Light Combat Aircraft-Tejas Testing

Written by Air Marshal P. Rajkumar, PVSM, AVSM, VM (Retd.) in 2005 for Aerospace and Marine International.

Since completion of the technology demonstration phase on March 31, 2004 the LCA program has commenced the Full Scale Engineering Development (FSED) phase in right earnest. The aim of the program now is to achieve Initial Operational Clearance (IOC) with the Multi Mode Radar (MMR) integrated with a weapons suite which will give the aircraft limited operational capability by the end of 2007 i.e. in about three years time. This article will give the reader an insight into the current status of the program while also tracing the evolution of the two Technology Demonstrator aircraft TD-1 and TD-2, and the two Prototype Vehicles PV-1 and PV-2.

The first aircraft to be built, TD-1, suffered from all the ills that could beset any aircraft attempting to make a technology leap spanning two decades. It must be remembered that the aircraft industry in Bangalore had not attempted to design and develop a state of the art fighter since the Marut program in the early 60’s of the last century. Almost the entire workforce had their first exposure to new technologies like the fly by wire system, the glass cockpit and the composite structure while manufacturing this aircraft. There was a learning curve involved because most of the workers had to learn on the job. Numerous mistakes were made and the fuselage wing integration had to be done more than once to get things right. It was therefore not very surprising that the aircraft tipped the scales at 6,780 kg with Flight Test Instrumentation (FTI) against a targeted weight of around 6,300kg. Program managers very wisely decided to launch a weight reduction exercise.

TD-1 has the first generation glass cockpit configuration based on an Intel 80386 processor based mission computer and a dedicated display processor to drive the two Active Matrix Liquid Crystal Multi Function Displays (MFD’s) and an imported Sextant Head Up Display (HUD). Redundancy has been provided with a Control and Coding Unit (CCU) and a second display processor. Bharat Electronics designed and developed Multi Function Keyboard (MFK), a Get You Home (GUH) panel which provides the pilot with essential flight information in case of an emergency, a Multi Function Rotary (MFR) switch which enables the pilot to select radio frequencies, set altimeter settings on the HUD, select IFF frequencies, time display etc, a digital fuel and rpm strip gauge, a Function Selection Panel (FSP), a Sensor Selection Panel (SSP) and a BAE Systems SCR-300 Crash Data Recorder (CDR) make up the major part of the avionics suite. Two units developed by the Electronics and Radar Development Establishment (LRDE), the Mission Preparation and Retrieval Unit (MPRU) and the Centralised Warning Panel (CWP) complete the avionics suite.
Communication is provided by a HAL Hyderabad developed INCOM V/UHF R/T set and a standby UHF set.

Four LRUs, the Environmental Control System Controller (ECSC) electronic unit (EU), Digital Fuel Monitoring System (DFM) EU, Engine and Electrical Monitoring System (EEMS) EU and the Digital Hydraulic (DH) EU which also has a brake management computer perform the utilities system monitoring function.

For the first block of flights it was decided to fly the aircraft with a fixed gain control law for the fly by wire system. This meant the control column to control surface deflection law had a fixed linear ratio .The leading edge slats and air brakes were non functional and the aircraft wing tanks were partially refuelled giving a total of 1,800 kg of fuel. Partial fuel in the wing tanks and approximately 140 kg of ballast weight in the nose kept the CG in the mid range. This was done to give the fixed gain control law an adequate margin of safety while stabilising the unstable aerodynamic configuration. The flight envelope was restricted to Mach 0.7,610 kmph Calibrated Air Speed (CAS), 8km altitude and normal acceleration +2g.The GUH was removed and round dial pressure instruments and an angle of attack indicator were fitted to give the pilot unprocessed air data to act as a cross check for the processed information put out by the mission computer. A calibrated chase Mirage 2000 aircraft provided the pilot with a completely independent check of air data and gave him the option of a shepherded landing in case of air data problems .The Mirage 2000 chase aircraft was used for all the 12 flights of the first block.

The almost trouble free completion of the first block flights flown by Wg Cdrs Rajiv Kothiyal and Raghunathan Nambiar between January 4, 2001 and June 2, 2001 did much to boost the confidence of both the designers and the flight test team.

The aircraft was extensively reworked after this phase to make the leading edge slats and airbrakes operational. Some fuel system modifications were also carried out to increase the amount of usable fuel to as high a figure as possible. The full scheduled gain control law wherein the control column to control surface deflection is made dependent on the flight condition of the aircraft was invoked and the aircraft flew again with Gp Capt Rakesh Bhaduria at the controls on February 3, 2003 just in time to be put on static display at Aero India 2003. HAL’s preoccupation with the Intermediate Jet Trainer programme had much to do with this protracted grounding of TD-1.A golden opportunity to fast track the program was thus lost forever.

Once scheduled gains were invoked for the flight control system, envelope expansion was commenced .The flutter envelope was cautiously explored and Wg Cdr Vikram Singh went supersonic in TD-1 for the first time on August 1, 2001.The aircraft has flown 120 flights to date.

