The present invention provides a multi-mode secondary power unit (14) comprising a first starter/generator unit (10) operably coupled to a shaft (20). The first starter/generator motor (10) is operable in a motor mode for causing rotational movement of the shaft (20) and is operable in a generator mode for generating electric power from rotational movement of the shaft (20). A thermal engine (22) is operably coupled to the shaft (20) and is operable in a fuel burning mode for causing rotational movement of said shaft (20) and is operable in a non fuel burning mode for receiving rotational movement from the shaft (20). A load compressor (25) is operably coupled to the shaft (20) for pressurizing air to a predetermined level.