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Aviation History
1954
1954 - 1696.PDF
11 June 1954 751 AFRICAN ACCIDENT REPERCUSSIONS Investigator's Criticisms Refuted by A.R.B. and SB. AC. CRITICISMS of the Air Registration Board, the Society of British Aircraft Constructors and Vickers-Arm-strongs, Ltd., are contained in the second of two official accident reports on the loss of a Central African Air ways Viking in Tanganyika last year. Replies to the charges were quickly issued by the A.R.B. and the S.B.A.C., and their statements are reproduced on this page. The extent to which the charges are justified, if at all, may be judged from a review of events leading to the present situation. The aircraft concerned, Viking VP-YEY, crashed at Mtara, in the Handeni district of Tanganyika, on March 29th, 1953, with the loss of all aboard—five crew and eight passengers. An inquiry into the accident was held at Dar-es-Salaam on April 28-29th, and an official report signed by Sir John Gray was published on June 1st, 1953. This report stated that the accident was caused by fracture of the starboard outer-plane spar boom as a result of metal fatigue cracks. These cracks, it was stated, had developed from corrosion nodes, due to the use of grease to specification D.T.D.577 during fitting of press-fit bolts to attach the outer to the inner spar-booms. Three assessors—Capt. C. W. Byas, Capt. G. D. Green and Capt. F. D. Greensted—added a postscript to the report in which they criticized the R.A.E. for anticipating the findings of the inquiry; they also urged that research into fatigue of aluminium alloys should be expedited by all possible means. The criticism of the R.A.E. referred to the suggestion that D.T.D.577 brought about corrosion. Subsequently it transpired that this suggestion did not necessarily represent the full official views of the R.A.E., and that it had been expressed only in an inter-departmental memorandum. It was therefore decided to hold a second inquiry and this was again conducted by Sir John Gray with the same three assessors on December 15th-16th, 1953. Meanwhile, attempts had been made to determine by chemical analysis whether D.T.D.577 had in fact been used, but the results of this analysis were inconclusive. However, official evidence was given by the R.A.E. mat the corrosion which led to fatigue was mainly due to the fact mat the bolts were not cadmium-plated. The court concluded that an approved compound, D.T.D.369, had been used rather than D.T.D.577. The conclusion reached in the second report was that the Viking had been struck by a sudden gust, resulting in fracture of the lower starboard outer boom outboard of station 41 due to weakening of the boom at the bolt hole by metal fatigue cracks, and that the onset of fatigue failure at this spot had been advanced considerably by corrosion at the common surface of the aluminium alloy boom and the steel bolt. REQUIREMENTS OF A.P.970 The report then asked, in effect, why the fitting of non-plated bolts was permitted, and attempted to fix responsibility for the insufficiency of the anti-corrosive measures taken. It noted that the Viking was built in 1946, and that in its construction Vickers were guided by the principles set out in A.P.970, the Air Ministry document specifying structural requirements for military aircraft. This contained a clause saying mat, "in general," steel detail parts should receive treatment against corrosion. The report said also that at this time the S.B.A.C. were to have drawn up a chart listing methods of protection against corrosion but that this chart was never produced. Finally, it was suggested that the only other source of guidance to the manufacturer was the A.R.B. Require ment D.18, issue 2, of September 14th, 1945, which specifies, inter alia, that "all members of the primary structure shall be suitably protected against deterioration or loss of strength in service due to weather, corrosion, abrasion or other causes." Sir John Gray complained that this document "comes near to being purely platitudinous," and added: "It was suggested that the Board could not be held responsible as the non-preparation of the proposed chart rested with the S.B.A.C. as well as the respon sibility for any consequences which may have resulted from this omission. I can see no great reason for this apparent self-com placency on the part of the Board. It appears perfectly obvious to me that either they should have insisted on the Society pro ducing such a chart or else have taken other measures to ensure that such a chart was forthcoming. Indirectly their omission has led to the very tragic consequences and the Board must accept its proper share of responsibility for this." The report absolved me manufacturers in the following terms:— "In the absence in 1946 of any detailed and definite instructions by the A.R.B. ... as to the protection to be applied, and within the