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A knowledge based aircraft-gate assignment advisor
Computers and Industrial Engineering Vol. 25, Nos 1..-4,pp. 123-126, 1993
Printed in Great Britain. All fights reserved
0360-8352/9356.00+0.( Copyright © 1993 PergamonPress L
A Knowledge Based Aircraft-Gate Assignment Advisor
Y.Y. Su Graduate Research Assistant K. Srihari, Ph.D. Department Of Mechanical And Industrial Engineering State University Of New York, Binghamton, New York 13902-6000.
Abstract An expert system based solution to the aircraft-gate assignment problem is discussed. The prototype system developed works as a planning tool. It assists in the strategic planning of schedules and with projected aircraft-gate assignments. Introduction Airport gate capacity is a major concern for the airline industry since the lack of gate facilities or inefficiencies in gate utilization can cause aircraft delays [1,2]. The use of large connecting hub operations, and the resulting volumes of passengers and baggage transferring between flights has increased the complexity of the aircraft-gate assignment task. Also, the number of factors to be considered have increased. Traditional approaches utilizing classic operations research techniques have difficulty with uncertain information and multiple performance criteria, and do not adapt well to the needs of real-time operations support [4,5]. Knowledge based tools could be used for the operational control of ramp activity [3,5]. This research designed and developed a prototype expert system framework that addresses the aircraft-gate assignment problem. It recommends gate allocation decisions considering the constraints imposed by the available facilities and personnel to handle the aircraft, and the consequences on downstream operations of particular assignment decisions. The aircraft-gate assignment system developed can be used as a planning tool and as an operational tool. It evaluates the results of a specific aircraft-gate assignment, and helps analyze the effectiveness of operational options to improve gate utilization through better aircraft-gate assignment. Problem Statement And Research Objective The lack of sufficient gates, particularly at busy airports, to accommodate the operations of both new entrants and incumbent airlines is a problem airlines need to deal with. While airlines are dynamically altering their operational strategies (aircraft used, routes served, frequency of service offered, fares charged, and hubs developed), the task of assigning arriving flights to the available gate is a key activity in airline station operations [1,2]. An airlines' operations planning needs to ensure that adequate resources are available to handle the expected traffic. These resources must be used cost effectively [3]. With the increased use of large connecting hubs in airline networks, the ability to handle large volumes of aircraft, passengers, and baggage in a relatively short time has become critical to the successful operation of the connecting banks of flights. Air traffic delays and severe weather can disrupt schedules and compound the difficulty of maintaining smooth station operations. In addition to ensuring that aircraft already on gates do not block later arrivals, the peaking created by late arrivals and the need to transfer passengers and large amounts of baggage between flights can delay subsequent departures or result in missed connections. The aircraft-gate assignment influences the deolovment of ramv oersonnel and eeuiament and the time reauired to transfer passengers and baggage between flights [2,4]. When the number of aircraft to be handled approaebes the number of gates available, poor gate assignment decisions may increase delays in getting aircraft on to a gate due to other aircraft that are obstructing access to available gates. When schedule disruptions result in the number of flights to be handled exceeding the available gates, then good assignment decisions can reduce the resulting delays and missed connections. The large amounts of connecting baggage at major hubs requires careful facilities design and deployment of ramp personnel and equipment. If inbound flights are delayed, connecting passengers may miss flights. Even if the passengers make their connections, their baggage may not. The transfer of passengers and baggage between two flights can be made more efficient through assigning the flights to adjacent gates, as well as appropriate allocation of personnel and equipment. If this is not possible, or not sufficient to avoid missed connections, a decision on whether to delay the outbound flight must be made while considering the downstream effects of the delay and the impact on the operation at the station. If another aircraft is scheduled into the gate, delaying a departure may delay that arrival or require the flight to be reassigned to another gate. The arrival schedule of aircraft and the subsequent assignment of arriving flights to gate positions affects several other aspects of station operations. The considerations that can affect gate assignment decisions include the aircraft size, passenger walking distance, baggage transfer, aircraft servicing requirements, ramp congestion, flight crew and aircraft rotation, and the use of remote parking stands. 123
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Proceedings of the 15th Annual Conference on Computers and Industrial Engineering
