There are many technical systems that operate with varying usage modes. The variable usage modes of technical systems in aircraft involve flexible adjustment and adaptation of systems to different flight conditions and types of tasks. This, in particular, allows for optimizing fuel consumption, improving flight efficiency, and increasing flight safety. Special attention is paid to the reliability of such systems. It is known that reliability refers to the ability of a technical system to maintain within specified limits all parameters that characterize its ability to perform required functions under given modes and conditions of use, maintenance, storage, and transportation. This paper discusses the technical systems of aircraft that operate in a two-shift mode, assuming that the time spent in each mode is non-random. It is also assumed that the exponential distribution law applies to each of the two modes. Taking into account these constraints and assumptions, and using a semi-Markov random process, a mathematical model has been developed for calculating the reliability of technical systems in aircraft that operate in variable operating mode.