A Tale of two hydraulic systems

Wg Cdr KS Suresh VrC (Retd) describes an unusual situation and how a Jaguar aircraft was saved with high degree of professionalism and quick thinking . This article first appeared in the magazine INDIAN AVIATION.


Jaguar a twin engined strike aircraft, jointly developed by British and French entered service in the mid seventies. Jaguars are operating in India, Ecuador and Oman besides UK and France. Indian Air Force (IAF) acquired the Jaguars in 1979 to meet the Air Staff Requirement (ASR) of Deep Penetration Strike Aircraft (DPSA). To speed up induction, pending manufacture and supply of "Jaguar International" meant for the IAF, aircraft from the Royal Air Force (RAF) were released to equip the first two squadrons of the IAF. As the strength of IAF's own Jaguars built up progressively, RAF aircraft were returned. Jaguars of the IAF are fitted with two Rolls Royce Adour Mk 811 engines. Hindustan Aeronautics Ltd (HAL) also started producing Jaguar aircraft and Adour Mk 811 engines, under a Licence Agreement. Aircraft produced within the country were inducted to augment the direct supply Jaguars and form additional squadrons. HAL has produced more than 100 aircraft so far and the production run continues. The very first innovative and bold move made by the IAF was, to replace the old generation Navigation Attack System with a state of the art system called the DARIN. The architecture and the software of the new system were entirely Indian.

IAF had no serious problems with the Jaguars, for over a decade, barring a few unavoidable accidents. There is an old saying, ?every aircraft, after about 10 to 15 years of use, irrespective of thought and care during the design stage, presents an unusual problem, that could not have been foreseen?. This statement is substantiated when history of different aircraft is examined. Serious problems had occurred on most of aircraft, world over, usually after about a decade of service exploitation. In the year 1993, after the IAF had operated the Jaguars for about 14 years, a very serious problem occurred. This emergent situation was something that had never been foreseen or anticipated and was also not listed as an emergency. Brief Description of Jaguar Hydraulic System. A brief description of the aircraft system is necessary to follow the events. Jaguar has two independent hydraulic systems, Hyd-1 and Hyd-2, each having a reservoir with a capacity of 6 litres. The two systems are energised by engine driven pumps of respective engines and are independently capable of operating all the services and flying controls. The services include, flaps, slats, airbrakes, undercarriage, wheel brakes, etc. System design is such, that the two systems do not meet and have no common pipelines. The hydraulic system also has necessary back up in accumulators and emergency electrical pump. In the event of hydraulic fluid leak, as the level in the reservoir drops to 1 litre, a solenoid-operated valve is energised electrically, to stop fluid supply to the services. This residual fluid of 1 litre is now available exclusively for the primary flying controls, to ensure that the aircraft can be flown safely away from populated areas and also avoid possible unusual attitude prior to ejection. In the Jaguar aircraft, the slats (leading edge high lift devices) can be selected to operate automatically and depending on the prevailing flight condition, slats take up appropriate position. They can also be operated manually. The slats are operated by an electric motor (slat motor) and its engagement and disengagement is through a hydraulic brake. The body of brake unit receives hydraulic fluid under pressure from both No 1 and No 2 hydraulic systems, to ensure uninterrupted operation of the slats, even if one of the hydraulic systems fails. It is this design feature that takes away the redundancy of the duplex hydraulic system in case of a mechanical failure, like breakage of the brake body of the slat motor.

First Accident.

On 22nd November 93, at Air Force Station Ambala, Jaguar No JS 120 (a direct supply aircraft) flown by Fg Offr Pareek was on a routine training flight. After about 35 minutes of flying, the pilot noticed that the hydraulic pressure for operating the "Services" was 'zero' in both the systems. However, the hydraulic pressure for operating the "Controls" was normal. He returned to base and found that the undercarriage could not be lowered and all other services like flaps, slats and airbrakes were inoperative. An emergency was declared and on advise from the ground, the pilot tried all known methods to lower the undercarriage. The wheels just did not come down. He had flown close to 90 minutes and was desperately short of fuel and all attempts to get undercarriage down had failed. Since belly landing of Jaguar is not permitted as per Pilot?s Operating Manual, the pilot ejected safely and was quickly picked up by the rescue helicopter. Aircraft was destroyed in the accident. Air Headquarters (Air HQ) ordered a court of inquiry and at their request, two HAL engineers were deputed to be members to assist the investigation. The inquiry team also had members from Government departments - Directorate of Aeronautics and Directorate of Technical Development & Production (Air), besides the regular complement of IAF officers.