An aircraft cannot be assigned to a gate position that is too small to accommodate it. The size of an aircraft in a particular position may also limit the use of adjacent positions, or even restrict access to more distant positions, by reducing clearances to apron taxilanes. These constraints can be expressed through a gate interaction matrix for each pair of gates, that represents the restrictions imposed on the use of each gate by every aircraft type considering the aircraft type using the other gate. It is advantageous to assign flights to gates so as to reduce the distance that passengers have to walk. Not only must the distance itself be considered, but also the time available to cover it, since this affects not only the walking speed required but the ability to arrange assistance for those who need it. Reducing the walking distance for passengers with tight connections may be more important than reducing total walking distance. Consideration should also be given to the dynamic aspects of gate assignment. Reassigning a late arriving flight will require passengers who have already gathered at the original gate to walk to the new gate. With centralized sorting systems, the time saved by assigning a flight to a gate closer to the sorting facility is not usually significant. However, with zone sorting systems, keeping flights with tight connections within the same gate group can significantly reduce transfer time. On the other hand, if ramp crews only work one zone and several flights arrive at that gate group over a short period, assigning a flight to another zone that is not busy may reduce the unload and transfer time. Aircraft servicing requirements must be considered along with the availability of ground handling equipment. Equipment is often aircraft type specific, and long moves from one gate position to the next will waste time and complicate crew supervision. The movement of aircraft and ground vehicles on the ramp and taxilanes can result in congestion, delaying arriving and departing flights. The rotation of flight crews and aircraft to flights must also be taken into consideration. Since flight crews have duties to perform before an aircraft is ready to leave the gate, delaying an arriving aircraft until an assigned gate is free may impact the departing flight using that aircraft or flown by that crew. When the gates available are not sufficient, remote parking positions may be considered. This will require vehicles to transport passengers to the terminal. Another option is to move aircraft off the gate positions once they have been unloaded and park them elsewhere until they can be moved onto a gate to load. Assignment of arriving international flights to appropriate gates must consider government inspection requirements (immigration, customs, health, etc.), which will generally restrict the available gates and require a longer turn around time. If the aircraft is continuing as a domestic flight, it may have to be moved to a different gate for loading. In response to the complexity of the aircraft-gate assignment task, and the need under operational conditions for a fast response to dynamic, changing circumstances, the research objective is to develop a heuristic reasoning based expert system as a solution methodology. The prototype system that has been developed is a planning tool which helps identify an initial aircraft-gate assignment, and assists with sensitivity analysis on input data such as departure and arrival information. The planning tool could evolve to be able to deal with dynamic aircraft-gate assignment, that is, it could become a dynamic tool. Since the system must be airport specific, it's knowledge base should be adaptable for use at any airport. That is, the system must be able to accommodate any schedule of arriving and departing flights with any airport terminal configuration. The aircraft-gate assignment expert system needs to be able to identify multiple solutions (if they exist). Also, the use of graphics and a user friendly interface was a primary concern. SvstemDesign This research is targeted for airports in the North American continent. Assumptions were made to reduce the problem's scale. The airline is assumed to occupy 10 gate positions. Some gates are limited in the size of the aircraft that they can accommodate. The use of some gates depends on the size of any aircraft in adjacent gates. The airline fleet comprises of six aircraft types, which may be grouped into some kind of size classes. The number and types of aircraft to be scheduled is at least approximately equal to the number and types of gates available. Gate restrictions include the clearance between adjacent gate positions which is determined by the specific constraining (or adjacent) gate(s), aircraft class, and gate size limit. It is assumed that there is sufficient clearance between each gate position and the apron taxilanes that no aircraft on a gate blocks access to any other gate. The gates are divided into two ramp zones, corresponding to the odd and even numbered gates. Baggage is assumed to be preloaded by zone at the origin station, and sorted for reloading in each zone. Therefore baggage transfer within a zone take less time than transfers between zones. It is also desirable to have an even distribution of arrival and departure times within each zone, to reduce peak workload. A gate position is the area of the terminal apron designated for the ground services such as refueling, baggage handling, cleaning, servicing, etc. The flight schedule is for a typical day begins at 07:50 hours and concludes at 22:00 hours. An hour is divided into three time slots. Within the first time slot, which is 20 minutes long, airplanes arrive. The