The paper presents identification of destructive processes that affect hydraulic servomechanisms of the aircraft control assist system and influence of these processes onto values of parameters that characterize structures and operation of these systems. Particular attention is paid to the description of essential properties attributable to operation of the distributing device within a hydraulic servomechanism with a spotlight to physical background of the process when friction equilibrium of a sliding hydraulic pair is disturbed, which leads to its destruction. More specifically the issue associated with violation of friction stability in a cylindrical, sliding hydraulic pair due to the impact of an unbalanced radial hydrostatic force as well as material contamination and a hydrodynamic axial force. Qualitative assessment of unbalanced radial forces are carried out for the case when cylindrical slider is placed inside a guiding sleeve with consideration of various positions of the slider against the sleeve. Consequently, a set of parameters was found out with the aim to characterize operation quality (dependability) of a hydraulic servomechanism. Finally, characteristics of a hydraulic servomechanism with a linear distributing slider or a rotary slider have been developed as a function of their lifetime.