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Aviation History
1981
1981 - 1244.PDF
1254 FLIGHT International, 2 May I9BI - design for maintenance Chief designer BOB GRIGG describes the philosophy behind the maintainability of the new airliner MAINTENANCE is a well known word which appears to need no ex planation, but the "airt" of maintain ing a modern transport aircraft is far more complex than the definition of the word implies. The object of aircraft maintenance is toi prevent deterioration of the in herent safety and reliability levels of the aircraft's structure and equipment and if possible to improve these levels by modification action as deficiencies are recognised, or by practical demon stration during the initial testing periods and during subsequent ser- vice operation of the' aircraft. In the past, whatever the "down time" required for maintenance, it was accepted as unavoidable. But the increased application of more com plex equipment, competition between similar types of aircraft and compe tition between short-haul aircraft and surface transport, and increasing utilisation of the aircraft have made "down time" much moire critical. In creases in complexity result in more scheduled and unscheduled mainte nance simply because there are more components to go wrong. So it can be seen that the maintenance engineers are faced with mounting pressure to' do a more complicated task in less time. It is vital that the proposed main tenance plan is first reduced to an absolute minimum and that it is thoroughly checked by the manufac turer before the airoraft enters ser vice. This short paper looks at methods employed by British Aerospace, based on experience with Trident, One- Eleven, 748 and Concorde to achieve this end, with particular reference to our new short-haul BAe 146. Design philosophy The will to reduce maintenance costs must start right at the beginning of the design stage and be monitored throughout. The three major 146 design philoso phies are: — • Design simplicity using well tried engineering principles using off-the- shelf components wherever possible, requiring minimum development • Using minimum number of compo nents and minimum number of different components • Establishing maintenance cost tar gets and monitoring during design stage • Including maintenance cost guaran tees or targets as part of the con tract with major equipment sup pliers. A determined effort has been made to simplify both structure and sys tems design by significantly reducing the number of parts, joints and fasteners throughout the aircraft. Where possible, off-the-shelf compo nents are used, and not new tech nology for the sake of new tech nology, but only where a positive gain in improved performance or reduction in weight or costs can be justified. Maintenance costs have been treated as a major design parameter, targetting and control being carried out by the design department in a similar fashion to weight and produc tion cost control. All major vendor suppliers have been subjected to the same mainten ance cost disciplines. All vendors tendering for equipment supply have been required to give maintenance- cost guarantees or maintenance cost targets supported by convincing evi dence that these targets would be met. These data were assessed very carefully during the vendor-equip ment decisions. Maintenance cost target The initial target for the airframe maintenance cost was derived by analysing the achieved maintenance cost levels of a range of similar aircraft doing a similar task and plotting these costs against aircraft max Zero Fuel Weight (ZFW) (Fig 1). A mean costs line was drawn through these results and then an improved cost line was determined by assuming a 15 per cent improve ment at the lower weight end (mainly older designs) and a 5 per cent im provement at the higher weight end (mainly later aircraft designs). A vertical line drawn at thei 146 ZFW gave the "improved" level for the direct airframe maintenance cost and the 146 target was set at 10 per cent below this figure. From a knowledge of maintenance costs for systems on aircraft fulfilling a similar role, this total airframe target cost was sub divided according to US Air Trans port Association methods. To ensure that these targets are met it is neces sary that a close control is continually maintained over the system and equipment design. This process is carried out by the design office relia bility department using maintenance cost control charts. These component- cost estimates are continually moni tored and any differences are examined with the supplier. At the same time, the product sup port department carries out qualita tive appraisals of the aircraft, its systems and components, approached from the aircraft operation viewpoint. These appraisals examine in detail line replaceable units (LRUs) and their installation, the overall systems,
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