Calculation and Analysis of Aircraft Pollutant Emission Based on Time Wake Separation Mode under Coastal Airport and Headwind Conditions

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Introduction
e aviation pollutants mainly include nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHC) [1]. e International Civil Aviation Organization (ICAO) stipulated that the engine performance and fuel consumption rate need to be improved continuously in the industry, and aircraft should reduce the emission of pollutants to meet the increasingly stringent airworthiness requirements [2].
When the headwind speed increases, the reduction of the ground speed of the aircraft leads to an increase in flight time and a decrease in the arrival rate, which not only affects the current stable airport capacity but also affects the predictability of operation, time, fuel efficiency, and environmental pollution [3]. e TBS had been evolved into a concept by the National Air Traffic Service (NATS) and Leidos (formerly Lockheed-Martin) as a separate system to fully unlock the runway capacity irrespective of wind conditions [4]. In the frame of SESAR Time-Based Separation (TBS) concept was used to provide a consistent time spacing between aircraft in airports to increase runway throughput according to Morris [5].
e Time-Based Separation concept was proposed as a logical way to solve possible conflicts between an RPAS and a civil aircraft by assigning a controlled time to overfly (CTO) to RPAS. In 2015, the system was operational at London's Heathrow Airport after over 150000 assessments on wake vortices of inbound flights. is system used time separation instead of distance separation to increase runway usage and ensure safety [6]. e Tribhuvan International Airport (TIA) used the time-based separation for two consecutive aircraft in approach. Nishan published an article in 2020 and concluded: An average daily time lag of 79 minutes is observed between DBS and TBS. Hence, implementation of the ETBS system can save TIA almost 20 days of operation annually [7]. e ICAO stipulated the emission standards of CO, UHC, and NOx in the takeoff, climb, approach, and ground taxiing stages of the standard LTO (landing and takeoff) cycle. Xia estimated the pollutant emissions during the takeoff and landing cycle of Chinese civil aviation airports according to the engine emission data published by ICAO, which provides the emission data of different pollutants for this paper [8]. Huang studied the NOx emission distribution over China by using the fuel flow method, which provides a reference for the establishment of the fuel flow method in this paper [9]. According to the actual flight parameters, Wei estimated the pollutant emissions of each flight stage by using the fuel flow method, which provides the engine thrust setting and flight time of each flight stage for this paper [10]. According to the schedule, route setting of CAAC, and the characteristics of aircraft and engine, Huang evaluated the NOx stock of flight emissions in 2001 by using the emission database published by ICAO and NASA, which provides a reference for the final calculation of the total pollutant emission in each stage of for this paper [11]. However, there were few studies on the direct estimation method and P3-T3 method based on pollutant generation mechanism in China.
ere are a direct method and relative method in the calculation of aviation pollutants. e direct method needs to be based on combustion chemical reaction dynamics. Since the calculation process involves the data of combustion chamber design parameters and fuel atomization characteristics, this method is difficult to model and calculate. e model is usually only applicable to specific working conditions, so the calculation error is significant under other working conditions [12]. In this study, the relative method uses the known emission indexes of the engine under various thrust conditions on the ground, then modifies the model according to the ratio of some key thermal parameters of the aircraft under high altitude and ground conditions, and finally calculates the emission indexes of the starting engine under various actual conditions. ese key parameters mainly include the total temperature (T3) and total pressure (P3) at the inlet of the combustion chamber.
e above key parameters are easy to obtain. erefore, this method is also called the P3-T3 method [13]. According to the P3-T3 emission index and Boeing Fuel Flow Method 2 (BFFM2) calculation model, this study calculates the reduced operation time based on TBS mode compared with the DBS mode under the same landing sorties and obtains the pollutant emissions under the two modes. It is further concluded that the TBS operation mode has an obvious effect on reducing pollutant emission compared with the DBS operation mode.
is paper theoretically studies the saved time by the TBS operation mode compared with the traditional DBS operation mode. Under the condition of the same number of approaching flights, the saved time by the TBS operation mode compared with the DBS operation mode is converted into fuel consumption reduction and pollutant emission.
Finally, the paper evaluates the TBS mode's significant energy-saving and emission reduction effect compared with the DBS mode. e TBS operation mode will significantly contribute to the energy conservation and emission reduction of the airport. e TBS mode can reduce fuel consumption by at least 30% and pollutant emissions by 25%.
e application of this method can reduce aircraft flight time, improve operational efficiency, reduce fuel consumption, and reduce pollutant emission for coastal airports under headwind conditions. e structure chart of the article is shown in Figure 1.

