For damping vertical oscillations in a helicopter having a rotor drive-unit and an airframe and carrying an external payload suspended via a lifting gear from a load attachment point on the airframe, oscillation sensors are arranged on the helicopter, and an actively operated linear actuator is provided between the load attachment point and the lifting gear. With the oscillation sensors excitations of a first mode of the vertical oscillations, in which the external load and the rotor drive-unit oscillate together relative to the load attachment point, and of a second mode of the vertical oscillations, in which the load attachment point oscillates relative to the external load and the rotor drive-unit, are sensed, and forces opposed to the vertical oscillations are applied with the actuator between the load attachment point and the lifting gear as a function of the phase and of the amplitude of the sensed excitations.