AIRCRAFT DESIGNER'S VIEWPOINT OF RELIABILITY AND MAINTAINABILITY .sx A J Hill .sx Westland Helicopters Limited .sx Yeovil , England .sx Abstract .sx Papers on Reliability and Maintainability ( R&M ) stress the importance of R&M design to optimise Life Cycle Costs and Availability .sx This view is reflected in the requirements placed upon aircraft manufacturers by Civil and MoD customers to the extent that R&M is frequently given equal priority with , for example , aircraft performance requirements .sx During the initial aircraft design phase and subsequent development phase , R&M engineers advise the designer on R&M requirements but these requirements are frequently in direct conflict with other requirements such as weight , aircraft performance and cost .sx Although the idealised view is to treat these requirements as equal , the reality is that aircraft performance and weight are frequently perceived to be more equal than other specialist requirements , including R&M , particularly if the aircraft experiences weight problems .sx The aircraft designer has to reconcile these conflicting requirements to achieve the overall aircraft design .sx Frequently , this results in rejection of the design recommendations of specialist departments and adopting alternative design solutions to meet the aircraft design objectives .sx This paper discusses an aircraft designer's view of R&M design , its relationship to other design requirements and the problems raised by the often conflicting requirements .sx Discussion .sx Introduction .sx Ladies and gentlemen , you will note from the Chairman's introduction that my background is in aircraft maintenance , R&M and Design to Cost .sx Therefore , I am only too aware that R&M is extremely important in aircraft design and the subsequent success of the aircraft in the market place , either in the military market or in the civil market .sx The necessity for Failure Mode Effect and Criticality Analyses , Direct Maintenance Cost Analyses and the many other R&M tools used in the design and development of an aircraft is acknowledged :sx however , these tools will not be discussed in this paper .sx I will put forward some views of an aircraft designer on R&M issues with an emphasis on maintainability .sx I have held the job of an assistant to a Chief Designer for some years and I am , therefore , familiar with R&M and other design requirements .sx Most papers on R&M are presented by R&M specialists who frequently exclude other disciplines .sx This paper will attempt to show that the aircraft designer has to reconcile the requirements of R&M with other , often conflicting , requirements .sx R&M Requirements .sx R&M has become increasingly important over the years with the MoD and civil customers requiring good R&M features to minimise Life Cycle Costs and to maximise the Availability of the aircraft .sx This is a typical statement which is included in aircraft specifications from the earliest stage of an aircraft project :sx Reliability and Maintainability shall be given equal priority to aircraft performance .sx To be given equal priority to aircraft performance means that R&M requirements must be clearly defined before the design begins and closely monitored during the design and development phases .sx Figure 1 shows a simple bar chart defining the phases and the activities addressed in each phase .sx In my view , the most important phase of a project is the definition of the R&M requirements .sx In addition to requirements expressed in rates , such as failures per flying hour and maintenance man hours per flying hour , the R&M design features must also be clearly defined .sx Loosely defined or ambiguous requirements frequently result in very expensive re-design activities when problems arise during the development phase .sx However , and this is a very important point , the requirements must not be excessive for the task .sx The next important activity is the initial design phase which must be given sufficient time for R&M specialists to advise designers and draughtsmen on the required R&M design features .sx Programme Managers must ensure that the initial design timescale allows the iteration of all designs before the drawings are required for manufacture .sx Appreciable cost savings are made by avoiding the re-design activities during the development phase which result from designing to tight project timescales in the initial design phase .sx The necessity for R&M specialist knowledge in the initial and subsequent design phases is considered essential .sx Careful personnel selection can ensure that R&M specialists provide valuable advice to designers .sx For example , R engineers with an extensive knowledge of past reliability problems , and M engineers with first hand knowledge of problems unfamiliar to the designer , such as maintaining naval aircraft at sea in cramped hangar conditions , servicing aircraft on flight decks in adverse sea and weather conditions and operating military aircraft from forward bases .sx Thus , the R&M specialists can make important contributions to the design process due to their on-the-job experience which the designer or draughtsman may not possess .sx However , it should be noted that , whereas maintainability features can be assessed with reasonable confidence from drawings , reliability assessment is often overturned during development due to unforeseen problems , such as higher than expected local vibration levels .sx At Westland , we have a very active R&M department influencing the design process .sx The result is large numbers of recommendations for initial design and subsequent design improvements throughout the design , manufacture/build and development phases in order to achieve optimum R&M design .sx This is only to be expected , as any R or M engineer worth his salt will strive to introduce the best features relating to his own discipline .sx However , R&M engineers are often not concerned with the adverse effects that the recommendations have on other design requirements and may be unaware of any adverse effects .sx The aircraft designer is , however , concerned with all design parameters and has to reconcile the often conflicting requirements of aircraft performance , weight and cost as shown in Figure 2 .sx Selecting the maintainability discipline as an example , many maintainability features lead to higher acquisition costs and increased weight .sx Whilst the cost aspect is important , an increase in weight beyond the aircraft specification dry weight has serious consequences to aircraft performance such as reductions in payload , range , endurance and time-on-station .sx Any recommendation , from whatever source , which compromises the aircraft specification dry weight and performance will not be looked upon favourably .sx