Aircraft Control for Flight Duration and Fuel Savings

Abstract

The aircraft autothrottle is configured to reduce engine power of the aircraft such that the aircraft decelerates incrementally from a previous airspeed to a reduced airspeed by a first amount, compare the energy consumption rate at the previous airspeed to the reduced airspeed, repeat the above operations in response to the energy consumption rate at the reduced airspeed being less than the energy consumption rate at the previous airspeed, and increase engine power such that the aircraft accelerates incrementally from the previous airspeed to the increased airspeed by a second amount in response to the increased airspeed corresponding to a more efficient operating point of the aircraft than any previous airspeed.

Description

Technical Field

[0001] (Related Applications) This application claims the benefit of U.S. Provisional Application No. 63 / 44,801, filed Jun. 26, 2020, entitled "METHOD OF OPERATING AN AIRCRAFT TO ACHIEVE A MINIMUM FUEL BURN FOR MAXIMUM FLIGHT ENDURANCE", the disclosure of which is incorporated herein by reference.

[0002] (Technical Field) The present invention relates generally to aeronautics and aircraft control, and more particularly to throttle control for maintaining efficient flight characteristics.

Background Art

[0003] The maximum endurance or fuel efficiency of an aircraft depends on many factors, including the size and shape of the aircraft, air density, and flight speed. For a given aircraft, the maximum endurance is achieved when the aircraft is flying in the most aerodynamically efficient state. This state is achieved when the ratio of the lift L generated by the aircraft to the drag D experienced by the aircraft, L / D, is maximized. If the airspeed relative to the air during flight can be adjusted so that the L / D ratio is maximized, the aircraft can operate with maximum endurance and optimal fuel efficiency under current conditions. Similarly, when L / D is at its maximum value, the range of the aircraft can also be optimized.

[0004] One way to maximize L / D is to use aerodynamic analysis to determine the standard relationship between the indicated airspeed (IAS) and the aircraft weight at various density altitudes for the aircraft. In practice, it is difficult and unrealistic to perform such aerodynamic analysis. For example, the calculations are affected by weight measurement errors, engine performance variations, and changes to the aircraft fuselage and equipment.

Summary of the Invention

Problems to be Solved by the Invention

[0005] Some aspects of the present subject matter relate to systems and methods for controlling an operating point of an aircraft using closed-loop throttle control for fuel-efficient flight. **Means for Solving the Problem**

[0006] According to one aspect, a system for controlling an aircraft's automatic throttle includes an automatic throttle control device having a processing circuit, a memory, and an input / output mechanism. The automatic throttle control device is operable to execute instructions including a duration program. The automatic throttle actuator is operable to perform a throttle setting that controls the engine output of the aircraft in response to an output of the automatic throttle control device. The system includes an input that receives a signal from an airspeed sensor arranged to measure the current airspeed of the aircraft and supply the current airspeed to the automatic throttle control device. The system also includes an input that receives a signal from an energy consumption sensor arranged to measure the energy consumption rate of at least one engine of the aircraft.

[0007] When executed, the duration program causes the automatic throttle control device to reduce the engine output via the automatic throttle actuator such that the aircraft decelerates stepwise by a first amount of change from a previous airspeed to a reduced airspeed, and compare the energy consumption rates at the previous airspeed and the reduced airspeed. In response to the energy consumption rate at the reduced airspeed being less than the energy consumption rate at the previous airspeed, the above operation is repeated. In response to the energy consumption rate at the reduced airspeed not being less than the energy consumption rate at the previous airspeed, the engine output is increased via the automatic throttle actuator such that the aircraft accelerates stepwise by a second amount of change from the previous airspeed to an increased airspeed, and the increased airspeed corresponds to a more fuel-efficient operating point of the aircraft than any previous airspeed.

[0008] Another aspect of the present disclosure relates to an automated method for controlling an aircraft's automatic throttle. According to this method, the automatic throttle system reduces the engine output of the aircraft such that the aircraft decelerates stepwise by a first amount of change from a previous airspeed to a reduced airspeed. The automatic throttle system compares the energy consumption rates at the previous airspeed and the reduced airspeed. In response to the energy consumption rate at the reduced airspeed being less than the energy consumption rate at the previous airspeed, the above operation is repeated. In response to the energy consumption rate at the reduced airspeed not being less than the energy consumption rate at the previous airspeed, the automatic throttle system increases the engine output such that the aircraft accelerates stepwise by a second amount of change from the previous airspeed to an increased airspeed, where the increased airspeed corresponds to a more efficient operating point of the aircraft than any previous airspeed.

