Patent Publication Number: US-2021189977-A1

Title: Efficient Engine Start

Description:
BACKGROUND INFORMATION 
     1. Field 
     The present disclosure relates generally to aircraft and, more specifically, to a system for reducing jet engine start time prior to takeoff. 
     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. 
     Currently, the timing of engine startup is left to the discretion of the pilot. Typically, engine startup is initiated at the gate or while being pushed back from the gate. The jet engines are then relied upon to taxi. 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 the 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 well as other possible issues. 
     SUMMARY 
     An illustrative example of the present disclosure provides a method of automated timing of engine startup for an aircraft. The method comprises receiving data inputs regarding a number of factors influencing a time to departure for the aircraft and receiving data inputs regarding a number of factors influencing time required to start and set takeoff power for a number of engines on the aircraft. An engine startup countdown is calculated based on a comparison of a nominal time to departure with a nominal minimum time required to start and set takeoff power for the engines, wherein the nominal time to departure is based on the first data inputs and the nominal minimum time to start and set takeoff power is based on the second data inputs. Upon completion of the countdown an engine start signal is sent. 
     Another illustrative example provides a system for automated timing of engine startup for an aircraft. The system comprises a bus system, a storage device connected to the bus system, wherein the storage device stores program instructions, and a number of processors connected to the bus system, wherein the number of processors execute the program instructions to: receive first data inputs regarding a number of factors influencing a time to departure for the aircraft; receive second data inputs regarding a number of factors influencing time required to start and set takeoff power for a number of engines on the aircraft; calculate an engine startup countdown, wherein the countdown is based on a comparison of a nominal time to departure with a nominal minimum time required to start and set takeoff power for the engines, wherein the nominal time to departure is based on the first data inputs and the nominal minimum time to start and set takeoff power is based on the second data inputs; and upon completion of the countdown, send an engine start signal. 
     Another illustrative example provides a method for preparing an aircraft for departure. The method of comprises loading a payload onto the aircraft, performing a partial taxi to a point of departure with a number of engines on the aircraft turned off, receiving an engine start signal at a specified time before a nominal time of departure, wherein the specified time provides a minimum sufficient interval before departure to start and set takeoff power for the engines, and starting the engines in response to the start signal. 
     The features and functions can be achieved independently in various examples of the present disclosure or may be combined in yet other examples 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 examples are set forth in the appended claims. The illustrative examples, 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 example 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 with an efficient engine start system in accordance with an illustrative example; 
         FIG. 2  is an illustration of a plurality of aircraft waiting to taxi to or from a runway in accordance with an illustrative example; 
         FIG. 3  depicts a flowchart for a process of starting a jet engine in accordance with an illustrative example; 
         FIG. 4  illustrates a partial cross-section view of a turbofan engine compressors with which illustrative examples can be implemented; 
         FIG. 5  depicts a flowchart for a process of reducing delay time between jet engine startup and takeoff in accordance with illustrative examples; 
         FIG. 6  depicts an engine power setting gauge with warm up limit in accordance with an illustrative example; 
         FIG. 7  is an illustration of an aircraft manufacturing and service method in a form of a block diagram in accordance with an illustrative example; 
         FIG. 8  is an illustration of an aircraft in a form of a block diagram in which an illustrative example may be implemented; and 
         FIG. 9  depicts a block diagram of a data processing system in accordance with an illustrative example. 
     
    
    
     DETAILED DESCRIPTION 
     The illustrative examples recognize and take into account one or more different considerations. The illustrative examples recognize and take into account that as engines idle on an aircraft, fuel is expended. The illustrative examples 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, fewer emissions are released for each flight, and non-renewable resources are conserved. In addition, by reducing engine idle/run time less noise is created at airports and less braking is required to counteract thrust created from idling/running engines, thereby reducing brake wear. 
     The illustrative examples 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 examples recognize and take into account that operating an aircraft engine produces engine noise and emissions. The illustrative examples recognize and take into account that it may be desirable to reduce the cumulative engine noise at an airport. The illustrative examples recognize and take into account that it may be desirable to reduce the cumulative emissions at an airport. 
     The illustrative examples recognize and take into account that jet engine startup is a multi-step procedure that is typically performed manually by the pilot. The illustrative examples also recognize and take into account that a minimal amount of startup time is necessary for a jet engine to reach idle speed and a specified operating temperature prior to setting takeoff power, where “specified operating temperature” means one of or a combination of: one or more air temperatures in the engine; one or more metal temperatures in the engine; exhaust gas temperature; and compressor case temperature, further where each temperature is based on a measurement and/or a calculation. 
     The illustrative examples provide an efficient engine start system for an aircraft that allows delay of engine startup closer in time to takeoff. A computer system concurrently calculates an estimate time to departure and a time to turn on the engines that is close enough to estimated takeoff to reduce unnecessary engine use but far enough in advance to enable reliable start and allow the engines to reach idle speed and a specified temperature before takeoff. This calculation takes multiple factors into account such as position of the aircraft relative to the runway, taxi speed, weather, engine temperature, etc. 
