Patent Publication Number: US-2021192964-A1

Title: Electric storage and electric taxiing system for an aircraft

Description:
BACKGROUND INFORMATION 
     1. Field 
     The present disclosure relates generally to aircraft and, more specifically, to generation and use of electricity in an aircraft. The present disclosure relates to an electric taxiing system for an aircraft. 
     2. Background 
     While an aircraft is on the ground, movement of the aircraft is either performed by a tug or under the aircraft&#39;s own power. Towing and push-back of the aircraft is performed by a tug. Movement of the aircraft under its own power is called taxiing. 
     During taxi, the aircraft&#39;s engines generate more energy than is used to propel the aircraft. In some instances, an aircraft waits to approach a destination within an airport such as a runway or a gate. As an aircraft waits, the aircraft idles with its engines running. 
     Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as weil as other possible issues. 
     SUMMARY 
     An illustrative embodiment of the present disclosure provides a taxiing system for an aircraft. The taxiing system comprises energy storage locations electrically connected to a number of electric motors, the energy storage locations including at least two of a number of engines of the aircraft, a number of batteries, and a number of auxiliary power units of the aircraft; and the number of electric motors connected to wheels of the aircraft to at least one decelerate the wheels by transferring the kinetic energy of the aircraft into electric energy or drive the wheels using electric energy provided by at least one energy storage location of the energy storage locations. 
     Another illustrative embodiment of the present disclosure provides an aircraft. The aircraft comprises carbon brakes, engines, landing gear having wheels, and a taxiing system configured to propel and decelerate motion of the aircraft. The taxiing system comprises energy storage locations electrically connected to a number of electric motors, the energy storage locations including at least two of a number of engines of the aircraft, a number of batteries, and a number of auxiliary power units, and the number of electric motors connected to wheels of the aircraft to at least one of decelerate the wheels by transferring the kinetic energy of the aircraft into electric energy or drive the wheels using electric energy provided by at least one energy storage location of the energy storage locations. 
     Yet another illustrative embodiment of the present disclosure provides a method of taxiing an aircraft. Electric energy is directed from an energy storage location of energy storage locations selected from one of an engine of the aircraft, an auxiliary power unit, or a battery to an electric motor to generate kinetic energy. Wheels of the aircraft are driven using the kinetic energy generated by the electric motor. Movement of the aircraft is decelerated using electric energy from one of the energy storage locations directed to electric motor brakes of the taxiing system. 
     Another illustrative embodiment of the present disclosure provides a method of taxiing an aircraft. Energy from landing an aircraft is harvested by a number of electric motors connected to landing gear of the aircraft. The energy harvested from landing the aircraft is sent to an engine core of the aircraft and converted to kinetic energy. The aircraft is taxied by sending electric energy generated by extracting the kinetic energy from the engine core to the number of electric motors. motors 
     Yet another illustrative embodiment of the present disclosure provides a method of powering auxiliary systems of an aircraft. Energy from landing an aircraft is harvested by a number of electric motors connected to landing gear of the aircraft. The energy harvested from landing the aircraft is sent to an auxiliary power unit of the aircraft. Auxiliary operations are powered by the auxiliary power unit using the energy. 
     The features and functions can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments in which further details can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the illustrative embodiments are set forth in the appended claims. The illustrative embodiments, however, as well as a preferred mode of use, further objectives and features thereof, will best be understood by reference to the following detailed description of an illustrative embodiment of the present disclosure when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is an illustration of a block diagram of an aircraft in which a taxiing system is present in accordance with an illustrative embodiment; 
         FIG. 2  is an illustration of a plurality of aircraft waiting to taxi to or from a runway in accordance with an illustrative embodiment; 
         FIG. 3  is an illustration of a diagram of an aircraft having a taxiing system in accordance with an illustrative embodiment; 
         FIG. 4  is an illustration of an operational diagram of power flow in a taxiing system in accordance with an illustrative embodiment; 
         FIG. 5  is an illustration of a flowchart of a method of taxiing an aircraft in accordance with an illustrative embodiment; 
         FIG. 6  is an illustration of a flowchart of a method of taxiing an aircraft in accordance with an illustrative embodiment; 
         FIG. 7  is an illustration of a flowchart of a method of powering auxiliary systems of an aircraft in accordance with an illustrative embodiment; 
         FIG. 8  is an illustration of an aircraft manufacturing and service method in a form of a block diagram in accordance with an illustrative embodiment; and 
         FIG. 9  is an illustration of an aircraft in a form of a block diagram in which an illustrative embodiment may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     The illustrative embodiments recognize and take into account one or more different considerations. For example, the illustrative embodiments recognize and take into account that when a jet powered aircraft is taxiing under its own power, the method of jet propulsion is inefficient in terms of fuel consumption. 
     The illustrative embodiments recognize and take into account that as engines idle on an aircraft, fuel is expended. The illustrative embodiments recognize and take into account that it is desirable to reduce fuel waste for environmental and cost reasons. By reducing fuel waste, operating an aircraft is less expensive. By reducing fuel waste, fewer emissions are released for each flight. By reducing fuel waste, non-renewable resources are conserved. 
     Additionally, increase in aircraft engine ground idle speed involves frequent use of wheel brakes. Frequent use of wheel brakes increases the frequency of brake use during taxi operations. Increasing the frequency of brake use is undesirable for brake thermal management. 
     The illustrative embodiments recognize and take into account that fuel burn during taxiing can be a significant amount of fuel expenditures for an aircraft. By reducing fuel burn during taxiing, the fuel used by an aircraft is advantageously reduced. 
     The illustrative embodiments recognize and take into account that operating an aircraft engine produces engine noise and emissions. The illustrative embodiments recognize and take into account that it may be desirable to reduce the cumulative engine noise at an airport. The illustrative embodiments recognize and take into account that it may be desirable to reduce the cumulative emissions at an airport. 
     Additionally, taxiing frequently requires stopping the aircraft and then re-starting the motion. Taxi stops are accomplished currently using the aircraft brakes. Each taxi stop leads to additional heat in the brake, and each stop creates additional wear. A substantial fraction of total brake wear occurs during taxi, because the initial contact of the brake elements creates a contact wear event which removes some of the brake surface material. The illustrative embodiments recognize and take into account that for aircraft brakes, an amount of brake wear is not correlated to the energy dissipated by the braking. The illustrative embodiments recognize and take into account that braking during taxiing can add considerable wear to aircraft brakes. The illustrative embodiments recognize and take into account that in some cases, braking during taxiing may be substantially the same amount of wear as multiple landings. 