[0009] In another aspect, at least one non-transitory machine-readable medium includes instructions that, when executed by an aircraft's automatic throttle control device, cause the automatic throttle control device to perform the method summarized above.

[0010] Optionally, the system and method can further include comparing the energy consumption rates at the previous airspeed and the increased airspeed and increasing the engine output such that the aircraft accelerates stepwise by a third amount of change from the previous airspeed to the increased airspeed in response to the energy consumption rate at the increased airspeed being less than the previous airspeed, where the increased airspeed corresponds to a more efficient operating point of the aircraft than any previous airspeed.

[0011] In some examples, the first amount of change is 2 knots, the second amount of change is 4 knots, and the third amount of change is 2 knots, although various other values for these amounts of change are contemplated.

[0012] Each of the energy consumption rates can be the fuel flow rate, or, in the case of an electric aircraft, the current supplied to the engine(s).

[0013] In some examples, the takeoff airspeed is a predetermined airspeed corresponding to the minimum energy consumption rate of the aircraft at the maximum gross weight.

[0014] A more efficient operating point of the aircraft may correspond to an airspeed at which the ratio of the lift to the drag of the aircraft is substantially its maximum value under the current operating conditions of the aircraft.

[0015] Many advantages will become apparent from the following detailed description.

[0016] The present invention can be more fully understood by considering the following detailed description of various embodiments of the invention in connection with the accompanying drawings.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Modes for Carrying Out the Invention

[0018] The present invention is applicable to various modification examples and alternative forms, and its specific content is exemplarily shown in the drawings and will be described in detail. However, it should be understood that the purpose is not to limit the present invention to the specific embodiments described. Rather, the purpose is to cover all modifications, equivalents, and alternative forms that fall within the spirit and scope of the present invention as defined by the appended claims.

[0019] It should be noted that aspects of the present disclosure are applicable to any powered aircraft, such as conventional fuel-burning aircraft (propeller-driven, turboprop, jet, etc.), electric aircraft (battery-driven, solar cell-driven, or fuel cell-driven), or hybrid-driven aircraft. In the following description, various embodiments are described in relation to one or several types of propulsion or propulsion-energy-delivery systems, but it should be understood that the principles of the described embodiments can be appropriately applied to other types of aircraft having other propulsion or propulsion-energy-delivery systems with appropriate adaptations within the skill of an aircraft engineer.

[0020] FIG. 1 is a simplified diagram showing an aircraft 100 and the basic forces involved in its flight. The aircraft 100 generates lift 102 from its forward motion by directing air downward, mainly by utilizing the shape and orientation of the main body of the aircraft 100 (e.g., its wings, fuselage, and control surfaces). Also, lift depends on air density, the square of the velocity, air viscosity and compressibility, and the surface area over which the air flows. The dependence on the outer shape of the aircraft 100 is complex and difficult to mathematically model. The effects of the tilt of the aircraft 100, air viscosity (e.g., due to air temperature, humidity, altitude), and compressibility on the lift are variable and difficult to derive for a given operating condition.

[0021] Drag force 104 is the force that resists the forward movement of the aircraft 100. Drag force 104 has many components, such as, in particular, the aerodynamic friction (skin friction) between the air and the surface of the aircraft 100, the aerodynamic resistance (form drag) to the movement of the aircraft 100 through the air, and the drag force (induced drag) caused by lift. These are also difficult to consider for predicting drag force computationally. Similar to lift force 102, drag force 104 depends on a number of complex factors, including the size, shape, and weight of the aircraft 100, the surface characteristics of the aircraft 100, the fluid characteristics of the air, and other parameters. In particular, different parameters of drag force 104 become dominant at different airspeeds. At low airspeeds, the main component of drag force 104 is induced drag. As the aircraft 100 increases its airspeed, lift force 102 is more easily generated and induced drag actually decreases. However, as the airspeed increases, other drag components (collectively referred to as parasite drag) increase.

