDP), forward looking infra-red (FLIR) and other opto-electronic sensors provide accurate target information to enhance kill probabilities. A ring laser gyro (RLG)-based inertial navigation system (INS), provides accurate navigation guidance to the pilot. An advanced electronic warfare (EW) suite enhances the aircraft survivability during deep penetration and combat. Secure and jam-resistant communication systems, such as IFF, VHF/UHF and air-to-air/air-to-ground data link are provided as a part of the avionics suite. All these systems are integrated on three 1553B buses by a centralised 32-bit mission computer (MC) with high throughput which performs weapon computations and flight management, and reconfiguration/redundancy management. Reversionary mission functions are provided by a control and coding unit (CCU). Most of these subsystems have been developed indigenously.

The digital FBW system of the LCA is built around a quadruplex redundant architecture to give it a fail op-fail op-fail safe capability. It employs a powerful digital flight control computer (DFCC) comprising four computing channels, each powered by an independent power supply and all housed in a single line replaceable unit (LRU). The system is designed to meet a probability of loss of control of better than 1x10-7 per flight hour. The DFCC channels are built around 32-bit microprocessors and use a safe subset of Ada language for the implementation of software. The DFCC receives signals from quad rate, acceleration sensors, pilot control stick, rudder pedal, triplex air data system, dual air flow angle sensors, etc. The DFCC channels excite and control the elevon, rudder and leading edge slat hydraulic actuators. The computer interfaces with pilot display elements like multifunction displays through MIL-STD-1553B avionics bus and RS 422 serial link.

The digital FBW system of the LCA is built around a quadruplex redundant architecture to give it a fail op-fail op-fail safe capability. It employs a powerful digital flight control computer (DFCC) comprising four computing channels, each powered by an independent power supply and all housed in a single line replaceable unit (LRU). The system is designed to meet a probability of loss of control of better than 1x10⁷ per flight hour. The DFCC channels are built around 32-bit microprocessors and use a safe subset of Ada language for the implementation of software. The DFCC receives signals from quad rate, acceleration sensors, pilot control stick, rudder pedal, triplex air data system, dual air flow angle sensors, etc. The DFCC channels excite and control the elevon, rudder and leading edge slat hydraulic actuators. The computer interfaces with pilot display elements like multifunction displays through MIL-STD-1553B avionics bus and RS 422 serial link.

Multi-mode radar (MMR), the primary mission sensor of the LCA in its air defence role, will be a key determinant of the operational effectiveness of the fighter. This is an X-band, pulse Doppler radar with air-to-air, air-to-ground and air-to-sea modes. Its track-while-scan capability caters to radar functions under multiple target environment. The antenna is a light weight (< 5 kg), low profile slotted waveguide array with a multilayer feed network for broad band operation. The salient technical features are: two plane monopulse signals, low side lobe levels and integrated IFF, and GUARD and BITE channels. The heart of MMR is the signal processor, which is built around VLSI-ASICs and i960 processors to meet the functional needs of MMR in different modes of its operation. Its role is to process the radar receiver output, detect and locate targets, create ground map, and provide contour map when selected. Post-detection processor resolves range and Doppler ambiguities and forms plots for subsequent data processor. The special feature of signal processor is its real-time configurability to adapt to requirements depending on selected mode of operation.

Seven weapon stations [plus a centerline hardpoint] provided on LCA offer flexibility in the choice of weapons LCA can carry in various mission roles. Provision of drop tanks and inflight refueling probe ensure extended range and flight endurance of demanding missions. Provisions for the growth of hardware and software in the avionics and flight control system, available in LCA, ensure to maintain its effectiveness and advantages as a frontline fighter throughout its service life. For maintenance the aircraft has more than five hundred Line Replaceable Units (LRSs), each tested for performance and capability to meet the severe operational conditions to be encountered.

In May 2020 Indranil Roy sought to Remove Some Media Fallacies About The Capabilities of The LCA Tejas Fighter. Some critics charge that the Tejas combat “endurance” is barely an hour compared to 3 hours for the JAS 39 Gripen and almost 4 hours for the US-made F-16. But Roy crunched the numbers, and concluded "it is easy to see why the endurance of the Tejas Mk1 and Mk1A is above 3 hours, pretty much like that of the Gripen C/D and the F-16, with whom it is being compared."

The second falacy Roy addresses is the charge that "the Tejas can carry half the weapons payload — of 3 tonnes — vis-a-vis the Gripen (6 tonnes) or the F-16 (7 tonnes), means twice the number of Tejas aircraft will need to be deployed in combat". Roy counters that the LCA Tejas LSP with an all up weight of about 14 tons has a payload of about 3.5 tons, and has been flight tested with even higher payloads of 4.0 tonnes. The clean and maximum take-off weights of the Gripen C/D are 10 t and 14 t respectively, so the payload would be 4 t (i.e. similar to that of the Tejas). The Gripen C/D can take off with 5.3 t of payload, only if it significantly sacrifices on the carriage of internal fuel, and thus endurance.

Another facly is the charge that "The lifespan of the Tejas is 20 years against the 40 years of the Gripen and the F-16." Roy notes that the lifespan of aircraft is not measured in years, but flight hours. An initial conservative estimate of 9000 flying hours was established for Tejas, which is probably going to be revised upwards. "When the F-16 first flew, its service life was estimated to be 6,000 hours. It was revised upwards to 8,000 hours subsequently and this has only recently been further pushed up to about 12,000 hours. One can expect the Tejas to also see similar upward revisions in its estimated service life over the years."

The fourth fallacy is the contention that "The service time (hours per 1 hour of flying) is 20 hours for the Tejas, 6 hours for the F-16 and 3.5 hours for the Gripen." Buy Roy nots that operations to date signified "a flying tempo which is at par or above any of the IAF’s in-service aircraft types".