All of us hold Group Capt Suranjan Das in great esteem. Apart from being a wonderful person, he was an exceptionally competent test pilot. But it would not be right to assume that Groupie Das could never make a mistake during test flying. Absolutely no one is infallible at all times.
More than one person confirmed at HAL that in fact he did keep the canopy of the HF-24 Mk IR unlocked and held it up slightly to get some ventilation during taxiing. There is little doubt that the clamshell canopy used for the first time on the HF-24 was unlocked and opened up to vertical position as speed built up. This was the main cause of the crash. My personal part in attending to the accident’s aftermath follows.
Having taken charge of A&ATU on 1 Jan 1970 after returning from Egypt, I got the news of Groupie Das’ crash on the 10th itself from Air Hq. The phone call included an order from Air Mshl OP Mehra, DCAS (holding the post now designated as VCAS) to report to him as soon as possible. The Air Marshal asked me to proceed to HAL Bangalore and see what action must be taken to prevent a recurrence. On arrival at HAL, after some preliminary talk with a number of people, the sequence of events was confirmed as I have already reported in my first account of the crash. At that time I did not know or guess that either of the two engines had failed and no one then mentioned it to me.
The Chief Designer of HAL SC Das explained the system as arranged for this aircraft with the first clamshell canopy installation on a Marut. It was locked by T-Bolts which are about the surest way to ensure a good firm retention with no chance of failure as shown later in the Kiran. In Marut MkI and its derivatives, the sliding canopy had a reinforcing metal strip from front to rear in its middle. This was the hard metal which the pilot’s head would hit If ejection was attempted without first getting rid of the canopy. Although I never actually saw the clamshell canopy of the Marut MIR, I am not sure if there was any similar metal in the middle. But its metal frame holding the perspex was thick and heavy. Its front edge was likely to be very dangerous for a pilot to hit despite the breakers fitted on the ejection seat on both sides at the top. HAL then made it impossible to eject through the canopy. Neither the top blind nor the handle between the legs would operate unless a pin was pulled out to arm the seat and fire the cartridge. A lanyard fitted to the canopy was intended to pull out the pin as it was jettisoned. Only after this had been done, the seat was armed and ready.
Getting back to the fatal crash, the canopy was seen to open up to upright position. This would have been impossible if the T-bolts had been turned to the locking position. The hinges, theoretically referred to as sheer pins did not sheer. The seat was not armed and the drag from the open canopy was excessive. According to AVM Roy Choudhary, Director GTRE, under these conditions the fatal accident was inevitable whether or not an engine failed. But this became a huge bone of contention.
Since the MkIR was a prototype, it had been instrumented in considerable detail. There was much discussion about the recorded traces showing the failure of an engine, perhaps cutting off the reheat leaving the exhaust nozzle open. The loss of thrust would have been large but there was no direct way to know of it. The reheat system on the Orpheus had been developed by GTRE with the active assistance of Rolls Royce and some other companies of which Dowty was a prominent partner with Full Authority Digital Engine Control (FADEC). Both GTRE and Rolls Royce protested very strongly to the allegation of engine failure. However, I recently checked with Air Mshl Mally Wollen who mentioned that this had been concluded and recorded at least as a possibility in the findings of the enquiry.
Let us get to the aftermath of the accident: I asked SC Das to change the canopy to side opening. He instantly replied, “But Kopeel. That will mean a design change!” My suggestion, that this was the least designers were expected to do, did not give him the incentive to take up the challenge. Meanwhile, for the MkIR, and earlier MkIA, the aft fuselage had just been fattened to accommodate the larger diameter of the engines with reheat. The after body drag was very high. In a personal duel between HAL’s chief engine designer and Director GTRE, both accused each other of doing poor scientific work. HAL alleged that the engines were losing too much thrust in the installation while GTRE said the loss of performance was due to excessive drag resulting from the poor design of the after body. The project was in dire trouble and had been carrying on only because of the unwavering support of Groupie Das, His death killed the project. I decided to recommend to Air Mshl Mehra officially abandoning it under these conditions.
Two prototypes of Maruts referred to as MkIA had reheated engines. I heard but never confirmed it that Sqn Ldr BD Jayal taking off a Marut MkIA almost failed to get airborne as the reheat cut out during the take off. I recall being told that he had at least two drop tanks installed. However, this story could just as easily be about a MkI in very hot conditions. Perhaps he can now confirm this tale, if true, and in any case describe it for us.
There is one reason I am willing to believe that the reheat of at least one engine could have failed. At A&ATU Kanpur we got a Marut MkIA for performance measurements. I flew it for some sorties and was impressed by its performance. Even with two drop tanks at low level, I had to throttle back in level flight at 540 knots to not go any faster. The limit was imposed apprehending failure of the fuel pump drive shafts. During test flights, I found the GTRE staff acting strangely. They fiddled with the engines depending on whether we were measuring the take off ground roll or climb performance. On investigation I found them adjusting a Temperature Monitoring Unit (TMU) which would cut off the reheat if the temperature went up too high. It was being adjusted for better performance for each intended test. Otherwise it would do its job, putting the aircraft and the pilot at risk. I stopped the trials and told the AVM that we had been given the aircraft for measuring performance, not for its development. We never saw it again.
The final part of this saga was almost amusing. The original HJT-16 canopies were also sliding ones. They were being replaced with clamshells. In view of this, I asked SC Das to design the sheer pins so that during ground roll they would sheer off at 90 knots, the speed at which it was permissible to use the ejection seat. The job was done in over one year. Some aircraft were delivered with clamshell canopies with the correctly designed sheer pin (hinges). But during towing an aircraft at FIS, the canopy flew off due to a minor gust of air. A little later exactly the same thing happened at A&ATU, with a parked aircraft of which the canopy had not been locked in anticipation of some work yet to be done. Gp Capt PG Joshi came to do the enquiry and told me that he had to find the unit at fault. I said,” Not on your life! Let us go to HAL and see what the cause is”. We confronted SC Das. I asked him how he had designed the sheer pins. SC Smiled and said that not only had he designed it well, he had actually tested the sheering by actual canopy jettisoning on to a net at various speeds to get it exactly right at 90 knots. I then asked him how he arrived at the load on the pins. He looked at me as if I was an utter fool. He said, “You know it very well, half rho v squared s gives me the load”. I asked him what happens to the shock load when the canopy opens and hits the stops. Wouldn’t the load be very much higher? He admitted that they had forgotten to allow for it. My unit was not blamed by Groupie Joshi. At Air Hq I explained the visit to the VCAS Air Mshl Shiv Dev Singh and suggested that we ask HAL to redesign the sheer pins. He smiled and said that they had taken more than year for the first lot. Now if we were to ask them to redesign the sheer pins, IAF would not have the Kiran for at least another two years. He said that it would be better to accept the loss of a few canopies and not delay the aircraft any more. Apparently, strict instructions were issued that the retaining lever must be engaged, or else!
Received from Gp Capt Kapil Bhargava on email.