Estimation of Pollutant Emission Index of Aeroengine
A. All flights operate in the international standard atmosphere, and the changes of humidity and temperature are ignored in a day. B. In this study, the standard LTO cycle specified by ICAO is adopted, and the defined emission is 915 m (3000 feet) from the surface to the top of the atmospheric boundary layer regardless of the aircraft engine emission under cruise conditions. e approach, taxi, takeoff, and climb times are taken from the standard LTO cycle specified by ICAO. e approach process requires 30% of the engine thrust and lasts for 4 minutes [10]. C. e flight speed and engine fuel flow remain unchanged during the approach. D. e average approach speeds of different aircraft types are shown in Table 1. E. e chemical reaction between pollutants and atmospheres is ignored.

Calculation of Fuel Consumption and Pollutant Emission of Aeroengine.
e LTO can describe all the aircraft activities cycle at the airport [14]. is study uses the specified standard cycle, which ranges from the surface to the top of the atmospheric boundary layer (3000 feet). e operational status, thrust, and operation time of each stage are shown in Table 2. e fuel consumption and emission index of the aircraft in each operation mode are obtained from the engine emission database of ICAO. e ideal LTO (landing and takeoff) cycle is divided into four stages in the international standard atmosphere, and each has different engine power settings. is paper only studies the approach stage and takes only the approach process data of various types of engines.
ere is a different type of engine matching among the aircraft teams of civil aviation airlines [16]. e matching data between the aircraft model and the engine are from the annual report of the airworthiness approval department of CAAC aircraft, and the time is up to October 1, 2020. e details are shown in Table 3. e civil aviation engine emission database is shown in Table 4.
MES (Manufacturing Execution System) system is a realtime information acquisition system, which develops rapidly in recent years. e total fuel consumption of an aeroengine equipped with an MES system is calculated by collecting relevant engine operation information in real time as follows: Among them, N a is the engine number of a aircraft, T a,m is the approach and landing time of a aircraft under standard conditions, and NFF a,m is the single-engine fuel flow (kg/s) of a aircraft under standard conditions. e pollutant j emission NE a,j,m is calculated as follows: e NEI a,m,j is the pollutant j emission index of an aircraft engine under standard conditions.

Correction of Aeroengine Fuel Flow and Pollutant Emission Index.
e relative method uses the known engine emission indexes under various thrust conditions on the ground, then modifies the model according to the ratio of some key thermal parameters of aircraft under high altitude and ground conditions and finally calculates the operational engine emission indexes under various actual conditions. e engine pollutant emission data by the ICAO at 30% thrust level on the ground are shown in Table 3. e engine emission index of the actual environment is obtained by correcting the P3-T3 emission index and Boeing Fuel Flow Method 2 (BFFM2) [17]. e study uses BFFM2 to calculate the fuel flow and pollutant emission index. Under international standard atmospheric conditions, the fuel flow and pollutant emission indices are based on the ideal LTO cycle. e BFFM2 model needs to correct the approach stage's fuel flow, and the approach stage's correction coefficient is 1.020. erefore, the revised fuel flow is linear with the thrust level of the engine [18] .
Under the international standard atmosphere (ISA) condition, the modified single-engine fuel flow model converts sea-level fuel flow into actual fuel flow. e singleengine fuel flow correction expression is stated as follows: Among them, NFF a0 is the single-engine fuel flow correction value (kg/s). e NFF a is the fuel flow under sealevel conditions (kg/s). δ is the ratio between airport ground ambient pressure (P) and sea-level pressure (101.325 kPa) under International standard atmosphere (ISA). θ is the ratio between airport ground ambient temperature (T + 273.15) and the ISA sea-level temperature (288.15 K). V Is the aircraft speed of approach (Mach).

P3-T3 Emission Index Correction Model of Various
Pollutants. NOx emission index estimation model: e NOx is mainly generated in the high-temperature flame zone of the primary combustion zone. It is related to the central combustion zone temperature, combustion chamber inlet temperature, combustion chamber pressure, fuel atomization quality, and residence time. e gas residence time in the high-temperature zone has an important influence on the generation of NOx [19]. e modified model E 1 of NOx emission index is as follows: where dλ is obtained by ICAO data fitting, as shown in Figure 2. T 1 and T 2 are the primary combustion zone and flame temperature. e variable with subscript "r" is the reference value of the corresponding variable underground reference state. T 4 is the temperature when the primary combustion zone equivalence ratio is exactly 1. T(φ) Is the primary combustion zone temperature when the equivalence ratio is φ. R is the weight factor, generally 0.25.
CO and UHC emission index estimation model. Both CO and UHC are generated by incomplete combustion of fuel.
e CO is the hydrocarbon oxidation intermediate product.
e UHC is mainly composed of incompletely burned fuel particles, fuel vapor and small molecular fuel cracked during combustion [20]. e calculation model of CO and UHC emission index (E2, E3) is as follows: where ω Air is the airflow; a, b, and c are the emission parameter obtained by ICAO data fitting. e CO and UHC variation trend with θ at a ground standstill is shown in Figures 3 and 4, respectively.
According to the reference emission data of the engine ground test published by ICAO, with the help of the engine performance model, we obtain the relevant thermodynamic parameters under actual operational conditions and the estimation model of the above three engine pollutant emission indexes. e model can calculate the engine pollutant emission index under the actual operational conditions and then estimate the engine total pollutant emission under the actual flight conditions [21]. e pollutant emission index of taxing aircraft engines is also related to airport ambient pressure, ambient temperature, saturated vapor pressure, and atmospheric relative humidity. erefore, the correction results of the pollutant emission index calculated by formulas (4) and (5) e humidity correction factor P V is derived from the Magnus Hedden equation: where NEI a,j,0,0 is the final modified value of NEI  [23]. e EDMS version 5.0.2 also changes the exponential equation of humidity correction coefficient to equation (7), where P V is the saturation pressure of water vapor (kPa), T is the ambient temperature (°C), a � 7.5, b � 237.3, ϕ is the relative humidity of the atmosphere, p is the total ambient pressure (kPa), and H is the humidity correction factor.

