The safe dispatch of aircraft with known faults

Time-limited dispatch (TLD) allows the dispatch of aircraft with faults present in their control systems for limited time periods. In order for TLD to be applied to an aircraft system it is first necessary to demonstrate that the relevant safety and certification requirements are being met by modelling the system in question. To do this existing modelling techniques use variations of fault tree analysis and Markov analysis with various simplifying assumptions, made to assist in the analytical process. Monte Carlo simulation is presented here as an alternative method of analysis, which can deal well with the potential difficulties that may present themselves when modelling TLD, such as the complex architectures of aircraft systems and dependencies that are introduced when applying TLD. In this paper a simple example system is introduced and the application of TLD to it is modelled using the existing variation of Markov analysis and a Monte Carlo simulation technique. The results obtained using the different techniques are seen to differ and a number of reasons are suggested for this difference.