second time slot, which is about 35 minutes long, all aircraft are at their gates ensuring that passengers can transfer. The last (third) time slot is 15 minutes long. It is used by aircraft to depart. There could be an overlap between the last time slot of a specific hour and the first time slot of the next hour. The entire set of flights are divided into three segments. The first segment is the flights between any two cities east of the Mississippi River or between any two cities west of the Mississippi. The second segment would be flights that cross the Mississippi, and the third segment would be overseas flights. Every flight route has a specific aircraft type assigned to it depending upon the distance traversed and the volume of traffic. Aircraft are assigned to gate on 'firstarrived-first-assigned' basis but with priority given to larger aircraft in order to minimize passenger delays. Other factors used in the assignment of gates to aircraft include the time of aircraft arrival and departure, operational factors such as the aircraft-gate size compatibility, the gate's access to government inspection services, the maximization of the usage of certain gates, and the minimization of aircraft delays. Other user controllable parameters which influence
Su and SmHAVa:Aircraft-Gate Assignment Advisor
125
gate allocation include the time separation between successive assignments of the gates, the maximum allowable gate occupancy times by aircraft size and flight sector, and the maximum 'tolerable' aircraft delay for gate access. The factors considered in the aircraft-gate assignment problem can be summarized into the following categories: aircraft characteristics, airport area and construction, gate characteristics, government inspection and customs service needs, weather, airline policy and passenger service levels, and the marketing strategy. For an aircraft being assigned to a gate, the prototype aircraft-gate assignment expert system considers the type and size of an aircraft, servicing regulations and requirements for the flight, and flight and aircraft changes. Aircraft size would impact the assignment of an aircraft to a particular gate, and could impact the assignment of aircraft to adjacent gates. For the purposes of this research, the prototype system developed assumes that the movement of aircraft on apron taxiway is not affected by aircraft assigned to gates. An aircraft cannot be assigned to a gate that is too small for it. The amount of the time that a flight is on the ground and it actually needs to be on the ground is another factor. Also, the regulations and requirements applicable when an aircraft is assigned to a particular gate are considered. These could include the availability of fuel hydrants, ground power and air, ground handling equipment, passenger transportation, etc. The aircraft-gate assignment system considers the use of remote parking bays to service aircraft if the number of aircraft to be assigned exceeds available gate capacity. Also aircraft could be withdrawn from gates and parked elsewhere until they are ready to board. The characteristics of gates/ramps are considered in the problem of aircraft-gate assignment. The factors considered include the characteristics of individual gates, capacity and capability of individual gates, the gate needs of individual aircraft, constraints imposed by aircraft parked at neighboring gates in the same time slot, and gate/ramp congestion. The aircraft-gate assignment system considers the special constraints including the immigration and customs requirements of international flights. These requirements restrict the number of available gates. International flights also have a longer turnaround time. The level of service provided to the passenger is a primary concern. Costs can be reduced by reducing staffing levels or facilities often at the price of more delays, greater crowding, and missed flights [2]. The aircraft-gate assignment should attempt to meet established service levels in the most cost efficient manner. Severe weather could adversely affect airline schedules, and as a result make aircraft-gate assignment more difficult. Several of these above factors are inter-related. Information Used. Inouts. And Outouts The prototype aircraft-gate assignment system (Figure 1) is implemented as an interactive microcomputer program using Prolog. It simulates the arrivals and departures of commercial aircraft at the terminal apron as described by an input flight schedule and assigns gates to these aircraft. The information used by the aircraft-gate assignment system include the flight schedules, aircraft service times required on the ground, aircraft classes, international flight requirements, gate characteristics including description and capacity, and the government inspection service. The flight schedule is used by the aircraft-gate assignment to get each flight's airline code and flight number, aircraft size, time of arrival, time of departure, requirement for government inspections both upon the aircraft's arrival and before its departure, and the aircraft service times required on the ground. For each aircraft type the data required includes the airline code, flight number, aircraft size, and aircraft class. A description of each available gate in terms of the maximum aircraft size it can accommodate, its accessibility to government inspection facilities for both emplaning and deplaning passengers, and the strategy adopted by the airport operator in assigning that gate are some other pieces of information required by the aircraft-gate