     In an illustrative example, the efficient start system provides an indication to the pilot to the begin engine startup procedure at the calculated time before takeoff. In another illustrative example, quick start technology enables automatic engine startup in response to a signal from the computer system at the calculated time. 
     In an illustrative example, the efficient start system provides feedback to the throttle to avoid premature throttle up before the engine has reached the specified temperature for takeoff power, thereby ensuring a more efficient and reliable engine start. 
     Turning now to  FIG. 1 , an illustration of a block diagram of an aircraft with an efficient engine start system is depicted in accordance with an illustrative example. Aircraft  100  comprises a number of turbofan engines  102  and a computer system  152  that can calculate an engine startup time. 
     Each turbofan engine  104  among turbofan engines  102  comprises a bypass fan  112  and engine core  116 . Bypass fan  114  produces a bypass airstream that flows around the engine core  116  and provides the majority of thrust in the case of a high-bypass turbofan engine. Engine core  116  produces exhaust thrust from fuel combustion, which is also used to generate power to turn bypass fan  114 . 
     At the front of engine core  116  is compressor  118  that compresses intake air before it is mixed with fuel in combustion chamber  134 . Compressor  118  comprises case  120  inside of which is the compressor core  122 . Compressor core  122  comprises a number of compressor stages that can be grouped into low-pressure  124 , intermediate-pressure  126 , and high-pressure  128 . As air moves through compressor stages  124 ,  126 ,  128  it is progressively compressed to higher pressure levels by each subsequent compressor stage. 
     Of particular importance for the present discussion, compressor case has a temperature  132  that might differ considerably from compressor core temperature  130 , depending on the operational state of the turbofan engine  104 . Because the compressor stages  124 ,  126 ,  128  comprise disks, blades, and drums, the compressor core  122  has more mass and therefore heats more slowly that the thinner compressor case  120 . This differential between core temperature  130  and case temperature  132  is most pronounced during initial startup from a cold state, which can lead to problems during throttle up, discussed in more detail below. 
     Compressed air from compressor  118  enters combustion chamber  134 , where it can be mixed with fuel  166  from fuel tanks  164  and ignited by igniter  136 . 
     Exhaust from combustion chamber  134  passes through turbines  138  before exiting turbofan engine  104  as exhaust thrust. The expanding exhaust drives the rotation of high-pressure turbines  140  and low-pressure turbines  142 . In the case of a multi-shaft turbofan engine, high-pressure turbines  142  rotate high-pressure shaft  142 , which in turn rotates intermediate-pressure compressor stages  126  and high-pressure compressor stages  128 . Low-pressure turbines  142  rotate low-pressure shaft  146 , which in turn rotates bypass fan  112  and low-pressure compressor stages  124 . 
     When a turbofan engine such as engine  104  is up to speed, the process of compressing air, combusting it with fuel, and using energy released from the combustion to continue compressing air via the turbines is a self-sustaining cycle. However, before turbofan engine  104  reaches self-sustaining speed, it needs help getting started. 
     Each turbofan engine  104  comprises a starter system  106  that initiates engine start and helps the engine  104  reach self-sustaining speed. Starter system  106  uses compressed air  108  to initiate rotation of compressor  118  through accessory gearbox  110 . Compressed air  108  can be supplied from auxiliary power unit (APU)  150 . Compressed air  108  can also be supplied as bleed air from another of engines  102  that is already started and up to operating speed. 
     Bypass fan  112  has a fan speed N1  162 , and engine core  116  has engine a core speed N2  148 , which refer to their respective rotational speeds (RPM). During engine startup, N2  148  has to reach a specified percentage of maximum speed, ensuring proper airflow through engine core  116  before igniter  136  is activated and fuel  166  is introduced into combustion chamber  134 . N2  148  then reaches a higher percentage before turbofan engine  104  is self-sustaining, and starter system  106  can be turned off. 
     Computer system  152  can employ quick start technology to enable efficient startup of turbofan engines  102  prior to takeoff. Computer system  152  can be implemented in software, hardware, firmware or a combination thereof. When software is used, the operations performed by computer system  152  can be implemented in program code configured to run on hardware, such as a processor unit. When firmware is used, the operations performed by computer system  152  can be implemented in program code and data and stored in persistent memory to run on a processor unit. When hardware is employed, the hardware can include circuits that operate to perform the operations in computer system  152 . 
     In the illustrative examples, the hardware can take a form selected from at least one of a circuit system, an integrated circuit, an application specific integrated circuit (ASIC), a programmable logic device, or some other suitable type of hardware configured to perform a number of operations. With a programmable logic device, the device can be configured to perform the number of operations. The device can be reconfigured at a later time or can be permanently configured to perform the number of operations. Programmable logic devices include, for example, a programmable logic array, a programmable array logic, a field programmable logic array, a field programmable gate array, and other suitable hardware devices. Additionally, the processes can be implemented in organic components integrated with inorganic components and can be comprised entirely of organic components excluding a human being. For example, the processes can be implemented as circuits in organic semiconductors. 