[0022] Thrust 106 is the propulsive force generated by the aircraft 100 to overcome the drag force 104. Generation of thrust requires the consumption of fuel or other onboard energy sources (e.g., the amount of electricity in the case of a battery-powered aircraft). The magnitude of the thrust depends on a number of parameters related to the propulsion system of the aircraft 100, such as the type and quantity of engines, and throttle setting(s). Weight 108 is the combination of the gravitational force and the mass of the airframe, plus the mass of the fuel (which is a time-varying quantity in the case of a fuel-consuming aircraft), plus any payload on the aircraft 100 (such as people, cargo, etc., which can also vary dynamically as in the case of an aerial delivery operation). The dynamic variation of weight 108 means that the magnitudes of the lift force 102 and drag force 104 also change over time during the flight of the aircraft 100.

[0023] Since both the lift force 102 and the drag force 104 are complex and variable, it is difficult for the pilot of the aircraft 100 to select the airspeed relative to the current altitude, weight, and air conditions in order to fly at the L / D ratio optimal for maximum flight time. Additionally, since air density depends strongly on altitude, the maximum L / D ratio of the aircraft 100 varies based on altitude. FIG. 2 shows the variation of the L / D ratio of a given aircraft 100 at various altitudes as a function of indicated airspeed. As shown, the L / D ratio depends on the indicated airspeed and has a peak value. At high altitudes where the air has a relatively low density r1, the L / D ratio has a maximum value at the indicated airspeed IAS3. At intermediate altitudes with air density r2, the corresponding L / D curve has a maximum value at the indicated airspeed IAS2. At low altitudes corresponding to high air density r3, the L / D curve has its maximum value at a relatively slow indicated airspeed IAS1.

[0024] According to some embodiments, an automatic throttle control system is employed in an aircraft to dynamically determine the optimal (or near-optimal) airspeed to achieve the maximum (or near-maximum) L / D ratio under dominant conditions. In some embodiments, an automatic throttle configuration as shown and described in U.S. Patent No. 10,099,795, the disclosure of which is incorporated herein by reference, can be used. In other embodiments, other suitable automatic throttle configurations can be used.

[0025] FIG. 3 is a block diagram illustrating an automatic throttle control system 300 according to some embodiments. As shown, the system 300 includes a manual throttle input 302 that can take the form of a conventional output control lever (or set of levers in a multi-engine aircraft), and a suitable operating connection configuration for each engine(s) and fuel supply system(s) of the aircraft 100. For example, the operating connection configuration can be a mechanical system that adjusts engine output based on the positioning of each output control lever(s). The engine output can be changed by varying the flow rate of fuel or combustion air to the engine(s). In another example, as in the case of a fly-by-wire configuration, the operating connection configuration can include an electrical system that transmits a command signal to an actuator that adjusts engine output (e.g., the flow rate of fuel or combustion air, or in the case of an electric aircraft, the supply of power to the engine(s)) from a pilot control input based on a set value provided to the pilot control input.

[0026] The automatic throttle actuator 304 is a subsystem in parallel with the manual throttle input 302. The automatic throttle actuator 304 automatically adjusts the engine output based on a command signal 305 generated by the automatic throttle controller 310. In one embodiment, the automatic throttle actuator 304 includes a motor and a motor controller, such as a servo motor system, and the rotating part of the motor is mechanically connected to the manual throttle input 302. In another example, the automatic throttle actuator 304 is an actuator coupled to the engine(s) or fuel system of the aircraft 100 and can include one or more valves for controlling the flow rate of fuel or combustion air to the engine(s). In another example, the automatic throttle actuator 304 includes one or more switches, transmission gates, or signal amplifiers interfaced with the engine control system of the aircraft 100.

[0027] The outputs of the manual throttle input 302 and the automatic throttle actuator 304 are combined to produce a throttle setting 306, which is supplied as an engine output input to the engine(s) and fuel system as appropriate. The combination of the manual throttle input 302 and the output of the automatic throttle actuator 304 can be achieved mechanically, electromechanically, or electronically, depending on the various embodiments.

[0028] In some embodiments, when the automatic throttle is operating, the automatic throttle actuator 304 controls the throttle setting 306 in the absence of the manual throttle input 302, but when the manual throttle input 302 is present, it controls the throttle setting 306 preferentially over the automatic throttle actuator 304. In other embodiments, the throttle setting 306 can implement different combinations of the manual throttle 302 and the automatic throttle 304 when both inputs are supplied simultaneously. For example, the automatic throttle actuator 304 can be supplied to the throttle setting as a relatively low-weighted input, while the manual throttle input 302 can be supplied as a relatively high-weighted input. In related embodiments, sufficient force is required at the manual throttle input 302 to override the automatic throttle actuator 304. In other related embodiments, the throttle setting 306 supplies a mechanical force or an electronic signal as feedback 307 to the manual throttle input 302, and the action of the automatic throttle actuator 304 is such that it can be sensed by the pilot at the manual throttle input 302 or observed in other ways.