assignment expert system. This research assumes the exclusive use of a gate by an airline. For each gate/ramp, the following data is accessed and used: aircraft type, size, and class; gate size, limits, and use policies; constraining gate number; aircraft servicing requirements; aircraft and flight changes; use of remote parking stands; and government inspection needs. Also required is a specification of the maximum time an aircraft of a given size and flight sector (domestic, trans-border, other international) is allowed to occupy a gate. The aircraft-gate assignment system's outputs include the gate usage and utilization information over a specific time period, aircraft changeover conditions (number of aircraft using a specific gate) over a specific time period, and the graphical output of the status of gates at a specific time. An output that describes the status of all gates indicates the aircraft-gate assignment (including remote parking positions) and operational characteristics of assigned flights. A description of all flights considered by the prototype aircraft-gate assignment expert system can be obtained. Information provided includes a specification of the gate assigned to each aircraft and the delay, if any, in obtaining that gate, and a listing of unassigned flights (remote parking) for which certain arrangements may be required. Gate utilization is expressed as the percentage of time the gate has been in use relative to the analysis time period, and the total number of aircraft handled over the course of that period of time. These measures complement each other as, for example, an aircraft could needlessly occupy a gate for an extended period of time (i.e. using it merely for parking) thus resulting in artificially high gate utilization values. For each type of event, rules were developed to determine if the change will cause a problem. In the case of a late arrival, these rules cheek if the expected turn-around time will result in a late departure, and if this will block the gate for a subsequent arrival. Delayed departures are checked for conflicts with subsequent arrivals. If there is an aircraft/flight change, the rules cheek the suitability of the gate for the new aircraft type and adjacent gate restrictions. The output provided by the system with respect to aircraft changeovers shows how many aircraft occupied a gate during the analysis time period. The graphical output that depicts the gates occupied at a specific time period displays CAIE 2,f~l/4~J
126
Proceedings of the 15th Annual Conference on Computers and Industrial Engineering
Flight Schedules
I
Aircraft service times
AircrafL dasses
International flight
Gate eharaeteldstiea description and capacity
~
~ [ ' I
~
~'~: "-I~ M M
xI~'.'I~'~__~ [~ " 1
~ n ~ ~ 1~ AIHM:MAIrI'.GA~ AdlillGMIIIglVlr m
~
~
I
Gate usage information
I
Gate utilization value over a speeific time period
[ [
~ i
i
Aircraft changeover conditions[
b.-I Graphical OUtput of the status ~ of a gate at a specific time
Government inspection services
Fieure 1: Expert System Based Aircraft-Crate Assignment Framework the airport terminal configuration along with the specific information on aircraft and the gates they occupy. Conclusion Aircraft-gate assignment is a major operational issue for airlines, especially at high aircraft density airports. The use of computer models offers a method to deal with huge quantities of knowledge and data. It provides the user with the capability to conduct efficient planning and operational management of airport capacity. Expert systems are an ideal solution methodology in a domain such as the aircraft-gate assignment problem. Identifying the relevant information and defining the constraints are not particularly difficult tasks. Although managing the data flows is not difficult in principle, careful attention needs to be paid to system architecture because of the large amount of knowledge and data involved and the need to continuously adjust to changing circumstances. The most difficult part of the problem is identifying the rules to guide the assignment process, because of the large number of factors to be taken into account. A prototype expert system has been developed to gain some understanding of the issues involved. It demonstrates the ability to take some of the relevant factors (mentioned above) into account in solving a restricted set of aircraft-gate assignment problems. In addition, the system integrates the inference process with the operational database. In particular, the system will allow end users to modify the knowledge base to tailor the system to local conditions and changing requirements. Acknowled~,ements The authors are thankful to Dr. R. Muthukrishan, Corporate Research And Development, United Airlines, Chicago, Illinois for his input. References 1. Gosling, G.D., 'Application Of Expert Systems In Air Traffic Control', Journal of Transoortation Engineering, Vol. 113, No.2, 1987, pp. 139-154. 2. Gosling, G.D., 'Design Of An Expert System For Aircraft Gate Assignment', Trans_oortation Research Part A, Vol.24A, No.I, 1990, pp.59-69. 3. Hamzawi, S.G., 'Management And Planning Of Aircraft Gate Capacity: A Microcomputer-based Gate Assignment Simulation Model', Trans_nortation Planning, and Technology, Vol. 11, 1986, pp. 189-202. 4. Su, Y.Y., 'An Expert System Approach To Aircraft-Gate Assignment', Masters Thesis. State University Of New York, Binghamton, New York, 1991. 5. Tobias, L., & Scoggins, J.L. Jr., 'Time-Based Air-Traffic Management Using Expert Systems', IEEE Control Systems Magazin¢, Vol.2, No.2, 1987, pp.23-29.
