Abstract Planetary gearboxes nowadays serve as the heart of complicated structures such as helicopters, presenting a number of advantages. They have compact size and low weight, achieving high power density and high efficiency. Moreover the gearing can be very accurate with virtually no backlash and modular, as most stages can be stacked. Furthermore, the last years planetary gearboxes have been introduced in the area of air breathing engines, helping to deliver improved efficiency over a wide range of thrusts. Planetary gearboxes consist of various shafts, gears and bearings and operate under varying conditions under excessive friction, heat and high mechanical forces. Their use in such critical applications demands for monitoring systems that can track the health condition of different components, such as bearings and gears, focusing towards early and accurate fault detection with limited rate of false alarms and missed detections, and on trending of the system degradation targeting to prognostics. Therefore the aim of this paper is the analysis of a dedicated dataset using a basic diagnostic indicator named Squared Envelope Spectrum. The indicator is applied and evaluated on a comprehensive dataset that has been generated in 1992 by Westland Helicopters Ltd (WHL), at their Universal Transmission Test Rig, testing a CH46 aft transmission, for the United States Navy.