[0029] The throttle setting sensor 308 is arranged to detect the state of the throttle setting 306 and supply a signal 309 representing the detected state to the automatic throttle control device 310. In particular, the signal 309 represents the effect of the manual throttle input 302 on the throttle setting 306 when the manual throttle input 302 is activated. In some embodiments, the throttle setting sensor 308 is not necessary and can be omitted, for example, if the throttle setting 306 is output as an electronic signal (in this case, the throttle setting 306 can be supplied directly to the automatic throttle control device 310). The throttle setting sensor 308 can be used in embodiments where the throttle setting 306 is realized as a mechanical force or movement (such as the movement or positioning of an output control lever, or the movement or positioning of a throttle control cable or associated linkage).

[0030] The automatic throttle control device 310 generates a command signal 305 for the control of the automatic throttle actuator 304 based on a plurality of inputs. Other inputs to the automatic throttle control device 310 can include an energy consumption sensor 314 and an airspeed sensor 316. The automatic throttle mode input 312 is controlled by the pilot of the aircraft 100 and includes parameters such as the activation / deactivation of the automatic throttle control device 310 and the selection from one or more available automatic throttle programs that define the operation or operation target of the automatic throttle.

[0031] Other inputs to the automatic throttle control device 310 may include an energy consumption sensor 314 and an airspeed sensor 316. The energy consumption sensor 314 can include one or more fuel flow sensors, engine torque and speed sensors, sensors (multiple possible) arranged to detect the volumetric output of a fuel pump, or a current measuring device that measures the current drawn by one or more electric motors functioning as the engine(s) of the aircraft 100. The airspeed sensor 316 can include a pitot tube differential pressure measuring device, an anemometer-thermometer measuring device, or other suitable sensor types.

[0032] FIG. 4 is a simplified block diagram showing the components of an automatic throttle control device 310 according to an exemplary embodiment. The automatic throttle control device 310 includes a central processing unit (CPU) 410, which can include one or more processor cores 412. The memory circuit 414 can include a static or dynamic random access memory (RAM) and a memory control circuit interfaced with the CPU 410. Instructions 416 can be stored on an electrically erasable read-only memory (EEPROM) device such as a read-only memory (ROM) device or a flash EEPROM device interfaced with the memory control circuit of the CPU 410 or the memory 414. The input / output (I / O) control device 418 includes an interface to the various inputs and command signal outputs 305 described above. In some embodiments, the I / O control device 418 can include a universal asynchronous receiver / transmitter (UART) for serial communication, a parallel port, an analog / digital (A / D) converter, or a digital / analog converter (D / A). The I / O control device 418 can interface with the memory control device of the CPU 410 or the memory 414.

[0033] The automatic throttle control device 310 is operable to execute instructions 416 to perform the functions of the automatic throttle control system 300. FIG. 5 is a simplified block diagram showing a portion of the instructions 416 according to some embodiments. When operating the automatic throttle control system 300 via the automatic throttle mode input 312, the pilot of the aircraft 100 can select from among the available programs. The airspeed maintenance program 510 is a set of instructions executable by the automatic throttle control device 310 to operate the automatic throttle control system 300 to implement basic fixed airspeed control. The program 510 receives a pilot input for setting a specific airspeed that can be set via the manual throttle input 302. Thereafter, the airspeed maintenance program 510 operates the automatic throttle actuator 304 to increase the engine output when the indicated airspeed drops below the setpoint and to reduce the engine output when the indicated airspeed rises above the setpoint.

[0034] The maximum endurance program 512 is a set of instructions executable by the automatic throttle control device 310 to operate the automatic throttle control system 300 to implement a dynamic airspeed control algorithm for determining and maintaining an efficient operating point of the aircraft 100, and the aircraft is adapted to operate at or near the maximum L / D ratio under dominant conditions. FIG. 6 is a flowchart showing the operation by the program 512 according to some embodiments. At 602, the engine output is adjusted via the throttle setting 306 by the automatic throttle actuator 304 to set a first airspeed. The first airspeed can be an initial airspeed designated for the aircraft 100 to result in minimum energy consumption at the maximum gross weight of the aircraft 100. The first airspeed can be maintained using the airspeed maintenance program 510, for example, using the adjustment of the airspeed sensor 316 and the automatic throttle actuator 304.