Printed in Great Britain. All fights reserved
0360-8352/9356.00+0.( Copyright © 1993 PergamonPress L
A Knowledge Based Aircraft-Gate Assignment Advisor
Y.Y. Su Graduate Research Assistant K. Srihari, Ph.D. Department Of Mechanical And Industrial Engineering State University Of New York, Binghamton, New York 13902-6000.
Abstract An expert system based solution to the aircraft-gate assignment problem is discussed. The prototype system developed works as a planning tool. It assists in the strategic planning of schedules and with projected aircraft-gate assignments. Introduction Airport gate capacity is a major concern for the airline industry since the lack of gate facilities or inefficiencies in gate utilization can cause aircraft delays [1,2]. The use of large connecting hub operations, and the resulting volumes of passengers and baggage transferring between flights has increased the complexity of the aircraft-gate assignment task. Also, the number of factors to be considered have increased. Traditional approaches utilizing classic operations research techniques have difficulty with uncertain information and multiple performance criteria, and do not adapt well to the needs of real-time operations support [4,5]. Knowledge based tools could be used for the operational control of ramp activity [3,5]. This research designed and developed a prototype expert system framework that addresses the aircraft-gate assignment problem. It recommends gate allocation decisions considering the constraints imposed by the available facilities and personnel to handle the aircraft, and the consequences on downstream operations of particular assignment decisions. The aircraft-gate assignment system developed can be used as a planning tool and as an operational tool. It evaluates the results of a specific aircraft-gate assignment, and helps analyze the effectiveness of operational options to improve gate utilization through better aircraft-gate assignment. Problem Statement And Research Objective The lack of sufficient gates, particularly at busy airports, to accommodate the operations of both new entrants and incumbent airlines is a problem airlines need to deal with. While airlines are dynamically altering their operational strategies (aircraft used, routes served, frequency of service offered, fares charged, and hubs developed), the task of assigning arriving flights to the available gate is a key activity in airline station operations [1,2]. An airlines' operations planning needs to ensure that adequate resources are available to handle the expected traffic. These resources must be used cost effectively [3]. With the increased use of large connecting hubs in airline networks, the ability to handle large volumes of aircraft, passengers, and baggage in a relatively short time has become critical to the successful operation of the connecting banks of flights. Air traffic delays and severe weather can disrupt schedules and compound the difficulty of maintaining smooth station operations. In addition to ensuring that aircraft already on gates do not block later arrivals, the peaking created by late arrivals and the need to transfer passengers and large amounts of baggage between flights can delay subsequent departures or result in missed connections. The aircraft-gate assignment influences the deolovment of ramv oersonnel and eeuiament and the time reauired to transfer passengers and baggage between flights [2,4]. When the number of aircraft to be handled approaebes the number of gates available, poor gate assignment decisions may increase delays in getting aircraft on to a gate due to other aircraft that are obstructing access to available gates. When schedule disruptions result in the number of flights to be handled exceeding the available gates, then good assignment decisions can reduce the resulting delays and missed connections. The large amounts of connecting baggage at major hubs requires careful facilities design and deployment of ramp personnel and equipment. If inbound flights are delayed, connecting passengers may miss flights. Even if the passengers make their connections, their baggage may not. The transfer of passengers and baggage between two flights can be made more efficient through assigning the flights to adjacent gates, as well as appropriate allocation of personnel and equipment. If this is not possible, or not sufficient to avoid missed connections, a decision on whether to delay the outbound flight must be made while considering the downstream effects of the delay and the impact on the operation at the station. If another aircraft is scheduled into the gate, delaying a departure may delay that arrival or require the flight to be reassigned to another gate. The arrival schedule of aircraft and the subsequent assignment of arriving flights to gate positions affects several other aspects of station operations. The considerations that can affect gate assignment decisions include the aircraft size, passenger walking distance, baggage transfer, aircraft servicing requirements, ramp congestion, flight crew and aircraft rotation, and the use of remote parking stands. 123
124
Proceedings of the 15th Annual Conference on Computers and Industrial Engineering