     When more than one data processing system is present in computer system  152 , those data processing systems are in communication with each other using a communications medium. The communications medium can be a network. The data processing systems can be selected from at least one of a computer, a server computer, a tablet computer, or some other suitable data processing system. 
     Efficient startup timing reduces the time of engine startup before takeoff while still ensuring adequate time for efficient engine start. To calculate an engine start, computer system  152  uses a plurality of data comprising external data  154  and internal data  156 . 
     External data  154  relate to information and conditions external to aircraft  100 . Example of external data  154  include weather conditions, global positioning system (GPS) position, schedules departure, runway selection, known takeoff priority, the position speed of other taxing aircraft received via automatic dependent surveillance broadcast (ADS-B), etc. Internal data  156  relate to information and conditions internal to aircraft  100 . Example of internal data  156  include engine characteristics, type of start, engine conditions (temperature, etc.), current taxiing speed, etc. 
     Computer system  152  might also incorporate artificial intelligence (AI)  158  to assist in processes and interpreting external data  154  and internal data  156 . AI  158  might be internal to computer system  152  inboard aircraft  100  or an external resource accessible by computer system  152  through a wireless communications link. AI  158  can correlate vast amounts of data that can potentially affect estimated departure time and engine startup time, including not only current external data  154  and internal data  156  but also historical data regarding air traffic at different airports, weather conditions, flight schedules and delays, etc., sometimes referred to “Big Data.” 
     AI  158  is able to determine patterns within such Big Data and can correlate current external data  154  and internal data  156  to assist countdown calculator  160  in estimating an engine start time with an accuracy and precision not possible through manual calculation by a human pilot. A human pilot will almost invariably start the engines too soon, resulting in unnecessary fuel consumption, engine wear, emissions, and noise. 
     Computer system  152  might also include throttle feedback  162 , which provides feedback to pilots to prevent attempts by the pilots to increase the throttle prematurely before the engine has reached a specified temperature. 
     Aircraft  100  might optionally include an automated engine controller  172  that can automatically initiate engine start in response to a signal from computer control system  152  at a calculated engine start time prior to estimated time of departure to ensure timing of engine start and warmup. Automatic engine controller  172  can also set engines  102  to takeoff power in response to a signal from computer control system  152  when the engines have reached a specified operating temperature for takeoff power. 
     Delay of engine start might be applied to all turbofan engines  102  on aircraft  100  or just one or more, depending on how aircraft  100  taxis on the ground. For example, many airlines use a single-engine-taxi procedure to save fuel while taxiing to the runway. The time to start the remaining engine(s) is dictated by the engine operation manual, which requires a certain time operating at idle speed prior to setting takeoff power. For example, the value might be three minutes at idle. 
     As an alternative to single-engine-taxi, aircraft  100  might optionally comprise an onboard taxiing system  168  that can propel aircraft  100  on the ground without energy from turbofan engines  102 . As another option, an external tow vehicle  170  can provide pushback and towing of aircraft  100  on the ground. 
     In situations when taxiing is provided by an onboard taxiing system  168  or tow vehicle  170 , delayed engine start can be applied to all turbofan engines  102  on aircraft  100  since none of them are needed for taxi. 
     The illustration of aircraft  100  in  FIG. 1  is not meant to imply physical or architectural limitations to the manner in which an illustrative example 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 example. 
     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 example. In view  200 , aircraft  202  is present on runway  204  and is preparing for takeoff. Aircraft  202 , therefore, has its engines set to takeoff power. 
     Each of aircraft  206 , aircraft  208 , and aircraft  210  are present on taxiway  212  and are waiting to taxi to another location in airport  211 . Aircraft  214  might be an aircraft that has just landed. Any of aircraft  202 ,  206 ,  208 ,  210 , or  214  might be an example aircraft  100  shown in  FIG. 1 . 
     For aircraft  206 ,  208 , and  210  on taxiway  212  to reduce fuel consumption during taxi to the runway  204 , it is desirable to reduce the time needed between engine start and setting takeoff power. All things being equal, aircraft  206  should begin engine startup and set takeoff power before aircraft  208 , and aircraft  208  in turn should start engines and set takeoff power before aircraft  210 . Of course, things are rarely equal, and engine start times for aircraft  206 ,  208 , and  210  can vary for a number of reasons including engine type and engine temperature. 
       FIG. 3  depicts a flowchart for a process of starting a jet engine in accordance with an illustrative example. Process  300  might be applied to a turbofan engine, such as turbofan engine  104  in  FIG. 1 , by a computer system such as computer system  152 . 
     Process  300  begins with turning on compressed bleed air for the starter system (step  302 ). The bleed might be supplied by an APU such, as APU  150  in  FIG. 1 , or might be provided from another turbofan engine on the aircraft that is already at operational speed. The bleed air fed into the starter drives an accessory gearbox, which in turn begins rotating the compressors in the engine core. 
