1. A method of aircraft performance predictions for descent and approach phases in a flight management system (FMS), comprising: stopping cruise computation substantially around a default distance from a destination by an aircraft performance prediction module residing in a memory of the FMS; determining current predicted aircraft state using a total energy of the aircraft starting from the default distance by the aircraft performance predictions module; computing descent and approach phase profiles including a flight path using the determined current predicted aircraft state by the aircraft performance prediction module, comprising: selecting a starting top of descent distance from the destination while the aircraft is in cruise altitude by the aircraft performance predictions module; performing an iterative backward computation from the destination to the cruise altitude by the aircraft performance predictions module to compute the descent and approach phase profiles including the flight path using the determined current predicted aircraft state; and obtaining the aircraft performance predictions along the flight path based on the computed descent and approach phase profiles by the aircraft performance predictions module that comprises: obtaining a first top of descent distance from the obtained descent and approach profiles; determining whether a difference between the starting top of descent distance and the first top of descent distance is within a design value; and if so, determining the aircraft performance predictions along the flight path using the descent and the approach phase profiles in a forward computation from the cruise altitude to the destination; and if not, considering the first top of descent distance to be the starting top of descent distance; and using the considered starting top of descent, repeating the steps of performing the iterative backward computation, obtaining the first top of descent, and determining whether the difference between the starting top of descent distance and the first top of descent distance is within the design value, until the difference is within the design value; and guiding the aircraft through the descent and approach phase profiles based on the aircraft performance predictions.