     The illustrative embodiments recognize and take into account that braking during taxiing generates heating due to stopping. The additional heat absorbed by the brakes during taxiing can sometimes cause a reduction in the available brake energy capability below the allowed limit allowed for takeoff. Takeoff is not allowed unless there is sufficient brake energy capacity to perform an aborted takeoff. Each stop is followed by an additional accelerate to taxi speed event, which consumes additional fuel and creates additional pollutants. 
     Additionally, the illustrative examples recognize and take into account that there is a limit on the brake temperature prior to takeoff. The illustrative examples recognize and take into account that the limit on brake temperature is to prevent taking off with an undesirably high temperature of brakes. The illustrative examples recognize and take into account that the brakes must have enough “reserve energy” capacity in the brakes to perform a rejected takeoff (emergency stop). The illustrative examples also recognize and take into account that withdrawing hot brakes into the wheel well could potentially damage components of the aircraft. The illustrative examples recognize and take into account that keeping the landing gear extended into the airstream to allow the brakes to cool would lead to a limitation on the airplane&#39;s ability to climb. 
     The illustrative examples recognize and take into account that an aircraft carries extra weight to keep the brakes cool so that the brakes are maintained at a desirable temperature. The illustrative examples recognize and take into account that it would be desirable to reduce the weight of an aircraft to improve fuel efficiency of the aircraft. 
     The illustrative embodiments recognize and take into account that it is desirable to reduce fuel consumption between flights. The illustrative embodiments recognize and take into account that it may be desirable to power down the engines as soon as possible after a flight to conserve fuel. 
     The illustrative embodiments recognize and take into account that some ground vehicles include regenerative braking. In regenerative braking, kinetic energy of the ground vehicle is converted into a form that can be stored, such as electric energy. Regenerative braking is most advantageous for ground vehicles that will frequently have in-city driving due to the frequency of stopping in-city. 
     The illustrative embodiments recognize and take into account that aircraft have different considerations than ground vehicles. Aircraft not only have significantly higher speeds, higher energy consumption, and higher mass, but are also subject to Federal Aviation Regulations. Further, aircraft do not spend a substantial amount of time stopping and starting motion on the ground. Additionally, aircraft system designs take into account weight of the aircraft. An increase in the weight of an aircraft reduces fuel efficiency and decreases the amount of passengers and cargo that can be transported by the aircraft. Accordingly, the illustrative embodiments recognize and take into account that weight considerations also are different between ground vehicles and aircraft. 
     The illustrative examples present a taxiing system for an aircraft. The taxiing system comprises energy storage locations electrically connected to a number of electric motors, the energy storage locations including at least two of a number of engines of the aircraft, a battery, and an auxiliary power unit of the aircraft; and the number of electric motors connected to wheels of the aircraft to at least one decelerate the wheels by transferring the kinetic energy of the aircraft into electric energy or drive the wheels using electric energy provided by at least one energy storage location of the energy storage locations. Using a number of electric motors in the taxiing system to drive or decelerate the wheels of the aircraft reduces at least one of fuel emissions, fuel waste, braking wear, or engine noise. 
     As used herein, a first component “connected to” a second component means that the first component can be connected directly or indirectly to the second component. In other words, additional components may be present between the first component and the second component. The first component is considered to be indirectly connected to the second component when one or more additional components are present between the two components. When the first component is directly connected to the second component, no additional components are present between the two components. 
     Turning now to  FIG. 1 , an illustration of a block diagram of an aircraft in which a taxiing system is present is depicted in accordance with an illustrative embodiment. Aircraft  100  has taxiing system  102 . Taxiing system  102  comprises energy storage locations  104  and number of electric motors  106 . As used herein, a “number of” items is one or more items. For example, number of electric motors  106 , is one or more electric motors. 
     Energy storage locations  104  are electrically connected to number of electric motors  106 . Energy storage locations  104  include at least two of number of engines  107 , number of batteries  109 , and number of auxiliary power units  111  of aircraft  100 . As depicted, number of engines  107  includes engine  108 , number of batteries  109  includes battery  110 , and number of auxiliary power units  111  includes auxiliary power unit  112 . Energy storage locations  104  include engine  108  of aircraft  100 , battery  110 , and auxiliary power unit  112  of aircraft  100 . Engine  108  includes rotating elements of engine core  114 , including the high-pressure compressor, high pressure shaft, and high-pressure turbine. Engine  108  also includes engine fan  116  comprising the fan which mechanically couples to the low-pressure compressor, shaft, and low-pressure turbine. Components of engine fan  116  are a second separate set of rotating elements from the rotating elements of engine core  114 . Engine core  114  refers to the high-pressure rotating elements of engine  108 . Engine fan  116  refers to the low-pressure rotating elements of engine  108 . 
     Although energy storage locations  104  as depicted includes one of each of engine, battery, and auxiliary power unit, energy storage locations  104  is not limited to engine  108  of aircraft  100 , battery  110 , and auxiliary power unit  112 . Energy storage locations  104  includes any desirable quantity of at least two of engine, battery, and auxiliary power unit. In other non-depicted examples, energy storage locations  104  includes at least one of more than one engine, more than one battery, or more than one auxiliary power unit. 
     Engine  108  conventionally only operates in a single direction of rotation. In taxiing system  102 , engine  108  generator/motors operate in two quadrants of control. The two quadrants include a motoring mode in which electric energy  127  contributes to adding rotational energy to engine  108  and a generating mode in which the rotating energy is used to produce electric energy  127 . Number of engines  107  of aircraft  100  is configured to receive electric energy  127  from the number of electric motors  106  and rotate components of the number of engines  107  using the electric energy  127  in motoring mode. The generating mode is a conventional mode of operation of an engine mounted generator. 
     Engine  108  motor/generators can be of any type. In some illustrative examples, engine  108  is a wound field three phase generator with an auxiliary permanent magnet exciter which powers the field winding, and is electronically controlled. 
     Battery  110  is a battery which can be electrically charged or discharged. The state of charge of battery  110  can be measured. Although a battery charge controller is not shown, a battery charge controller can be part of battery  110 . 
