The invention relates to a method for inserting a segment (Tins) of flight plan into an initial flight plan (Pini) of an aircraft, performed by a flight management system (FMS) of the said aircraft, the initial flight plan (Pini) comprising an ordered series of initial legs (Sini), the said fixed initial legs being indexed with an index i that varies from 1 to n, the method comprising the steps involving: identifying (110), using a first iterative calculation on the index i, in the segment to be inserted (Tins), the fixed legs to be inserted that have a position identical to the position of the leg of index i Sini(i)), the said legs thus determined being referred to as occurrences of the leg of index i, the said occurrences (O1, O2) being ordered by rank k varying from 1 to m, as a function of their position in the segment that is to be inserted (Tins), and searching, among the identified occurrences, for the occurrence of lowest index i and lowest rank k (Oi0(k0)) that has a type and attribute values identical to the segment of index i, referred to as equivalent point, when the said equivalent point exists, inserting the segment that is to be inserted (Tins) from the said equivalent point, otherwise, inserting the segment that is to be inserted (Tins) from the identified occurrence of lowest index i and lowest rank k (Oi1(k1)) referred to as a pseudo equivalent point, when the said pseudo equivalent point exists.