Flight Testing

Proceedings of the international seminar

On Flight testing and flight test training
03 - 04 february, 2004
Aircraft and systems testing establishment, Vimanapura post, Bangalore - 560017

Development Flight Testing of the Tejas Light Combat Aircraft

Gp Capt RKS Bhadauria VM,
Offg Project Director (Flight Test) NTFC, ADA, India

Development Flight Testing of the Tejas Light Combat
Aircraft -

Tejas has been designed as a compound delta configuration at near midwing with three leading edge slats and two segment elevons that span the entire trailing edge of each wing .

The design challenge was to have materials with high specific strength and stiffness that can take complex shapes and provide adequate fatigue and corrosion resistance. Extensive work in this field resulted in the first two aircraft, TD- 1 and TD2 having nearly one third of the airframe ( by weight ) built in composites




1. Aeronautical Development Agency ( ADA ) was set up to design a state of art fighter aircraft against the Indian Air Force Air Staff Requirements ( ASR ) projected in 1985. The IAF desired a light weight, agile, multi role, supersonic aircraft capable of carrying smart weapons, close combat and beyond visual range ( BVR ) missiles , conventional weapons and a modern self protection EW suite. This design effort resulted in the Light Combat Aircraft ( LCA ) now named .TEJAS.. (Fig.1). Wg Cdr (retd) Rajiv Kothiyal, KC, took off on the maiden flight of TD-1on 04 Jan 01. The last fighter aircraft prototype HF-24 Marut flew on 24 June 1961 from HAL, Bangalore. These four decades saw rapid technological advances in the aeronautical field . The Indian Industry had unwittingly taken a long break in design effort at the most crucial time. Therefore, the successful first flight of Tejas was extremely significant.

TEJAS Programme Overview

2. The objectives of the Tejas programme were to build a state of the art fighter aircraft against IAF ASRs and also to bridge the large technology gap that had developed.

3. Configuration : Tejas has been designed as a compound delta configuration at near midwing with three leading edge slats and two segment elevons that span the entire trailing edge of each wing . The Y- duct air intakes shielded by the wing are designed for buzz free performance at all speeds and high angles of attack. Tejas incorporates latest technologies in contemporary fighter aircraft design in terms of quadruplex Digital Fly-by-wire control system, all glass cockpit, microprocessor based management of utility system ( USMS ) and use of composites in primary structures.

4. Integrated Flight Control System ( IFCS ) : Integrated Flight Control System (IFCS) forms the heart of the Tejas aircraft. It has a quadruplex digital fly-by-wire control system with no back up either mechanical or analogue. Lack of technology combined with high-risk levels forced the programme to adopt a cautious incremental approach to development and testing of critical technologies of which IFCS is the most significant. Accordingly, the first block of flights of the first two prototypes was done in fixed gain Control Law (CLAW). The primary objective of these fixed gain flights was air data calibration to ensure acceptable calibration status of air data and flow angle sensors, before scheduled gains could be invoked. For the subsequent block of flights CLAW gains were scheduled as a function of altitude and Mach No with air data and AOA brought in the feed back loop. Thus CLAW development would progressively move towards the full up CLAW, which would give the aircraft capability for unrestricted manoeuvre in the full design flight envelope. It is important to note that unlike in the case of conventional aircraft, air data information is a vital input to the safety critical flight control system of Tejas. Accordingly efforts were made to locate the sensors where the errors were likely to be minimum over a wide range of AOA, as established by CFD studies.

5. Control Law : Tejas configuration is aerodynamically unstable in the pitch axis and directionally stable except at high angles of attack (AOA). The instability level in pitch depends upon the flight conditions (Alt, M , AOA) and configuration (slats, mass and cg). National Control Law team was formed to develop the Tejas control laws. They designed a stability and command augmentation system configuration for an AOA / normal acceleration (nz) command system in the longitudinal axis . The directional stability and Dutch roll damping have been augmented. The roll rate response on Tejas show significant Dutch roll oscillations due lateral and directional coupling. The two axes are decoupled from the input in roll by the control law. The aircraft rolls about the velocity vector to suppress the kinematic coupling between AOA and side slip angle(b ) especially at high AOA. The control law is designed to give roll rate demand in roll and ß demand from rudders.

