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Aviation History
1953
1953 - 0787.PDF
FLIGHT, 19 June 1953 781 THE COMET ACCIDENT Report from Calcutta LAST Monday was published the report of the Court Investiga-* tion instituted by the Government of India into the accident to Comet G-ALYV which occurred shortly after take-off from Calcutta on May 2nd. The cause of the accident is not definitely established, but probable causes are suggested, and one assessor, Shri N. Srinivasan of the design department of Hindustan Aircraft, Ltd., gives in Appendix 7 to the report some personal deductions regarding the probable cause of structural failure. On the recommendation of »he Court and at the request of the manufacturers of the Comet, the wreckage is being transported as soon as possible to this country in order that a detailed technical examination may be undertaken with a view "to determine the primary failure and to consider if any modification in the structure of the Comet aircraft is necessary." The Court of Inquiry consisted of N. S. Lokur, K. M. Raha, N. Srinivasan and T. R. Nelson. It was assisted by Shri Y. R. Malhotra, inspector of accidents, and by D.H. and B.O.A.C. experts from England. The accident occurred at Jagalgori, a village about 24 miles fromDumDum Airport on a bearing of 277 degrees T. The Comet was on a scheduled return flight from Singapore to London, carrying 37 passengers and a crew of six. Six minutes after take-off, which was normal, radio contact with the aircraft was lost, and it was last heard by Calcutta at 1105 hours G.M.T. calling Delhi. About that time, cultivators residing in villages around Jagalgori saw the aircraft coming down in a blaze of fire through severe thunderstorms and rain. The report gives details of weather conditions which included scattered thunderstorms and cu-nim and a forecast of moderate turbulence. The report goes on to indicate that navigational aids were adequate for the flight, as was the fire-fighting equipment. The weather conditions at take-off were well above the minima, and the captain was duly briefed and cleared. The report states that "the captain was not only well qualified but had considerable experience of weather conditions on this route. He was therefore fully competent to judge the weather forecast en route and the warning given, and make up his mind whether to take off or not." The report continues : "The storm which the Comet encountered, as the eye witnesses from the neighbouring villages state, was unusually severe." Before stating briefly its findings, the Court adds under the heading "Discussion of the Evidence" : "We have no evidence before us to indicate sabotage or a stroke of lightning, or faulty workmanship or defective material. There being thus no direct evidence as to the cause of the failure of the Comet to get safely through the storm, we have to infer it from the state of the wreckage, which was distributed over a large area. There is no doubt that, as an expert witness, Mr. Lett, has stated, the aircraft suffered a complete structural failure in the air and thereafter the aircraft was on fire in the air." The Court states the belief that the conclusions given by the one assessor are quite plausible, but thinks that a further pro longed technical study of the wreckage will be necessary to verify his deduction and determine the sequence of failures. This may take nine to twelve months. The Court's findings indicate that at take-off all was in order with the aircraft regarding C. of A. licences, all-up weight, flight information and clearance. Item V of the findings states : "The aircraft encountered a nor'wester squall with thunderstorm shortly after take-off when climbing to its cruising altitude, and suffered structural failure in the air, which caused fire." In the Court's opinion the failure was due to over-stressing resulting from either severe gusts or from over-controlling, or loss of control, by the pilot when flying through the thunderstorm. In addition to recommending a detailed technical examination, the Court feels that consideration should be given to the desir ability of modifying the flying-control system of the Comer to give the pilot positive feel of air loads on the control surfaces. The report gives in considerable detail the condition of all major components and pieces among the wreckage. The under carriage and flaps were retracted; the throttle levers were broken and jammed in the half-open position; fuel cocks were on; the flying-control system change-over levers were in their normal position; elevator and aileron trim was normal; the cabin was being pressurized; the fire extinguishers had not been operated, nor was there any evidence of any emergency procedures. Some extracts from the statement of damage to main airframe units are as follows : Both outer wings had failed at a station outboard of rib No. 7. Both tailplanes had suffered impact damage in the air. There was no structural damage to the fin panels. The lower fin and rudder had suffered extensive impact damage. The port elevator had been cut in two pieces, and the spar showed bending failure at a station in between No. 3 and No. 4 hinge brackets. It indicated a compression failure on the top flange and a tension failure at the bottom, that is, a down load failure. The starboard