     Number of electric motors  106  is connected to wheels  118  of the aircraft to at least one of drive or decelerate wheels  118  using power provided by at least one energy storage location of energy storage locations  104 . Decelerating wheels  118  includes all decrease of deceleration. In some illustrative examples, decelerating wheels  118  slows aircraft  100 . In some illustrative examples, decelerating wheels  118  stops aircraft  100 . In some illustrative examples, decelerating wheels  118  additionally includes keeping aircraft  100  stationary. 
     Number of electric motors  106  is connected to wheels  118  of landing gear  119  of aircraft  100 . Number of electric motors  106  is configured to at least one of: harvest energy  124  from landing  126 , harvest energy  124  from decelerating wheels  118  during taxiing  140 , drive wheels  118 , or decelerate wheels  118 . In some illustrative examples, number of electric motors  106  is configured to drive wheels  118  using electric energy in taxiing system  102 . In some illustrative examples, number of electric motors  106  act as electric motor brakes  121  to decelerate wheels  118  of aircraft  100 . 
     In some illustrative examples, number of electric motors  106  operates in all four quadrants of operation. Two quadrants of applying a positive torque in either the forward direction or reverse direction, used when propelling aircraft  100  forwards. Two opposite quadrants, when retarding aircraft  100 , which is accomplished by absorbing electric energy  127  produced by the stopping action of number of electric motors  106 , in the form of increased motor bus voltage. 
     Number of electric motors  106  take any desirable form. In some illustrative examples, each of number of electric motors  106  is a permanent magnet three phase brushless DC motor in which the motor phases are electronically commutated using control electronics. 
     Number of electric motors  106  includes one or more motors connected to one or more of wheels  118 . In some illustrative examples, number of electric motors  106  includes a first motor connected to first wheel  120  and a second motor connected to second wheel  122 . In other illustrative examples, a single electric motor is connected to both first wheel  120  and second wheel  122 . 
     In some illustrative examples, number of electric motors  106  harvest energy  124  from landing  126 . In these illustrative examples, number of electric motors  106  is electrically connected to energy storage locations  104  such that energy  124  from landing  126  aircraft  100  is stored in energy storage locations  104 . When aircraft  100  is landing  126 , number of electric motors  106  is set to direct electric energy  127  generated from energy  124  into energy storage locations  104 . In other illustrative examples, number of electric motors  106  are disengaged during landing  126  and do not harvest energy  124  from landing  126 . In some illustrative examples, electro-mechanically actuated clutches can be used to engage and disengage number of electric motors  106 . 
     In some illustrative examples, number of electric motors  106  harvest energy  123  from taxiing  140 . In some of these illustrative examples, excess electric energy  127  generated by decelerating movement of aircraft  100  during taxiing  140  using number of electric motors  106  is directed to at least one of energy storage locations  104 . Energy  123  harvested during taxiing  140  can be directed into non-running engine core  114  or engine fan  116  in a single-engine taxi approach, into running engine  108 , or into battery  110 . 
     During landing  126 , thrust reversers in an engine, such as engine  108 , are employed. In a propeller driven aircraft, reversing propellers is employed during landing  126 . Electric energy  127  generated by number of electric motors  106  in a regenerative mode can be supplied to engine  108  to create the reverse thrust to slow aircraft  100 . Providing electric energy  127  to generate reverse thrust by engine  108  during landing  126  reduces fuel consumption. 
     Energy storage locations  104  take any desirable form. In some illustrative examples, energy storage locations  104  include auxiliary power unit  112 . When energy storage locations  104  includes auxiliary power unit  112 , number of electric motors  106  is electrically connected to auxiliary power unit  112 . In some illustrative examples, number of electric motors  106  is electrically connected to auxiliary power unit  112  bidirectionally. When number of electric motors  106  is bidirectionally electrically connected to auxiliary power unit  112 , number of electric motors  106  can both send electric energy  127  to and receive electric energy  127  from auxiliary power unit  112 . In some illustrative examples, auxiliary power unit  112  spins to absorb electric energy  127 . In some illustrative examples, a motor (not depicted) of auxiliary power unit  112  spins to absorb electric energy  127 . Auxiliary power unit  112  uses the electric energy  127  received from number of electric motors  106  to perform operations for which auxiliary power unit  112  conventionally provides power. In some illustrative examples, auxiliary power unit  112  uses the electric energy  127  received from number of electric motors  106  to perform auxiliary operations  135 . In some illustrative examples, number of auxiliary power units  111  is configured to receive electric energy  127  from number of electric motors  106  and rotate components of number of auxiliary power units  111  using the electric energy  127 . In some illustrative examples, electric energy  127  is sent to auxiliary power units  111  to power auxiliary systems  136  to perform auxiliary operations  135 . As depicted, auxiliary systems  136  include environmental system  137  and entertainment units  138 . Environmental system  137  provides heating and cooling to a passenger cabin of aircraft  100 . Entertainment units  138  include screens, audio, or other types of entertainment systems provided to passengers within aircraft  100 . The depiction of auxiliary systems  136  is not limiting, auxiliary systems  136  include any desirable types of systems. 
     Auxiliary power unit  112  generator/motor acts only in a single direction of rotation, but operates in both quadrants. Auxiliary power unit  112  acts as a generator in conventional operation to convert mechanical rotary power into electrical power or as a motor, in which electric energy  127  is converted to rotating power. 
     In some illustrative examples, energy storage locations  104  includes battery  110 . When energy storage locations  104  includes battery  110 , number of electric motors  106  is electrically connected to battery  110 . In some illustrative examples, number of electric motors  106  is electrically connected to battery  110  bidirectionally. When number of electric motors  106  is bidirectionally electrically connected to battery  110 , number of electric motors  106  can both send electric energy  127  to and receive electric energy  127  from battery  110 . Battery  110  stores electric energy  127  generated by number of electric motors  106 . Battery  110  stores electric energy  127  for powering number of electric motors  106  in taxiing  140 . 
     In some illustrative examples, energy storage locations  104  includes engine  108 . When energy storage locations  104  includes engine  108 , number of electric motors  106  is electrically connected to engine  108 . In some illustrative examples, number of electric motors  106  is electrically connected to engine  108  bidirectionally. When number of electric motors  106  is bidirectionally electrically connected to engine  108 , number of electric motors  106  can both send electric energy  127  to and receive electric energy  127  from engine  108 . In some illustrative examples, engine  108  spins to absorb electric energy  127 . In some illustrative examples, a motor (not depicted) of engine  108  spins to absorb electric energy  127 . The motor may be a part of either engine core  114  or engine fan  116 . Engine  108  uses the electric energy  127  received from number of electric motors  106  to operate with reduced fuel. In some illustrative examples, engine  108  uses the electric energy  127  received from number of electric motors  106  to keep engine  108  powered with reduced fuel usage. 
