During the flight of the aircraft (AC) in many cases, there is the problem of determining linear and angular parameters of its motion in the inertial (fixed to inertial space) coordinate system. Lately increasingly strapdown inertial navigation system (SINS) based on three accelerometers and three angular velocity sensors of the sensitivity axis with the axes associated with the body of the aircraft coordinate system. Issues of assessment of accuracy of determining navigation parameters AC, these systems cover a lot of work. However, they do not allow to assess the accuracy of the determination of motion parameters of the aircraft with a strapdown inertial navigation system in the inertial basis. This article presents the method of solving the problem of estimation of accuracy of definition of linear and angular motion parameters of the aircraft with a strapdown inertial navigation system in the inertial basis. A block diagram for explaining the principle of operation of the described the SINS. To describe the motion of AC and estimation of accuracy of definition of parameters of its motion, the SINS in the inertial basis is used two coordinate systems: inertial, the associated. As the main factors determining the errors, the SINS, are considered dead zones, errors of implementation and scale factors of reorthogonalize the installation of measuring devices. On the basis of the developed technique obtained numerical values of accuracy of calculation of parameters of motion of the aircraft. The proposed method allows to evaluate the accuracy of the determination of motion parameters of AC with the SINS in the inertial basis. This gives you the opportunity to decide on the satisfaction of specified requirements on the accuracy of sins with certain characteristics of measuring devices in each case, application of the АС to the destination. This technique can also be used in the error analysis of SINS used in the different types of АС, in determining the errors that have the most significant impact on the determination of motion parameters of АС, when forming recommendations on reducing the negative impact of these errors and validating requirements for strapdown inertial navigation systems of aircraft.