An aircraft cannot be assigned to a gate position that is too small to accommodate it. The size of an aircraft in a particular position may also limit the use of adjacent positions, or even restrict access to more distant positions, by reducing clearances to apron taxilanes. These constraints can be expressed through a gate interaction matrix for each pair of gates, that represents the restrictions imposed on the use of each gate by every aircraft type considering the aircraft type using the other gate. It is advantageous to assign flights to gates so as to reduce the distance that passengers have to walk. Not only must the distance itself be considered, but also the time available to cover it, since this affects not only the walking speed required but the ability to arrange assistance for those who need it. Reducing the walking distance for passengers with tight connections may be more important than reducing total walking distance. Consideration should also be given to the dynamic aspects of gate assignment. Reassigning a late arriving flight will require passengers who have already gathered at the original gate to walk to the new gate. With centralized sorting systems, the time saved by assigning a flight to a gate closer to the sorting facility is not usually significant. However, with zone sorting systems, keeping flights with tight connections within the same gate group can significantly reduce transfer time. On the other hand, if ramp crews only work one zone and several flights arrive at that gate group over a short period, assigning a flight to another zone that is not busy may reduce the unload and transfer time. Aircraft servicing requirements must be considered along with the availability of ground handling equipment. Equipment is often aircraft type specific, and long moves from one gate position to the next will waste time and complicate crew supervision. The movement of aircraft and ground vehicles on the ramp and taxilanes can result in congestion, delaying arriving and departing flights. The rotation of flight crews and aircraft to flights must also be taken into consideration. Since flight crews have duties to perform before an aircraft is ready to leave the gate, delaying an arriving aircraft until an assigned gate is free may impact the departing flight using that aircraft or flown by that crew. When the gates available are not sufficient, remote parking positions may be considered. This will require vehicles to transport passengers to the terminal. Another option is to move aircraft off the gate positions once they have been unloaded and park them elsewhere until they can be moved onto a gate to load. Assignment of arriving international flights to appropriate gates must consider government inspection requirements (immigration, customs, health, etc.), which will generally restrict the available gates and require a longer turn around time. If the aircraft is continuing as a domestic flight, it may have to be moved to a different gate for loading. In response to the complexity of the aircraft-gate assignment task, and the need under operational conditions for a fast response to dynamic, changing circumstances, the research objective is to develop a heuristic reasoning based expert system as a solution methodology. The prototype system that has been developed is a planning tool which helps identify an initial aircraft-gate assignment, and assists with sensitivity analysis on input data such as departure and arrival information. The planning tool could evolve to be able to deal with dynamic aircraft-gate assignment, that is, it could become a dynamic tool. Since the system must be airport specific, it's knowledge base should be adaptable for use at any airport. That is, the system must be able to accommodate any schedule of arriving and departing flights with any airport terminal configuration. The aircraft-gate assignment expert system needs to be able to identify multiple solutions (if they exist). Also, the use of graphics and a user friendly interface was a primary concern. SvstemDesign This research is targeted for airports in the North American continent. Assumptions were made to reduce the problem's scale. The airline is assumed to occupy 10 gate positions. Some gates are limited in the size of the aircraft that they can accommodate. The use of some gates depends on the size of any aircraft in adjacent gates. The airline fleet comprises of six aircraft types, which may be grouped into some kind of size classes. The number and types of aircraft to be scheduled is at least approximately equal to the number and types of gates available. Gate restrictions include the clearance between adjacent gate positions which is determined by the specific constraining (or adjacent) gate(s), aircraft class, and gate size limit. It is assumed that there is sufficient clearance between each gate position and the apron taxilanes that no aircraft on a gate blocks access to any other gate. The gates are divided into two ramp zones, corresponding to the odd and even numbered gates. Baggage is assumed to be preloaded by zone at the origin station, and sorted for reloading in each zone. Therefore baggage transfer within a zone take less time than transfers between zones. It is also desirable to have an even distribution of arrival and departure times within each zone, to reduce peak workload. A gate position is the area of the terminal apron designated for the ground services such as refueling, baggage handling, cleaning, servicing, etc. The flight schedule is for a typical day begins at 07:50 hours and concludes at 22:00 hours. An hour is divided into three time slots. Within the first time slot, which is 20 minutes long, airplanes arrive. The second time slot, which is about 35 minutes long, all aircraft are at their gates ensuring that passengers can