     In some illustrative examples, it is desirable to speed up at least one of engine core  114  or engine fan  116  during landing  126  to provide reverse thrust. In these illustrative examples, electric energy  127  can be supplied to engine  108  during thrust reverser deployment. Reverse thrust helps with providing deceleration during landing  126 . In other illustrative examples, it would be desirable to direct electric energy  127  to propellers of a propeller aircraft with the propellers reversed. 
     Aircraft  100  has control system  128  configured to send commands  130  to flow control switch system  131  to direct electric energy  127  between energy storage locations  104  and number of electric motors  106 . In some illustrative examples, flow control switch system  131  is referred to as a power/energy flow control switch system. Control system  128  may also be referred to as a controller. 
     Control system  128  is configured to direct electric energy  127  generated by number of electric motors  106  to at least one battery of number of batteries  109 . Control system  128  is configured to direct electric energy  127  generated by number of electric motors  106  to at least one of an engine of number of engines  107  of aircraft  100  or an auxiliary power unit of number of auxiliary power units  111  when battery  110  reaches a charge capacity. 
     Flow control switch system  131  may also be referred to as a “power switch matrix”. Flow control switch system  131  can be referred to as a combination of switches  133  which connect electrical power generating sources to loads. While a system can be implemented used electrical switches  133 , in another embodiment, these switches  133  can be implemented within the electrical motor/generator controls themselves, such that the switching function is implemented in a logical fashion without requiring actual physical switches. 
     Engine  108  and auxiliary power unit  112  each have motor devices. At the controller for each of these motor devices, operation in regenerative mode will tend to raise the electrical voltage of the connected electrical bus, which can be detrimental to the connected loads. Consequently, the electrical power control system  128  signals to one or more of another rotating element motor/generator controllers a request to increase the speed of that element, e.g. to convert from generating mode to motoring mode. At the same time, any fuel supplied to the rotating element&#39;s engine core  114  is reduced, to prevent the element speed from rising to an excessive level. Rotation can also be added to rotating elements of engine  108  which have no fuel consumption and are simply rotating masses used as an energy storage system. 
     In the case where the regenerative energy is sent to battery  110 , it simply acts as a charging current, but a protection means is provided to limit the charging current to a safe level. If the energy exceeds the capacity of battery  110  to absorb it, control system  128  activates additional power sinks by changing the control of rotating elements from generating mode to motoring mode. 
     When the retarding force available from operating number of electric motors  106  in regenerative (generating mode) is insufficient to slow aircraft  100 , conventional aircraft friction brakes, such as carbon brakes  151 , may be used to provide additional retarding force. 
     Motor/generator outputs are typically three phase, but number of electric motors  106  operate over a wide speed range, their control electronics converts the AC input to a DC bus voltage. Therefore, when an AC system is used, a feature of the motor controllers will have to include the ability to phase synchronize to the applied power such that current will flow out of the device in phase with the bus voltage. 
     Control system  128  sends commands  130  to flow control switch system  131  depending upon a desired operation of operations  132 . Commands  130  are sent to set switches  133  to perform at least one operation of operations  132 . Switches  133  direct the movement of electric energy  127  within taxiing system  102 . Switches  133  direct the movement of electric energy  127  between number of electric motors  106  and energy storage locations  104 . As depicted, commands  130  include auxiliary command  134 , drive command  139 , brake command  146 , neutral command  152 , and back-up command  156 , and charge command  158 . 
     Although not depicted in  FIG. 1 , a pilot can be present in aircraft  100 . When aircraft  100  is piloted, brake command  146  is generated when the pilot applies their feet to brake pedals of aircraft  100 , which are conventionally mounted on top of the rudder pedals. Application of pressure to the brake pedals indicates a request to retard or halt the motion of aircraft  100 , and the control system  128  does so. In some illustrative examples, drive command  139  to move aircraft  100  forward is generated when a pilot provides physical pressure or movement of a new pilot control, such as a taxi control, to provide an input. In other illustrative examples, in an unmanned or unpiloted aircraft, drive command  139  is generated based on alternative input. 
     In some illustrative examples, brake command  146  is generated based on input from an autobrake system. An autobrake system automatically applies and controls braking during landings. In some illustrative examples, when aircraft  100  is an unmanned or unpiloted aircraft, commands  130  are generated as part of a centralized aircraft control system and are not generated based on physical input by a pilot present on aircraft  100 . 
     In some illustrative examples, taxiing system  102  is operated during landing  126  to store energy  124  from landing  126  in at least one of energy storage locations  104 . In some illustrative examples, during landing  126 , energy  124  of landing  126  is converted to electric energy  127  by number of electric motors  106 . Number of electric motors  106  is connected to energy storage locations  104  such that energy  124  from landing  126  aircraft  100  is stored in energy storage locations  104 . In some illustrative examples, to store energy  124 , control system  128  sends charge command  158 . 
     Number of electric motors  106  harvests a fraction of the total energy in landing  126 . Number of electric motors  106  harvests a fraction of the total energy in landing  126  based on threshold force  149  of electric motor brakes  121 . 
     Control system  128  sends drive command  139  to taxiing system  102  to perform taxiing  140 . Drive command  139  includes instructions to set switches  133  to direct electric energy  127  to number of electric motors  106 . Drive command  139  instructs number of electric motors  106  connected to wheels  118 . Driving  144  of wheels  118  of aircraft  100  during taxiing  140  is performed using kinetic energy  145  generated by number of electric motors  106  from electric energy  127  from at least one of energy storage locations  104 . 
     Control system  128  sends brake command  146  to number of electric motors  106  to perform braking  148  of wheels  118  during taxiing  140 . During braking  148 , aircraft  100  is decelerated using electric energy  127  provided by energy storage locations  104  in taxiing system  102 . In some illustrative examples, braking  148  is performed by energy stored from landing  126  aircraft  100 . Snubs  150  of carbon brakes  153  during taxiing  140  are reduced or eliminated when taxiing system  102  performs braking  148  of wheels  118 . In some illustrative examples, when electric motor brakes  121  decelerate aircraft  100 , aircraft  100  may be decelerated without engaging carbon brakes  153 . Although taxiing system  102  performs braking  148  of wheels  118 , carbon brakes  153  are still available if braking  148  using carbon brakes  153  is desired. In some illustrative examples, electric motor brakes  121  and carbon brakes  153  are engaged to decelerate aircraft  100 . 
