The HF-24 Flight testing claimed the lives of four test pilots in the years 1970-71. The author revisits these accidents and the events leading upto them.
The first prototype of the Hindustan Aircraft Limited’s (HAL) HF-24 made its maiden flight on 24 June 1961. Some fans of the aircraft, presumably from one of its three squadrons, are keen to celebrate its Golden Anniversary next year. One of them asked me about the fatalities of test pilots in the HF-24. My response to him is this note. Kindly note that the facts mentioned by me are as I know them, many of them without any confirmation. They might not all be accurate.
Of the 23 test pilots of IAF killed (so far), the record of fatalities of test pilots in the HF-24 is similar to the Gnat. In the Gnat we lost three test pilots and one in Ajeet Trainer.
In the HF-24 the following accidents occurred that claimed the lives of test pilots. The name of the unit to which the Test Pilot belonged follows his own name. After the aircraft Mark is the name of the organisation owning the aircraft. All four aircraft were still HAL’s when they crashed.
1. 10th January 1970 – HF-24 Mk 1R (HAL) flown by Gp Capt Suranjan Das (HAL).
Prototype aircraft canopy open; right engine failure suspected.
Groupie Suranjan Das’ crash was perhaps partially pilot error. He was the greatest supporter of the HF-24 Mk IR with reheated Orpheus engines. It had the prospect of being earlier and better than the Jaguar. Its performance was less than it would have been if the rear fuselage had not simply been enlarged by HAL to house the larger engines and their nozzles. The nice area rule of the original design had been vitiated badly. All the same, the performance of the aircraft was impressive. All it needed was to get the right avionics. According to me, the other design work should have been for the addition of a second hydraulic system, and of course streamlining the fuselage a lot better than the fat end.
While taxiing out to the take-off, Groupie Das used to keep the clam shell canopy unlocked and hold it up slightly to get some fresh air. There was no retaining lever. It seems that on the fateful day, he forgot to lock it prior to beginning the take-off run. The canopy opened during the ground roll. The hinges were too strong for it to fly off. Ejection through the canopy, as in all Maruts, would almost certainly have led to killing the pilot. Hence, the canopy had to be jettisoned. Ejection, till the canopy was not in the way, was prevented by a locking pin. This pin was pulled out by a lanyard which would be extracted during jettisoning of the canopy.
For Groupie Das, with the canopy already open, its jettisoning was no longer possible. Ejection was also impossible as the seat was not armed till the pin was pulled out. The drag from the canopy was large and the aircraft did not get airborne. There was a debatable possible failure of the reheat of one engine. In short Groupie Das had no options left and died in the crash.
2. 12th May 1970 – HF-24 Mk.1 (HAL) flown by Sqn Ldr K L Narayanan (HAL)
First Prototype aircraft. Went into Goa Bay in bad weather
The crash of Sqn Ldr (Kiddo) Narayanan was somewhat mysterious. During the monsoon. he ferried the very first HF-24 prototype to Goa and flew over Dabolim airfield. A request to make another run to show off the aircraft was passed to him on the R/T. He came round heading west again but was seen to be well to the left of the runway. From about 1000 feet or so his aircraft suddenly dived into the Goa Bay. The drizzle and low clouds present at the time may have caused disorientation, or the tail trim had run away. But no cause was determined, since the aircraft was never recovered from the sea.
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The first HF-24 Marut prototype was lost at sea off Goa on 12th May 1970. |
3. 27th July 1971 – HF-24 Mk 1 (HAL) flown by Wg Cdr J K Mohlah (HAL)
Loss of longitudinal control. Possibly trim ran away?
Wingco Jagat Kishore Mohlah was flying the aircraft at 10,000 feet heading west in the area north of Bangalore airfield. Suddenly his aircraft dived into the ground. It hit granite rocks where the engines embedded deep into them. There was almost nothing left of the pilot. A trim runaway was suspected but never proved. (His son Gp Capt Sridhar Kishore Mohlah is also a test pilot and is currently commanding the Air Force Test Pilots School).
4. 21st Nov 1971 – HF-24 Mk 1 (HAL) flown by Sqn Ldr A K Sapre (A&ATU)
Went into Gulf of Kutch during four-gun firing tests.
In the last HF-24 crash involving a test pilot, Sqn Ldr Sapre was patently lost for no fault of his. With the Bangladesh War looming on the distant horizon, HAL had tried to clear the aircraft for four-gun firing. The vibration level in the aircraft when firing four guns was frighteningly high, almost violent, during butt tests. The traces of flight test records showed very high amplitudes. Yet HAL had cleared it.
I was then commanding Aircraft & Armament Testing Unit (A&ATU). During the Steering Committee of the HF-24 at Air Hq in the conference room of the Chief of Air Staff (CAS), the point came up that A&ATU needed to test this to approve it for service. The CAS, Air Chief Marshal P C Lal, in his usual sarcastic manner said that it had already been cleared by IAF test pilots in HAL.
To this I piped up with my comment, “Sir what is acceptable to the manufacturer may not be acceptable to the customer”. At this he lost his temper. I had forgotten that along with being CAS he was still Chairman HAL.
He said, “I suppose the customer means you”. I promptly replied. “No Sir. The customer is you. But if Air Headquarters were happy with the results presented by HAL, A&ATU had no cause to test anything”. This was followed by a coffee break.
After the coffee break, with a sneer, ACM Lal said, “Well, if A&ATU want to play with it, they can do so”. Just before the trails came up in Jamnagar, Sqn Ldr Sapre (Saper in our parlance) came to see me in Army Hospital in Palam where I had been admitted for investigations as a confirmed diabetic. He said that he was very scared to do these trials. The only advice I could give him was to do absolutely nothing which was not first demonstrated by HAL on the same aircraft. As far as I know, he was in an unlucky breach of this casual briefing.
During several tests of firing two-second long four gun firing, no trouble was encountered. Then it was decided to do a four-second or maybe longer burst. Saper volunteered to do this, though it was not demonstrated by HAL, at least on that aircraft. Over Sarmath Range during gun firing, the pawl engaging the aileron linkage came out and caused a sudden bank, probably to the right. Those who had encountered this problem after practice manual flying might know that re-engaging hydraulic power often caused a sudden and high roll to one side. The natural tendency was to fight the roll, but the way to engage it was to roll into the roll. This was obviously not possible for Saper, even if he knew of it. The aircraft went into the Gulf of Kutch and was never found again.
The aftermath of this accident was quite unpleasant. When after the 1971 War, Air Chief Marshal Lal came to Kanpur to thank the Station, we had gathered for lunch. Finding him alone for a short while I spoke to him and said that testing this had cost us an IAF test pilot. He just looked down and I could see that he was as upset as I was.
Another aftermath of the accident was that HAL did not even undertake two gun firing till some preventive mods were done. A test pilot of HAL had encountered the same problem perhaps at a higher height and had recovered. But this fact was never communicated to IAF, neither to A&ATU nor to Air Hq. I still hold strong views on this but you can draw your own conclusions.