Vibration diagnostics that assume a “stationary” monitored signal are not effective for main and engine accessory gearboxes because the operating conditions (speed and/or load) of these components are often non-stationary, which leads to spectral smearing and erroneous analysis. Traditionally, this is avoided by defining steady state operating conditions in which to perform the analysis. However, most engine accessories are only highly loaded during transients and, in the case of rotorcraft gearboxes, non-stationary loading is often prevalent due to rotor effects. In addition, the progression of certain faults can lead to non-stationary vibration signals. Therefore, the authors have developed a novel health monitoring methodology that is applicable during non-stationary operation through application of Joint Time-Frequency Analysis (JTFA). This paper highlights the results obtained from applying JTFA techniques to very high frequency (VHF) vibration data collected from healthy and damaged components. When the accuracy of these approaches is compared with conventional techniques, the JTFA approaches significantly outperform the conventional methods.