     The starter continues to increase the engine core speed N2 and determines when this speed reaches a specified minimum percentage of maximum RPM (step  304 ). In many jet engines, the required N2 is in the range of 15-25% of maximum, depending on the specific engine design. The minimum required percentage ensures that the airflow through the engine core is sufficient to prevent overheating of the combustion chamber when the fuel is ignited (i.e. “hot start”), which can damage the engine. 
     Once N2 reaches the specified minimum percentage of maximum, the igniter in the combustion chamber is activated (step  306 ), and fuel is injected into the combustion chamber to mix with the compressed air and ignite (step  308 ). Fuel combustion then begins contributing to rotating the turbines, thereby increasing engine core speed N2. 
     Initially, the energy released from combustion is not sufficient to make the compression and combustion cycle self-sustaining. Therefore, the starter system continues to assist the engine to increase core speed until N2 reaches self-sustainability. Many jet engines have a specified starter cutout speed for N2 that is slightly higher than self-sustaining speed. This cutout speed might be, for example, 55% of maximum N2. Therefore, after combustion is initiated, N2 is continually monitored until it reaches starter cutout speed (step  310 ). 
     Once the engine core speed N2 reaches starter cutout speed, the starter system and igniter are turned off (step  312 ). The engine then continues accelerating under its own self-sustaining power until it reaches idle speed (step  314 ). In many modern turbofan engines, idle speed is in the range of 60% of maximum N2. 
     After the engine reaches idle speed it should be held at idle for a sufficient time to allow the engine to reach specified starting temperature before setting takeoff power (step  316 ). One limitation that results in a minimum engine startup time is the danger of a stall of one of the axial compressors in the engine (e.g., low-pressure compressor, intermediate-pressure compressor, high-pressure compressor). 
       FIG. 4  illustrates a partial cross-section view of a turbofan engine compressors with which illustrative examples can be implemented. Compressor  400  might be an example of compressor  118  in  FIG. 1 . 
     Compressor  400  comprises case  404  inside of which a number of rotor blades  406  rotate about a central shaft  402 . Positioned between the rotor blades  406  are stator vanes  408 . 
     The rotor blades  406  increase the speed of the air for the following stator vanes  408 , which decrease the air speed and increase pressure. As compressor  400  compresses the air over subsequent stages, the air density increases, and the air takes up less place. Therefore, the compressor duct  410  gets narrower and the rotor blades  406  and stator vanes  408  become shorter. Air passing over the rotor blades  406  is flowing against an increasing pressure gradient as it moves through compressor  400 . Therefore, rotors blades  406  are working against progressively increasing resistance. A typical pressure gain from one rotor/stator to the next is 30% or higher. 
     As explained briefly above, the compressor core (comprising rotor blades  406 , disks, and drums) has more mass and warms up more slowly than the case  410 , which is thin and has less mass. Because the compressor case  410  warms up more quickly than the core, it expands more quickly in the radial direction than rotor blades  406 . As a result, the clearance gap between the end of the rotor blades  406  and the case  410  increases. If a sudden increase in engine acceleration occurs with the compressor in this uneven warmup state, the increased pressure gradient from downstream compressor stages can cause airflow back through the gaps between the blade tips and case, causing the rotor blades  406  to stall. 
     Referring back to  FIG. 3 , process  300  monitors engine function until the engine is at the proper operating temperature (step  318 ). This might occur through direct measurement of compressor temperature and/or indirectly through indicators of compressor function. If engine temperature is not yet at specified operating temperature, the engine is held at idle speed to prevent stall from premature throttle up. 
     Once the engine reaches operating temperature, it set to takeoff power (step  320 ). 
       FIG. 5  depicts a flowchart for a process of reducing delay time between jet engine startup and takeoff in accordance with illustrative examples. Process  500  might be applied to a turbofan engine, such as turbofan engine  104  in  FIG. 1 , by a computer system such as computer system  152  applied to an engine startup process such as process  300  in  FIG. 3 . 
     Process  500  essentially comprises two concurrent calculations. One is a countdown until takeoff time. The other is a count-up from engines off to engines ready to set takeoff power. The goal of process  500  is to synchronize the countdown with the count-up as close as possible under real world operating conditions. 
     Process  500  begins with multiple data inputs regarding a number of factors influencing when the aircraft will be able to take off using best available data (step  502 ). These inputs include factors that affect the time it will take the aircraft in question to reach the point of takeoff and might comprise a combination of external data  154  and internal data  156 . Example of such input factor include GPS position of the aircraft, taxiing speed of the aircraft, runway selection assigned to the aircraft, scheduled departure of the aircraft, current weather conditions, and known takeoff priority of the aircraft. Another possible input influencing time to departure comprises the GPS/GNSS (Global Navigation Satellite System) position and speed data of other taxiing aircraft at the airport received via ADS-B. ADS-B adds GPS/GNSS position data and ground speed to the digital code pulse time delay method of transponders used by airport secondary radar and by traffic collision avoidance systems (TCAS). Therefore, any transponder-equipped aircraft can maintain positional awareness of other on-ground, transponder-equipped aircraft in the vicinity on the same taxiway. The taxi speeds, and hence taxi times, of these other aircraft can be used in calculating departure time. 