     In some illustrative examples, control system  128  sends neutral command  152  when taxiing system  102  is not actively driving  142  or braking  148  wheels  118 . In some illustrative examples, neutral command  152  sets taxiing system  102  such that number of electric motors  106  do not apply force to wheels  118 . In some illustrative examples, control system  128  sends neutral command  152  prior to aircraft  100  flying  154 . In some illustrative examples, neutral command  152  and charge command  158  are the same. 
     Neutral command  152 , places taxiing system  102  into a neutral condition. Neutral command  152  disengages number of electric motors  106  from wheels  118 . A traction motor providing high performance at low speed may be adversely affected by high speed operation. By neutral command  152  mechanically disengaging number of electric motors  106  from wheels  118 , number of electric motors  106  are not engaged during high speed operation. 
     In some illustrative examples, taxiing system  102  is used for backing aircraft  100  from a gate. In some illustrative examples, control system  128  sends drive command  139  regardless of the direction of movement of aircraft  100 . In these illustrative examples, control system  128  sends drive command  139  for backing up aircraft  100  from a gate. In these illustrative examples, flow of power within taxiing system  102  to number of electric motors  106  is the same for taxiing  140  and backing-up of aircraft  100 . In these illustrative examples, additional non-depicted mechanical components may reverse rotational direction of wheels  118 . In other illustrative examples, control system  128  sends back-up command  156  to back up aircraft  100  using taxiing system  102 . 
     In some illustrative examples, taxiing system  102  performs taxiing  140  without energy from engines  143 . By using taxiing system  102  to perform taxiing  140 , fuel used by engines  143  during taxiing  140  is reduced. In some illustrative examples, taxiing system  102  allows aircraft  100  to perform taxiing  140  without engaging engines  143 . In these illustrative examples, taxiing system  102  uses electric energy  127  from one of battery  110  or auxiliary power unit  112 . 
     In some illustrative examples, taxiing system  102  performs taxiing  140  using engines  143  and energy  124  from landing  126  sent to engine core  114 . Engine  108  is one of engines  143 . By sending energy  124  from landing  126  to engine core  114 , fuel used by engines  143  during taxiing  140  is reduced. 
     The illustration of aircraft  100  in  FIG. 1  is not meant to imply physical or architectural limitations to the manner in which an illustrative embodiment may be implemented. Other components in addition to or in place of the ones illustrated may be used. Some components may be unnecessary. Also, the blocks are presented to illustrate some functional components. One or more of these blocks may be combined, divided, or combined and divided into different blocks when implemented in an illustrative embodiment. 
     For example, although not depicted in  FIG. 2 , motors connected to each of auxiliary power unit  112 , engine core  114 , engine fan  116  may be present. For example, separate motors may be present for each of engine core  114  and engine fan  116  to absorb amounts of energy  124  from landing  126 . 
     Turning now to  FIG. 2 , an illustration of a plurality of aircraft waiting to taxi to or from a runway is depicted in accordance with an illustrative embodiment. In view  200 , aircraft  202  is present on runway  204 . Aircraft  202  is preparing to takeoff on runway  204 . Aircraft  202  has its engines running in preparation for takeoff. 
     Each of aircraft  206 , aircraft  208 , and aircraft  210  are present on taxiway  212 . Each of aircraft  206 , aircraft  208 , and aircraft  210  are waiting to taxi to another location in airport  211 . If any of aircraft  206 , aircraft  208 , or aircraft  210  are conventional aircraft, the respective conventional aircraft will idle with the respective conventional aircraft&#39;s engines running. 
     Any of aircraft  202 , aircraft  206 , aircraft  208 , or aircraft  210  may be aircraft  100  of  FIG. 1 . Any of aircraft  202 , aircraft  206 , aircraft  208 , or aircraft  210  may have taxiing system  102  of  FIG. 1 . 
     When aircraft  206  has a taxiing system with electric motors connected to the wheels of the landing gear, aircraft  206  is configured to taxi without operating the engines of aircraft  206 . When aircraft  208  has a taxiing system with electric motors connected to the wheels of the landing gear, aircraft  208  is configured to taxi without operating the engines of aircraft  208 . When aircraft  210  has a taxiing system with electric motors connected to the wheels of the landing gear, aircraft  210  is configured to taxi without operating the engines of aircraft  210 . When the engines of an aircraft are not operated, fuel consumption of the respective aircraft is reduced. For each of aircraft  206 , aircraft  208 , and aircraft  210  that has a taxiing system with electric motors connected to the wheels of the landing gear, electric energy from an energy storage location other than the engines of the respective aircraft powers the taxiing of the aircraft. 
     In some illustrative examples, one of a battery or an auxiliary power unit powers electric motors connected to the landing gear of aircraft  206  to drive aircraft  206  during taxiing. In some illustrative examples, one of a battery or an auxiliary power unit powers electric motors connected to the landing gear of aircraft  208  to drive aircraft  208  during taxiing. In some illustrative examples, one of a battery or an auxiliary power unit powers electric motors connected to the landing gear of aircraft  210  to drive aircraft  210  during taxiing. 
     For each aircraft of aircraft  206 , aircraft  208 , and aircraft  210  that has a taxiing system electric motors connected to the wheels of the landing gear, idling emissions in airport  211  are reduced. Idling emissions in airport  211  can be reduced by at least one of taxiing using electric motors or operating the aircraft engines using electric energy generated from harvesting kinetic energy from decelerating the respective aircraft. For each aircraft of aircraft  206 , aircraft  208 , and aircraft  210  that has a taxiing system with electric motors connected to the wheels of the landing gear, engine noise in airport  211  is reduced. 
     When aircraft  206  has a taxiing system with electric motors connected to the wheels of the landing gear, the taxiing system decelerates movement of aircraft  206  using the electric motors in the taxiing system. In some illustrative examples, when aircraft  206  has a taxiing system with electric motors connected to the wheels of the landing gear, aircraft  206  is configured to taxi without engaging the carbon brakes of aircraft  206 . When aircraft  206  is configured to taxi without engaging the carbon brakes of aircraft  206 , the carbon brakes of aircraft  206  may be replaced less frequently, thus reducing maintenance costs of aircraft  206 . Although aircraft  206  is configured to decelerate without engaging the carbon brakes of aircraft  206 , carbon brakes are still available and prepared for braking if desired. 
