The specified life of an aircraft and its components is determined on the basis of fatigue bench tests. In addition to the traditional methods it is offered to use the method of acoustic emission. The fatigue cracks were grown from stress concentrators introduced into the most loaded parts of the undercarriage. It is demonstrated that the development of a fatigue crack occurs irregularly: the periods of accelerated growth are followed by the periods of slowdown. The peculiarities of crack development in different stages are analyzed. The method of fractographic analysis was used to confirm the moment of fatigue crack initiation as well. The possibility of indicating the moment when a microcrack is transformed into a macrocrack, which is dangerous for further operation, is considered.