The article discusses the design and principles of operation of three diagnostic testers constructed by the authors of this study, intended for three, different aircraft propulsion units. These design have one common way of processing the original diagnostic signal - they use one FAM-C method. This method is based on natural processing of the rotational speed fluctuations of individual kinematic cells of the propulsion unit, by the on-board generator into frequency modulations. The method, thanks to its properties, is extremely beneficial for the automatic digital diagnostic processing. The tester can be connected at any, convenient for the crew, location of the power grid, away from dangerous zones of the aircraft.