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Aviation History
1955
1955 - 0320.PDF
320 FLIGHT, 11 March 1955 AIRWORTHINESS . . . '\ contained in the old A.P. 1208 were very meagre. The perform-ance requirements were similarly sparse, and were purely arbitrary. On take-off, aircraft had to be able to clear 20m (66ft) withina distance of 600m (1,968ft); landing distance was fixed at 250m (820ft) for aircraft weighing under 6,600 1b, and 300m (984ft) forall heavier aircraft. There were no "engine-out" requirements. During the 1930s, there began the complete replacement of earlytransports by cantilever monoplanes. The latter introduced numerous refinements such as flaps, retractable undercarriages,variable-pitch airscrews and a complete absence of external brac- ing, and they also caused a great acceleration of the trend towardslower power loading and higher wing loading. Such machines demanded a more comprehensive airworthiness performance codethan had hitherto existed. In Great Britain, World War II began before such a revisedcode had been formulated, but the U.S.A. were ahead of us in introducing "modern" transports and they were accordingly thefirst to expand their performance code. Originally each State of the Union had its own ideas upon air safety, but a uniform systemwas obvious (readers may like to ponder upon the consequences of the former) and this was provided by the formation of the CivilAeronautics Administration (see p. 314). One of the characteristics of the original U.S. code was theestablishment of an absolute limit upon Vs, the stalling speed. When the new monoplanes came, V8 had to be allowed to rise—and it has kept on rising, in easy stages, ever since, as the latest American transports show. At an earlier time, the length of fieldrequired for take-off increased to the point at which a pilot could no longer study the strip by eye and decide whether or not it wassufficient. For the first time, therefore, account had to be taken of the route flown and the conditions experienced. The increasein V. also made necessary the introduction of certain safeguards in engine-out performance of a sufficiently comprehensive natureto supersede the code based on V s itself.A major characteristic of the U.S. code was that rate of climb was made a function of stalling speed—actually 0.035 Vs2. Noaccount whatever was taken of climb gradient. As a result, manu- facturers tended to meet the requirements by employing variousdevices which did not necessarily effect any real improvement in safety. Taking the Boeing 377 (Stratocruiser) as an example,elevator power in this aircraft is insufficient to achieve V s withthe e.g. fully forward, so a restriction was placed keeping the e.g. near the aft limit. The rate of climb curve on page 319 underlinesthe difference between British and American aircraft now in ser- vice; the British aircraft retains a positive climb right down tothe stall, whereas the American aircraft, with highlift flaps and its nose well up, begins to sink at an air speed greater than the stall.Again, U.S. transports are all air-cooled, and a standard for a hot day was specified as 100 deg F, at which the en route per-formance was adequate for appropriate cooling. Nevertheless, a new requirement was later introduced establishing the mini-mum climb speed as 1.21 Vs to meet the requirements for the standard atmosphere. This speed, known as the take-off safetyspeed, increased the field length needed for a particular aircraft. At the same time, automatic safety features were introducedto ease the pilot's task. Most important of these devices was the automatic feathering airscrew; auto-feathering is generallyinitiated by a signal whenever torquemeter pressure falls below a datum. Such equipment represented the first attempt to relatereliability with performance; previously, there was no guarantee that performance requirements would improve detail engineering.Concurrently with the introduction of the new C.A.A. perform- ance code went the first systematic mapping of the obstructionsaround airfields. This was rendered doubly necessary by the fact that the code took no account of climb gradient. In orderto get economic field length, engine-out performance and (in particular) directional control were greatly improved; it is appro-priate to cite the Constellation in comparison with the earlier twin-engined Lockheed transports. Naturally enough, one ofthe chief aims of the U.S. manufacturers was to reduce stalling speed, and this can all too easily be done at the expense of a reduc-tion in stall quality. As a result, the U.S. code had to introduce a number of very detailed requirements regarding stall quality.Under B.C.A.Rs., a full examination is made of the stall itself, irrespective of stall-warning. This period up to about 1951—which might justifiably betermed die "American era"—gave, for the first time, reasonable attention to the several factors mentioned. On the other hand,temperature and humidity of the air were still unaccounted for and there were other ill-effects. For example, pilots were pre-sented with complex flight manuals involving varying e.g. limits and varying safety speed. A more serious trend resulted directlyfrom the engine-out climb requirements; power loading was decreased and the failure rate rose. In fact, the more likely anengine was to fail, the more climb was necessary, so forming a vicious circle. Broadly