I being in-charge of Flight Operations and Safety at HAL Corporate Office, responsible for interaction and co-ordination with operators on safety matters, was in touch with our specialists on the progress of the investigation. After about two weeks, the investigation was still inconclusive but I learnt that several improbable theories were being put forward. It was necessary to correct these distortions. At that stage, I was confident to the extent of saying that the accident certainly had not happened the way it was being visualised by the court. I immediately mobilised support of my colleague, Head of Quality Assurance at HAL Corporate Office, Mr GS Jamadagni (Jam). He and I had been working as a team for over five years and had tackled many problems. We complemented each other's efforts, had excellent coordination and working relationship. The very first thing we did was to visit the factory and study the Jaguar Systems thoroughly. We then requested Air Vice Marshal (AVM) Bharat Kumar (Bharat), Officiating Inspector General IAF at Air HQ to permit us to interact with the court of inquiry, mainly to correct distortions and facilitate possible explanations for the accident. Bharat readily agreed to our proposal, as he was also not happy with the progress of the investigation. Thus the entire inquiry team (about to submit its report) was held back at Ambala (weekend ruined). They were asked to await our arrival and have brainstorming sessions with us.

Suspense at Air HQ.

Jam and I left Bangalore for Delhi on Monday, 13th December 93 by the morning flight. We were to meet Bharat at Air HQ and then proceed to Ambala in the evening. As we reached Air HQ at about 1130 hrs, we were asked to rush to the office of Assistant Chief of the Air Staff (Operations) (ACAS (Ops)). On reaching there we found that the atmosphere was very tense and ACAS (Ops), AVM Nadkarni, was talking to Ambala. Bharat and other senior Air Force officers connected with Jaguar were also present. There was a communication patch up with Ambala. It transpired that another Jaguar aircraft (JS 139 - HAL manufactured aircraft) flown by Sqn Ldr K Palit (Palit), Flight Commander of the squadron and a test pilot, had a problem exactly like that of JS 120; "Services pressure zero", "Controls pressure normal" and not able to get the undercarriage down. At that very moment, he was still in air, having flown close to 85 minutes and making attempts to get the wheels down. Air Officer Commanding (AOC), Air Force Station, Ambala and his team were at the Flying Control advising the pilot to try various methods. Air HQ had even contacted the Original Equipment Manufacturer (OEM), British Aerospace (BAe) for a possible solution. ACAS (Ops) had kept the Chief of the Air Staff (CAS) informed about the situation. As the time passed, fuel state was getting lower and lower. AOC Ambala called to say that the pilot was being advised to eject and that the rescue helicopter was already airborne. After this, there was an eerie silence; unbearable suspense and seconds appeared to be hours before the next communication. All of us heard with joy that the aircraft had landed safely with minor damage. There was no time to get the details, as we had to rush to Ambala along with Bharat and Air HQ team. At Ambala, we learnt the sequence of events leading to the relatively safe recovery of the aircraft.


At the end of 95 minutes, the situation was desperate, as all attempts to lower the undercarriage had failed. The AOC who was at the flying control, in sheer desperation sent word for the HAL specialists (part of JS 120 investigation team) told them that "the aircraft has about 3 minutes of fuel left, suggest anything, even the most ridiculous thing, I will ask the pilot to try it before he ejects". One of the HAL engineers, Mr Jayamohan who had his thinking cap on, quickly came out with a solution, "advise the pilot to put OFF the battery, explaining that this would de-energise the solenoid-operated valve and the trapped fluid would then be available to operate the services. There would be no instruments, lights and radio for a short duration and aircraft may yaw a bit. Battery could be put ON after hearing the thud of the undercarriage coming down". This was precisely what the pilot was asked to do. At this stage, Palit had about 170 kgs of fuel and in terms of duration, just about 3 minutes. He carried out the drill as advised and heard the thud of the undercarriage coming down. Aircraft, which was very light at that fuel state, developed a pronounced yaw (due to small differential in wheels coming down) and Palit put the battery on. The main wheels had locked down, but the nose wheel, which was in the process of coming down, remained half cocked, as the fluid reverted back to controls once the battery was put on. The nose wheel of the Jaguar extends forward, against the airflow and thus takes a while longer. Palit had no fuel to try the procedure once more; he did a quick turn and with his experience and finely honed skill of a test pilot, landed the aircraft safely on the reciprocal runway. He held the nose up as long as he could, used the tail chute and then gently lowered it on to the runway. Aircraft came to a stop with minimum damage to the nose area. One of the engines flamed out during landing run, as the aircraft ran out of fuel.

