This article is devoted to observation results of gear wheel wear of two different types of helicopter transmission: reduction gear of Mi-2 helicopter engine as well as accessory gearbox (AGB) of Mi-24 helicopter using proprietary FAM-C and FDM-A methods. The substantial sensitivity and complementarity of the observed phenomena was established – it is possible to simultaneously observe a given pair of a gear wheel and also other mechanical points, such as, e.g. the worn out bearing of the main shaft of helicopter rotor. The authors based their deliberations on the results of laboratory tests and complex tests on helicopters during their operation. The article is addressed at subsequent destruction stages of a gear wheel in helicopter transmission. The authors paid high attention to the observed wear due to the so-called structural resonance observed in some helicopters, where the worn out bearing of helicopter rotor results in the destruction of the pair of gear wheels. The gear wheel equalizes its frequency of free vibrations with the damaged rolling bearing in the line of rotation axis. Due to this, occurs the synchronous acceleration and deceleration of rotational speed of the wheel under consideration, which leads to material fatigue wear at the root of two teeth. Such wear may contribute to gear teeth breakage and helicopter crash.