Method for obtaining a predictive vertical speed v AP of a rotorcraft which the rotorcraft is going to reach after a given time without modification of its trajectory, the following two conditions being satisfied: - the calibrated airspeed in level flight being less than the minimum-power speed, and - the calibrated airspeed being decreasing, characterised in that the following steps are performed in succession: a) the instantaneous vertical speed (v) of the rotorcraft is measured, b) a corrective vertical speed (v CORR) is determined with the aid of at least one predictive term (C P1), c) the corrective vertical speed (v CORR) is added to the instantaneous vertical speed (v) to obtain said predictive vertical speed (v AP), and in that, in the course of step b), during a step b1) said predictive term (C P1) is determined such that: C P 1 = v + k V PP − V P 2 V γ − V PP with: V P = instantaneous proper speed of the rotorcraft, VPP = predictive proper speed of the rotorcraft determined by the following relationship where t and Δt designate respectively the time and a time interval: V PP = V P + dV P dt Δt, V Y = predetermined minimum-power speed of the rotorcraft, of a given type of rotorcraft, k = predetermined characteristic constant of the rotorcraft from a linear approximation which corresponds to a proportionality ratio between powers of the rotorcraft and its vertical speed v, the characteristic constant k being such that: v = k W W n − 1 with the approximation: W n Wv Υ = 2 − V P V Υ, Wv Y and W n being the powers needed to fly level respectively at the minimum-power speed V Y and at the proper speed V P, W being the instantaneous available power of the rotorcraft. - the calibrated airspeed in level flight being less than the minimum-power speed, and - the calibrated airspeed being decreasing, a) the instantaneous vertical speed (v) of the rotorcraft is measured, b) a corrective vertical speed (v CORR) is determined with the aid of at least one predictive term (C P1), c) the corrective vertical speed (v CORR) is added to the instantaneous vertical speed (v) to obtain said predictive vertical speed (v AP), V P = instantaneous proper speed of the rotorcraft, VPP = predictive proper speed of the rotorcraft determined by the following relationship where t and Δt designate respectively the time and a time interval: V PP = V P + dV P dt Δt, V Y = predetermined minimum-power speed of the rotorcraft, of a given type of rotorcraft, k = predetermined characteristic constant of the rotorcraft from a linear approximation which corresponds to a proportionality ratio between powers of the rotorcraft and its vertical speed v, the characteristic constant k being such that: v = k W W n − 1 with the approximation: W n Wv Υ = 2 − V P V Υ, Wv Y and W n being the powers needed to fly level respectively at the minimum-power speed V Y and at the proper speed V P, W being the instantaneous available power of the rotorcraft. V Y = predetermined minimum-power speed of the rotorcraft, of a given type of rotorcraft, k = predetermined characteristic constant of the rotorcraft from a linear approximation which corresponds to a proportionality ratio between powers of the rotorcraft and its vertical speed v,