6. Composites : Low weight and low RCS requirements demanded extensive use of advanced composites. The design challenge was to have materials with high specific strength and stiffness that can take complex shapes and provide adequate fatigue and corrosion resistance. Extensive work in this field resulted in the first two aircraft, TD- 1 and TD2 having nearly one third of the airframe ( by weight ) built in composites. Third aircraft (PV-1) airframe comprises of 45% composites and approximately 90% of the surface area is covered with advanced composites.

7. USMS : The micro.processor based utility management systems (USMS) is responsible for Engine and Electrical Monitoring System, Environmental Control System, Digital Fuel Monitoring System and Digital Hydraulic Electronic unit. Satisfactory functioning of the USMS forms an important part of the technology demonstration .

8. Avionic Systems : The major avionics components on the prototype Tejas TD1, TD2 and PV1 aircraft include a Mission Computer, Mission Preparation and Retrieval Unit (MPRU), RLG based INS, HUD, and two 5. x 5. color MFDs, multi function keyboard, a Get you Home (GUH) and two display processors. The ac has a radio altimeter, two R/T sets (INCOM and Stby UHF set). Figure 3 shows the cockpit configuration of the first three ac. The entire avionics system is configured around multiplexed MIL-STD 1553 B digital bus in a distributed process environment. The MC comprises of a dual Intel 80386 processor with three dual redundant 1553 B bus controllers. In the event of MC failure, CCU takes over the functions of MC in reversionary mode of avionics operation. In addition a comprehensive warning management system has been implemented in the avionics suite, which includes warning captions, action pages, audio tones and a hard-wired CWP. The navigation guidance functions, preparation of certain functions etc are performed directly through the Function Selection Panel (FSP). On the first three technology demonstrators only navigation modes have been implemented in the MC.

9. Weapon Capability : Tejas is designed to carry a mix of Air . Air and Air- Ground weapon loads on seven hard points with an additional pylon under the intake for sensor like LDP. Tejas weapon load would comprise of CCMs / BVR missiles and a wide variety of LGB and conventional weapons. With integration of multimode radar, HMDS and LDP as sensors, we are certainly looking at a very potent platform.

10. Organisations involved : HAL is the principal partner with ADA. ADE and NAL are the major work centers in respect of the Fly by Wire Flight Control System ( FCS ) which are the two main constituents of Integrated Flight control system, handled by NAL and ADE respectively. Apart from these, LCA related work is being done at over 100 work centers spread across the length and breadth of the country. This alone should be indicative of the enormity of coordination and other management problems the programme is faced with.

Flight Test

11. This brief description of the aircraft, the design intent, new technologies involved and the number of agencies involved would have given a clear idea of the complexities of flight test task. National Flight Test Center was set up in 1994 by pooling resources from IAF, HAL and ADA for the purpose of undertaking development flight testing of the Tejas.

12. Design and Development : Flight test team interacted with the designers at the design stage to steer the programme to successful flight. Risk management was the highest priority of the flight test team. A lot of activities were undertaken at this stage, some important areas are covered below:-

a) Pilot vehicle interface ( PVI ) Cockpit evaluation facility (CEF ) at ADA was extensively used to evaluate PVI issues before finalisation. The facility was also used for formulating and practicing normal as well as emergency procedures which is an evolving and continuous process.

(b) Generation of normal and emergency operating procedures was another important task handled by flight test. These procedures and profiles were thoroughly practiced in CEF , RTS and in Mirage trainer, which was subsequently used in the chase role during the actual flight test phase.

