BACKGROUND 
     The present disclosure relates generally to fuel systems for gas turbine engines. In particular, the present disclosure relates to fuel systems utilizing fuel flow to operate engine actuation systems. 
     In conventional fuel systems, a main fuel pump is used to deliver fuel to a fuel metering valve that provides fuel directly to fuel injectors in the combustion section of the engine. Additionally, some of the fuel flow from the main fuel pump is circulated through actuators that operate other engine or aircraft systems. Such a system is described in U.S. Pat. No. 4,487,016, which is assigned to United Technologies Corporation. In some systems, fuel flow is metered using a servo valve-controlled torque motor that provides fuel based on engine requirements for different speeds. Additionally, servo valve-controlled valves are used to regulate airflow to active clearance control systems and variable vane systems based on fuel flow. The servo valves utilize fuel flow from the main fuel pump to provide actuation. The main fuel pump needs to be sized at a minimum to provide flow to the servo valves and to the injectors at idle engine speed, and at a maximum to provide flow to the servo valves and to the injectors under transient engine conditions, such as during take-off. Thus, the main fuel pump must have a large capacity to accommodate the entire engine operating envelope and to provide fuel to other various aircraft systems. The large pump capacity produces inefficiencies in the engine, such as consuming excess system horsepower and generating waste heat. Furthermore, the servo valve-controlled actuators need to be sized to withstand the elevated pressures generated during transient conditions and the associated fatigue stress with such a wide operating envelope. There is, therefore, a need for a more efficient fuel and actuation system for gas turbine engines. 
     SUMMARY 
     The present disclosure is directed to systems and methods for delivering fuel in an aircraft. In one embodiment, a fuel system for an aircraft comprises a boost pump, a main fuel pump and a motive pump. The boost pump receives fuel from a storage unit. The main fuel pump receives fuel from the boost pump and delivers fuel to a distribution system. The motive fuel pump receives fuel from the boost pump, routes fuel through the storage unit, and delivers fuel to an actuator. 
     In another embodiment, the present disclosure is directed to a method for delivering fuel in an aircraft. The method comprises pumping fuel from a fuel tank to a distribution system using a main pump, and pumping fuel from a fuel tank to an actuator using a motive pump. The method also comprises routing fuel from the actuator to the main pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The sole FIGURE shows a fuel and actuation system utilizing a main fuel pump and a motive fuel pump to deliver fuel flows to a combustion system and to actuators. 
     
    
    
     DETAILED DESCRIPTION 
     The sole FIGURE shows fuel and actuation system  10  utilizing main fuel pump  12  and motive fuel pump  14  to deliver fuel flows to distribution system  16  and actuators  18 A and  18 B. The present disclosure is described with reference to a fuel system for gas turbine engine  19  that utilizes fuel flow to operate aircraft system actuators. However, the fuel and actuation system described herein may be used with other aircraft or non-aircraft systems. 
     Fuel located in a storage unit, such as fuel tank  20 , is pumped out by boost pump  22 . Boost pump  22  provides fuel flow to main fuel pump  12  and motive pump  14 . Heat exchanger  24  and filter  26  are fluidly positioned between boost pump  22  and main fuel pump  12  in series connection. Main fuel pump  12  delivers fuel to distribution system  16 , which comprises manifold  28 , valves  30 A and  30 B, and injectors  32 A,  32 B,  34 A and  34 B. Main fuel pump  12 , through distribution system  16 , provides fuel to combustor  35  in gas turbine engine  19  where a combustion process that consumes fuel occurs. Motive pump  14  provides fuel to ejector  36  to circulate or distribute fuel within fuel tank  20 , and also provides fuel to actuators  18 A and  18 B to control various operations related to gas turbine engine  19 . Main fuel pump  12  and motive pump  14  are mechanically operated based on shaft speed within gas turbine engine  19 . Thus, at higher shaft speeds, pumps  12  and  14  provide higher volumetric flow rates of fuel. 
