Modelling and optimisation of maintenance intervals to realize structural health monitoring applications on aircraft

Commercial airliners have continually voiced the need to improve the availability of their fleet and with current financial pressures facing many airlines this need has even become more significant. SHM can contribute to enhancing aircraft availability through substituting routine heavy structural maintenance inspections with automation. However, specific aircraft locations and the impact of SHM alternatives are yet to be defined. The paper presents a two stage approach centred on analysing maintenance intervals, and areas where improvements to an aircraft's availability can be achieved. This approach is explained with reference to real aircraft maintenance scenarios where critical areas are identified and the impact of implementing such technologies are assessed. The analysis requires initially sourcing information via maintenance documentation and then mapping sequences of steps from the data collected. As part of a study to highlight the approach two types of scheduled intervals were taken for analysis. Conventional DCheck (still adopted today) was modelled with a discrete event simulation tool called Arena. Savings of 3 hours per D-check interval could be realised by placing hotspot monitoring to a set of locations highlighted in the paper. This would equate to 6 hours saved over the life, which may be insufficient to justify investment in SHM technologies. It also proves that aircraft maintenance processes and damage tolerant design are well optimised from an initial design point of view and that the high degree of an aircraft disassembly required may only provide a very limited potential for SHM within the frame of current aircraft initial structural design. However aircraft structures are likely to fracture at locations other than those predicted initially. This effect is not critical from a safety point of view since those fractures are detected well before they become critical. However those damages being called the drop outs lead to specially directed inspections which are likely to fall outside of the conventional maintenance schedule and hence can become comparatively costly. The example of a 2400 FC interval only provided here has three items of which the critical item is a detailed inspection that always lies on the critical path, despite fluctuations with uncertainties. Substituting for a SHM centred process could save up to 280 hours over the aircraft life. With the cost of operation at a few thousand EUR per hour, application of SHM to drop out intervals to further optimise maintenance processes could certainly justify future investment.