The invention relates to a process and an on-board system for evaluating flight strategies aboard an aircraft, linked with a flight plan (13) of the aircraft, including: calculation means (5) for determining relations between the flight parameters including parameters of time, of fuel quantity, of altitude and of speed, interface means (9) for providing at least one envelope of values (25a-25d) representative of a domain of definition of a plurality of flight strategies, interface means (9) for providing at least one determined strategy defined by determined values assumed by the flight parameters at a reference point, interface means (9) to allow the selection and setting of the value of a first parameter among said flight parameters, generating and supplying another envelope of values allowing exploration of another plurality of possible strategies, and interface means (9) to allow the selection and setting of the value of a second parameter, generating and supplying another determined strategy.