[0035] At 604, to ensure that the operating state of the aircraft 100, including energy consumption rates such as the airspeed and fuel burn rate, is in a steady state, a sufficient period of time is allowed to elapse. This time can be on the order of 1 - 5 minutes and is nominally 2 minutes. Thereafter, the current energy consumption rate E CUR is measured via the energy consumption sensor 314. At this point, E CUR represents the reference measurement value of energy consumption.

[0036] At 606, the automatic throttle control device 310 causes the automatic throttle control system 300 to reduce the airspeed by a first amount of change. The first amount of change can be a predefined airspeed such as -2 knots, for example. In various other embodiments, the first amount of change can be predefined as other values or determined dynamically based on other factors such as altitude, wing loading, etc. Accordingly, the engine output is reduced via the throttle setting 306 by the automatic throttle actuator 304, and the airspeed sensor 316 can be monitored under closed - loop control in conjunction with any necessary adjustments to the throttle setting 306 to reach a new steady state at the newly reduced airspeed. At 608, the operating stability of the steady state is verified (e.g., by waiting for an appropriate duration of 1 - 5 minutes, nominally about 2 minutes, and performing continuous measurements of the airspeed via the airspeed sensor 316 to ensure that the airspeed has stabilized at the desired setpoint within the allowable fluctuation limits). The new energy consumption rate E NEW is measured via the sensor 314.

[0037] At 610, the program 512 tests whether the new energy consumption rate E NEW is less than the previous (reference) measurement value E CUR . If this comparison result is positive, it indicates that the operating point of the aircraft 100 was at an airspeed exceeding the speed related to the maximum L / D ratio, suggesting that it may be necessary to further reduce the airspeed to determine the maximum L / D ratio. Accordingly, the program 512 branches to operation 612, where the new energy consumption rate E NEW is E CURIt is copied to become the new reference value. Further, the program 512 returns to operation 606, where the airspeed is again reduced by the first change amount and proceeds to operations 608 - 610.

[0038] Operations 606 - 612 are repeated until the measured energy consumption rate increases with the recent reduction in airspeed. This result indicates that the airspeed has been reduced to a point below the speed corresponding to the maximum L / D ratio. Therefore, the process proceeds to 614, where E NEW is copied to E CUR to set a new reference value. Next, at 616, the airspeed is increased by the second change amount. The second change amount can be a predefined value such as +4 knots or a dynamically determined value. Therefore, the engine output increases via the throttle setting 306 by the automatic throttle actuator 304, and the airspeed sensor 316 can be monitored under closed-loop control in conjunction with any necessary adjustment of the throttle setting 306 to reach a new steady state at the newly increased airspeed. At this point, the aircraft 100 will operate at substantially its minimum energy utilization point. In this context, substantially at the minimum energy utilization point means within ±5% of the minimum energy utilization point under the current operating conditions of the aircraft 100.

[0039] As an optional further optimization according to some embodiments, at 618, the steady-state operation stability is verified (for example, by waiting for an appropriate duration of 1 - 5 minutes, nominally about 2 minutes, and continuously measuring the airspeed via the airspeed sensor 316 to ensure that the airspeed stabilizes at the desired set point within the allowable fluctuation limit). Also, the new energy consumption rate E NEW is measured via the sensor 314.

[0040] At 620, the program 512 determines that the new energy consumption rate E NEW at the higher airspeed is the previous (reference) measured value E CURTest whether it is smaller. If this comparison result is negative, it indicates that the operating point of the aircraft 100 is at the airspeed corresponding to approximately the maximum L / D ratio, which means that the aircraft 100 is at or near its minimum energy utilization rate, and thus at the operating point of the maximum endurance. However, if the test result at 620 is positive, this indicates that the operating point of the aircraft 100 was at an airspeed less than the speed associated with the maximum L / D ratio, and the airspeed may need to be further increased to determine the maximum L / D ratio. Therefore, at 622, the airspeed is increased by a third amount of change, such as +2 knots. At this point, the aircraft 100 will operate substantially at its minimum energy utilization point. In this regard, substantially at the minimum energy utilization point means within ±5% of the minimum energy utilization point under the current operating conditions of the aircraft 100.

