Systems reliability for phased missions

The concept of a phased mission has been introduced as a sequential set of objectives that operate over different time intervals. During each phase of the mission, the system may alter such that the logic model, system configuration, or system failure characteristics may change to accomplish a required objective. A new fault tree method has been proposed to enable the probability of failure in each phase to be determined in addition to the whole mission unreliability. Phase changes are assumed to be instantaneous, and component failure rates are assumed to be constant through the mission. For any phase, the method combines the causes of success of previous phases with the causes of failure for the phase being considered to allow both qualitative and quantitative analysis of both phase and mission failure. A new set of Boolean laws is introduced to combine component success and failure events through multiple phases so that the expression for each phase failure can be reduced into minimal form. [Continues.

Phased mission modelling using fault tree analysis

Many types of system operate for missions which are made up of several phases. For the complete mission to be a success, the system must operate successfully during each of the phases. Examples of such systems include an aircraft flight, and also many military operations for both aircraft and ships. An aircraft mission could be considered as the following phases: taxiing to the runway, takeoff, climbing to the correct altitude, cruising, descending, landing and taxiing back to the terminal. Component failures can occur at any point during the mission but their condition may only be critical for one particular phase. As such it may be that the transition from one phase to another is the critical event leading to mission failure, the component failures resulting in the system failure may have occurred during some previous phase. This paper describes a means of analysing the reliability of non-repairable systems which undergo phased missions. Fault Tree Analysis has been used as the method to assess the system performance. The results of the analysis are the system failure modes in each phase (minimal cut sets), the failure probability in each phase and the total mission unreliability. To increase the efficiency of the analysis the fault trees constructed to represent the system failure logic are analysed using a modularisation method. Binary Decision Diagrams (BDD’s) are then employed to quantify the likelihood of failure in each phase

Phased mission modelling using fault tree analysis

Many types of system operate for missions that are made up of several phases. For the complete mission to be a success, the system must operate successfully during each of the phases. Examples of such systems include an aircraft flight, and also many military operations for both aircraft and ships. An aircraft mission could be considered as the following phases: taxiing to the runway, take-off, climbing to the correct altitude, cruising, descending, landing and taxiing back to the terminal. Component failures can occur at any point during the mission, but their condition may only be critical for one particular phase. As such, it may be that the transition from one phase to another is the critical event leading to mission failure, and the component failures resulting in the system failure may have occurred during some previous phase. This paper describes a means of analysing the reliability of non-repairable systems that undergo phased missions. Fault tree analysis (FTA) has been used as a method for assessing the system performance. The results of the analysis are the system failure modes in each phase (minimal cut sets), the failure probability in each phase and the total mission unreliability. To increase the efficiency of the analysis, the fault trees constructed to represent the system failure logic are analysed using a modularization method. Binary decision diagrams (BDDs) are then employed to quantify the likelihood of failure in each phase