In this paper, the longitudinal dynamics of a fast-morphing, variable-wing-sweep micro air vehicle are investigated from a flight dynamics perspective. The time scales over which the morphing occurs are of the same order as the flight dynamics of the micro air vehicle, due to which the time-varying aspects of the dynamics must be carefully evaluated. The time-varying characteristic equation (which represents a generalization of the standard linear time-invariant characteristic equation) for this micro air vehicle is presented in analytic form, and this enables a characterization of the influence of different morphing trajectories on the flight dynamics. The concept of a time-varying pole is subsequently adopted and the flight dynamic properties of the time-varying modes are determined. A tool to properly evaluate the sensitivity of the time-varying modes to different morphing trajectories is demonstrated.