[0041] Since the dominant flight conditions may change due to fuel usage and weight reduction of the aircraft 100, altitude changes, or other factors, the process can return to 604 and re - execute the optimization routine. This additional cycle of the program 512 can be executed continuously or periodically after a predetermined time has elapsed.

[0042] The above - described embodiments are intended to be illustrative and non - limiting. Additional embodiments are within each claim that does not explicitly exclude such subject matter. In addition, although aspects of the present invention are described with reference to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the scope of the present invention as defined by the claims.

[0043] One of ordinary skill in the art will recognize that the present invention may have fewer features than those illustrated in any of the individual embodiments described above. The embodiments described herein are not intended to be an exhaustive presentation of all ways in which the various features of the invention may be combined. Thus, the embodiments are not mutually exclusive combinations of features; rather, the invention can be composed of different combinations of different individual features selected from different individual embodiments, as will be understood by one of ordinary skill in the art.

[0044] Any incorporation by reference of the above references is limited to exclude any subject matter that would be contrary to the explicit disclosure herein. Any incorporation by reference of the above references is further limited to exclude any claims contained in the reference from being incorporated by reference into the claims of this application. However, the claims of any reference are incorporated as part of the disclosure herein, unless specifically excluded. Any incorporation by reference of the above references is further limited to exclude any definitions presented in the reference from being applied only to the incorporated subject matter and not to any subject matter that exists directly in this specification.

[0045] For purposes of interpreting the claims of the present invention, it is expressly intended that the provisions of 35 U.S.C. § 112(f) shall not apply unless the specific terms "means for" or "step for" are recited in a claim.

Claims

1. An automated method for controlling an aircraft's automatic throttle, comprising: (a) reducing the engine output of the aircraft such that the automatic throttle system causes the aircraft to decelerate in steps by a first amount of change from a previous airspeed to a reduced airspeed; (b) the automatic throttle system comparing the energy consumption rates at the previous airspeed and the reduced airspeed; (c) in response to determining in (b) that the energy consumption rate at the reduced airspeed is less than the energy consumption rate at the previous airspeed, repeating (a) and (b); (d) in response to determining that the energy consumption rate at the reduced airspeed is not less than the energy consumption rate at the previous airspeed, increasing the engine output such that the aircraft accelerates in steps by a second amount of change from the reduced airspeed to an increased airspeed; comprising: an airspeed within the range of the second amount of change between the reduced airspeed and the increased airspeed corresponds to a more efficient operating point of the aircraft than any airspeed lower than the range; (e) repeating (a) through (d) to control the airspeed to track the more efficient operating point. A method.

2. (f) in response to (d), the automatic throttle system comparing the energy consumption rates at the previous airspeed and the increased airspeed; (g) in response to determining in (f) that the energy consumption rate at the increased airspeed is less than the energy consumption rate at the previous airspeed, increasing the engine output such that the aircraft accelerates in steps by a third amount of change from the increased airspeed to a further increased airspeed; further comprising The method according to claim 1.

3. The method according to claim 2, wherein the first change amount is 2 knots, the second change amount is 4 knots, and the third change amount is 2 knots.

4. The method according to claim 1, wherein each of the energy consumption rates is a fuel flow rate.

5. The method according to claim 1, wherein the previous airspeed in (a) is a predetermined airspeed corresponding to the minimum energy consumption rate of the aircraft at the maximum gross weight of the aircraft.

6. The method according to claim 1, wherein the more efficient operating point of the aircraft corresponds to an airspeed at which the ratio of lift to drag of the aircraft is substantially maximized under the current operating conditions of the aircraft.

7. The method according to claim 1, further comprising, after (a) and before (b), the automatic throttle system waiting for a time sufficient for the aircraft to stabilize energy consumption at the reduced airspeed.

