Papers by Cynthia Barnhart
P lagued by high labor costs, low profitability margins, airspace and airport congestion, high ca... more P lagued by high labor costs, low profitability margins, airspace and airport congestion, high capital and operating costs, security and safety concerns, and complex and large-scale management and operations decisions, the airline industry has armed its planners with sophisticated optimization tools to improve decision making and increase airline profits. In this paper, we describe optimization approaches for airline schedule planning, demonstrating how optimization can facilitate the management of a diverse and finite set of expensive, highly constrained resources. We focus on the art and science of modeling and solving these problems, providing illustrative examples of the associated impacts and challenges, and highlighting effective techniques that might be applicable to problems arising in other industries.
SSRN Electronic Journal, 2017
International Series in Operations Research & Management Science, 2012
, except for brief excerpts in connection with reviews or scholarly analysis. Use in connection w... more , except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights.
In scheduled passenger air transportation, airline profitability is critically influenced by the ... more In scheduled passenger air transportation, airline profitability is critically influenced by the airline's ability to construct flight schedules containing flights at desirable times in profitable markets. In this dissertation, we study two elements of the schedule generation process, namely, schedule design and fleet assignment. The schedule design problem involves selecting an optimal set of flight legs to be included in the schedule, while the fleet assignment problem involves assigning aircraft types (or fleets) to flight legs to maximize revenues and minimize operating costs simultaneously. With the fleet assignment problem, we investigate the issues of network effects, spill, and recapture. On a constrained flight leg in which demand exceeds capacity, some passengers are not accommodated, or spilled. When passengers travel on two or more constrained legs, flight leg interdependencies or network effects arise because spill can occur on any of these legs. In most basic fleet assignment models, simplistic modeling of network effects and recapture leads to sometimes severe, miscalculations of revenues. Recapture occurs when some of the spilled passengers are re-accommodated on alternate itineraries in the system. In this dissertation, we develop new fleet assignment models that capture network effects, spill, and recapture. Another benefit of one of our models is its tractability and potential for further integration with other schedule planning steps. Our study shows that the benefits of modeling these elements can be as large as $100 million annually for a major U.S. airline. In addition, we show that modeling flight leg interdependence is more important than demand stochasticity for hub-and-spoke fleet assignment problems. We develop two models for schedule design, one assuming that the market share of an airline remains constant with schedule changes; and the other assuming that market share varies with schedule changes. The constant market share model, while less precise in its modeling, is much easier to solve than the variable market share model. We estimate that the potential benefits of these models range from $100 to $350 million annually.
Handbooks in operations research and management science, 2007
Publisher Summary This chapter discusses problems related to the management of air traffic and ai... more Publisher Summary This chapter discusses problems related to the management of air traffic and airline operations for minimizing the impact and cost of disruptions. Flight and crew schedules and passenger itineraries have become increasingly fragile because of the growing complexity of the air transportation system and the tight coupling of its various elements. The resulting direct and indirect economic costs are very large, certainly amounting to several billion dollars annually. The airline industry has a vital stake in research aimed at mitigating the effects of severe weather and other disruptive events and at expediting recovery from irregular operations. A significant body of recent and ongoing work has led to major progress toward these objectives. Two breakthrough developments have been the primary drivers behind this progress. First, collaborative decision making has made it possible to apply the principles of information sharing and distributed decision making to air traffic flow management (ATFM) by expanding the databases available to airline and Federal Aviation Administration (FAA) traffic flow managers, by creating common situational awareness, and by introducing shared real-time tools and procedures. Second, there is growing recognition in the airline industry of the fact that planning for schedule robustness and reliability may be just as important as planning for minimizing costs in the complex, highly stochastic, and dynamic environment of air transportation.
Abstract: Many of the existing methods for evaluating an airline's on-time performance are b... more Abstract: Many of the existing methods for evaluating an airline's on-time performance are based on flight-centric measures of flight delay. However, recent research has demonstrated that as much as 50% of passenger delays are caused by passenger travel disruptions, either flight cancelations or missed connections. The propensity for disruptions varies significantly across airports and carriers, based on key factors such as scheduling practices, network structures, and passenger connections. In this paper, we analyze the causes and costs of ...
Logistics is the cost aware planning, design, and control of material flow and related informatio... more Logistics is the cost aware planning, design, and control of material flow and related information flow (persons, energy, money, information, ...) in production processes. The notion is often used as a synonym for transportation, distribution, or warehouse management. The topic is of a rich variety, has great practical importance , and attracts researchers from the computer science (CS), mathematical programming (MP), and the operations research (OR) communities alike. Today, problems from logistics are widely studied as parts of the disciplines of mathematical programming and operations research; algorithmics and theoretical computer science; and computer systems. The specific models and methods , as well as the objectives to be optimized, differ in the various disciplines; nevertheless, there are remarkable similarities (as well as significant differences) in the general framework adopted by researchers in logistics in these disparate disciplines. The primary objectives of the sem...