(c) Development of CLAW from the initial fixed gain sets to the final full up scheduled gain CLAW with all the limiters and carefree manoeuvring features in place is a gradual and phased developmental process . With each higher version of CLAW, flight envelope and manoeuvring capabilities of the aircraft expand. The various steps involved in CLAW development wherein flight test interacted closely are listed below:

  • Off line simulation
  • ELS ( CLAW development at NAL )
  • In flight simulation ( IFS at CALSPAN )
  • RTS at ADE ( Normal and failure cases )
  • Iron Bird tests at ARDC , HAL ( PIL and fault free tests )
  • On aircraft checks ( PBIT PLBIT system level tests}.
  • Model validation from flight test results
  • Gain / phase margins monitoring

(d) Claw team of NAL has for the first time developed the capability to monitor phase and gain margins available in real time as the flight test was going on. With this facility in place at the telemetry, CLAW development and validation process is expected to progress in a safe and efficient manner. Fig.5 shows typical result of phase and gain margins (tested along with PIDs) monitored in real time.

(e) Iron Bird test facility where IFCS functionality can be tested. Extensive PIL and fault free testing of IFCS were done by the flight test team before any version of FCS software was cleared for flight.

13. Flight test instrumentation

An exclusive telemetry was set up at NFTC which was located with HAL . Important flight test instrumentation details are covered below:

(a) On board FTI system : The on-board FTI system of the LCA is a high-speed performance system for Data Acquisition, recording limited cockpit display and transmission to the ground station through a L-band telemetry channel. Presently about 2000 parameters are acquired by the on board FTI system. The system adheres to IRIG standard and is based on distributed architecture to facilitate future enhancement. The on board FTI system is also capable of acquiring MIL 1553B bus and high speed RS 422 data in addition to analog and discrete data. All these data are time synchronized using GPS time code reader.

(b) Ground Telemetry Station : Telemetry ground station is configured to perform the following functions

  • Auto tracking
  • Data de-commutation
  • Real time data processing and display
  • Post processing of recorded data
  • Audio video support

(c) Data processing and real time display are performed by a high end alfa server and twenty workstation/ terminals that are networked. In addition there is a dedicated FM system and server to handle flutter and vibration data. The ground station also supports archival of post-processed data on a jukebox, which is optically linked to the design team at ADA and HAL design centre.

Planning and conduct

14. Number of safety critical prototype technologies packed into this prototype increased risk factor significantly. Therefore, a cautious, incremental approach was followed in the conduct of flight test. Comprehensive test plan documents were prepared in consultation with the designers. Test schedules are derived from the document for each flight. All test schedules are coordinated with CEMILAC, the Military Aircraft Airworthiness Authority. Each test sortie briefing is attended by the pilot, entire telemetry team, aircraft preparation & safety group and concerned designers. The flight test is conducted from the control room at the telemetry monitoring room. The test director, who conducts the test, is in constant radio contact with the pilot. A safety pilot monitors the flight to provide assistance in tackling emergency recovery. All the critical sorties (initial sorties of a new prototype, envelope expansion, new CLAW etc) are chased. Each sortie is planned meticulously and practiced on Real Time Simulator (RTS). This permits accurate packaging of test points to the extent that pilots have been usually landing within ± 10kg of planned landing fuel. Independent test frequencies and a very co-operative Air Traffic Control contributes immensely towards achieving this. Each sortie is followed by a hot debrief to the same group usually within 15 min of landing. Detailed data debrief is conducted on the next day. Next sortie on a aircraft is planned only after the data debrief is completed.

15. Objectives : The Tejas programme is currently in the Technology Demonstration phase also known as Full Scale Engineering Demo (FSED.I) phase I. The objective of this Tech Demo phase is to demonstrate the four core technologies . quadruplex DFBW, glass cockpit, composite structures and micro processor controlled utility management system. To achieve this, the specific flight test objectives are akin to any prototype development viz handling qualities assessment in all phases, envelope expansion, performance, avionics assessment and systems assessment.