8. A system for controlling an automatic throttle of an aircraft, comprising a processing circuit, a memory, and an input / output mechanism, and an automatic throttle control device operable to execute instructions including a flight duration program; an automatic throttle actuator operable to perform a throttle setting for controlling the engine output of the aircraft in response to an output of the automatic throttle control device; an input operable to receive a signal from an airspeed sensor arranged to measure the current airspeed of the aircraft and supply the current airspeed to the automatic throttle control device; an input operable to receive a signal from an energy consumption sensor arranged to measure the energy consumption rate of at least one engine of the aircraft; comprising When the endurance time program is executed, it causes the automatic throttle control device to (a) reduce the engine output through the automatic throttle actuator so that the aircraft decelerates stepwise at a first change amount from the previous airspeed to the reduced airspeed; (b) compare the energy consumption rates at the previous airspeed and the reduced airspeed; (c) in response to being determined in (b) that the energy consumption rate at the reduced airspeed is less than the energy consumption rate at the previous airspeed, repeat (a) and (b); (d) in response to the energy consumption rate at the reduced airspeed not being less than the energy consumption rate at the previous airspeed, increase the engine output through the automatic throttle actuator so that the aircraft accelerates stepwise at a second change amount from the reduced airspeed to the increased airspeed; The airspeeds within the range of the second change amount between the reduced airspeed and the increased airspeed correspond to an operating point of the aircraft that is more efficient than any airspeed lower than the range; (e) repeat (a) to (d) to control the airspeed so as to track the more efficient operating point. System. **Claim 9** When the endurance time program is executed, it causes the automatic throttle control device to (f) in response to (d), compare the energy consumption rates at the previous airspeed and the increased airspeed; (g) in response to being determined in (f) that the energy consumption rate at the increased airspeed is less than the energy consumption rate at the previous airspeed, increase the engine output through the automatic throttle actuator so that the aircraft accelerates stepwise at a third change amount from the increased airspeed to a further increased airspeed. The system according to claim 8. **Claim 10** The system according to claim 9, wherein the first change amount is 2 knots, the second change amount is 4 knots, and the third change amount is 2 knots.

11. The system according to claim 8, wherein the energy consumption sensor is a fuel consumption sensor, and the energy consumption rate is a fuel flow rate.

12. The system according to claim 8, wherein the previous airspeed in (a) is a predetermined airspeed corresponding to the minimum energy consumption rate of the aircraft at the maximum gross weight of the aircraft.

13. The system according to claim 8, wherein the more efficient operating point of the aircraft corresponds to an airspeed at which the ratio of lift to drag of the aircraft is substantially maximized under the current operating conditions of the aircraft.

14. The system according to claim 8, wherein when the endurance program is executed, the automatic throttle control device is made to wait for a time sufficient for the aircraft to stabilize energy consumption at the reduced airspeed after (a) and before (b).

15. When executed by an automatic throttle control device of an aircraft, the automatic throttle control device is caused to (a) reduce the engine output of the aircraft so that the aircraft decelerates stepwise by a first change amount from the previous airspeed to the reduced airspeed, (b) compare the energy consumption rates at the previous airspeed and the reduced airspeed, (c) in response to being determined in (b) that the energy consumption rate at the reduced airspeed is less than the energy consumption rate at the previous airspeed, repeat (a) and (b), (d) in response to the energy consumption rate at the reduced airspeed not being less than the energy consumption rate at the previous airspeed, increase the engine output so that the aircraft accelerates stepwise by a second change amount from the reduced airspeed to the increased airspeed, including instructions. The airspeed within the range of the second change amount between the reduced airspeed and the increased airspeed corresponds to an operating point of the aircraft that is more efficient than any airspeed lower than the range, (e) Repeating (a) through (d) to control the airspeed to track the efficient operating point. At least one non-transitory machine-readable medium.

16. When executed, the instructions cause the automatic throttle control device to (f) Compare the energy consumption rates at the previous airspeed and the increased airspeed in response to (d), (g) Increase the engine output so that the aircraft gradually increases its speed in a third change amount from the increased airspeed to a further increased airspeed in response to being determined in (f) that the energy consumption rate at the increased airspeed is less than the energy consumption rate at the previous airspeed. Further comprising The at least one non-transitory machine-readable medium according to claim 15.

17. The first change amount is 2 knots, the second change amount is 4 knots, and the third change amount is 2 knots, for the at least one non-transitory machine-readable medium according to claim 16.

18. Each of the energy consumption rates is a fuel flow rate, for the at least one non-transitory machine-readable medium according to claim 15.

19. The previous airspeed in (a) is a predetermined airspeed corresponding to the minimum energy consumption rate of the aircraft at the maximum takeoff weight of the aircraft, for the at least one non-transitory machine-readable medium according to claim 15.

20. The more efficient operating point of the aircraft corresponds to an airspeed at which the ratio of lift to drag of the aircraft is substantially at a maximum under the current operating conditions of the aircraft, for the at least one non-transitory machine-readable medium according to claim 15.

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