A control system for controlling a flight path during descent of a vehicle, said system comprising: a flight management system operable to calculate a speed margin, ΔV, using an estimated uncertainty of forecast winds and temperatures, a desired Required Time-of-Arrival accuracy, a distance or time to a next RTA waypoint, the presence or lack of other performance entries, and a characteristic of an RTA algorithm; a processor communicatively coupled to said FMS to receive the calculated speed margin, ΔV, said processor being programmed to: receive a desired flight speed, the desired flight speed representing a desired vehicle speed along the flight path; determine a shallower flight path of the vehicle that is shallower than a flight path during the idle segment of the vehicle descent trajectory, during which idle segment the vehicle throttle is at an idle setting, characterized in that during the shallower flight path an amount of thrust is determined using: T = D + V T − Δ ⁢ V V T ⋅ T − Δ ⁢ V − D − Δ ⁢ V where: T is the amount of thrust, D is a drag at the desired flight speed, V T is the true airspeed equivalent of the desired flight speed, V T-ΔV is the TAS equivalent of the desired flight speed less the speed margin ΔV, T -Δ V is idle thrust at the desired flight speed less the speed margin ΔV, and D - Δ V is drag at the desired flight speed less the speed margin ΔV; and generate a thrust control signal using the determined amount of thrust and transmit the thrust control signal to control the thrust of a propulsion engine; generate a flight control surface control signal configured to maintain the determined flight path using the speed margin; said system further comprising: an output device communicatively coupled to said processor, said output device is configured to transmit the flight control surface control signal to a flight control system of the vehicle. a flight management system operable to calculate a speed margin, ΔV, using an estimated uncertainty of forecast winds and temperatures, a desired Required Time-of-Arrival accuracy, a distance or time to a next RTA waypoint, the presence or lack of other performance entries, and a characteristic of an RTA algorithm; a processor communicatively coupled to said FMS to receive the calculated speed margin, ΔV, said processor being programmed to: receive a desired flight speed, the desired flight speed representing a desired vehicle speed along the flight path; determine a shallower flight path of the vehicle that is shallower than a flight path during the idle segment of the vehicle descent trajectory, during which idle segment the vehicle throttle is at an idle setting, characterized in that during the shallower flight path an amount of thrust is determined using: receive a desired flight speed, the desired flight speed representing a desired vehicle speed along the flight path; determine a shallower flight path of the vehicle that is shallower than a flight path during the idle segment of the vehicle descent trajectory, during which idle segment the vehicle throttle is at an idle setting, characterized in that during the shallower flight path an amount of thrust is determined using: T is the amount of thrust, D is a drag at the desired flight speed, V T is the true airspeed equivalent of the desired flight speed, V T-ΔV is the TAS equivalent of the desired flight speed less the speed margin ΔV, T -Δ V is idle thrust at the desired flight speed less the speed margin ΔV, and D - Δ V is drag at the desired flight speed less the speed margin ΔV; and generate a thrust control signal using the determined amount of thrust and transmit the thrust control signal to control the thrust of a propulsion engine; generate a flight control surface control signal configured to maintain the determined flight path using the speed margin; said system further comprising: an output device communicatively coupled to said processor, said output device is configured to transmit the flight control surface control signal to a flight control system of the vehicle.