Jaguar JS-139 at rest in Ambala after landing without the nose wheel fully extended. Sqn Ldr D K Palit got the Vayu Sena Medal (Gallantry) for this feat.

Observations on JS 139.

We found that the aircraft had been cleared off the runway and parked in the dispersal. It had been quarantined for investigation by specialists from BAe, who had been specially requested to come down to India. In order to preserve the evidence, we were allowed to make external observations only. We found that there were telltale marks of profuse hydraulic fluid leak on the left hand side of the fuselage. Origin of leak could be traced to the panel housing the slat motor. On opening the panel, we found that the brake unit body of the slat motor had cracked resulting in profuse hydraulic fluid leak. Slat motors had been manufactured by AVIACA France, a sub-contractor of BAe. As mentioned in the previous paragraph, the aircraft had minor damage to its nose section.

Four specialists from BAe arrived at Ambala on the 19th of December and left on the 23rd to be back in UK for Christmas. During their stay, they made extensive notes on all their observations and spoke to the pilot and maintenance crew. They were noncommittal on the possible cause of twin hydraulic failure, an unusual problem. BAe team stated that they would study the problem after they got back to UK and then respond. IAF had to take stock of the situation and introduce interim measures as it was clear that breakage of brake unit body of slat motor could cause rapid leakage of hydraulic fluid from both the systems (operating at a pressure of 3000 psi), leading to the situation encountered in both the accidents. Since the failed brake units had done more than 1000 hrs (About 1700 and 1400 hours respectively), as a first step, flying on aircraft fitted more than 1000 component hours was suspended. Action was initiated to procure and replace these slat motors. This measure reduced the availability of aircraft. Air HQ also issued instructions to manage the emergency, should it occur, by resorting to putting off the battery. This severely limited operations at night and in bad weather, as putting off the battery would take away the instruments, lights and radio.

Indian Efforts.

As explained above, since there were serious limitations to unrestricted operation of the aircraft and no tangible solution was forthcoming from BAe, we had to do something. There were several insinuations from BAe: IAF aircraft were being subjected to excessive number of cycles of slat operation. Strictures were passed on IAF maintenance practices, permitting entry of air into the system, during the process of charging hydraulic fluid. The first allegation of excessive slat operation was negated after our Air Advisor in London checked with the RAF and found that their slat operation cycle was identical to that of IAF. The second allegation was negated after it was pointed out that the Jaguar had a self-bleeding hydraulic system and there was no scope for air remaining within the system.

Bharat from Air HQ exhorted HAL to study the whole problem and come up with quick solutions, so that the aircraft could be cleared for unrestricted operations at the earliest. The damaged slat motor of JS 139 was brought to Bangalore for detailed metallurgical examination at National Aerospace Laboratory (NAL). By now it was clear that the accident of JS 120 could also be due to the same reason. On our request the wreckage (of JS 120) was searched again and the damaged slat motor located and brought to Bangalore. Jam put in relentless effort to analyse the cause of failure by interacting continuously with metallurgists of NAL and scientists of Aeronautical Development Agency (ADA). Detailed analysis was made using ELFINI, structural analysis software. It was found that the brake body of the slat motor had inadequate corner radius (by design) in the body unit, at the circlip holding groove, resulting in a sharp corner, leading to be a stress raiser. This component was prone to early failure even with normal load and duty cycles. The corner radius of 0.01 mm to 0.1 mm was permitted as per design. Jam and his team of scientists then worked out the duty cycles obtainable at different radii. Calculations revealed that even under normal operating conditions, units with corner radius of less than 0.03 mm were likely to fail prematurel,y around 1500 flying hours. This was precisely what had happened in both the accidents.

Jam presented the detailed analysis to BAe who were very reluctant to accept the findings. After a number of meetings and presentation of accurate facts and figures, BAe did accept the shortcoming. In the meantime, BAe had also examined RAF aircraft with about 1500 flying hours and found that the brake bodies of slat motors with lower corner radius showed signs of impending failure. BAe then took action to correct the design requirements and advise AVIACA accordingly. The reluctance on the part of BAe was probably to avoid any liability or legal hassles. We had learnt our lesson at the cost of one aircraft and almost another one.


