Abstract In this work, we address the influence of period variation on the Westland CH-46E helicopter vibration data. The estimation of the time-varying shaft speed by using a newly developed constrained AR(2) estimator and the well-known EKF estimator will be discussed. The influence that the period variation captured by these two estimators have on the periodic time–frequency analysis will be illustrated via numerical simulation. This simulation clearly illustrates a potential spectral smearing if the period variation is not properly accommodated. A data resampling scheme will then be proposed to eliminate the spectral smearing effect. The potential of this scheme will be demonstrated via the analysis of the helicopter vibration data.