     When aircraft  208  has a taxiing system with electric motors connected to the wheels of the landing gear, the taxiing system decelerates movement of aircraft  208  using the electric motors in the taxiing system. In some illustrative examples, when aircraft  208  has a taxiing system with electric motors connected to the wheels of the landing gear, aircraft  208  is configured to taxi without engaging the carbon brakes of aircraft  208 . When aircraft  208  is configured to taxi without engaging the carbon brakes of aircraft  208 , the carbon brakes of aircraft  208  may be replaced less frequently, thus reducing maintenance costs of aircraft  208 . Although aircraft  208  is configured to decelerate without engaging the carbon brakes of aircraft  208 , carbon brakes are still available and prepared for braking if desired. 
     When aircraft  210  has a taxiing system with electric motors connected to the wheels of the landing gear, the taxiing system decelerates movement of aircraft  210  using the electric motors in the taxiing system. In some illustrative examples, when aircraft  210  has a taxiing system with electric motors connected to the wheels of the landing gear, aircraft  210  is configured to taxi without engaging the carbon brakes of aircraft  210 . When aircraft  210  is configured to taxi without engaging the carbon brakes of aircraft  210 , the carbon brakes of aircraft  210  may be replaced less frequently, thus reducing maintenance costs of aircraft  210 . Although aircraft  210  is configured to decelerate without engaging the carbon brakes of aircraft  210 , carbon brakes are still available and prepared for braking if desired. 
     Aircraft  214  is also present in view  200 . Aircraft  214  is a larger aircraft than any of aircraft  206 , aircraft  208 , or aircraft  210 . In some illustrative examples, aircraft  214  may be aircraft  100  of  FIG. 1 . In some illustrative examples, aircraft  214  has taxiing system  102  of  FIG. 1 . When aircraft  214  has a taxiing system with electric motors connected to the wheels of the landing gear, aircraft  214  is configured to taxi without operating the engines of aircraft  214 . When the engines of aircraft  214  are not operated, fuel consumption of aircraft  214  is reduced. When aircraft  214  has a taxiing system with electric motors connected to the wheels of the landing gear, idling emissions in airport  211  are further reduced. When aircraft  214  has a taxiing system with electric motors connected to the wheels of the landing gear, engine noise in airport  211  is further reduced. 
     Turning now to  FIG. 3 , an illustration of a diagram of an aircraft having a taxiing system is depicted in accordance with an illustrative embodiment. View  300  depicts locations of components of taxiing system  302  in aircraft  304 . Taxiing system  302  is a schematic representation of taxiing system  102  of  FIG. 1 . Taxiing system  302  may be implemented in any of aircraft  202 , aircraft  206 , aircraft  208 , aircraft  210 , or aircraft  214 . 
     Taxiing system  302  comprises number of electric motors  306 , controller  308 , and energy storage locations  310 . Number of electric motors  306  is connected to wheels  312  of landing gear  314 . As depicted, number of electric motors  306  includes electric motor  316  connected to wheel  318  and electric motor  320  connected to wheel  322 . 
     Number of electric motors  306  are configured to drive or decelerate wheels  312  using electric energy provided by one of energy storage locations  310 . In some illustrative examples, electric energy is provided to number of electric motors  306  by one of energy storage locations  310  to drive wheels  312 . In some illustrative examples, electric energy is provided to number of electric motors  306  by one of energy storage locations  310  to decelerate wheels  312 . In these illustrative examples, number of electric motors  306  act as electric motor brakes. 
     Decelerating wheels  312  slows aircraft  304 . In some illustrative examples, decelerating wheels  312  stops aircraft  304 . In some illustrative examples, decelerating wheels  312  additionally includes keeping aircraft  304  stationary. 
     Energy storage locations  310  includes at least two of a number of engines, a number of batteries, and a number of auxiliary power units of aircraft  304 . Energy storage locations  310  include engine  324 , battery  326 , and auxiliary power unit  328 . Engine  324  includes engine core  330  and engine fan  332 . Engine core  330  has motor  334 . Motor  334  may also be referred to as a generator or a motor/generator. Engine fan  332  has motor  336 . Motor  336  may also be referred to as a generator or a motor/generator. In some illustrative examples, at least one component of engine  324  provides energy to operate at least one motor of number of electric motors  306 . In some illustrative examples, at least one component of engine  324  provides energy to number of electric motors  306  to drive at least one wheel of wheels  312 . In some illustrative examples, at least one component of engine  324  provides energy to number of electric motors  306  to decelerate at least one wheel of wheels  312 . 
     In some illustrative examples, at least one component of engine  324  absorbs energy generated by number of electric motors  306  as wheels  312  are decelerated. In one illustrative example, energy is sent from number of electric motors  306  to spin engine fan  332 . In another illustrative example, energy is sent form number of electric motors  306  to engine core  330 . 
     Auxiliary power unit  328  has motor  338 . Motor  338  may be referred to as motor/generator. In some illustrative examples, auxiliary power unit  328  provides energy to operate at least one motor of number of electric motors  306 . In some illustrative examples, auxiliary power unit  328  provides energy to number of electric motors  306  to drive at least one wheel of wheels  312 . In some illustrative examples, auxiliary power unit  328  provides energy to number of electric motors  306  to decelerate at least one wheel of wheels  312 . 
     Controller  308  may also be referred to as a control system. Controller  308  is an implementation of control system  128  of  FIG. 1 . Controller  308  is configured to send commands to a flow control switch system to direct electric energy between energy storage locations  310  and number of electric motors  306 . 
     The flow control switch system comprising a plurality of switches configured to direct electric energy between energy storage locations  310  and number of electric motors  306 . The plurality of switches is configured to change flow of electric energy within taxiing system  302  to perform at least one of moving aircraft  304  using number of electric motors  306 , braking aircraft  304  using taxiing system  302 , or storing electric energy in at least one of energy storage locations  310 . 
     Controller  308  is configured to direct electric energy generated by number of electric motors  306  to battery  326 . Controller  308  is configured to direct electric energy generated by number of electric motors  306  to at least one of engine  324  of aircraft  304  or auxiliary power unit  328  when battery  326  reaches a charge capacity. 
     Although not depicted, controller  308  receives inputs from different sources to control taxiing system  302 . In one non-limiting example, controller  308  can generate commands such as drive commands, brake commands, neutral commands, or other commands to taxiing system  302  based on input from a pilot. In other illustrative examples, controller  308  can generate commands based on input provided by an autobrake system or other automated system. 
