Flight control system always use high level hardware redundancy configuration to improve mission reliability for big aircraft, but it is not reasonable to install triple or quadruple sensor redundancy on small aircraft due to the limitations on space, weight and cost. It is very difficult to identify the failure with dual or single sensor. Also,conventional fault detection and isolation method cannot isolate multiple faults in a triple redundancy system. To solve this problem, a kind of non-model based fault diagnosis which combines a parity equation approach with the wavelet based technique were introduced in this paper.