elevator spar had failed in bending, significantly at the same point as on the port elevator. The deduction of Shri N. Srinivasan, to which we have referred, contained the following passages :— "A close examination of the spar in either elevator shows a bending failure at a station in between the No. 2 and No. 3 outboard hinges. It is a down-load bending with compression at the top flange and tension at the bottom. It is significant that this failure is of a localized nature with no damage over the surrounding area either in the tailplane or elevator skin, in spite of the subsequent impact damage observed on other portions of the structure. This elevator down-load failure may have been due to a "pull-up." The down-load on the tail-unit seems to have caused a fuselage failure in bending at bulkhead No. 26. The top panels have failed in tension and the bottom panel in com pression. "It is understood during the investigation that the wing was subjected to a static test by the manufacturing firm during the development stage of the aircraft. On one test-piece static and fatigue tests were conducted alternately. The wing failed in fatigue test and after modifications was subjected to a static test. The wing failed again at 90 per cent of the ultimate load. The failure was attributed to the fatigue test conducted before. Modifications were carried out again, and without a re-test, it was found satisfactory for the ultimate load on theoretical considera tions. The fatigue failure during static test occurred at Rib No. 7 where the cross-section changes from two heavy spars to an outboard shell construction. In this accident, again the wings have significandy failed at Rib 7. Whatever the load may be the failure at Rib 7 may indicate the lack of proper diffusion of the wing loads on to the two spars at Rib 7. In the absence of design data no definite comments can be made on the wing failure, but a further investigation on the above subject of load transfer at Rib 7 will be helpful. "It is extremely difficult during this short period of investigation with limited facilities and data to substantiate the primary failure with all details, but there are strong indications on the wreckage to suggest the primary failure of the elevator during a "pull-up." The Comet has got an elevator control system operated with booster power with no feed-back arrangement for pilot feel. It is quite probable that the pilot, who is accustomed to a sort of "feel" on the controls during manoeuvres had overcontrolled the aircraft beyond the limit that would impose the design loads on the aircraft." Following the publication of the report, the following comments were made jointly by the de Havilland Aircraft Co., Ltd., and B.O.A.C.:— "The report is agreed in that the aircraft met severe gusts in thunder storms. The force of the gusts has not been possible to determine, but it is probable that they were of an unusually high order. "It is not possible until the detailed examination, now under way, of the aircraft wreckage has been completed at the Royal Aircraft Establishment to determine the sequence of structural failure. Until such a sequence is determined it is only possible to theorise on the cause of the accident. "The very considerable flying experience including many flights in turbulent conditions over the last three years by B.O.A.C. and de Havilland's with Comet aircraft does not suggest that over-control or loss of control by the pilot was likely." REFRIGERATING TUNNEL B ELIEVED to be the largest commercially owned wind-tunnel ever built, a new low-temperature tunnel is now being erected by Lockheed Aircraft at Burbank, California, and is scheduled for completion by next August. It will be about 73ft long and will have a working section of about a square metre. The refrigeration system will have a capacity of 1,165,000 B.Th.U./hr and at zero temperature it will be possible to test components at a simulated speed of 200 m.p.h.; higher speeds will be obtainable in the upper freezing ranges, or lower speeds down to — 50 deg C. The airflow will be chilled by a massive "gridiron" of refriger ating tubes, and for icing tests it will be possible to introduce a water spray of any required droplet-size. Oddly enough, some of the directing-vanes in the tunnel will be steam-heated; the reason of course, is that they would otherwise become iced up. Normal aircraft de-icing methods would not be effective in the conditions under which the tunnel will operate. FOKKER S BRAZILIAN PROJECT REPRESENTATIVES of the Fokker Company have recently visited Brazil in connection with the organization of a concern which is to build a factory and manufacture aircraft in that country. The capital will be subscribed by the Brazilian Govern ment and public and by the Dutch company. According to the preliminary agreement the factory will produce 100 S.ii basic trainers, 50 S.I2S (nosewheel version of the same type) and 50 S.14 jet trainers in four years, the Fokker group supplying machinery and technical staff. The cost of the 200 aircraft, if all are taken by the Brazilian Air Force, will amount to about £7,100,000. The factory may be built on Governador Island, in the Guanabara Bay, or at SSo Jose dos Campos, Sao Paulo, near the Technical Aeronautical Centre.
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