     The computer system then calculates a nominal estimated time to departure (ETD) for the aircraft according to the data inputs of the departure factors (step  504 ). The ETD calculation can be made with assistance of Big Data and artificial intelligence such as AI  158  in  FIG. 1 . The current data input at step  502  can be correlated with historical data to determine a more accurate ETD. For example, if the aircraft is in Chicago in winter, and it begins snowing while the aircraft is taxiing, AI  158  can access historical data related to weather conditions and air traffic delays in Chicago and weigh current conditions against it to help calculate nominal ETD and the likelihood and probably length of delay. 
     The nominal ETD calculation is then fed into a statistical uncertainty (step  506 ). The statistical uncertainty calculator provides a confidence margin interval for the nominal ETD calculation. 
     Concurrent with the ETD calculation process is the count-up to start the engines and get them ready to set takeoff power. This concurrent process begins with multiple data inputs regarding a number of factors influencing startup time for a number of engines on the aircraft using best available data (step  508 ). Examples of such factors include the model of the engines on the aircraft, the type of start required (e.g., cold start, warm start), engine conditions such as engine temperature as well as temperature and pressures at the compressor exit and entrance, and current weather conditions. 
     The computer system calculates a nominal minimal time required to start the engine(s) and set the engines to takeoff power according to the data inputs of factors influencing startup time (step  510 ). The computer system controlling engine startup can compute the time remaining before allowing takeoff power setting from a combination of real-time measurements and the recent history of the engine. 
     As an example of how the recent history of the engine could be used, on the first flight of the day the engine core will be at ambient temperature, so the compressor disks and drums may take a relatively long time to heat up and match the compressor case temperature sufficiently. But on a quick turnaround at a gate, the engines may be shut down for only 30 minutes. In this case the compressor disks and drums may take a relatively short time to heat up and match the compressor case temperature sufficiently. 
     As an example of the real-time measurements that the engine control might use, the pressure and temperature at the compressor exit (P3, T3) and entrance (P25, T25) are commonly available, as is the rotational speed of the compressor (N2). As another example, if the taxi operation includes operation at above-idle conditions, then the warmup time could be shorter. The engine control would account for this in its calculations and in the signals it sends to the aircraft. 
     Using a combination of the above example, the engine control computer would record the pressures, temperatures and rotor speeds experienced by the compressor, as well as the time the engine was shut down and started. 
     The method used by the engine control computer to calculate the time remaining before setting takeoff power could be based on empirical correlations of the input parameters to the time remaining or could be based on a real-time model of compressor tip clearances derived from the input parameters. As with the ETD calculation, the engine start time calculation can take advantage of Big Data regarding operational histories of similar model engines. This historical data can be correlated with current and recent history of the engine in question. 
     Other time-based limitations might beyond compressor stall might also affect the engine&#39;s ability to operate at takeoff power. For example, engine start up time might be limited to prevent thermal stresses in the engine turbines. In cases in which there are multiple limitations on engine start time the control computer can calculate the earliest allowable time for each limitation, then report the longest time to the aircraft. 
     The engine startup time calculation is then fed into its own statistical certainty compensator, which calculates a confidence margin for the nominal startup time (step  512 ). Because the consequence of having to wait to set takeoff power is higher than the consequence of an undesirable idle time (e.g., the aircraft waits on the runway until engine is ready, delaying other traffic versus somewhat higher fuel consumption), the calculations would bias the predicted time for setting takeoff power to account for uncertainties and errors. This could be expressed as a confidence interval, for example: The recommended time for setting takeoff power is in 33 seconds; there is a 1% chance that it could be as long as 38 seconds and a 0.1% chance that it could be as long as 45 seconds. 
     The results of the estimated TED and engine start time are then fed into a countdown calculator such as countdown calculator  160  in  FIG. 1 . The countdown calculator calculates an engine startup countdown based on a comparison of the nominal time to departure with the nominal minimum time required to start the engines and set takeoff power to determine the time to begin engine start within the respective degrees of confidence (step  514 ). 
     Prior to completion of the countdown, the system continues an iterative loop of updating the nominal time to departure, the nominal minimum time required to start the engines and set takeoff power and reviving the countdown according to continually updated new input data. Upon completion of the countdown, the control computer sends an engine start signal (step  516 ). This start signal might be sent to the flight crew of the aircraft or sent to an automatic engine controller such as controller  172 . 
     If changes in input data lead to significant changes it ETD, the engine controller can adjust its idle setting to meet that time. For example, if the ETD changes to sooner than predicted, the controller can set a higher idle speed so that the compressor will warm up more quickly. Alternatively, if a delay causes the ETD to be later than predicted, the engine control could reduce the idle setting. 