Aircraft Classification and Wake Separation in China.
China's current RECAT-CN (Re-categorization-CHINA, RECAT-CN) experimental standard has appropriately reduced the CAAC's (China Administrative Association of Cat) radar wake separation standard. According to the maximum takeoff weight and wingspan length, the RECAT-CN classification standard is shown in Table 5. Under the MTOW (maximum take-off weight) standard of CAAC, the B767 (Boeing-767) belongs to heavy-duty aircraft, and the wake separation standard between B767 and ARJ21 (Advanced Regional Jet for twenty-first century) is 9.3 km. Under the RECAT-CN standard, B767 belongs to type C, similar to the wake separation standard of ARJ21. e classification of domestic operational aircraft based on the RECAT-CN standard is shown in Table 6 [24].
At present, the domestic wake separation standard based on RECAT-CN is shown in Table 7 [25]. e TBS separation mode recovers the loss of landing rate caused by DBS separation mode under strong headwind, improves landing rate, and reduces approach landing time.

Journal of Advanced Transportation
Some studies show that the headwind has little effect on landing efficiency when the headwind is below 15 kt. If the headwind is above 15 kt, it is recommended that the TBS separation mode replaces the DBS separation mode. e TBS is equivalent to the time required to fly the actual distance separation without wind [26]. e TBS separation mode is derived from the DBS separation mode, and the equivalent time separation standard is given by the following formula (8).
Suppose D is a given distance separation, and GS is the aircraft ground speed, and T is the time required to fly this distance.
Under headwind conditions: where IAS (Indicated Airspeed) represents the aircraft's indicated airspeed, WS (Windspeed) is the wind speed (positive against the wind). α is the angle between wind direction and aircraft course. e time separation at different headwind speeds is as follows: During the final approach, the airspeed is set at 160 kt; the headwind speed of coastal airports often occurs between      Table 8. Table 9 shows the reduction of landing efficiency using DBS mode landing under 15 kt, 25 kt, and 35 kt headwind conditions compared to the reference landing rate at 160 kt without headwind conditions. It is found that the aircraft needs more time to fly at a given distance under strong headwind conditions so that airport will lose the stable runway capacity.
Under different headwind speed conditions, the same time separation will reduce the longitudinal distance separation. e details are shown in Figure 5, which help to maintain a constant runway capacity. e wake distance stipulated by ICAO establishes the time separation standard. e detailed data are shown in Table 10. e minimum separation is 60 seconds to provide sufficient runway occupation time for aircraft ahead [28]. e detailed time separation standards of coastal airports in China are listed in Table 11.
Considering the constant distance separation, when the IAS is 136 kt, the time required to flight given distances is evaluated under constant headwind conditions of 0 kt, 15 kt, and 25 kt. e results are shown in Table 12. When the headwind is stronger, the flight time required for a given distance is longer, and the flight arrival rate will be reduced [30]. Figure 6 shows the change of runway theoretical capacity (TC) with the change of ground speed at constant distance separations of 3, 4, 5, and 6 nautical miles, where TC represents the maximum number of aircraft continuity landing per hour without other interference [31].

Runway Arrival Capacity Model Based on Time Separation Standard.
Under TBS mode, the separation of two aircraft is measured by time [32]. erefore, no matter how the ground speed of the aircraft changes, the relative speed of the two aircraft will not affect the time separation between the two aircrafts [33]. e runway arrival capacity model is established under TBS mode as follows: where λ a is the airport landing capacity. t a ij is the time separation of aircraft combination i and j (Leader is i and Follower is j) passing through the runway entrance    successively. p ij is the proportion of front and rear aircraft of type i and j. v i and v j are the average speed of final approach and landing of type i an j aircraft. R ai is the average time of i aircraft runway occupation. δ ij is the wake separation of different types of aircraft. ws is the headwind speed, and α is the angle between the wind direction and the aircraft heading.