limits of the manufacturer's licence and of the discretion allowed under British Civil Airworthiness Requirements as set out in the leaflet D.18. . . . Vickers-Armstrongs, Ltd., were justified in their choice of design and assembly and anti-corrosion protection of the joint at station 41 [the point at which the spar boom failed]." On the basis of these findings, the report urged the cessation of the system whereby the A.R.B. "delegates responsibility as to the fulfilment of the airworthiness requirements . . ." saying that the board should take "immediate steps to issue detailed explicit and mandatory instructions as to the manufacture of British civil aircraft." When Central African Airways' Vikings were grounded after the accident, an inspection was made of the bolts used to attach outer wings to the inner spar-boom fitting. It was found that eleven wings had plated bolts top and bottom; seven had un plated bolts top and bottom; and two had both plated and unplated bolts. Since the drawing called for the use of unplated bolts, the manufacturers believed that the plated bolts had been fitted in error. In their postscript to the report, the assessors suggested that the fact that a high percentage of non-standard bolts were fitted in Vikings was a grave reflection on Vickers-Armstrongs' production methods. They also criticized the operator, C.A.A., for not showing a more active interest in problems of fatigue failure. On May 26th, 1954, the Director-designate of Civil Aviation for the Federation of Rhodesia and Nyasaland, published his comments on the two accident reports, and pointed out a number of alleged technical errors contained in them—e.g., that D.T.D.369 is not a grease as stated but a pigmented-varnish joint compound. The Director-designate's comments also referred to a previous occurrence of fatigue in a wing of a Central African Airways' Viking, evidendy in order to prevent over-emphasis of the rela tionship between the two incidents. INBOARD SPAR BOOMS The first instance was in October 1951, when VP-YEW flew from Blantyre to Dar-es-Salaam in turbulent weamer. During the flight a loud crack was heard but nothing was found when the aircraft was inspected on landing. Later, during a more thorough inspection at Salisbury, one of die inboard lower spar booms was found to be cracked. The component was flown back to England for inspection by Vickers-Armstrongs. On December 28m a notice was sent to Viking owners ordering an immediate inspec tion of all aircraft which had flown more than 5,000 hours and an inspection of other Vikings at the earliest possible moment. On January 17th, 1952, the life of the inboard lower spar boom of Vikings operated in tropical conditions was fixed at 5,000 hours, and a permanent system of precautionary inspections was initiated. All these precautions referred to the inboard spar booms and no life had been fixed for the outboard booms. It may be noted that the Viking outboard spar has two booms, of which the lower —being subjected in flight (particularly in tropical conditions) to a large number of alternating tensile loads—is the more subject to fatigue failure. Ironically enough, it transpires that at the time of the accident the R.A.E. was conducting fatigue tests to deter mine the life of this very component, and the A.R.B. was on the point of fixing a provisional limit of 10,000 hours. The failure to VP-YEY occurred when it had flown 8,815 hours—clearly as a result of the increased stress concentrations, and consequent vul nerability to fatigue failure, produced by corrosion. In focusing attention on the fatigue problem, and on the value of the experience gained from accident investigation, Sir John Gray's reports have been valuable. But, however deep the sin cerity of his motives, the author shows a tendency to reach sweep ing conclusions without, it would appear, having digested a mass of complex evidence, much of it highly technical. It would be no exaggeration to say that the Air Registration Board was astounded by the criticisms made in the second report. The Board's state ment is as follows: — "The findings and recommendations of the Court appear to indicate a considerable misconception of the functions and methods of the A.R.B. and a very understandable lack of knowledge of the Board's require ments, which is shown by the recommendation that 'The A.R.B. should produce at once the most detailed, explicit and mandatory instructions as to the manufacture of civil aircraft to ensure attainment of sound standards of manufacture, operation and perform ance of civil aircraft,' "It must be observed that the only A.R.B. publication before the Court was Requirement D.18 which is in general terms stating the object, but leaving the method to basic engineering practice. "On the other hand, in other fields such as structures and perform-(Concluded on page 773)
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