The method of generating a secure flight plan portion in the event of depressurization corresponding to a time-conditioned descent profile that may include a succession of levels, comprises: a step of defining a first point of the flight plan, called non-return point, denoted NRP, and a second point of the flight plan, called EOZ, the two points delimiting a portion of the flight plan plotted above an area identified as “with risks”; a step of defining two radials in the horizontal plane respectively passing through the first and the second points; a step of determining a first flight plan portion comprising the definition of a lateral offset angle from the original flight plan; a step of determining a vertical descent profile; a step of displaying time markers indicating the exhaustion of the oxygen reserve.