16. Tests conducted : The following tests have been conducted so far:-

(a) HQ tests in fixed gain and scheduled gain in the envelope cleared.

(b) Performance tests -T/o, landing , climb and cruise

( c ) Calibration of air data and flow angle sensors

(d) Parametric Identification ( PID ) tests to validate aero data set.

(e) Flutter tests in order to expand envelope to current envelope of 1.4M/ 1100 CAS.

(f) In-flight load test comprising of strain monitoring at selected locations as ride along tests with HQ and envelope expansion.

(g) Integrated system performance as ride along tests.

(h) Avionics tests, which have usually been planned as ride along tests.

17. Flight Envelope : The flight envelope was expanded in three blocks Fig . 7 shows the flight envelope details . In first block, envelope was expanded to 612 CAS/ 0.7 M/8 km /2g in fixed gain. In the second block , flight envelope was expanded in scheduled gains to 742 CAS / 1.15M / 12km / 3.5 g / 20° a. In the third block, the flight envelope is cleared up to 1100 CAS / 1.4 M / 12 km / 3.5 g / 20° a. The flight envelope was expanded in small steps of 50 kmph / 0.1M. The handling qualities, in-flight loads and damping measurement through flutter test were closely monitored. Enormous amount of preparatory work in terms of theoretical analysis, validation of test


techniques, experimental verification and discussions on test procedures preceeded flutter testing. The testing was achieved through a flight test panel ( FTP ) with provision to inject excitation signals through the flight control computer.The telemetry consoles were configured to monitor structural frequencies and damping in real-time. The real time analysis was achieved in about 2 min for clearance to commence next test point involving a critical mode. The test procedures, on line analysis itself was streamlined to the extent that in subsonic zone up to 27 test points were executed in one sortie. The test point matrix itself was rationalized after establishing the baseline.

18. Test results

(a) Flight envelope : The flight envelope permitted by the current CLAW version has been completed up to the extent shown in Fig 8. During this process one of the slats panel had to be redesigned because of signs of distortion. Rest of the results from structures, relevant systems and handling view point were satisfactory.

(b) Handling qualities : Handling quality results in various phases are summarized in the table at Fig 9. More than one HQR is indicative of ratings from different pilots or ratings in different aspects of that phase. Summary of Handling Qualities

Sl No Task Handling Qualities Rating

1 Ground handling HQR 2,3

2 Take-off (fixed gain) HQR 2

3 UP and Away subsonic HQR 2,3

4 Supersonic handling HQR 3,4

5 Landing HQR 2,3

6 Brake ( modified software) HQR 3

In general, the aircraft exhibited very good handling qualities in the entire envelope tested so far. Handling qualities in high gain tasks like close formation are excellent in the mid and low speed range tested. Based on flight test feed back the brake-by-wire brake system has already been modified with two software upgrades which have improved the handling from HQR 5 to HQR 3 in the current standard.

(c) PID Results : PID tests were conducted in the current envelope covering the zones shown in Fig 10. All the results were analysed by a dedicated PID group. Apart from conventional techniques, large amplitude PIDs were devised and executed. The results so far have indicated that all derivatives are as predicted except for small deviations established in Cma and Cnß. Typical results are shown graphically at Figure 10. The drag polar established from flight test shown at figure 10 shows an exceptionally good match with the prediction. The model validation route (Fig. 11) is well in place and is expected to result in substantial reduction in flight test effort.

(d) Flutter : Initial flutter tests on TEJAS TD1 conducted at 9 km/ 0.65M (CAS 455), 9 km/ 0.77M (CAS 545) and 7 km/ 0.77M (CAS 625) . The data obtained from these tests were used to ascertain the functionality of flutter accelerometers in real time (15 channels) and the procedure verification for real time flutter monitoring and clearance. The formal flutter tests were started at different Mach numbers on the constant CAS line of 700 kmph to maintain the dynamic pressure constant. The test points No. of points No. of points flown in the flight envelope are shown in Figure 12. Points indicated on the flight envelope on the CAS 700 line were cleared before going supersonic. First supersonic flight of TEJAS TD1was flown on August 1, 2003. All results obtained so far upto 1.2 M and 900 CAS have indicated adequate damping. Typical results are a (deg) a (deg) shown graphically in Figure 13.