speaking, most transports in service could at one time (1948-50) be divided into those which could meet require-ments but had unreliable engines, and those which had reliable engines but which could not climb. The DC-3 is an obviousexample of the latter; and it must be noted that such aircraft as the DC-3 and Rapide are still in service, although they haveno hope of meeting modern performance requirements. Per- mission for them to do so results from the fact that it is reason-able to assume that the majority of their troubles have been ironed out years ago, and today they are exceptionally reliable. Anyaircraft intended to replace them has to meet modern require- ments fully and, although this places the newer design at a greatdisadvantage, it is logical in order to ensure safety of the untried machine.The revised C.A.A. code was largely written around the actual performance of certain aircraft then in use, and it incorporatedaccountability for airfield height, size, obstructions and en route terrain; it did not, however, account for temperature, and thisparameter has still not been fully allowed for in the American code. A Rational Code As previously related, the United Kingdom set about establish-ing a completely new code of performance requirements after World War II. The proposals for full temperature-accountabilitywere made by this country at the first I.C.A.O. session of 1946, and a rational method of allowing for the random variation in air-craft performance was made available in 1948. A new code em- bodying these principles was proposed by the U.K. at the thirdAir and Operations session in 1949; it was intended to establish a certain minimum safety on each flight and to err on the safeside where circumstances were unknown. Basic minima were calculated, below which a pilot might be in trouble; there mightbe random advantageous features associated with a particular case, but, for example, if an aircraft were allowed to deceleratebelow its safety speed, a pilot could not reasonably be blamed for losing control. Suppose that the likelihood of engine failure is 1 : 1,000; theaircraft then needs an engine-out performance such that there is a 1 : 100,000 chance of going below the minimum datum, havingregard to all factors. This datum is defined as the flight path below which the aircraft should not, by reason of lack of per-formance, be brought except on the occasions allowed by the design incident probability. For example, on take-off the datumis an obstacle clearance of 35ft at the end of the runway, which includes a certain amount for missing the obstacle and a smalleramount to allow for the height which might be lost due to the effect of small corrective turns. This concept of a design incident probability (of 10"5) is one ofthe basic criteria of the rational concept of aircraft performance aimed at in British certification. In assessing the incident prob-ability, the contributions of all the possible cases are summated, although the cases with one or two engines out probably accountfor over 90 per cent of the whole. Such statistical investigation emphasizes engine reliability and also correlation between onefailure and another (for example, a loose airscrew blade from No. 3 engine might cause complete failure in No. 4). Correlation betweenfailures cannot be ignored and the aircraft systems have to be so arranged that, apart from the possibility of dirty fuel or oil, orthat of a ground engineer making a similar mistake on each engine, no combined engine failures are likely. Reliability of flaps, brakes, and similar components all affectperformance, and the consequences of unreliability can be evalu- ated. In the design stage it is impossible to state the reliability of,say, a hydraulic system, but intelligent engineering can ensure that the system will not cause a catastrophe. Thus, it is common-sense to ensure that, even with two engines out, a long-distance aircraft will retain full control, full radio facilities, ice protectionand other essential services; control must also be retained even with all engines failed. Naturally, some loss of services has to betolerated but an aircraft is difficult enough to land with two engines out in any case, and it is essential that the pilot's task should beeased as far as possible. In all cases, engines must be made re-startable in the air. If anaircraft were to develop an en route vibration the pilot would like to be able to shut down each engine in turn to find the source ofthe trouble. With a piston engine re-starting is generally quite straightforward, the only likely difficulty being the possibility ofcoring of the oil cooler and freezing of the airscrew hub in the feathered position. With a gas turbine, however, there may bea definite maximum re-light altitude and, although not usually a critical consideration, this should be taken into account whenapplying en route performance requirements. This type of problem, which was first met with the Comet 1,demanded the establishment of a scheduled all-power-units- operating height which should not be greater than the re-lightaltitude. This permitted flexibility in the accepted re-light altitude by ensuring that the aircraft was properly matched to its route.At the same time, the drift-down technique was employed to enable the aircraft to clear all obstacles by an adequate safety Continued on page 336)
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