     After engine start, the system monitors engine temperature to determine when the engines reach the specified operating temperature to set takeoff power (step  518 ). The computer controller can provide a signal to the aircraft to communicate the time remaining before takeoff power can be set. A second signal could indicate the maximum allowed throttle position that can be set while the engines warm up to a temperature required to set takeoff power. These signals would be updated frequently, e.g., twice per second. The aircraft can use these signals to display the time remaining to the crew and/or show the limit of allowed throttle position on the engine control display of the flight deck. 
       FIG. 6  depicts an example of an engine power setting gauge with active warmup limit. Gauge  600  in an example of a display provided to the crew in response to the start signal from step  520  in  FIG. 5 . In this example gauge  600  displays a warmup limit  602  and time remaining of 32 second before setting takeoff power. 
     The computer system might automatically limiting throttle motion to the maximum allowed throttle position. Alternately, the throttle could provide a high mechanical resistance to crew movement of the throttle lever as feedback to indicate that the engine is not ready for an acceleration but would not prevent throttle motion. 
     When the engines reach the specific operating temperature to set takeoff power, the system sends a set takeoff power signal (step  520 ). Again, this signal might be sent to the flight crew of the aircraft or sent to an automatic engine controller such as controller  172 . 
     Illustrative examples of the present disclosure may be described in the context of aircraft manufacturing and service method  700  as shown in  FIG. 7  and aircraft  700  as shown in  FIG. 8 . Turning first to  FIG. 7 , an illustration of an aircraft manufacturing and service method is depicted in accordance with an illustrative example. During pre-production, aircraft manufacturing and service method  700  may include specification and design  702  of aircraft  800  in  FIG. 8  and material procurement  704 . 
     During production, component and subassembly manufacturing  706  and system integration  708  of aircraft  800  takes place. Thereafter, aircraft  800  may go through certification and delivery  710  in order to be placed in service  712 . While in service  712  by a customer, aircraft  800  is scheduled for routine maintenance and service  714 , which may include modification, reconfiguration, refurbishment, or other maintenance and service. 
     Each of the processes of aircraft manufacturing and service method  700  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. 8 , an illustration of an aircraft is depicted in which an illustrative example may be implemented. In this example, aircraft  800  is produced by aircraft manufacturing and service method  500  of  FIG. 7  and may include airframe  802  with plurality of systems  804  and interior  806 . Examples of systems  804  include one or more of propulsion system  808 , electrical system  810 , hydraulic system  812 , and environmental system  814 . Any number of other systems may be included. Although an aerospace example is shown, different illustrative examples 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  700 . One or more illustrative examples may be manufactured or used during at least one of component and subassembly manufacturing  706 , system integration  708 , in service  712 , or maintenance and service  714  of  FIG. 7 . 
     The illustrative examples might provide a taxiing system for an 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. 
     Turning now to  FIG. 9 , an illustration of a block diagram of a data processing system is depicted in accordance with an illustrative example. Data processing system might be an example of computer system  152  in  FIG. 1 . Data processing system  900  might be used to implement one or more computers to carry out the process steps shown in  FIGS. 3 and 5 . In this illustrative example, data processing system  900  includes communications framework  902 , which provides communications between processor unit  904 , memory  906 , persistent storage  908 , communications unit  910 , input/output unit  912 , and display  914 . In this example, communications framework  902  may take the form of a bus system. 
     Processor unit  904  serves to execute instructions for software that may be loaded into memory  906 . Processor unit  904  may be a number of processors, a multi-processor core, or some other type of processor, depending on the particular implementation. In an example, processor unit  904  comprises one or more conventional general-purpose central processing units (CPUs). In an alternate example, processor unit  904  comprises a number of graphical processing units (CPUs). 
     Memory  906  and persistent storage  908  are examples of storage devices  916 . A storage device is any piece of hardware that is capable of storing information, such as, for example, without limitation, at least one of data, program code in functional form, or other suitable information either on a temporary basis, a permanent basis, or both on a temporary basis and a permanent basis. Storage devices  916  may also be referred to as computer-readable storage devices in these illustrative examples. Memory  906 , in these examples, may be, for example, a random access memory or any other suitable volatile or non-volatile storage device. Persistent storage  908  may take various forms, depending on the particular implementation. 
     For example, persistent storage  908  may contain one or more components or devices. For example, persistent storage  908  may be a hard drive, a flash memory, a rewritable optical disk, a rewritable magnetic tape, or some combination of the above. The media used by persistent storage  908  also may be removable. For example, a removable hard drive may be used for persistent storage  908 . Communications unit  910 , in these illustrative examples, provides for communications with other data processing systems or devices. In these illustrative examples, communications unit  910  is a network interface card. 
     Input/output unit  912  allows for input and output of data with other devices that may be connected to data processing system  900 . For example, input/output unit  912  may provide a connection for user input through at least one of a keyboard, a mouse, or some other suitable input device. Further, input/output unit  912  may send output to a printer. Display  914  provides a mechanism to display information to a user. 
     Instructions for at least one of the operating system, applications, or programs may be located in storage devices  916 , which are in communication with processor unit  904  through communications framework  902 . The processes of the different examples may be performed by processor unit  904  using computer-implemented instructions, which may be located in a memory, such as memory  906 . 
