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Aviation History
1967
1967 - 0050.PDF
50 FLIGHT International, 12 January AIR TRANSPORT . . . Airworthiness for the Future THE AIRWORTHINESS COMMITTEE has a somewhat unusualstatus within ICAO. It is less than a division, butmore than a panel, and it functions as a committee of experts rather than as a meeting of representatives of States or organisations. Since its sixth meeting, which was held in Paris during June 1964, it has conducted its continuing busi- ness by correspondence between members and by the exchange of working papers in preparation for the seventh meeting. By this means it was able to recommend to the Air Navigation Commission the issuance of a PAMC (Pro- visional Acceptable Means of Compliance) on aeroplane flying qualities in July 1965—a point in time approximately mid-way between the sixth and seventh meetings. The work of the AIR Committee is aimed at the adoption of specifications leading toward international uniformity between the member States of ICAO. PAMCs and AMCs are the means used to indicate to both States and organisa- tions the standard to which their own regulations should be directed. All the ICAO aircraft manufacturing states, IATA and IFALPA provide members of the committee and the PAMCs and so on do therefore have a more substantial role to play in airworthiness matters than their somewhat tentative titles would indicate. The seventh meeting of the Committee was held in Mon- treal from November 22 to December 15, 1966, and discus- sions were held and some recommendations made on a wide range of airworthiness subjects. Altimeters/Static Systems One task assigned to the Com- mittee arises directly from the difficulties experienced in providing safe separation of air traffic—particularly in North Atlantic airspace. The vertical separation standard above Flight Level 290 is now 2,000ft, but some consideration is being given to establishing the need for continuing this stan- dard of separation. The Committee was therefore charged with the task of developing a uniform static-pressure system calibration method and also to develop uniform test pro- cedures for Type 3 altimeters (pressure-sensitive servo-driven altimeters or pressure-sensitive altimeters with equivalent correction facilities). The recommendations made on these two agenda items are an important prerequisite for an examination by other bodies of the problems inherent in any consideration of the possibilities of reducing vertical separa- tion. The static-system calibration method given most attention was the trailing-cone method which has been operationally used in the USA. The advantages of this system as compared with the other methods which also provide the required accuracy (tower fly-past and precision radar tracking) were stated to be: (a) the difference between the pressure measured by the aeroplane static system and the free-stream pressure, is measured direotly by a differential-pressure instrument; (b) the accuracy is very high; (c) the test procedure is easily repeatable; (d) the test instrumentation and procedure are simple; and (e) the operation is practical and economical— no ground or other equipment is necessary. For these reasons the trailing-cone method may become the most widely adopted means of calibrating static-pressure systems. A PAMC was recommended to the Air Navigation Commission. In the discussion on test procedure for Type 3 altimeters such good progress was made that the Committee hopes to reach complete agreement by means of correspon- dence between the seventh and eighth meetings. Turbine Engine Type Tests It was noted that existing national codes are very similar except in respect of con- tingency ratings. (The United Kingdom is the only state which has amassed considerable experience with contingency- rated turbine engines in aeroplanes). It proved possible to recommend a revised PAMC which provides type-test schedules for turbine engines without contingency ratings for use in fixed-wing aircraft, and for turbine engines with contingency ratings for use in multi-engined helicopters. Two alternative test schedules for turbine engines in multi-engined helicopters are detailed in the PAMC but it is hoped that further studies may enable them to be combined into a single test schedule. Bird Strikes The discussion on airworthiness problems arising from bird strikes revealed that 29 States are now supplying reports on such strikes. The bird programmes now being adopted by some States show signs of producing bene- ficial results and in one programme the rate of bird strikes per thousand aircraft movements has been almost halved. Various research programmes in different countries have provided valuable data. The ability of those aircraft struc- tures designed to fail-safe criteria, to withstand bird strikes has been an encouraging item of information to be revealed. The recommended PAMC specifies the requirement for wind- shields and engines to withstand bird strikes, and it is intended to examine the vulnerability of other parts of the airframe to strike damage and to add suitable criteria to the document in order to provide the necessary guidance. Structural Problems—Gust Criteria The PAMC recom- mended as a result of the committee's work on this agenda item adheres to the traditional concept of ultimate loads, and states that a factor of safety of not less than 1.5 be used. One member pointed out that the ultimate load concept docs not provide the same level of safety in both positive and negative directions and that this is of great significance to those short-range jets which operate at speeds very near to the design cruising speed. The alternative "maximum" load concept gives improved protection equal to 0.6g against nega- tive loads and has been adopted by one State. Further studies in this and other fields will be carried out by members of the Committee between the seventh and eighth meetings. There was also some valuable discussion on flexibility effects and the response of the aeroplane to gusts. Performance The draft PAMC on this item had been dis- cussed by the Committee at previous meetings and it had been expected that it would have been concluded at the sixth meeting. In the event, the item consumed a good deal of the time allocated to the seventh meeting, and some keen arguments and compromises were necessary before it was pos- sible to reach agreement. It was decided that stalling speed Vs could be determined by means other than the lg method, provided that appropriate factors are used to provide the intended level of safety. The specifications which provide wet-runway accountability in the take-off case were removed from Part 1 of the PAMC and included in Part 2—the flight manual section—where they become information to the pilot rather than specified requirements to be demonstrated before certification. The landing-distance specifications were another cause of difficulty, for it was not possible to achieve unani- mous agreement on the original "rational" method. The PAMC now contains both the "rational" and "arbitrary" specifications used in the UK and USA regulations respec- tively. It was agreed that further consideration will be given to providing crosswind accountability in the landing case and to the applicability of the PAMC for all-weather opera- tions. Flying Qualities The Committee had what were perhaps its most productive discussions on this agenda item. The range of discussions included dynamic stalls, high angle of attack stalls, unconventional stall identification (stick pushers), speed stability, out of trim conditions, Dutch roll, high speed characteristics and longitudinal control stick force per g.
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