We use an approximate input reconstruction algorithm to reconstruct unknown inputs, which are then used as a basis for fault detection. The approximate input reconstruction algorithm is a least squares algorithm that estimates both the unknown initial state and input history. The estimated inputs are then compared to the commanded values and sensor values to assess the health of actuators and sensors. This approach is applied to the longitudinal and lateral dynamics of an aircraft. The input reconstruction algorithm can be used for minimum-phase or nonminimum-phase zeros; however, zeros on the unit circle yield persistent estimation errors and thus poor input reconstruction.