     Aircraft electrical loads  340  are also present on aircraft  304 . Aircraft electrical loads  340  are conventional electrical loads that are powered by either engine  324  or auxiliary power unit  328 . Aircraft electrical loads  340  are still powered when taxiing system  302  is present in aircraft  304 . Controller  308  continues to power aircraft electrical loads  340  with stable power. 
     In some illustrative examples, engine  324  of aircraft  304  is configured to receive electric energy from number of electric motors  306  and rotate components of engine  324  using the electric energy. For example, at least one of motor  334  or motor  336  may be rotated using the electric energy from number of electric motors  306 . In some illustrative examples, auxiliary power unit  328  is configured to receive electric energy from number of electric motors  306  and rotate components of auxiliary power unit  328  using the electric energy. 
     Turning now to  FIG. 4 , an illustration of an operational diagram of power flow in a taxiing system is depicted in accordance with an illustrative embodiment. Flow diagram  400  is an example of flow of electric energy  127  in taxiing system  102  of  FIG. 1 . Flow diagram  400  may be an example of flow of electric energy in a taxiing system of any of aircraft  202 , aircraft  206 , aircraft  208 , aircraft  210 , or aircraft  214 . Flow diagram  400  may be implemented in taxiing system  302  of  FIG. 3 . 
     Flow diagram  400  includes power switch matrix  402  configured to direct flow of electric energy within taxiing system  404 . Power switch matrix  402  directs electric energy flow to and from each of engine  406  motor/generators, auxiliary power unit  408  motor/generators, battery  410 , number of electric motors  412  attached to the wheels of the aircraft, and aircraft electrical loads  414 . 
     As depicted, when engine  406  does not receive electric energy from power switch matrix  402 , engine  406  is powered by fuel  416 . When powered by fuel  416 , engine  406  provides  418  electric energy to power switch matrix  402 . As depicted, when auxiliary power unit  408  does not receive electric energy from power switch matrix  402 , auxiliary power unit  408  is powered by fuel  416 . When powered by fuel  416 , auxiliary power unit  408  provides  420  electric energy to power switch matrix  402 . 
     In taxiing system  404 , power switch matrix  402  can direct electric energy into either of engine  406  or auxiliary power unit  408 . As depicted, power switch matrix  402  can direct electric energy from either number of electric motors  412  or battery  410  into engine  406  or auxiliary power unit  408 . When engine  406  receives  422  electric energy, engine  406  operates with less fuel  416 . When auxiliary power unit  408  receives  424  electric energy, auxiliary power unit  408  uses less fuel  416 . 
     Number of electric motors  412  attached to wheels of the aircraft can be used to either drive or brake the wheels. When number of electric motors  412  drive the wheels of the aircraft, number of electric motors  412  receive  426  electric energy. Power switch matrix  402  directs electric energy from one of battery  410 , engine  406 , or auxiliary power unit  408  to power number of electric motors  412 . 
     In some illustrative examples, number of electric motors  412  generate  428  electric energy. In these illustrative examples, number of electric motors  412  harvest kinetic energy of an aircraft while decelerating the aircraft. In these illustrative examples, number of electric motors  412  convert kinetic energy to electric energy. 
     Power switch matrix  402  distributes electric energy generated  428  by number of electric motors  412  to at least one of battery  410 , engine  406 , auxiliary power unit  408 , or aircraft electrical loads  414 . In some illustrative examples, power switch matrix  402  sends  430  electric energy to battery  410 . In some illustrative examples, power switch matrix  402  sends  432  electric energy from battery  410  to power number of electric motors  412 . 
     Turning now to  FIG. 5 , an illustration of a flowchart of a method of taxiing an aircraft is depicted in accordance with an illustrative embodiment. Method  500  may be performed using taxiing system  102  of  FIG. 1 . Method  500  may be implemented in at least one of aircraft  202 , aircraft  206 , aircraft  208 , aircraft  210 , or aircraft  214  of  FIG. 2 . Method  500  may be performed using taxiing system  302  of  FIG. 3 . Method  500  may be performed using taxiing system  404  of  FIG. 4 . 
     Method  500  directs electric energy from an energy storage location of energy storage locations selected from one of an engine of the aircraft, an auxiliary power unit, or a battery to an electric motor to generate kinetic energy (operation  502 ). Method  500  drives wheels of the aircraft using the kinetic energy generated by the electric motor (operation  504 ). Method  500  decelerates movement of the aircraft by transferring the kinetic energy of the aircraft into electric energy by operating the electric motor as electric motor brakes of a taxiing system of the aircraft (operation  506 ). Decelerating movement of the aircraft slows the aircraft. In some illustrative examples, decelerating movement of the aircraft stops the aircraft. Afterwards, method  500  terminates. 
     In some illustrative examples, movement of the aircraft is decelerated by applying electric motor brakes up to a threshold force (operation  508 ). The threshold force is a maximum force that can be applied by the electric motor brakes. 
     In some illustrative examples, energy from landing the aircraft is stored in the energy storage location prior to taxiing (operation  510 ). In these illustrative examples, energy stored from landing the aircraft can be only a portion of the kinetic energy from landing the aircraft. In these illustrative examples, regenerative energy from landing the aircraft can be used in taxiing the aircraft. 
     In some illustrative examples method  500  directs excess electric energy generated by decelerating movement of the aircraft using the electric motor to at least one of the energy storage locations (operation  512 ). By directing the excess electric energy to at least one of the energy storage locations, energy from braking the aircraft during taxiing is harvested. This regenerative energy reduces fuel consumption of the aircraft. 
     Turning now to  FIG. 6 , an illustration of a flowchart of a method of taxiing an aircraft is depicted in accordance with an illustrative embodiment. Method  600  may be performed using taxiing system  102  of  FIG. 1 . Method  600  may be implemented in at least one of aircraft  202 , aircraft  206 , aircraft  208 , aircraft  210 , or aircraft  215  of  FIG. 2 . Method  600  may be performed using taxiing system  302  of  FIG. 3 . Method  600  may be performed using taxiing system  404  of  FIG. 4 . 
     Method  600  harvests energy from landing an aircraft by a number of electric motors connected to landing gear of the aircraft (operation  602 ). Method  600  sends the energy harvested from landing the aircraft to an engine core of the aircraft and converts the energy to kinetic energy (operation  604 ). Method  600  taxis the aircraft by sending electric energy generated by extracting the kinetic energy from the engine core to the number of electric motors (operation  606 ). Afterwards, method  600  terminates. 