transfer. The last (third) time slot is 15 minutes long. It is used by aircraft to depart. There could be an overlap between the last time slot of a specific hour and the first time slot of the next hour. The entire set of flights are divided into three segments. The first segment is the flights between any two cities east of the Mississippi River or between any two cities west of the Mississippi. The second segment would be flights that cross the Mississippi, and the third segment would be overseas flights. Every flight route has a specific aircraft type assigned to it depending upon the distance traversed and the volume of traffic. Aircraft are assigned to gate on 'firstarrived-first-assigned' basis but with priority given to larger aircraft in order to minimize passenger delays. Other factors used in the assignment of gates to aircraft include the time of aircraft arrival and departure, operational factors such as the aircraft-gate size compatibility, the gate's access to government inspection services, the maximization of the usage of certain gates, and the minimization of aircraft delays. Other user controllable parameters which influence
Su and SmHAVa:Aircraft-Gate Assignment Advisor
125
gate allocation include the time separation between successive assignments of the gates, the maximum allowable gate occupancy times by aircraft size and flight sector, and the maximum 'tolerable' aircraft delay for gate access. The factors considered in the aircraft-gate assignment problem can be summarized into the following categories: aircraft characteristics, airport area and construction, gate characteristics, government inspection and customs service needs, weather, airline policy and passenger service levels, and the marketing strategy. For an aircraft being assigned to a gate, the prototype aircraft-gate assignment expert system considers the type and size of an aircraft, servicing regulations and requirements for the flight, and flight and aircraft changes. Aircraft size would impact the assignment of an aircraft to a particular gate, and could impact the assignment of aircraft to adjacent gates. For the purposes of this research, the prototype system developed assumes that the movement of aircraft on apron taxiway is not affected by aircraft assigned to gates. An aircraft cannot be assigned to a gate that is too small for it. The amount of the time that a flight is on the ground and it actually needs to be on the ground is another factor. Also, the regulations and requirements applicable when an aircraft is assigned to a particular gate are considered. These could include the availability of fuel hydrants, ground power and air, ground handling equipment, passenger transportation, etc. The aircraft-gate assignment system considers the use of remote parking bays to service aircraft if the number of aircraft to be assigned exceeds available gate capacity. Also aircraft could be withdrawn from gates and parked elsewhere until they are ready to board. The characteristics of gates/ramps are considered in the problem of aircraft-gate assignment. The factors considered include the characteristics of individual gates, capacity and capability of individual gates, the gate needs of individual aircraft, constraints imposed by aircraft parked at neighboring gates in the same time slot, and gate/ramp congestion. The aircraft-gate assignment system considers the special constraints including the immigration and customs requirements of international flights. These requirements restrict the number of available gates. International flights also have a longer turnaround time. The level of service provided to the passenger is a primary concern. Costs can be reduced by reducing staffing levels or facilities often at the price of more delays, greater crowding, and missed flights [2]. The aircraft-gate assignment should attempt to meet established service levels in the most cost efficient manner. Severe weather could adversely affect airline schedules, and as a result make aircraft-gate assignment more difficult. Several of these above factors are inter-related. Information Used. Inouts. And Outouts The prototype aircraft-gate assignment system (Figure 1) is implemented as an interactive microcomputer program using Prolog. It simulates the arrivals and departures of commercial aircraft at the terminal apron as described by an input flight schedule and assigns gates to these aircraft. The information used by the aircraft-gate assignment system include the flight schedules, aircraft service times required on the ground, aircraft classes, international flight requirements, gate characteristics including description and capacity, and the government inspection service. The flight schedule is used by the aircraft-gate assignment to get each flight's airline code and flight number, aircraft size, time of arrival, time of departure, requirement for government inspections both upon the aircraft's arrival and before its departure, and the aircraft service times required on the ground. For each aircraft type the data required includes the airline code, flight number, aircraft size, and aircraft class. A description of each available gate in terms of the maximum aircraft size it can accommodate, its accessibility to government inspection facilities for both emplaning and deplaning passengers, and the strategy adopted by the airport operator in assigning that gate are some other pieces of information required by the aircraft-gate assignment expert system. This research assumes the exclusive