The problem of inertial navigation accuracy with using of accelerometers in inertial measurement unit are discussed. It is shown that a strict successive analysis of accelerometer measurement procedure makes it possible to understand probable new source of errors in the value estimation of the disturbing force magnitude. If do not take into account these new possible errors any self-adjusting system of an airplane may be functionally unreliable. These errors arise only when temporal duration of disturbing force is less than some critical time τ, which depends on the given characteristics of an aircraft Inertial Navigation System and aircraft itself. The method of finding τ is proposed. A presence of a damping force in the proof mass motion equation is examined.