While an aircraft engine is being used, its wear takes place. Its gain coefficients decrease and at the same time its time constants increase (engine becomes „weak“ and „slow“). Therefore, the rigorously definite adjustment of engine according to the principles of automatics and manufacturer recommendations is carried out periodically during the ground service of engine. The practice shows that during ground tests an engine works in „easy“ surrounding, however, during flight an engine works in „difficult“ surrounding. After good adjustment of an engine from changing the set value w during the ground service, this engine may not meet requirements of the work quality by variable disturbances z during the flight. Therefore arises a problem of the synthesis of quantification of the control possibility of engine which will result in connecting the operation status of engine (automatics) with the maintenance status of engine (diagnostic). The measure of this relationship can be named „the control potential“ of the engine. The control potential can be determined from the equation of state: , where: y – operational signal of engine, u – diagnostic signal of engine, b – coefficient of intensity of influence of the maintenance status of engine on its control possibilities, a – control potential of engine, - operation time (Bergson’s time). The parameter „a“ calculated with help of static and dynamic identification methods will permit to estimate the functional value of engine during its ground tests.