'(e) General Systems : All the systems were closely monitored right from the ground run stage. Systems integrated performance was evaluated and discussed at great length in the data debriefs. Several issues like spurious warnings ( normally due reasons like persistence or minor transients in system behaviour ) and control logic flaws were identified. Many systems like environmental control system and brake management system have already undergone many software revisions to refine the system functioning. Each system fault or design improvement identified is raised as a request for action ( RFA ) and tracked regularly by flight test at the management level. Some system designers were quick to respond with upgrades and sit peacefully in the data debriefs now. Rest of the system designers are in the process of upgrades to address the issues raised . This cohesive and coordinated approach between flight test and designers has been one of the most significant achievements of this flight test programme.

19. Avionic Systems

Avionics assessment has been a huge task involving flight test inputs from requirements and documentation stage through CEF evaluations/ cockpit evaluation to flight evaluation. During the 150 flights Tejas has flown so far a number of software upgrades have been carried out on the avionics system to improve on the overall performance and PVI issues. Based on the evaluations done so far and the technological advancements in this field, the requirements have been reviewed along with the Avionic designers to finalise the production version architecture which would be integrated in the fourth ac (PV-2) scheduled to fly this year. The Tejas production version would have an Open Architecture Computer(OAC). This is Power PC 7400 based and will also do the functions of Display processors. The ac will have three colour MFDs ( 5. x 5. ) , HUD, MFUFCP and two Smart Standby Display Units (SSDU). Figure 14 shows the cockpit layout. The ac will also feature a Function Selection panel (FSP) and a Sensor Selection panel(SSP). All the inputs for the avionics input functions, sensor mode selections, sensor control etc will be through the HOTAS controls. The displays have been organized such that the LMFD is used for navigational guidance and operational functions whereas the RMFD and CMFD are used for ac environment, sensor and tactical systems functions. The global specification for the PV2 avionics architecture has been finalized and cockpit interface LRUs have been implemented in the Cockpit Environment Facility (CEF). The navigation and weapon mode symbologies, logics and HOTAS controls are getting refined through a series of pilot evaluations. A unique concept of Display of Interest (DOI) has been implemented through a four way switch provided on the control hand grip to allow pilot to dedicate the HOTAS controls to different sensors. This allows the pilot to effectively manage different sensors simultaneously through the multiplexed HOTAS controls.


20. Important highlights of the programme are covered below:

(a) First flight of TD-1 on 04 Jan 2001 followed by a total of 12 flights in Block I up to June 01.

(b) Second aircraft TD-2 took off on 06 Jun 02, flight envelope of Block I was covered in fixed gain.

(c) TD1 with scheduled gain CLAW, operational slats and airbrakes took off on 03 Feb 03. Scheduled gain envelope expansion started in Apr 03.

(d) Naming ceremony of the aircraft by Prime Minister on 04 May 03, both aircraft took off in a pair for 2g formation and individual 3.5 g limited demonstration, aircraft was named .Tejas..

(e) First supersonic flight on TD-1 on 01 Aug 03. The aircraft handling in transonic to supersonic was very good and air data performance was close to prediction.

(f) Initial scheduled gain envelope of 1.15 M / 742 CAS / 3.5 g / 20° a covered in Aug 03.

(g) Third aircraft PV-1 joined the flight line on 25 Nov 03.

(h) Current envelope achieved is 1.2 M / 1000 CAS / 3.5 g / 20° a. One hundred and fifty flights completed on 22 Jan 04.

(i) The cautious incremental approach resulted in a slow sortie rate in the beginning which has now increased to 15 sorties on an aircraft in one month, which is good by any aircraft industry standard. Many potential safety hazards were identified in time and dealt with appropriately, more on this in the next paragraph.