Results and Discussion
e direction and speed of wind have a significant impact on the aircraft approach process. e data of Heathrow Airport confirmed that the aircraft's ground speed decreases under the condition of increasing headwind. Under the same approach distance, the aircraft needs more time to complete the given separation distance than the condition of calm wind, resulting in the reduction of runway capacity.
In this paper, Shanghai Pudong Airport is selected as an example to analyze the landing data of one hour during peak hours. e Pudong Airport is a coastal airport, and windy weather is common. In this paper, using the landing data of Pudong Airport analyze the impact of headwind weather on the approach. e headwind speed data of Pudong Airport are shown in Table 13. e broken line diagram of the runway time-varying headwind component is shown in Table 14. e direction of the Pudong Airport runway is 167°and 347°, respectively. e velocity adopts the headwind component in the runway direction.
According to the average headwind speed from January 11 to February 13, the result shows a significant headwind speed on January 31 and February 7, and the average    According to the experimental data of laser radar at Heathrow airport, the TBS time is reduced by about 65 s when the wind speed reaches 20 kt. e relationship between headwind speed and time-saved is shown in Figure 8.
Considering A320 and B737 series models, the radar separation between them is 6 km, their approach speed is 136 kt, and the time separation is 86 s under the static wind. Under headwind, the airspeed is constant, and the ground speed is reduced. According to the experimental data of Heathrow airport, the average time separation calculated is the 40 s. Under the strong headwind condition, the efficiency is increased by TBS mode. e minimum time separation is defined as the 60 s to provide enough runway occupancy time for the front aircraft. e theoretical bearing capacity of the 35 L runway in those periods is calculated according to the measured landing aircraft data. Table 15 shows the runway capacity by using TBS mode in different periods. Table 16 contains the fuel consumption and pollutant emission reduction of landing aircraft in different periods. e fuel consumption reduction in different periods is shown in Figure 9(a), and the pollutant emission reduction is shown in Figure 9(b).
From the above tables, compared with the DBS operation mode in three different periods, it can be concluded that the fuel saving of TBS operation mode is 858.64 kg, 493.26 kg, and 784.02 kg; the emission reduction of CO is 2.58 kg, 1.48 kg, and 2.35 kg; the emission of HC is 0.53 kg, 0.31 kg, and 0.49 kg; and the emission reduction of NOx is 7.91 kg, 4.54 kg, and 7.22 k. e TBS operation mode can save many fuel costs for airlines and reduce many pollutant emissions for airports all over the country.       Combined with Table 17, the statistical analysis in the three periods shows that the fuel saving is 32.52%, 19.12%, and 30.41%; the reduction of CO pollutant emissions is 28.93%, 17.9%, and 29.29%; the reduction of HC pollutant emission is 31.02%, 19.36%, and 33.78%; the reduction of NOx pollutant emission reduction is 30.85%, 16.42%, and 28.67%. For large coastal airports, we should pay more attention to energy conservation and emission reduction. [24]

Conclusion
(1) Compared with the DBS operation mode in the approach phase, the TBS operation mode will reduce fuel consumption by 10679.6 kg, CO emission by 32.05 kg, HC emission by 6.65 kg, and NOx emission by 98.35 kg. (2) e TBS operation mode will make a significant contribution to the airport's energy conservation and emission reduction. ey can reduce fuel consumption by at least 30% and pollutant emissions by 25%.
(3) Considering the stronger headwind weather conditions, some coastal airports have greater fuel consumption and pollutant emissions, such as Qingdao Liu-ting Airport, Shanghai Pu-dong Airport, Xiamen Gao-Qi Airport, Guangzhou Bai-Yun Airport, and Dalian Zhou-Shuizi Airport. e airport or airline may consider implementing the TBS approach operation mode to reduce fuel and pollutant emissions. (4) e further research content of this paper will take into account the influence of the changes of humidity and temperature in the actual atmospheric environment on the fuel consumption and pollutant emissions of the aeroengine. At present, this paper is based on the average approach speed of the same type of aircraft in the approach, and the next step study introduces the actual speed of aircraft in the process of approach into the model for calculation, which will improve the accuracy and objectivity of calculation. Finally, the aircraft's actual engine thrust and time are used to calculate to improve the accuracy of the results.
Data Availability e data used in this paper are weather forecast data and airport monitoring data of Shanghai Pudong airport. e current research project provides these data. It is confidential and cannot be directly disclosed. It can be used for paper writing after processing.
is paper uses the data measured by Morris et al. At Heathrow Airport about the relationship between headwind speed and time interval reduction.

Conflicts of Interest
No conflict of interest.