     These instructions are referred to as program code, computer-usable program code, or computer-readable program code that may be read and executed by a processor in processor unit  904 . The program code in the different examples may be embodied on different physical or computer-readable storage media, such as memory  906  or persistent storage  908 . 
     Program code  918  is located in a functional form on computer-readable media  920  that is selectively removable and may be loaded onto or transferred to data processing system  900  for execution by processor unit  904 . Program code  918  and computer-readable media  920  form computer program product  922  in these illustrative examples. In one example, computer-readable media  920  may be computer-readable storage media  924  or computer-readable signal media  926 . 
     In these illustrative examples, computer-readable storage media  924  is a physical or tangible storage device used to store program code  918  rather than a medium that propagates or transmits program code  918 . Alternatively, program code  918  may be transferred to data processing system  900  using computer-readable signal media  926 . 
     Computer-readable signal media  926  may be, for example, a propagated data signal containing program code  918 . For example, computer-readable signal media  926  may be at least one of an electromagnetic signal, an optical signal, or any other suitable type of signal. These signals may be transmitted over at least one of communications links, such as wireless communications links, optical fiber cable, coaxial cable, a wire, or any other suitable type of communications link. 
     The different components illustrated for data processing system  900  are not meant to provide architectural limitations to the manner in which different examples may be implemented. The different illustrative examples may be implemented in a data processing system including components in addition to or in place of those illustrated for data processing system  900 . Other components shown in  FIG. 9  can be varied from the illustrative examples shown. The different examples may be implemented using any hardware device or system capable of running program code  918 . 
     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. 
     As used herein, the phrase “a number” means one or more. 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, without limitation, “at least one of item A, item B, or item C” may include item A, item A and item B, or item C. This example also may include item A, item B, and item C or item B and item C. Of course, any combinations of these items may be present. In some illustrative 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 examples illustrate the architecture, functionality, and operation of some possible implementations of apparatuses and methods in an illustrative example. In this regard, each block in the flowcharts or block diagrams may represent at least one of a module, a segment, a function, or a portion of an operation or step. For example, one or more of the blocks may be implemented as program code. 
     In some alternative implementations of an illustrative example, 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 performed 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. 
     The description of the different illustrative examples has been presented for purposes of illustration and description and is not intended to be exhaustive or limited to the examples in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different illustrative examples may provide different features as compared to other illustrative examples. The example or examples selected are chosen and described in order to best explain the principles of the examples, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various examples with various modifications as are suited to the particular use contemplated. 
     Clause 1: A method comprising receiving, by a processor, first data inputs regarding factors influencing a time to departure for an aircraft; receiving, by the processor, second data inputs regarding factors influencing time to start and set takeoff power for at least one engine on the aircraft; calculating, by the processor, an engine startup countdown, wherein the engine startup countdown is based on a comparison of a nominal time to departure with a nominal minimum time to start and set takeoff power for the engines, wherein the nominal time to departure is based on the first data inputs and the nominal minimum time to start and set takeoff power is based on the second data inputs; and upon completion of the engine startup countdown, sending, by the processor, an engine start signal. 
     Clause 2: The method of Clause 1, further comprising, prior to completion of the countdown, iteratively: updating, by the processor, the nominal time to departure according to new first data inputs; updating, by the processor, the nominal minimum time to start and set takeoff power according to new second data inputs; and revising, by the processor, the engine startup countdown according to the new first and second data inputs. 
     Clause 3: The method of Clause 1 or 2, wherein the engine start signal is sent to at least one of a flight crew of the aircraft or an automatic engine controller. 
     Clause 4: The method of any of Clauses 1-3, further comprising sending a set takeoff power signal when the engines reach a specified operating temperature. 
     Clause 5: The method of any of Clauses 1-4, wherein the set takeoff power signal is sent to at least one of a flight crew of the aircraft or an automatic engine controller. 
     Clause 6: The method of any of Clauses 1-5, wherein factors influencing the time to departure comprise at least one of: GPS position of the aircraft; taxiing speed of the aircraft; runway selection assigned to the aircraft; scheduled departure of the aircraft; current weather conditions; known takeoff priority of the aircraft; or position and speed data of other taxiing aircraft received via automatic dependent surveillance broadcast. 
     Clause 7: The method of any of Clauses 1-6, wherein the factors influencing engine startup and time to set takeoff power comprise at least one of: model of the engines on the aircraft; type of start; engine temperature; temperature and pressures at a compressor exit; temperature and pressures at a compressor entrance; or current weather conditions. 
     Clause 8: The method of any of Clauses 1-7, further comprising calculating, by the processor, a confidence margin for nominal time to departure and/or the nominal minimum time to start the engines and set takeoff power. 
     Clause 9: The method of any of Clauses 1-8, further comprising indicating, by the processor, a maximum allowed throttle position that can be set while the engines warm up to a temperature associated with set takeoff power. 