use of a gate by an airline. For each gate/ramp, the following data is accessed and used: aircraft type, size, and class; gate size, limits, and use policies; constraining gate number; aircraft servicing requirements; aircraft and flight changes; use of remote parking stands; and government inspection needs. Also required is a specification of the maximum time an aircraft of a given size and flight sector (domestic, trans-border, other international) is allowed to occupy a gate. The aircraft-gate assignment system's outputs include the gate usage and utilization information over a specific time period, aircraft changeover conditions (number of aircraft using a specific gate) over a specific time period, and the graphical output of the status of gates at a specific time. An output that describes the status of all gates indicates the aircraft-gate assignment (including remote parking positions) and operational characteristics of assigned flights. A description of all flights considered by the prototype aircraft-gate assignment expert system can be obtained. Information provided includes a specification of the gate assigned to each aircraft and the delay, if any, in obtaining that gate, and a listing of unassigned flights (remote parking) for which certain arrangements may be required. Gate utilization is expressed as the percentage of time the gate has been in use relative to the analysis time period, and the total number of aircraft handled over the course of that period of time. These measures complement each other as, for example, an aircraft could needlessly occupy a gate for an extended period of time (i.e. using it merely for parking) thus resulting in artificially high gate utilization values. For each type of event, rules were developed to determine if the change will cause a problem. In the case of a late arrival, these rules cheek if the expected turn-around time will result in a late departure, and if this will block the gate for a subsequent arrival. Delayed departures are checked for conflicts with subsequent arrivals. If there is an aircraft/flight change, the rules cheek the suitability of the gate for the new aircraft type and adjacent gate restrictions. The output provided by the system with respect to aircraft changeovers shows how many aircraft occupied a gate during the analysis time period. The graphical output that depicts the gates occupied at a specific time period displays CAIE 2,f~l/4~J
126
Proceedings of the 15th Annual Conference on Computers and Industrial Engineering
Flight Schedules
I
Aircraft service times
AircrafL dasses
International flight
Gate eharaeteldstiea description and capacity
~
~ [ ' I
~
~'~: "-I~ M M
xI~'.'I~'~__~ [~ " 1
~ n ~ ~ 1~ AIHM:MAIrI'.GA~ AdlillGMIIIglVlr m
~
~
I
Gate usage information
I
Gate utilization value over a speeific time period
[ [
~ i
i
Aircraft changeover conditions[
b.-I Graphical OUtput of the status ~ of a gate at a specific time
Government inspection services
Fieure 1: Expert System Based Aircraft-Crate Assignment Framework the airport terminal configuration along with the specific information on aircraft and the gates they occupy. Conclusion Aircraft-gate assignment is a major operational issue for airlines, especially at high aircraft density airports. The use of computer models offers a method to deal with huge quantities of knowledge and data. It provides the user with the capability to conduct efficient planning and operational management of airport capacity. Expert systems are an ideal solution methodology in a domain such as the aircraft-gate assignment problem. Identifying the relevant information and defining the constraints are not particularly difficult tasks. Although managing the data flows is not difficult in principle, careful attention needs to be paid to system architecture because of the large amount of knowledge and data involved and the need to continuously adjust to changing circumstances. The most difficult part of the problem is identifying the rules to guide the assignment process, because of the large number of factors to be taken into account. A prototype expert system has been developed to gain some understanding of the issues involved. It demonstrates the ability to take some of the relevant factors (mentioned above) into account in solving a restricted set of aircraft-gate assignment problems. In addition, the system integrates the inference process with the operational database. In particular, the system will allow end users to modify the knowledge base to tailor the system to local conditions and changing requirements. Acknowled~,ements The authors are thankful to Dr. R. Muthukrishan, Corporate Research And Development, United Airlines, Chicago, Illinois for his input. References 1. Gosling, G.D., 'Application Of Expert Systems In Air Traffic Control', Journal of Transoortation Engineering, Vol. 113, No.2, 1987, pp. 139-154. 2. Gosling, G.D., 'Design Of An Expert System For Aircraft Gate Assignment', Trans_oortation Research Part A, Vol.24A, No.I, 1990, pp.59-69. 3. Hamzawi, S.G., 'Management And Planning Of Aircraft Gate Capacity: A Microcomputer-based Gate Assignment Simulation Model', Trans_nortation Planning, and Technology, Vol. 11, 1986, pp. 189-202. 4. Su, Y.Y., 'An Expert System Approach To Aircraft-Gate Assignment', Masters Thesis. State University Of New York, Binghamton, New York, 1991. 5. Tobias, L., & Scoggins, J.L. Jr., 'Time-Based Air-Traffic Management Using Expert Systems', IEEE Control Systems Magazin¢, Vol.2, No.2, 1987, pp.23-29.