The air intake duct was redesigned for this aircraft to make it easy to manufacture. Some weight reduction was also attempted which resulted in a weight saving of 110 kg. The airframe weighed 6,670 kg when manufactured.

The other significant change in the aircraft was the installation of the Central Scientific Instruments Organisation (CSIO) Chandigarh designed and developed HUD with 25 x 20 degrees field of view (FOV). The display processor was developed by the Aeronautical Development Establishment (ADE). The HUD is Night Vision Goggles (NVG) compatible. All the round dialled instruments were removed and the GUH was brought into operation. The aircraft originally scheduled to fly by the end of 2001 finally flew on June 2, 2002 with Wg Cdr Tarun Banerjee at the controls. The aircraft flew 61 flights with the fixed gain control law before it was grounded to make the slats and airbrakes operational. The aircraft flew with scheduled gains for the flight control system in October 2003 and has flown 150 flights to date.

Major weight reduction was attempted during the manufacture of this aircraft’s airframe. Carbon fibre composites were extensively used in the fuselage taking the overall composite content to 45 per cent by weight and 95 per cent by surface area. The part count, which was 10,000 for TD-1’s airframe, was reduced to 7,000 in this case. The airframe weighed 6,430kg when complete which meant the weight reduction exercise had reduced 350kg of weight, a praise worthy achievement.

PV-1 represents the production standard airframe. Of the structural material used the proportion of carbon composites account for 45 per cent by weight, aluminium alloys 43 per cent, titanium alloys 5 per cent, steels 4.5per cent and other materials 2.5 per cent.
The avionics suite is the same as that of the two TD aircraft. The aircraft first flew on November 25, 2003 with Sqn Ldr Sunit Krishna at the controls and has completed 80 flights to date.

There is a big difference between the avionics suite of the first three aircraft and the prototypes from the fourth aircraft PV-2 onwards. The distributed, integrated avionics suite in this aircraft is configured around three dual redundant MIL-STD-1553B data and two dedicated weapons buses. Central data processing is done by the open architecture computer (OAC) which is Power PC/VME64 based. It has a mezzanine card based MIL-STD-1553B, RS422 master and cursive graphics modules. Dual redundant OACs combine the functions of the mission computer, the two display processors, the CCU and the video switching unit replacing five of the LRUs on the Technology Demonstrator aircraft. The OAC has modular software written in the ADA language complying with MIL-STD-1521 and 2167A standards and will be able to generate digital maps without a separate module.

The production standard cockpit has no electro mechanical standby instruments. The cockpit is dominated by three 5”x 5” AMLCD MFD’s, two Smart Standby Display Units (SSDU) and the indigenous HUD. The HUD has an Up Front Control Panel (UFCP) which is a significant man machine interface (MMI) enhancement which allows the pilot to program, initialize the avionics and enter mission and system critical data through an interactive soft touch keyboard. Although the FOV of this HUD is slightly less than that of contemporary units on other aircraft of this generation it is not considered significant because the ELBIT, Israel furnished DASH helmet mounted display and sight (HMDS) will form an integral part of the avionics suite.

The four utilities system monitoring LRUs have been reduced to two dual redundant units. These units perform the control, monitoring, data logging for fault diagnosis and maintenance functions.

A HAL Korwa developed Crash Data Recorder will be fitted after the initial flights.
The Multi Mode Radar (MMR) jointly developed by LRDE and HAL Hyderabad will be fitted in the nose after redistributing the FTI carried in the first three aircraft. The MMR features LPRF, MPRF and HPRF modes, platform motion compensation, MTI and Doppler filtering, CFAR detection, range-Doppler ambiguity resolution, scan conversion, display of target and ground map data on MFDs and on line diagnostics to identify faulty processor modules.

The aircraft has the ADA developed Stores Management System (SMS) which will provide fully integrated control of weapon systems, external stores and fuel tanks. The SMS is based on a 32 bit, single chip micro controller with dual redundant architecture .Its main components include the single Stores Interface Box (SIB) and multiple pylon interface boxes(PIB) for each hard point.

A state of the art EW suite will be integrated and tested later in the program. Primary responsibility for development of the EW suite is that of the Defence Avionics Research Establishment (DARE), Bangalore

The aircraft is undergoing final integration checks and is expected to fly by the end of May 2005.

Program Update

The flight test program has logged 350 flights without encountering any major design deficiency. Due the complexities of the quadruplex digital fly by wire system it is clear that the flight test team and program managers are opening the flight envelope cautiously. At the time of writing(February 8, 2005) the flight envelope has been expanded to Mach 1.4,1150 kmph CAS,15 km altitude, +4.5 g, and an angle of attack of 23.


A host of daunting tasks like full envelope expansion after flutter testing, MMR tests, weapons integration, weapon delivery and environmental tests of the full aircraft have yet to be attempted by Team LCA They certainly have the nation’s good wishes to back them while they go about their onerous task.