A method for computing a required speed profile for an aircraft to meet a required time of arrival (RTA) for a waypoint of a current flight is provided. During flight, the method calculates a fuel-efficient speed profile for the aircraft to meet the RTA for the waypoint, by a processor of a computing device communicatively coupled to one or more avionics systems onboard the aircraft; activates the fuel-efficient speed profile to fly the aircraft to the waypoint, by the processor; determines a priority between fuel efficiency of the fuel-efficient speed profile and time reliability, by the processor; and when the priority is the time reliability, switches from the fuel-efficient speed profile to a guidance margin control strategy to fly the aircraft to the waypoint, wherein the guidance margin control strategy increases the time reliability by enabling the aircraft to satisfy constraints of the RTA.