     In some illustrative examples, method  600  decelerates movement of the aircraft using the number of electric motors acting as electric motor brakes of the aircraft (operation  608 ). In some illustrative examples, decelerating movement slows the aircraft. In some illustrative examples, decelerating movement stops the aircraft. In some illustrative examples, movement of the aircraft is decelerated by applying the electric motor brakes up to a threshold (operation  610 ). 
     Turning now to  FIG. 7 , an illustration of a flowchart of a method of operating an auxiliary system of an aircraft is depicted in accordance with an illustrative embodiment. Method  700  may be performed using taxiing system  102  of  FIG. 1 . Method  700  may be implemented in at least one of aircraft  202 , aircraft  206 , aircraft  208 , aircraft  210 , or aircraft  216  of  FIG. 2 . Method  700  may be performed using taxiing system  302  of  FIG. 3 . Method  700  may be performed using taxiing system  404  of  FIG. 4 . 
     Method  700  harvests energy from landing an aircraft by a number of electric motors connected to landing gear of the aircraft (operation  702 ). Method  700  sends the energy harvested from landing the aircraft to an auxiliary power unit of the aircraft (operation  704 ). Method  700  operates auxiliary operations by the auxiliary power unit using the energy (operation  706 ). Afterwards, method  700  terminates. 
     As used herein, the phrase “at least one of,” when used with a list of items, means different combinations of one or more of the listed items may be used, and only one of each item in the list may be needed. In other words, “at least one of” means any combination of items and number of items may be used from the list, but not all of the items in the list are required. The item may be a particular object, a thing, or a category. 
     For example, “at least one of item A, item B, or item C” may include, without limitation, item A, item A and item B, or item B. This example also may include item A, item B, and item C, or item B and item C. Of course, any combination of these items may be present. In other examples, “at least one of” may be, for example, without limitation, two of item A, one of item B, and ten of item C; four of item B and seven of item C; or other suitable combinations. 
     The flowcharts and block diagrams in the different depicted embodiments illustrate the architecture, functionality, and operation of some possible implementations of apparatus and methods in an illustrative embodiment. In this regard, each block in the flowcharts or block diagrams may represent a module, a segment, a function, and/or a portion of an operation or step. 
     In some alternative implementations of an illustrative embodiment, the function or functions noted in the blocks may occur out of the order noted in the figures. For example, in some cases, two blocks shown in succession may be executed substantially concurrently, or the blocks may sometimes be performed in the reverse order, depending upon the functionality involved. Also, other blocks may be added, in addition to the illustrated blocks, in a flowchart or block diagram. Some blocks may be optional. 
     Illustrative embodiments of the present disclosure may be described in the context of aircraft manufacturing and service method  800  as shown in  FIG. 8  and aircraft  900  as shown in  FIG. 9 . Turning first to  FIG. 8 , an illustration of an aircraft manufacturing and service method is depicted in accordance with an illustrative embodiment. During pre-production, aircraft manufacturing and service method  800  may include specification and design  802  of aircraft  900  in  FIG. 9  and material procurement  804 . 
     During production, component and subassembly manufacturing  806  and system integration  808  of aircraft  900  takes place. Thereafter, aircraft  900  may go through certification and delivery  810  in order to be placed in service  812 . While in service  812  by a customer, aircraft  900  is scheduled for routine maintenance and service  814 , which may include modification, reconfiguration, refurbishment, or other maintenance and service. 
     Each of the processes of aircraft manufacturing and service method  800  may be performed or carried out by a system integrator, a third party, and/or an operator. In these examples, the operator may be a customer. For the purposes of this description, a system integrator may include, without limitation, any number of aircraft manufacturers and major-system subcontractors; a third party may include, without limitation, any number of vendors, subcontractors, and suppliers; and an operator may be an airline, a leasing company, a military entity, a service organization, and so on. 
     With reference now to  FIG. 9 , an illustration of an aircraft is depicted in which an illustrative embodiment may be implemented. In this example, aircraft  900  is produced by aircraft manufacturing and service method  800  of  FIG. 8  and may include airframe  902  with plurality of systems  904  and interior  906 . Examples of systems  904  include one or more of propulsion system  908 , electrical system  910 , hydraulic system  912 , and environmental system  914 . Any number of other systems may be included. Although an aerospace example is shown, different illustrative embodiments may be applied to other industries, such as the automotive industry. 
     Apparatuses and methods embodied herein may be employed during at least one of the stages of aircraft manufacturing and service method  800 . One or more illustrative embodiments may be manufactured or used during at least one of component and subassembly manufacturing  806 , system integration  808 , in service  812 , or maintenance and service  814  of  FIG. 8 . For example, taxiing system  102  may be manufactured or installed during component and subassembly manufacturing  806 . Taxiing system  102  may be connected to electrical system  910  of aircraft  900  during component and subassembly manufacturing  806 . As an example, method  500  may be used during in service  812  to taxi aircraft  900 . As another illustrative example, taxiing system  102  may be installed during maintenance and service  814 . Taxiing system  102  may reduce frequency of maintenance and service  814  for other components of aircraft  900 , such as carbon brakes. In some illustrative examples, method  500  may be used to operate portions of aircraft  900  such as portions of propulsion system  908 . In some illustrative examples, method  500  may utilize electric system  910  of aircraft  900 . Further, landing gear  119  of  FIG. 1  is part of or connected to airframe  902  of aircraft  900 . 
     The illustrative examples provide a taxiing system for an aircraft. The taxiing system uses electric energy to perform at least one of taxiing the aircraft or braking the aircraft. In some illustrative examples, the taxiing system generates electric energy from landing energy of the aircraft. 
     By employing the taxiing system, the aircraft may taxi without running the aircraft engines. The illustrative examples reduce the fuel consumed during taxiing of the aircraft. The illustrative examples reduce the fuel emissions generated by an aircraft during taxiing. The illustrative examples may reduce the wear on the carbon brakes of an aircraft due to taxiing. In some illustrative examples, the carbon brakes of an aircraft may be used for more flights due to braking using the taxiing system. 
     The description of the different illustrative embodiments has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different illustrative embodiments may provide different features as compared to other illustrative embodiments. The embodiment or embodiments selected are chosen and described in order to best explain the principles of the embodiments, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.