There were a number of brainstorming sessions at the Aircraft Division of HAL to develop and introduce suitable modifications. The aim being to clear the aircraft for unrestricted operations by being able to manage the emergency, should it recur. The first modification was the introduction of a warning light on the Central Warning Panel (also wired to the Master Warning Flasher), to come ON the moment the reservoir levels dropped, consequent to a hydraulic fluid leak This was relatively easy and Mr Veluswamy, Design Engineer came up with trumps in no time. The second and most important part was to be able to use the residual fluid to lower undercarriage without putting off the battery. Jam came up with a concept; ? why not selectively de-energise one of the solenoid-operated valves, so that the fluid trapped exclusively for controls from one of the systems could be diverted to operate services like undercarriage. Controls could still be operated with the fluid from the other system?. This idea took shape and with tremendous encouragement from Mr Haridas, then General Manager of Aircraft Division and Bharat from Air HQ. Veluswamy, Rajshekar, Jayamohan and the team of the Division came up with a viable modification scheme. An additional switch was provided in the cockpit, which enabled the pilot to selectively isolate one of the hydraulic systems and divert the fluid for services, while the other system took care of the controls. Aircraft could thus operate unrestricted, as it was not required to put off the battery.

Both the modifications were rigged on one aircraft in record time, (less than two weeks). These were assessed and evaluated by HAL test pilots. In early February 94, Bharat and a team from Air HQ came to Bangalore and assessed the modifications. Aircraft and Systems Testing Establishment (ASTE) was also involved at this stage. Air HQ gave a go ahead to fabricate the required components and introduce the modifications on the Jaguar fleet of the IAF. HAL undertook the task on priority, sent teams to Jaguar operating bases and the entire fleet was modified within six weeks. The limitations on Jaguar operations were removed by April 94. In June 94 it was found that BAe introduced these modifications as applicable to the Jaguar fleet all over the world. Interestingly the Company Notice of BAe was identical to that of HAL, word for word; expect that the letters BAe substituted letters HAL. There was a very fine print (visible only through a magnifying glass) that the modifications were based on ?work done by HAL? Contract.

Since the modifications were developed in India and adapted by BAe, it was decided to examine the possibility of claiming "Intellectual Property Rights" from them. Some interesting features of the Jaguar contract and licence agreement signed in 1978 by Government of India with the BAe emerged. The gist without actual legal terminology is given below: Operator (India) may introduce modifications on their own aircraft, the details of which are to be communicated to BAe. If we (BAe) have any technical observations, we will inform you within two months. In any case, the introduction of modification is at your own risk and cost.

If we (BAe) find that the modification is useful and should be introduced on the entire Jaguar fleet world over, we shall do so. However, this does not attract any "Intellectual Property Rights" or Commercial considerations.

It is amazing that the contract signed even before the aircraft were inducted had incorporated clauses as above. We learnt that a team of experts specialised in drawing up contracts draft these documents and bring to them the desired thoroughness.


Mr Jayamohan and Mr Jamadagni were commended by the CAS. HAL management gave a cash award to Jayamohan and very reluctantly promoted him (he had less than year?s service at that stage). Jayamohan?s role had been invaluable. It was his accurate analysis, commitment, intimate knowledge of the Jaguar systems and clear headed thinking during crisis, that saved an aircraft. It also provided material evidence for identifying the exact cause of the problem and take corrective actions. He for one certainly deserved a National Award. Sqn Ldr Palit kept his cool throughout the 97 minutes he flew and displayed a very high degree of professionalism and airmanship in bringing back the aircraft safely. He richly deserved the award of Vayu Sena Medal.

The others from Aircraft Division like Haridas, Veluswamy, Rajshekar who burnt mid night oil remained unsung heroes. An irony that has baffled me for years; HAL management did not recognise the role and contribution of Jamadagni in solving the unusual Jaguar problem, although he had earned a commendation from the CAS. It was only in April 2002, that the present Chairman, HAL honoured him at a small function.


There are a number of lessons to be learnt from the Jaguar crisis described in this article. These are: No design is foolproof. There would always be surprises in aviation.

No problem is insurmountable, provided it is tackled with purpose and synergy, setting aside narrow organisational loyalties. Jayamohan's timely advise during the crisis, has once again reinforced the fact that there is no substitute to thorough professional knowledge.

The thoroughness with which BAe drew up contracts is to be admired. Hopefully our experts have learnt to be as thorough.

Copyright © Wg Cdr K S Suresh. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of Wg Cdr K S Suresh is prohibited.