System for assisting in the landing of an aircraft on a landing runway of an airport according to a predetermined approach of said landing runway, said landing assistance system being characterized in that it comprises a processing unit configured to: during a first phase of a landing runway approach procedure according to said predetermined approach, when the aircraft flies above a predetermined height with respect to a landing runway threshold: - acquire information from a receiver of ILS guidance signals embedded onboard the aircraft; - acquire aircraft position and speed information from a signal receiver of a satellite navigation system embedded onboard the aircraft; - acquire aircraft position and speed information from an inertial unit embedded onboard the aircraft; - determine so-called merged aircraft position and speed information as a function of said information from the receiver of ILS guidance signals and said aircraft position and speed information from the signal receiver of a satellite navigation system; - determine a bias of the position and speed information from the inertial unit as a function of said merged position and speed information; and - control the guiding of the aircraft on the basis of the information from the receiver of ILS guidance signals, then during a second phase of the landing runway approach procedure according to said predetermined approach, when the aircraft flies below said predetermined height with respect to the landing runway threshold: - acquire aircraft position and speed information from the inertial unit; - determine so-called adjusted aircraft position and speed information by applying, to the aircraft position and speed information from the inertial unit, a correction corresponding to the bias determined during the first phase of the approach procedure; and - control the guiding of the aircraft along a landing runway approach axis corresponding to said predetermined approach, as a function of said adjusted position and speed information. during a first phase of a landing runway approach procedure according to said predetermined approach, when the aircraft flies above a predetermined height with respect to a landing runway threshold: - acquire information from a receiver of ILS guidance signals embedded onboard the aircraft; - acquire aircraft position and speed information from a signal receiver of a satellite navigation system embedded onboard the aircraft; - acquire aircraft position and speed information from an inertial unit embedded onboard the aircraft; - determine so-called merged aircraft position and speed information as a function of said information from the receiver of ILS guidance signals and said aircraft position and speed information from the signal receiver of a satellite navigation system; - determine a bias of the position and speed information from the inertial unit as a function of said merged position and speed information; and - control the guiding of the aircraft on the basis of the information from the receiver of ILS guidance signals, - acquire information from a receiver of ILS guidance signals embedded onboard the aircraft; - acquire aircraft position and speed information from a signal receiver of a satellite navigation system embedded onboard the aircraft; - acquire aircraft position and speed information from an inertial unit embedded onboard the aircraft; - determine so-called merged aircraft position and speed information as a function of said information from the receiver of ILS guidance signals and said aircraft position and speed information from the signal receiver of a satellite navigation system; - determine a bias of the position and speed information from the inertial unit as a function of said merged position and speed information; and - control the guiding of the aircraft on the basis of the information from the receiver of ILS guidance signals, then during a second phase of the landing runway approach procedure according to said predetermined approach, when the aircraft flies below said predetermined height with respect to the landing runway threshold: - acquire aircraft position and speed information from the inertial unit; - determine so-called adjusted aircraft position and speed information by applying, to the aircraft position and speed information from the inertial unit, a correction corresponding to the bias determined during the first phase of the approach procedure; and - control the guiding of the aircraft along a landing runway approach axis corresponding to said predetermined approach, as a function of said adjusted position and speed information. - acquire aircraft position and speed information from the inertial unit; - determine so-called adjusted aircraft position and speed information by applying, to the aircraft position and speed information from the inertial unit, a correction corresponding to the bias determined during the first phase of the approach procedure; and - control the guiding of the aircraft along a landing runway approach axis corresponding to said predetermined approach, as a function of said adjusted position and speed information.