An aircraft propulsion unit comprising a nacelle with a fan cowl locking and closing mechanism. The fan cowls of the nacelle can be closed by a single operator in accordance with the ergonomic standards defined for a single individual. A nacelle is provided with a lateral locking mechanism configured to keep one of the two fan cowls in the closed position, so as to reduce the spacing between the two lower sides of the fan cowls, whereas the presence of a stop between a first elastic member provided on one fan cowl and a second elastic member provided on the other fan cowl allows the operator to provide only that effort required to overcome the force exerted by a single spring at a time.