The operating cycle of planetary gear transmissions is composed by the run upregime, steady state regime with fixed speed and the run down regime. Run up andrun down are one of the main non stationary excitations for planetary gearboxes.This paper investigates, in these two regimes, the dynamic behaviour of aspecial configuration of two stages planetary gear which are mountedback-to-back. A dynamic model is developed. The variability of speed in the runup and run down regimes is included in the model. The computation of the dynamicresponse shows that additional sidebands instability zones are generated. Usingthe Short Time Fourier Transform, an amplitude-frequency modulation is observedin the non stationary condition. An experimental study done on a back-to-backplanetary gear test rig is conducted for validation of the mathematic model.