     Boost pump  22  draws in fuel from fuel tank  20  through shut-off valve  37  and pumps the fuel to heat exchanger  24 . Heat exchanger  24  is fluidly coupled to fluid flow  38  of another liquid, such as oil or another lubricant, that is at a different temperature than the fuel from boost pump  22 . In one embodiment, heat exchanger  24  comprises a fuel/oil cooler that transfers heat from oil used to lubricate various aircraft systems, such as bearings, to the fuel. From heat exchanger  24 , fuel is routed to filter  26 . Main fuel pump  12  provides fuel to distribution system  16 , which includes many small orifices, such as those in injectors  32 A- 34 B. Filter  26  removes contaminants from the fuel before being routed to main fuel pump  12  to avoid formation of blockages within main fuel pump  12  and distribution system  16 . In one embodiment, filter  26  comprises a screen with a bypass valve. 
     Main fuel pump  12  provides fuel directly to manifold  28 , which distributes fuel to a plurality of valves. Fuel valves  30 A and  30 B distribute fuel to fuel injectors  32 A- 34 B as needed by gas turbine engine  19 . In particular, injectors  32 A- 34 B provide fuel to combustor  35  within gas turbine engine  19  where combustion process is executed using the fuel. The combustion process, as is known in the art, operates gas turbine engine  19  to provide shaft power or thrust that drives an aircraft. In one embodiment of the invention, valves  30 A and  30 B are electronically operated metering valves controlled by a control system for gas turbine engine  19 . Injectors  32 A- 34 B may comprise primary and secondary fuel injectors that deliver fuel to different parts of combustor  35  at different times during the combustion process. 
     Fuel flow from main fuel pump  12  also includes pressure relief valve  40  and pressure regulating valve  42 . Pressure relief valve  40  allows fuel from the high pressure side of main fuel pump  12  to be returned to the low pressure side of main fuel pump  12 , such as at the inlet of filter  26 . Pressure relief valve  40  typically automatically opens when pressure at the high pressure side of main fuel pump  12  becomes higher than a system maximum to prevent system over-pressurization. 
     Pressure regulating valve  42  returns fuel unneeded by distribution system  16  to the low pressure side of main fuel pump  12 , such as at the inlet of filter  26 . For example, main fuel pump  12  operates to provide a steady flow of fuel to manifold  28  at different operating speeds of gas turbine engine  19 . Sometimes, such as during steady state cruise conditions of gas turbine engine  19 , main fuel pump  12  can provide more fuel than is needed by combustor  35 . In such scenarios, pressure regulating valve  42  returns unconsumed fuel back through fuel system  10 . In one embodiment, pressure regulating valve  42  comprises an electronically operated valve, such as a direct drive valve, that is controlled by a control system for gas turbine engine  19 . In another embodiment, pressure regulating valve  42  comprises a servo valve that operates based on fuel flow as do actuators  18 A and  18 B, which will be described in detail below. 
     In addition to providing burn flow to distribution system  16  and combustor  35  through main fuel pump  12 , boost pump  22  also pumps fuel from fuel tank  20  to motive pump  14 . Motive pump  14  provides fuel flow to ejector  36 , which is used to distribute fuel within fuel tank  20 . For example, ejector  36  transfers fuel from different partitions  20 A,  20 B within fuel tank  20 , such as those that are located in different wings of the aircraft. Such distribution and circulation of fuel within fuel tank  20  ensures that boost pump  22  will be adequately primed with fuel at different fuel level and at different aircraft orientations. Fuel from within fuel tank  20  is provided to boost pump  22  through shut-off valve  37 . Valve  37  can be closed to fluidly isolate fuel tank  20  from boost pump  22  such as for maintenance operations and the like. 
     Motive pump  14  additionally directly provides fuel flow to one or more actuators. As mentioned, motive pump  14  may also provide fuel flow to a servo valve for various embodiments of pressure regulating valve  42 . In the described embodiment, fuel and actuation system  10  includes two actuators  18 A and  18 B. In one embodiment, actuators  18 A and  18 B comprise servo valves that are operated by fuel flow from motive pump  14 . In one embodiment, actuators  18 A and  18 B include butterfly valves that are actuated based on the volume of fuel flow provided by motive pump  14 . Actuators  18 A and  18 B regulate airflows  44 A and  44 B, respectively, to other parts of gas turbine engine  19 . For example, actuator  18 A may actuate an active clearance control air valve that provides airflow  44 A to change the clearance gap in turbine section  46  of gas turbine engine  19 . Such an active clearance control system is described in U.S. Pat. No. 4,069,662 to Redinger, which is assigned to United Technologies Corporation and is incorporated herein by this reference. For example, actuator  18 B may actuate a bleed valve that controls bleed airflow  44 B from compressor section  48  of gas turbine engine  19  for various uses, such as clearance control systems. In another embodiment, actuators  18 A and  18 B may comprise a linear actuator that changes the position of a variable vane. In other embodiments, heat exchangers may be connected into system  10  upstream of actuators  18 A and  18 B to warm the fuel flow before interacting with airflows  44 A and  44 B. After providing actuation power to actuators  18 A and  18 B, fuel is returned to fuel and actuation system  10 , such as at the inlet of main fuel pump  12 . 
