Modern flight control systems design demands accurate computer models, able to reproduce the behavior of on-board equipment. These high-fidelity models have a large computational cost in terms of memory and time, since highly detailed. On the contrary, for development of diagnostic and prognostic strategies simplified models are needed, since they require smaller computational costs with an adequate level of accuracy. However these low-fidelity models with concentrated parameters, developed from the physic of the problem with different linearizations, are not sufficiently representative in several aspects. The current trend is to make adjust to this model, in order to simulate non-linearity such saturations, water hammer’s rise of pressure, oil leaks.