Inflight shutdown of one engine for twin-engine helicopters has proven beneficial for fuel consumption. A new flight mode is then considered, in which one engine is put into sleep mode while the second engine runs at nominal load. The ability to restart the engine in sleep mode is then critical for safety reasons. Indeed, the certification of this flight mode involves ensuring a close-to-zero failure rate for in-flight restarts and a fast restart capability of the shutdown engine (focus of this paper). Fast restart capability is necessary in case of a failure of the operating engine. Indeed, there is no more power available, and the helicopter can lose up to 15–20 meters per second during autorotation. The restart time becomes a critical parameter to limit the loss of altitude. The aim of the paper is to assess the potential restart time saving using an approach combining test rig data analysis and numerical results generated by a thermodynamic model able to simulate at low rotational speed. It is important to understand the detailed phenomenology of the startup process and the various subsystems involved, first to highlight the influencing parameters and then to establish an exhaustive listing of the possible time optimizations. The results of this study show that a fast restart going from sleep mode to max power speed can be up to 60% faster than a conventional restart going from sleep mode to idle speed, which is significantly faster.