In other words , aircraft performance requirements will take precedence over other requirements , excluding safety , as any approach to the customer on a reduction in aircraft performance would be rejected .sx In the final analysis , the fighting capability of a military aircraft or the commercial capability of a civil aircraft will be more important .sx Examples of Weight Increases .sx Examples of recommended maintainability features which have weight and cost penalties are as follows :sx Reposition hydraulic ground servicing points from the cabin roof to a position 5 feet from ground level .sx The maintainability advantages of such a feature are self-evident .sx Maintenance personnel would have easy access to the servicing points , connection and disconnection of hoses would be less risky to personnel , particularly on a ship rolling at sea , and the hydraulic hoses would not abrade the aircraft skin .sx However , such an improvement increases the hydraulic pipe lengths and introduces an additional panel , both of which increase weight and cost .sx The final judgement , taking into account such features as an on-board electro-hydraulic servicing pump , may be to reject the proposal .sx Introduce built-in steps for access to maintenance areas .sx Again , the maintainability advantages are obvious , but built-in steps also cause increased weight due to the steps and the required strengthening of the surrounding structure .sx An alternative and much lighter option would be to provide hard points on the aircraft for GSE ladders to access the maintenance areas .sx However , this alternative is always vetoed by customers on the grounds that GSE must be minimised and that the ladder would have to be carried in the aircraft , therefore providing no weight advantage .sx In my view , the requirement to carry ladders in the aircraft on all flights is questionable .sx Hard points and lightweight GSE ladders could provide greater safety for maintainers when accessing high areas of the aircraft and could be provided in more maintenance areas at a reduced weight penalty .sx In any event , if built-in steps are chosen , the number and position will be very carefully scrutinised to minimise the weight and drag impact .sx Modify the fuel pump installation to allow the pump to be changed without draining the fuel from the tank .sx Modern fuel pump installations provide the capability to change the fuel pump without entering the tank .sx Therefore , the advantage of not draining the tank is a reduction in the Time To Repair .sx However , this has to be weighed against the reliability of the pump and the time to drain the fuel .sx If the pump is highly reliable and the time to drain the tank is small , the additional weight and cost of such an improvement is questionable .sx Provide a single point opening/closing mechanism on the engine and gearbox panels .sx This recommendation reflects the customer's wish to ensure the safety of a maintainer when opening and closing panels , particularly in adverse weather conditions ashore and at sea .sx However , interconnected latches increase weight and cost and raise questions of safety .sx The loss of a panel , particularly on a helicopter , can be catastrophic and any design which interconnects panel latches must always incorporate a separate safety latch to guard against failure of the mechanism and loss of the panel .sx Provide sealed bearings in the landing gear to avoid routine greasing operations .sx Experience has shown that sealed bearings are not a 'fit and forget' maintenance feature when sealed bearings are used in areas exposed to the weather .sx Water ingress into the bearing , particularly salt water , causes the grease to deteriorate , damaging the bearings and reducing reliability .sx There have been reports that some operators have modified sealed bearings to introduce grease nipples .sx Provide a single point engine compressor washing facility with a motorised ball valve operated from the cockpit .sx On a multi-engined aircraft , this recommendation avoids the need for maintainers to move the compressor washing rig from one wash point to the other and reduces the time to wash engines .sx However , once again the penalties are the additional weight and cost of the motorised ball valve , electrical wiring and controls and the addition to the overall aircraft failure rate caused by the new system .sx The foregoing are typical examples of maintainability features which impact on weight and cost .sx Other examples are :sx Fit dummy plug stowages on the main rotor head for the blade heater mat plugs .sx Fit a cross spirit level in lieu of a plumb bob for aircraft levelling .sx Fit a tie-down socket in the main rotor blade for blade tethering .sx Fit lifting lugs to the transmission gearboxes .sx Lower the engine compressor wash panel to improve access .sx Introduce a maintenance 'bridge' to traverse the roof deck .sx Provide an in-flight compressor washing facility .sx Provide a drain pipe from each engine oil sump .sx Exclude locking wire from all aspects of design .sx Introduce quick release/self sealing couplings on hydraulic and fuel lines .sx Introduce remote oil level sensing and replenishment .sx Introduce Built In Test facilities for all avionic equipments .sx All the examples illustrated are accepted as good features from a maintainability point of view , but each of the recommendations has to be studied to assess its effect on weight and cost .sx If the features are essential , they will be introduced but non essential design features will be modified or rejected , particularly if the specification dry weight and the aircraft performance is compromised .sx Summary To summarise the points made :sx R&M requirements must be clearly defined before the initial design phase commences .sx There must be no ambiguity in the stated requirements and excessive design features should be avoided .sx R&M expertise is essential to the designer .sx Careful personnel selection is important to provide on-the-job experience to assist designers and draughtsmen .sx R&M must be an integral part of the design process .sx It must not be regarded as a fringe activity .sx Project Managers must ensure adequate time for iteration of the design in the initial design phase to avoid expensive re-design activities during development .sx R&M specialists must consider the cost and weight implications of R&M design improvements .sx Can the improvements be justified or are they excessive for the task ?sx The R&M disciplines lose credibility if excessive demands are placed on the aircraft designer .sx When aircraft specification dry weight is exceeded , a compromise on all design features is necessary to maintain weight and performance levels without adversely affecting aircraft safety .sx