A ladder is positioned between a first hatch of an upper level of an aircraft and a second hatch of a lower level of the aircraft. A bottom end of the ladder is mounted in rotation on a floor of the lower level. A top end of the ladder is unattached. The ladder can swivel between a first stable position and a second stable position. The top end of the ladder is provided with a safety device capable of the blocking an opening of the first hatch when the ladder is in the second stable position. For example, the safety device has a net. A first edge of the net is fixed to a first side of the first hatch. A second edge of the net is fixed to the top end of the ladder.