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Aviation History
1959
1959 - 0951.PDF
476 FLIGHT AIR COMMERCE... . r- , T.A.A. and the Friendship An Instructive Pre-Delivery Episode i • " NEXT week Trans-Australia Airlines are to take delivery of their firstFriendship—the first of a fleet of 12. This article discusses how T.A.A. and Fokker jointly and successfully tackled a difficult technical problemthat arose before delivery. AFEW months ago the airline industry grapevine was buzzingwith rumours to the effect that the Fokker Friendship was^ in fatigue-test trouble. Like most rumours, there was enough in it to prevent Fokker and the airlines most concerned—Aer Lingus and T.A.A., the first main recipients of the Dutch- built version of the aircraft—from issuing a denial. On the otherhand (again like most rumours) the "trouble" was much exaggera- ted. It is now cured, and everyone concerned has learnt some-thing more about fail-safe versus safe life. In particular, the air- lines most concerned will not forget the businesslike andresponsible manner in which Fokker faced up to the problem. What actually happened was that, one day last August, a seriesof cracks began to develop in a test specimen in the water tank at Schiphol. To be more precise, the lower (tension) skin of theFriendship's centre section—which was being tested attached to a pressurized fuselage immersed in the test tank—suffered seriouscracks'at between 16,000 and 20,000 "flights" of one hour each. These failures occurred when six production aircraft had beenbuilt and many centre-sections assembled, and when Aer Lingus and T.A.A. were both looking forward to taking delivery of theirfirst aircraft, well ahead of the contractual dates of November 30, 1958, and February 28, 1959, respectively. . The failures were serious because they could have meant thatthe certification authorities might not be prepared to approve a safe operational life of more than some 5,000 hr pending moreconclusive results from further tests. Modification action, as des- cribed below, was immediately planned by Fokker. Aer Lingus agreed to accept their first Friendships, unmodified,with a conservative and precautionary safe life and to return them to Fokker for modification before expiry of that life. T.A.A. (with 12 aircraft on order) felt that they could not acceptdelivery on this basis. The Australian operators live as far from the Netherlands as it is possible to get, and they also utilize theiraircraft at a very high rate. There was no point in taking delivery of aircraft which would probably need to be ferried twice moreacross the world within a couple of years of their acceptance. But T.A.A.'s reasons went deeper: it was their view that theycould not accept in any new aircraft the possibility of a major modification or replacement of structure in a period of less than30,000 flights (i.e., in T.A.A.'s Friendship operations, 30,000 hr). Modification Decision An engineering team under Mr. John Watkins, T.A.A.'s direc-tor of engineering, worked closely with Fokker in deciding the necessary modification action, and the Australian airline agreed toaccept a ^postponement of six weeks on the contractual date of its first deliveries, again with a good prospect of the revised contractdates being bettered. This action was influenced by T.A.A.'s strong view—held in thelight of very satisfactory Convair 240 experience—that fail-safe structural qualities, as well as safe life, must be demonstrated.Fokker agreed, particularly as they realized that a scatter factor of at least five was necessary to demonstrate safe life—a fact whichwas not, in their opinion, conducive to structural efficiency. The Australian civil aviation authorities, it should be added, havestrong views about the extreme difficulty of satisfactory safe life determination—views which might well some day be incorporatedin Australian airworthiness requirements. Mr. Peter Langford, Director of Airworthiness of the Australian D.C.A., accompaniedMr. Watkins to Holland for the discussions. The modification originally proposed by Fokker involved achange of material and a thickening of the skin by an amount calculated to reduce stress level by 20 per cent for a weight penaltyof 80 lb. The skin material was to be changed from 75 ST to 24 ST, and skin thickness was to be increased from 2 mm to 2.5 mm.At the same time the thickness of the stringer sheet material was to be increased from 1.6 mm to 2 mm. This would reduce nominallg stress levels in the area concerned from 12,300 lb/sq in to 10,000 lb/sq in, and would provide the slow crack-propagationqualities and high residential strength of 24 ST material. After considerable discussion and review of the many complexfactors involved, from both design and production viewpoints, it was decided to make even more certain of achieving the desiredresult. In the end, the final Fokker proposal was for a skin thick- ness of 3 mm, but without changing the stringer dimensions. This was estimated to reduce stress level by 30 per cent instead of the20 per cent offered by the 2.5 mm skin of the original proposal. This was agreed by T.A.A., who accepted the additional 35 lbweight penalty (i.e., 115 lb in all), and agreed also to the six-week extension of delivery beyond the contractual date. Mr. Watkins spoke of the "realistic and co-operative manner inwhich Fokker have received our views, and 'the cordiality with which the discussions have been conducted." In his opinion, themodification should eliminate the possibility of the need for re- placement or major repair within the 30,000 hr life agreed—"andthis is far in advance of anything demonstrated or achieved in service by any other known type of aircraft."As things stand now, the first T.A.A. aircraft for delivery is due to be handed over at Schiphol next Tuesday, April 7. Another willfollow at the end of this month, another in May, and three more in June. The contractual delivery dates of the second six remain un-altered; and T.A.A. expects to have its whole fleet of twelve by mid-October.The first aircraft, VH-TFA, flew on January 16 in unmodified form, and immediately began certification tests of the SmithsS.E.P.2 autopilot, the air conditioning and refrigeration, and the radio and electrics. These tests lasted well into March, when thecentre wing was retrospectively modified, and further production tests carried out. The disassembly and modification of the nextthree aircraft have already been accomplished. Hence -TFA will move back in order of delivery, and it is now expected to becomethe seventh acceptance. The other aircraft have been, or are being, modified in the assembly stage. In the meantime, Fokker are carrying out tests of a modifiedcentre section with the objectives of demonstrating not only a 30,000 hr safe life, but also fail-safe crack propagation-rates givingX-ray inspection intervals of not less than 1,000 hr. There will also be a limit-load test with one spar boom severed. So far as the rest of the Friendship's structure is concerned, thefuselage suffered no failures of any significant nature during no fewer than 48,000 simulated flights. There was some progressiveloss of pressure during the subsequent 1,000 "flights"; this was found to have been due to cracks developing close to the stiffenersof the rear pressure bulkhead. The Fokker designers were elated at the fact that none of these cracks had progressed beyond thebond between the skin and stiffeners and had thus proved the fail-safe nature of the design. Tests have since continued wellbeyond the equivalent of 100,000 flights. The story of the Friendship's trouble provides a model forthe ideal relationship between an airline and its supplier: recog- nition of a fault brought to light during an admirably early pre-service test programme; full and tough discussions of its implica- tions and the necessary action; and a willingness on the part of themanufacturer—despite the penalties to him—to accept the views of the customer. It might be added that, in this particular case, Fokker werefortunate to be dealing with an airline whose engineering stan- dards and experience are recognized throughout the operatingindustry as second to none. To put T.A.A.'s safety record down in print is tempting Providence—but anyone who cares to checkit cannot fail to be mightily impressed. There is one other moral: it will one day become very difficultto sell airliners unless their fail-safe design is not just claimed, b-it actually demonstrated. , J. M. R.
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