     Clause 10: The method of Clause 9, further comprising at least one of: showing the maximum allowed throttle position on a display; automatically limiting throttle motion to the maximum allowed throttle position; or providing mechanical resistance to throttle movement as feedback to indicate that the engines are not ready for an acceleration. 
     Clause 11: The method of any of Clauses 1-10, wherein at least one of nominal time to departure or the nominal minimum time to start the engines and set takeoff power is calculated by correlating the first data inputs and second data inputs to historical data. 
     Clause 12: A system comprising a bus system; a storage device connected to the bus system, wherein the storage device stores program instructions; and a processor connected to the bus system, wherein the processor executes the program instructions to: receive first data inputs regarding factors influencing a time to departure for the aircraft; receive second data inputs regarding factors influencing time to start and set takeoff power for at least one engine on the aircraft; calculate an engine startup countdown, wherein the countdown is based on a comparison of a nominal time to departure with a nominal minimum time to start and set takeoff power for the engines, wherein the nominal time to departure is based on the first data inputs and the nominal minimum time to start and set takeoff power is based on the second data inputs; and upon completion of the engine startup countdown, send an engine start signal. 
     Clause 13: The system of Clause 12, wherein, prior to completion of the countdown, the processor further executes instructions to iteratively update the nominal time to departure according to new first data inputs; update the nominal minimum time to start and set takeoff power according to new second data inputs; and revise the engine startup countdown according to the new first and second data inputs. 
     Clause 14: The system of Clause 12 or 13, wherein the engine start signal is sent to at least one of a flight crew of the aircraft or an automatic engine start controller. 
     Clause 15: The system of any of Clauses 12-14, wherein the processors further execute instructions to send a set takeoff power signal when the engines reach a specified operating temperature. 
     Clause 16: The system of Clause 15, wherein the set takeoff power signal is sent to at least one of a flight crew of the aircraft or an automatic engine controller. 
     Clause 17: The system of any of Clauses 12-16, wherein factors influencing time to departure comprise at least one of: GPS position of the aircraft; taxiing speed of the aircraft; runway selection assigned to the aircraft; scheduled departure of the aircraft; current weather conditions; known takeoff priority of the aircraft; or position and speed data of other taxiing aircraft received via automatic dependent surveillance broadcast. 
     Clause 18: The system of any of Clauses 12-17, wherein the factors influencing engine startup and time to set takeoff power comprise at least one of: model of the engines on the aircraft; type of start; engine temperature; temperature and pressures at a compressor exit; temperature and pressures at a compressor entrance; or current weather conditions. 
     Clause 19: The system of any of Clauses 12-18, wherein the processor further executes instructions to calculate a confidence margin for nominal time to departure and/or the nominal minimum time to start the engines and set takeoff power. 
     Clause 20: The system of any of Clauses 12-19, wherein the processor further executes instructions to indicate a maximum allowed throttle position that can be set while the engines warm up to a temperature to set takeoff power. 
     Clause 21: The system of Clause 20, wherein the processor further executes instructions to at least one of: show the maximum allowed throttle position on a display; automatically limit throttle motion to the maximum allowed throttle position; or provide mechanical resistance to throttle movement as feedback to indicate that the engines are not ready for an acceleration. 
     Clause 22: The system of any of Clauses 12-21, wherein at least one of the nominal time to departure or the nominal minimum time to start the engines and set takeoff power is calculated by correlating the first data inputs and second data inputs to historical data. 
     Clause 23: A method comprising loading a payload onto an aircraft; performing a partial taxi to a point of departure with an engine on the aircraft turned off; receiving an engine start signal at a specified time before a nominal time of departure, wherein the specified time provides a minimum interval before departure to start and set takeoff power for the engine; and starting the engine in response to the engine start signal. 
     Clause 24: The method of Clause 23, wherein the engine start signal is received by at least one of a flight crew of the aircraft or an automatic engine controller. 
     Clause 25: The method of Clause 23 or 24, further comprising: receiving a set takeoff power signal when the engine reaches a specified operating temperature; and setting takeoff power for the engine in response to the set takeoff power signal. 
     Clause 26: The method of Clause 25, wherein the set takeoff power signal is received by at least one of a flight crew of the aircraft or an automatic engine controller. 
     Clause 27: A computer program product for automated timing of engine startup for an aircraft, the computer program product comprising: a non-volatile computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor to cause a computer to: receive first data inputs regarding factors influencing a time to departure for the aircraft; receiving second data inputs regarding factors influencing time to start and set takeoff power for an engine on the aircraft; calculating an engine startup countdown, wherein the countdown is based on a comparison of a nominal time to departure with a nominal minimum time to start and set takeoff power for the engines, wherein the nominal time to departure is based on the first data inputs and the nominal minimum time to start and set takeoff power is based on the second data inputs; and upon completion of the engine startup countdown, sending an engine start signal. 
     Clause 28: The computer program product of Clause 28, further comprising instructions for sending a set takeoff power signal when the engine reaches a specified operating temperature.