1 Abstract The paper presents the analysis of the drive unit start-up process of training aircrafts during flight, which are equipped by the Dęblin's "School of Eaglets" and intended for a flight training of officer cadets. The analysis of the start-up process was conducted using data from on-board flight recorders recorded during a flight training, whose aim was an emergency launch of particular drive units, and with the help of the so-called phase diagrams of the selected operating parameters of the particular flight drive systems. The construction principle of typical start-up systems was presented schematically presenting their base subassemblies. The importance of proper selection of the start-up system for the particular aircraft was emphasized and the basic indicators, which must be taken into account in the process, were cited. The basic mathematical dependences necessary in designing and determining the basic characteristics of the engine start-up process were explained. At the end, the obtained results and the main problems that need intervention as well as the proposal of preventive activities were shortly commented. 1 Introduction The start-up process of drive units during the flight of training aircrafts, which are equipped by the Dęblin's "School of Eaglets" and intended for flight training of future pilots is a very important element in terms of flight safety. During use of the engine in the air, there is a possibility of its spontaneous turn-off. The causes of the turn-off, assuming that the fuel system operates correctly, are numerous, e.g.: execution of rapid maneuvers at an earlier reduction in engine speed, which can cause an asymmetric airflow around the engine intake, incorrect regulation of inlet diffusers, flow disturbances caused by e.g. firing of rockets, icing of the inlet, etc. The construction of the aircraft engine start-up system is often an indicator of ranges of the aircraft's possible applications. The start-up system – regardless of the type – consists of three main systems: start-up automatics (exercising also a control function), a basic generator, and a power supply (Fig. 1). The power source can be an integral part of the aircraft or engine (a dual generator). Fig. 1. Diagram of the start-up system [3] In modern turbine jet engines, start-up is automatic in accordance with the specially tailored program of the generator turn-on/turn- off, ignition and start fuel. The type of start-up system determines also its weight and dimensions, which is especially important for small aeroplanes, as well as the necessary actions to be performed by the pilot during its launch. This is especially important during the flight training in the air, of the future pilots and it is directly related to flights' safety. The task of the start-up system of the aero engine is to give the necessary rotational speed of the engine power transmission (an air compressor and turbine unit), at which the power developed by the turbine N t is greater RESTARTING OF A JET ENGINE DURING FLYING AND FLIGHT SAFETY Miroslaw KOWALSKI*, Antoni JANKOWSKI*, Jan RAJCHEL** * Air Force Institute of Technology, Ks. Boleslawa street 6, 01-494 Warsaw, Poland, **Polish Air Force Academy, Dywizjonu Street 303/35, 08-521 Deblin, Poland Keywords: engine start-up during flight, flight safety