     Motive pump  14  is fluidly connected within fuel and actuation system  10  with check valve  50 , pressure relief valve  52 , filter  54 A and filter  54 B. Filters  54 A and  54 B remove contaminants from the fuel before and after being routed to and from motive pump  14 , respectively, to avoid formation of blockage in passages within actuators  18 A and  18 B or ejector  36 . In one embodiment, filter  54 A comprises a screen with a bypass valve, and filter  54 B comprises a wash filter. 
     Pressure relief valve  52  allows fuel from the high pressure side of motive pump  14  to be returned to the low pressure side of motive pump  14 , such as at the inlet of filter  54 A. Pressure relief valve  52  typically automatically opens when pressure at the high pressure side of motive pump  14  becomes higher than a system maximum to prevent system over-pressurization. 
     Check valve  50  ensures that fuel from motive pump  14  remains above a baseline pressure in the fuel lines. In particular, fuel flow from motive pump  14  is divided between the needs of ejector  36  and actuators  18 A and  18 B. Ejector  36  is operable over a wide range of system pressures and need not continuously operate. Actuators  18 A and  18 B, however, require a minimum fuel pressure to be operable and need to operate over the entire operating envelope of gas turbine engine  19 . Check valve  50  ensures that flow from motive pump  14  to actuators  18 A and  18 B is at a minimum pressure to ensure functionality of actuators  18 A and  18 B. Specifically, check valve  50  establishes a restriction before ejector  36  that maintains a back pressure between motive pump  14  and actuators  18 A and  18 B. In one embodiment, check valve  50  comprises a minimum pressure valve, as is known in the art. In another embodiment, check valve  50  may comprise a computer controlled valve to modulate the minimum pressure for additional optimization of both pressure and temperature. 
     In view of the foregoing system, main fuel pump  12  is sized to provide only the fuel flow required by distribution system  16 . Typically, actuators within a gas turbine engine can consume 20%-30% of the output of the main fuel pump. Because motive pump  14  feeds actuators  18 A and  18 B, main fuel pump  12  need not be sized to provide additional fuel flows to actuators  18 A and  18 B. Furthermore, motive pump  14  need not be increased in capacity to accommodate actuator transients because short reductions in motive flow are acceptable. Thus, main fuel pump  12  can be 20%-30% smaller and lighter. Main fuel pump  12  also does not consume excessive system horsepower, such as by consuming shaft power of gas turbine engine  19 , or generate excess system heat, such as by pumping unnecessary volume of fuel through pressure regulating valve  42 . Furthermore, by de-coupling fuel flow to actuators  18 A and  18 B from main fuel pump  12 , actuators  18 A and  18 B and all fuel lines servicing actuators  18 A and  18 B, can be sized for lower pressures and lower cycle fatigue. Thus, actuators  18 A and  18 B and their respective fuel lines can be lighter and less expensive. Additionally, by having actuators  18 A and  18 B powered by motive pump  14 , transient disturbance from the actuators of pressure regulating valve  42  is eliminated. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
A fuel system for an aircraft comprises a boost pump, a main fuel pump and a motive pump. The boost pump receives fuel from a storage unit. The main fuel pump receives fuel from the boost pump and delivers fuel to a distribution system. The motive fuel pump receives fuel from the boost pump, routes fuel through the storage unit, and delivers fuel to an actuator. A method for delivering fuel in an aircraft comprises pumping fuel from a fuel tank to a distribution system using a main pump, pumping fuel from a fuel tank to an actuator using a motive pump, and routing fuel from the actuator to the main pump.