Developed and analyzed discrete choice models with latent variables/classes Clip-Air concept: Int... more Developed and analyzed discrete choice models with latent variables/classes Clip-Air concept: Integrated schedule planning for a new generation of aircraft, 2010-2013 Project manager Bilge (Küçük) Atasoy 2 Developed airline schedule planning and fleet assignment models for the new aircraft, compared its per-formance to standard aircraft. MyTosa: Simulation tool for the dimensioning, commercial promotion and case study set-up for ABB’s rev-
The Container Relocation Problem (CRP), also called Block Relocation Problem (BRP), is concerned ... more The Container Relocation Problem (CRP), also called Block Relocation Problem (BRP), is concerned with finding a sequence of moves of containers that minimizes the number of relocations needed to retrieve all containers, while respecting a given order of retrieval. The restricted CRP enforces that only containers blocking the target container can be relocated. We improve upon and enhance an existing binary encoding and using it, formulate the restricted CRP as a binary integer programming problem in which we exploit structural properties of the optimal solution. This integer programming formulation reduces significantly the number of variables and constraints compared to existing formulations. Its efficiency is shown through computational results on small and medium sized instances taken from the literature.
Manuf. Serv. Oper. Manag., 2022
Problem definition: Physical distancing requirements during the COVID-19 pandemic have dramatical... more Problem definition: Physical distancing requirements during the COVID-19 pandemic have dramatically reduced the effective capacity of university campuses. Under these conditions, we examine how to make the most of newly scarce resources in the related problems of curriculum planning and course timetabling. Academic/practical relevance: We propose a unified model for university course scheduling problems under a two-stage framework and draw parallels between component problems while showing how to accommodate individual specifics. During the pandemic, our models were critical to measuring the impact of several innovative proposals, including expanding the academic calendar, teaching across multiple rooms, and rotating student attendance through the week and school year. Methodology: We use integer optimization combined with enrollment data from thousands of past students. Our models scale to thousands of individual students enrolled in hundreds of courses. Results: We projected that ...
Journal of Scheduling, 2006
The mission of The Journal of General Physiology is to publish articles that elucidate basic biol... more The mission of The Journal of General Physiology is to publish articles that elucidate basic biological, chemical, and physical principles of broad physiological significance. Physiological significance usually means mechanistic insights, which often are obtained only after extensive analysis of the experimental results. The significance of the mechanistic insights therefore can be no better than the validity of the theoretical framework used for the analysis—and it is usually better to be vaguely right than precisely wrong. The uncertainties associated with data analysis are well illustrated in the Perspectives on Ion Permeation through membrane-spanning channels ( J . Gen . Physiol . 113:761– 794) and the related Letters-to-the-Editor in this issue. This exchange moreover identified a particular problem that can be resolved by a change in editorial policy. The problem is the graphic representation of the results of kinetic analyses of ion permeation based on discrete-state rate mo...
Transportation Science, 2018
The container relocation problem (CRP) is concerned with finding a sequence of moves of container... more The container relocation problem (CRP) is concerned with finding a sequence of moves of containers that minimizes the number of relocations needed to retrieve all containers, while respecting a given order of retrieval. However, the assumption of knowing the full retrieval order of containers is particularly unrealistic in real operations. This paper studies the stochastic CRP, which relaxes this assumption. A new multistage stochastic model, called the batch model, is introduced, motivated, and compared with an existing model (the online model). The two main contributions are an optimal algorithm called Pruning-Best-First-Search (PBFS) and a randomized approximate algorithm called PBFS-Approximate with a bounded average error. Both algorithms, applicable in the batch and online models, are based on a new family of lower bounds for which we show some theoretical properties. Moreover, we introduce two new heuristics outperforming the best existing heuristics. Algorithms, bounds, and ...
European Journal of Operational Research, 2018
The Container Relocation Problem (CRP), also called Block Relocation Problem (BRP), is concerned ... more The Container Relocation Problem (CRP), also called Block Relocation Problem (BRP), is concerned with finding a sequence of moves of containers that minimizes the number of relocations needed to retrieve all containers, while respecting a given order of retrieval. The restricted CRP enforces that only containers blocking the target container can be relocated. We improve upon and enhance an existing binary encoding and using it, formulate the restricted CRP as a binary integer programming problem in which we exploit structural properties of the optimal solution. This integer programming formulation reduces significantly the number of variables and constraints compared to existing formulations. Its efficiency is shown through computational results on small and medium sized instances taken from the literature.
