The paper overviews the state-of-art of aircraft powered by hybrid electric propulsion systems.The research status of the design and energy management of hybrid aircraft and hybrid propulsion systems are further reviewed.The first contribution of the review is to demonstrate that,in the context of relatively underdeveloped electrical storage technologies,the study of mid-scale hybrid aircraft can contribute the most to both theoretical and practical knowledge.Meanwhile,the profits and potential drawbacks of applying hybrid propulsion to mid-scale hybrid airplanes have not been thoroughly illustrated.Secondly,as summed in the overview of design methodologies,the multi-objective optimization transcends the single-objective one.The potential of the hybrid propulsion system can be thoroughly evaluated in only one optimization run,if several objectives optimized simultaneously.Yet there are few researches covering the conceptual design of hybrid aircraft using multi-objective optimization.The review of the most popular energy management strategies discloses the third research gap---current methodologies favoured in hybrid ground vehicles do not consider the aircraft safety.Additionally,both non-causal and causal energy management are needed for performing a complicated flight mission with several sub-tasks.