In this article, the fault detection problem for the networked flight control systems with packet dropout is investigated. The networked flight control systems with periodic packet dropouts are treated as a varying sampling rate system. Refer to the non-periodic packet dropout that could be described as a Markov process, the varying sampling rate networked control system could be farther modeled as a discrete-time Markov jump linear system. Based on this model, considering a more general situation that the transition probabilities are partially known, a fault detection filter is established whose robust performance index is optimal. Meanwhile the corresponding fault detection problem can be transformed to an LMI optimization problem, and sufficient conditions to solve this problem are simultaneity derived. Finally, a networked flight control system is considered to validate the effectiveness and potential of the proposed method.