European Journal of Operational Research, 2018
This paper introduces a novel optimization problem resulting from the combination of two major ex... more This paper introduces a novel optimization problem resulting from the combination of two major existing problems arising at storage yards in container terminals. The Yard Crane Scheduling Problem is typically concerned with routing the crane given a sequence of storage and retrieval requests to perform, while the Container Relocation Problem tackles the minimization of relocations when retrieving containers in a simpler setting. This paper is the first to consider a model that integrates these two problems by scheduling storage, retrieval and relocations requests and deciding on storage and relocation positions. We formulate this problem as an integer program that jointly optimizes current crane travel time and future relocations. Based on the structure of the proposed formulation and the linear programming relaxation of subproblems, we propose a heuristic local search scheme. Finally, we show the value of our solutions on both simulated instances as well as real data from a port terminal.
Operations Research Letters, 2016
The Container Relocation Problem (CRP) involves finding a sequence of moves of containers that mi... more The Container Relocation Problem (CRP) involves finding a sequence of moves of containers that minimizes the number of relocations needed to retrieve all containers in a given order. In this paper, we focus on average case analysis of the CRP when the number of columns grows asymptotically. We show that the expected minimum number of relocations converges to a simple and intuitive lower-bound for which we give an analytical formula.
International Series in Operations Research & Management Science
Transportation Science, 2017
Airlines and high speed rail are increasingly competing for passengers, especially in Europe and ... more Airlines and high speed rail are increasingly competing for passengers, especially in Europe and Asia. Competition between them affects the number of captured passengers and, therefore, revenues. We consider competition between airlines (legacy and low-cost) and high speed rail. We develop a new approach that generates airline schedules using an integrated mixed integer, nonlinear optimization model that captures the impacts of airlines’ decisions on passenger demand. We estimate the demand associated with a given schedule using a nested logit model. We report our computational results on realistic problem instances of the Spanish airline IBERIA and show that the actual airline schedules are found to be reasonably close to the schedules generated by our approach. Next, we use this optimization modeling approach under multimodal competition to evaluate multiple scenarios involving entry of high speed rail into new markets. We account for the possibility of demand stimulation as a res...
Transportation Research Part A: Policy and Practice, 2016
In this paper, we analyze the effectiveness of the 2010 Tarmac Delay Rule from a passengercentric... more In this paper, we analyze the effectiveness of the 2010 Tarmac Delay Rule from a passengercentric point of view. The Tarmac Delay Rule stipulates that aircraft lift-off, or an opportunity for passengers to deplane, must occur no later than three hours after the cabin door closure at the gate of the departure airport; and that an opportunity for passengers to deplane must occur no later than three hours after the touchdown at the arrival airport. The Tarmac Delay Rule aims to protect enplaned passengers on commercial aircraft from excessively long delays on the tarmac upon taxi-out or taxi-in, and monetarily penalizes airlines that violate the stipulated three-hour tarmac time limit. Comparing the actual flight schedule and delay data after the Tarmac Delay Rule was in effect with that before, we find that the Rule has been highly effective in reducing the frequency of occurrence of long tarmac times. However, another significant effect of the rule has been the rise in flight cancellation rates. Cancellations result in passengers requiring rebooking, and often lead to extensive delay in reaching their final destinations. Using an algorithm to estimate passenger delay, we quantify delays to passengers in 2007, before the Tarmac Delay Rule was enacted, and compare these delays to those estimated for hypothetical scenarios with the Tarmac Delay Rule in effect for that same year. Our delay estimates are calculated using U.S. Department of Transportation data from 2007. Through our results and several sensitivity analyses, we show that the overall impact of the current Tarmac Delay Rule is a significant increase in passenger delays, especially for passengers scheduled to travel on the flights which are at risk of long tarmac delays. We evaluate the impacts on passengers of a number of rule variations, including changes to the maximum time on the tarmac, and variations in that maximum by time-of-day. Through extensive scenario analyses, we conclude that a better balance between the conflicting objectives of reducing the frequency of long tarmac times and reducing total passenger delays can be achieved through a modified version of the existing rule. This modified version involves increasing the tarmac time limit to 3.5 hours and only applying the rule to flights with planned departure times before 5pm. Finally, in order to implement the Rule more effectively, we suggest the tarmac time limit to be defined in terms of the time when the aircraft begin returning to the gate instead of being defined in terms of the time when passengers are allowed to deplane.
Transportation Science, 1998
Given a schedule of flight legs to be flown by an airline, the fleet assignment problem is to det... more Given a schedule of flight legs to be flown by an airline, the fleet assignment problem is to determine the minimum cost assignment of flights to aircraft types, called fleets, such that each scheduled flight is assigned to exactly one fleet, and the resulting assignment is feasible to fly given a limited number of aircraft in each fleet. Then the airline must determine a sequence of flights, or routes, to be flown by individual aircraft such that assigned flights are included in exactly one route, and all aircraft can be maintained as necessary. This is referred to as the aircraft routing problem. In this paper, we present a single model and solution approach to solve simultaneously the fleet assignment and aircraft routing problems. Our approach is robust in that it can capture costs associated with aircraft connections and complicating constraints such as maintenance requirements. By setting the number of fleets to one, our approach can be used to solve the aircraft routing probl...
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Papers by Cynthia Barnhart