Autothrottle retard rate control system

In an autothrottle control system having automatic landing capability during which an engine throttle is reduced from a first position to an idle position, the improvement comprising a throttle rate reduction signal comprising:

• (a) a nominal retard rate signal;
• and
• (b) a retard rate adjustment signal calculated as a function of the difference between current airspeed and a commanded airspeed;
• wherein during an underspeed condition the retard rate adjustment signal decreases the rate of throttle reduction, and during an overspeed condition the retard rate adjustment signal increases the rate of throttle reduction;
• wherein the nominal retard rate signal is equal to the angular difference between a current engine throttle control lever position and an idle engine throttle control lever position, the difference being divided by an amount in the range of about 3 seconds to about 10 seconds.

In an autothrottle control system having the capability to reduce engine throttle setting from a first position to an idle position during landing by use of a nominal retard rate signal calculated as the angular difference between a current engine throttle control lever position and an idle engine throttle control lever position, the difference being divided by an amount of time to touchdown, an improvement comprising:

• calculating a retard rate adjustment signal as a function of the difference between current airspeed and a commanded airspeeds the retard rate adjustment signal being expressed as an angular amount per time;
• the calculating of the rate of throttle reduction being accomplished during a flare approach maneuver with an autopilot system engaged and the aircraft at or below about 24 feet;
• the retard rate adjustment signal being set to zero when the difference between current airspeed and commanded airspeed is at or between about −2.5 knots and +2.5 knots, the retard rate adjustment signal being sent to about +1.2 degree per second when the difference between current airspeed and commanded airspeed is less than about −8.5 knots;
• the retard rate adjustment signal being set to about −1.2 degree per second when the difference between current airspeed and commanded airspeed is greater than about +8.5 knots;
• and the retard rate adjustment signal varying linearly when the difference is between −2.5 to −8.5 and +2.5 to +8.5;
• combining the nominal retard rate signal with the retard rate adjustment signal to form a retard rate output signal, the retard rate output signal being limited to a value between about 0.0 and −3.5 degrees per second;
• and
• using the limited retard rate output signal to control the movement of engine throttles to idle;
• wherein the retard rate adjustment signal decreases the rate of throttle reduction during underspeed conditions and increases the rate of throttle reduction during overspeed conditions thereby helping to avoid hard and soft landings, respectively.