The article describes the onboard control-measurement system developed for recording the main technical parameters of vertical takeoff and landing (VTOL) micro-UAV motors and the data determining the airframe’s attitude during the flight. Using this system, the methodology of conducting a test flight has been developed to perform a diagnostic check of its motors and assess its condition using the VTOL micro-UAV in plane mode. Also, during the test flights, the number of revolutions, current consumption, and temperature of each motor, as well as angular changes along the three coordinate axes and acceleration of the airframe, were measured and recorded, and time graphs of the parameters were created based on these values. Analysis of the created time graphs allows for direct researching of the UAV in take-off, hover, flight and landing modes. The possibility of small-sized mini and micro unmanned aerial vehicles to diagnose their motors before, during, and after the flight, that is, to directly identify possible flight events in the air during the flight due to engine failure and to perform appropriate intervention has been shown by using of the developed onboard control - measuring system.