The ability to assess faults in a system, while it is operating, requires an appropriate set of measurements. Engine availability can be increased if the faults can be detected, isolated and assessed, so enabling an optimised shutdown of the plant for maintenance to ensue. Depending on the engine-power-setting parameter, the measurements required to diagnose the faults along the gas path of a gas-turbine vary. This study used a non-linear gas-path analysis (NLGPA) model to predict the required instrumentation set, which can be optimised with respect to the number and type of sensors and their locations for the considered engine-faults. A thermodynamic model of the behaviour of a 2-shaft engine is used as a case study. Redundancy in the sensor set is shown to be unnecessary.