The aim of this paper is to present the results of prepreg nacelles design and manufacturing for I-31T aircraft. The work was part of the European Union project ‘Efficient Systems and Propulsion for Small Aircraft’ (ESPOSA). The new engine cowling design was preceded by computational fluid dynamics numerical analyses stage. Use of TP100 engine was assumed, which was installed on the I-31T aircraft in the framework of the ESPOSA project. This article presents the process of external geometry adjustment, chosen results of numerical analyses, 3D model design, manufacturing process and tests results. The new cowlings underwent ground and in-flight tests. Monitoring during the test included external and internal cowling temperatures. The collected test data were further analysed. Throughout accumulated energy in hot parts of the engine the temperature inside the engine nacelles rises. ‘Hot’ composite nacelles were used in order to withstand the impact of high temperature. In majority of modern aircraft propulsion systems, cowlings are made of composite. Currently, there is a trend to manufacture initially solution treated materials – i.e. glass and carbon fiber pre-pregs in out-of-autoclave process. Such an approach allows to accelerate the fabric layup process and achieve highly repeatable structure. Materials that are currently available in the market allow to manufacture the cowlings and simultaneous weight minimization. Owing to the applied 3D software for both design and manufacture of machining tools, it is possible to shorten the time of manufacturing a complete element. The cowlings were designed in cooperation with the NLR – Netherlands Aerospace Centre, who was a partner in the ESPOSA project and has experience in prepreg elements design. NLR was responsible for material selection and final part manufacturing. Tooling has been designed and manufactured in the Institute of Aviation in Warsaw. The assembly works were carried out by Zakłady lotnicze Margański & Mysłowski.