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Aviation History
1964
1964 - 2069.PDF
90 fUGHT International, 16 July 196' AIR COMMERCE ... ICAO's Sixth Airworthiness Meeting PARIS was the venue for the sixth meeting of the ICAOAirworthiness Committee, held from June 11 to 30. This,the first formal meeting for two years, was attended by the technical experts of ten states and two organizations, IATA and IFALPA. As might be expected, the representatives of those countries which have active aircraft manufacturing industries were always prominent in the discussions. The agenda for the meeting included items on performance and flying qualities, each of which aspect was to be studied with a view to producing a PAMC (Provisional Acceptable Means of Compli- ance) for the consideration of ICAO's Air Navigation Commission. Although the title of such documents might imply that they are of little consequence, they do in fact exert a profound effect on aircraft design, certification, and operations, because they are the result of detailed studies by appointed experts, and are therefore not to be lightly disregarded. Previously agreed PAMCs have included such items as flight-manual standardization and turbine engine type tests and have produced notable improvements, particularly regarding international standardization. At this meeting it did not prove possible to reach final agreement on performance and flying qualities, but sufficient progress was made to justify the hope that final agreement on these two items may well be reached by informal meetings and correspondence within about a year. Aircraft Performance Final agreement on the performance PAMC will probably bring about significant changes in the certification and operation of new aircraft (the United Kingdom is already using a similar performance code). The proposed intro- duction of accountability for wet runways in the take-off and accelerate stop case, will be welcomed by many pilots who have long felt concern regarding this problem. It is proposed to introduce the concept of decision speeds Vxw (wet runway) and Vji> (dry runway) in such a manner that in the event of the critical engine failing at Vxw the aircraft would be able to stop on a wet runway within the distance available. If the same engine should fail at VXD the aircraft could, of course, safely continue the take-off. In the event of engine failure at a point between Viw and ViD on a wet runway, the take-off would be continued, but the normal margin of 35ft over all obstacles in the take-off path would be reduced. It is suggested that the maximum duration of exposure to these reduced safety margins should be limited to 4sec for either two, three or four engined aircraft. There is little doubt that pilots will welcome this additional guidance regarding the performance of their aircraft when operating from wet runways. The existence of the 4sec "risk" period in the statistically remote probability of the critical engine failing between V2D and Viw will probably be accepted as being only an acknowledgment of an already accepted risk. It is hoped by the airworthiness committee that the research work which has been in progress in several countries with the object of obtaining more information on tyre/runway friction, on both wet and dry runways will be continued to completion. This may eventually result in runway surface condition accountability becoming only another operational variable, like runway slope, wind velocity or temperature. One other major change which is proposed in the draft PAMC is to base the landing distance requirements on a maximum threshold speed (VTmax) which is the greater of 1.2 x Vs (lg stall speed) or minimum demonstrated threshold speed (VTMD) + 5kt—the selected value to be increased by 20kt or 0.2 Vs- This substitution of Vrmax f°r tne °ld 1.3 Vs, recognizes the difficulty in achieving the optimum threshold speed under operational conditions. The landing distance which is required by VTmax is to be increased by 15 per cent in the all-engine operating case, and by 10 per cent in the one-engine-inoperative case. The actual distance required is substantially greater than that provided by some previous codes. In order to encourage the development of runway arrester gear, the committee decided that it would be possible to "give credit" for such devices, when proved reliable, in both the landing and accelerate/stop case. This proposal would have beneficial economic considerations for both operators of aircraft and airport authorities. The IFALPA member of the committee considered that the piovision of such equipment should be primarily directed toward improving the existing level of safety, and so dissented from the majority decision. Flying Qualities A great deal of detailed work is reflected in the draft PAMC which the committee had before it for consideration. Attention was given to various possible means of improving the flying qualities of new aircraft. Among details which were discussed were proposals to improve longitudinal stability in baulked landings, and to reduce the maximum control forces necessary in such cases. The need to specify minimum control speeds for both the one engine inoperative landing (VMCLC) and one engine inoperative discontinued approach (VMCLD) were agreed. Discussion of the flying qualities required in stalling revealed the need for more work to be done on this problem. The use of "stick pushers" to produce desirable stall qualities was mentioned and will be fully discussed at future meetings. Dynamic stalls, buffeting, flight in turbulence, trimming problems, and the problems, likely to be met in the SST, STOL and VTOL aircraft of the future were all recognized as being in need of consideration. These work programme items were each accorded an appropriate degree of priority and will be treated in the appropriate fashion. During discussion it was encouraging to note the way in which the different states and research organizations are prepared to collaborate together on these problems. Other agenda items which received the attention of the meeting included:— Bird Strikes Working papers giving statistical data on bird- strikes were supplemented by the showing of films by the members for Canada and the United States. One film showed tests to determine the damage to structure caused by birds of various weights fired at high speed from an air gun designed for this purpose. Other films showed the effects of ingestion by engines of various birds. One film employed time-delay photo techniques to demonstrate the use of radar in correlating bird migration habits with meteorological pressure patterns, and the suggestion was made that radar techniques could be employed to obtain information on migrations and hence assist aircraft in avoiding large flocks of birds. After consideration of all the available information the Com- mittee concluded that the penalties involved in designing aircraft to withstand any bird (vultures weighing 201b have been struck) at all speeds up to cruising speeds, is impracticable. Aircraft engines are being tested by ingestion of "standard" birds of 41b weight and a number of smaller birds. In very large turbojets the ingestion of small birds appears to produce little effect, but with smaller engines there can be significant power reduction and even complete flame- out. There can also be potentially catastrophic effects from impacts by large birds; for instance, if the rotating assembly is sufficiently disturbed to produce large out-of-balance-loads causing loss of a compressor stage or of a turbine disc. Research in various States is aimed at design of engines to ingest birds without serious damage and at the design of guards and intakes which would prevent entry of birds into the engine, or other critical conditions. In all cases the problem is to achieve the desired results without unacceptable weight or performance penalties. The standard test for the regulations governing windscreen design is now also being applied to structural elements of some aircraft at present undergoing certification. These include tests on critical parts of the structure, attention being paid to the more vulnerable positions of tail-planes when mounted on top of the fin, and to aircraft systems where they may be vulnerable (e.g., controls running behind the leading edge of the fin). Both in
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