Important problem areas uncovered

21. Close monitoring of all systems and careful planning and sequencing of test points resulted in timely detection of many problem areas, some of which could have resulted in serious consequences. Important / interesting problem areas uncovered are covered briefly.

(a) High speed taxi runs upto nose rotation were planned as a build up to scheduled gain take off. During this test point, the nose oleo oscillations in pitch got magnified due coupling with flight controls and became divergent close to rotation. The rotation was stopped midway and aircraft decelerated safely to a stop. Graphs shown in Fig 15 show the rate of build up of nz, q and control rate saturation at critical point of rotation . After detailed analysis nose oleo design change for improved damping has been initiated and CLAW update to reduce the gains on ground has already been implemented. We expect to begin regular scheduled gain take off after the nose oleo modification.

(b) Environmental Control System ( ECS ) incorporates bleed air shut off valve ( BASOV ) which on closing shuts off the air supply resulting in ECS failure, loss of fuel pressurization ( implies only service tank fuel around 600 kg is available) and loss of cooling to avionics bay ( most importantly the flight control computers). BASOV closure was encountered at 11km altitude at 100 km from base. Aircraft was recovered safely and failure traced to spurious transient pressure spikes. The ECS software was modified and started flying on aircraft within a month.

(c ) Nose wheel judder was encountered early in the programme. A committee instituted for the study recommended the acceptance criteria and remedial action for overcoming the problem. Continuous monitoring and timely inspection/ rectification to meet acceptable criteria was followed till the design modification could be implemented. Regular checks have minimized the chances of any failure.

(d) One instance of uncommanded nose wheel movement on nose wheel steer (NWS) selection before taxiing out for a sortie was investigated thoroughly and a distinct malfunction possibility with serious consequences was identified. Remedial measure was undertaken before next flight in terms of an override and long term solution was identified and initiated.

(e) Abnormal pressure build up in the service tank close to leak limits of the tank resulted in some anxious moments in air . It was correctly identified as a venting problem and some unusual control inputs were resorted to in order to persuade the vent valve to work. The venting system is now modified.

(f) Small but unusual oil pressure fluctuations were noticed in accessory gear box. Precautionary recovery was initated. Low oil pressure warning came up later in descent to land. Aircraft was landed safely with near zero oil pressure.

Lessons learnt

22. Team LCA was eager to learn and apply. Crucial lessons were learnt along the way virtually with every flight. Some important lessons are summarized below:

(a) Suitable test rigs are crucial for all system testing. Proper testing on ground would reduce the number of surprises in air.

(b) Meticulous test planning and execution with close interaction with test director is extremely important for safe and efficient conduct of test .

(c ) Exhaustive data debriefs followed up with documented RFAs contribute immensely towards design improvement. . Fly and Flag. (tracking of RFAs at management level) is the most efficient way forward.

(d) Monitor all systems. Some failures in simple, proven or conventional designs can be equally catastrophic.

(e) Failures generally start after a while ( usually coincides with the rise in confidence ).

(f ) Do not succumb to programme pressures.

(g ) Optimize flight test effort through model validation route. Revisit and rationalize test matrix after analyzing initial results.

(h) Thorough knowledge of the system and test requirements are crucial towards understanding and managing risks.


23. The development flight-testing on Tejas has been a great learning experience for the entire team. We started with no background experience and recognized it. Nothing was taken for granted. Every step was planned and vetted for safety and risk levels. Smooth and safe progress of flight test so far and rapidly gathering pace are indicative of the learning curve and total team work wherein designers are virtually part of the flight test team effort. Operationalising the machine is only a matter of time.


The author wishes to gratefully acknowledge the contributions of NFTC members; all design groups of ADA, HAL, NAL, ADE; certification agencies CEMILAC and DGAQA; proactive support received from ASTE, ATC and IAM.