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Aviation History
1955
1955 - 0383.PDF
25 March 1955 383 MAINTENANCE of HELICOPTERS A B.E.A. Engineer Speaks to the Helicopter Association AT the seventh meeting of the Helicopter Association's 1954-55lecture programme, held in the library of the Royal>- Aeronautical Society on March 11th, a paper entitled Heli- copter Maintenance was read by Mr. J. H. Willans. Introducingthe speaker, the chairman, Mr. A. McClements said that he had been an aircraft maintenance engineer for 20 years and, since1951, had been works manager of the B.E.A. Helicopter Unit at Gatwick. Mr. Willans prefaced his remarks by refuting any possibleimpression that he was biased in favour of American helicopters. He thought, in fact, that the Bristol 171, on the maintenanceof which his comments were mainly based, was a better machine than all the others in most respects. That did not mean, however,that there was not room for improvement in certain details. The golden rule in helicopter maintenance was, he thought, toleave well alone when a component was working satisfactorily. Trouble always seemed to follow when a unit was subjected to aroutine examination to see whether it was functioning correctly. Sight, sound and feel were used to keep watch on the conditionof working parts and he had never known of a working-part failure which had given no warning of its condition. Fatigue, on the other hand, was a different matter, and wasentirely a designer's problem and responsibility. There was nothing the maintenance engineer could do about it other thanto realize which parts were subjected to a stress reversal whilst working and keep an eye on them for cracks. Most fatigue failuresseemed to occur in non-working parts where it was not appre- ciated that stress reversals were taking place before it was too late. The B.E.A. maintenance schedules were based on five inspec-tion periods: Check 1, daily; 2, at 42 hr; 3, at 125 hr; 4, at 250 hr; 5, at 500 hr. Check 4 was the one on which the serviceability of the machinewas based. At this period all the inspection panels and cowlings were removed and the helicopter was examined in detail through-out. An extensive programme of work was not necessarily called for. It was an examination to ensure a further 250 hours oftrouble-free running but it seldom happened that there was nothing to be attended to. Another side to the maintenance schedule was that associatedwith the overhaul requirements. Certain components with inter- nal working parts could not be classified as serviceable by anexternal examination and so the schedules listed all these items and defined the times at which they should be removed from thehelicopter for a complete strip down. The overhaul of these units was the most expensive part of the maintenance of acomplete helicopter. Using figures for the Bristol 171, there would be approximately3,800 man-hours expended on the maintenance checks per year, at the rate of a 2,000 hour/year utilization per aircraft. Theaverage so far obtained with these helicopters for total labour requirements was 12 man-hours per flying hour and so on thisbasis the total requirement for the year would be 24,000 man- hours. Since, out of this total, 3,800 man-hours would be takenup with the maintenance checks, it followed that the component overhauls would require 20,200 man-hours, or more than fivetimes the maintenance requirement. These figures were based on the low overhaul life (250 hours) for most of the componentsand showed clearly the urgent need for increasing this life. There was no reason why it should not be done, judging by the conditionof the components when examined. There was little complaint to make with regard to accessibilityand ease of removal from the airframe but there were many details where improvements could be effected. As far as gearboxes were concerned, the American method of using ball bear- ings and ball thrust races allowed a far simpler re-assembly afteroverhaul than did the British tapered roller races. Also, the American peel-off shims were better than the British solid steelshims which had to be ground down to size each time after an elaborate fitting process. Obtaining the correct fits and clearances could be an expensivejob as far as man-hours and equipment required were concerned and this point had been badly neglected in some helicopter designs.The lecturer continued on the question of rotor blades. There was a marked difference between the standards of finish in theAmerican and British designs of both main and tail rotor blades, the American finish being superior. In some cases, the thinsurface finish on British blades led to severe tracking troubles owing to moisture penetration. There was one type of Britishmetal blade in service, but it had not been in use for long enough to make comparisons. The need for tracking the main rotor hadto be avoided entirely if the helicopter was ever to become an economic commercial vehicle. It absorbed an enormous quantityof man-hours and none of the methods in use was completely satisfactory. The life of tail rotor bearings was too low andmeans should be provided to adjust the track of tail rotors, which were a constant source of vibration. The airframe structure had never given any trouble on eitherthe American or British designs but far more attention should be paid by the designer to the fitting of doors and their locks toensure that they were proof against draughts and rain. This was a problem not confined to rotating-wing aircraft but itdemanded more attention than in fixed-wing machines because of the increased vibration level. The undercarriage and wheel brakes had not given rise toany significant problems, and maintenance of these units could probably be considerably reduced without affecting safety. Therotor brake, on the other hand, was a most important item as it was necessary to be able to stop the rotor quickly for the sake ofpassenger safety and to prevent blade "saUing" in a high wind. For this purpose the rotor-brakes so far provided had been acomplete failure. They were too small and subject to constant unserviceability. There was now, for the first time, a rotor brakeof American design which might prove adequate but B.E.A. had not had it in service for long enough to be certain. Fuel and hydraulic systems could be said to be identical withthose in fixed-wing aircraft from a maintenance point of view and no special troubles peculiar to helicopters had been experienced.It might be expected, however, that leaks and broken pipes in the hydraulic system would be intensified, owing to vibration, unlessgreat attention was paid to proper support for all parts of the system. Instruments gave no more trouble in helicopters than they didin fixed-wing aircraft, but they could well be more reliable. The specifications used to manufacture and overhaul instruments madeno allowance for helicopter vibrations and, since the instrument panels in which they are mounted could not be brought withinthe fixed-wing standards of vibration, there was a gap between the two which spelt unreliability. This gap could not be closedby the maintenance engineer. Mr. Willans concluded by saying that careful planning couldreduce or vary the elapsed time required for a machine to be "off service" in order to carry out the maintenance schedule.The whole check could be done at the time it became due, or daily checks could be carried out with parts of all the other checksadded each day, so that the whole of a major check was com- pleted within the prescribed period. This latter practice, pro-gressive maintenance, had its advantages but had to be employed with care and experience, and then only as a necessity. Localconditions had a major influence on the way in which schedules were followed but if progressive maintenance was over-done itcould lead to an uneconomical use of man-hours and, therefore, a higher maintenance cost. The Discussion F. L. Swain (Westland) did not agree with the lecturer's state-ments on component lives. His company had had tail rotor gear boxes still in perfect condition after 600 hours' service and mainrotor gear boxes after 480-500 hours. On the question of tracking, he said that they had known their metal bladed rotors to keepperfect track for as long as 400 hours without adjustment in certain cases. Servicing could be made easier by using the unitreplacement system. G. E. Walker (Bristol) agreed that the lecturer's policy of leavingwell alone was a good one where it could be applied. The number of 250-hour inspections carried out on Bristol 171s was small bycomparison with the experience on American helicopters but his company thought that the experience so far obtained was sufficientto justify increasing the overhaul life of certain components to 500 hours in the near future. He thought that tapered rollerbearings were the most suitable for the application and said they were now almost universally used in motor-car wheel hubs anddifferential gears. He agreed that the tail rotor bearing life was low, but these components were inexpensive items which wereeasy to replace. He had not previously heard of any necessity to track tail rotor blades. E. Voss (Bristol) said that 400 hours was the present militaryoverhaul life for all components throughout the transmission system of the 171 helicopter and he also thought that the lecturer'sman-hour figures for overhaul times were too high. 500 man- hours should be ample for a complete transmission overhaul.The solid steel shims should only need grinding after 1,500-2,000 hours. Their correct fit was a good indication that the gear box
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