A Rescheduling Framework for Airline Scheduling

Birthe Gebhardt and Jürgen Sauer

Department für Informatik, Abt. Wirtschaftsinformatik, Carl von Ossietzky Universität Oldenburg,

Ammerländer Heerstr. 114-118, 26129 Oldenburg, Germany

Keywords: Reschedule, Framework, Airline.

Abstract: Airline scheduling is a sophisticated area. If a disruption occurs many tasks have to be taken into account. In

order to structure the rescheduling process a framework is useful. The existing framework for rescheduling

will extend with further information tasks to use various repair methods. The framework is cuts into two

parts. The first part includes the precondition of the domain. The second parts described the tasks of the

rescheduling process. The use of this framework allows to implement more than one rescheduling method.

1 INTRODUCTION

Airline scheduling is a sophisticated area. Several

schedules from different departments are

intermeshed, start with the flight schedule, followed

by the maintenance schedule, we have the crew

schedule and of course the human resource

schedules. The interaction between the schedules

depends on the business processes of each airline

therefore the used time lines in this paper could

differ from other airlines.

Disruptions could appear at every time. The

influence is more or less serious. This depends on

the connection to the other schedules and the time

when the disruption happens and therefore on the

time left to solve the problem. In (Love et al., 2002)

the authors discuss the separate schedules between

aircraft fleeting, routing and the crew schedule.

Several solutions were discussed for planning and

disruption management. Disruption events have

different sources. The European Organization for the

Safety of Air Navigation (EUROCONTROL)

publishes a delay analysis monthly including the

significant events. In February 2011 43,89% of the

delays were reactions of the primary delays (Central

Office for Delay Analysis / EUROCONTROL,

2011). In this paper an overview of the reactive

process will be given and expand the process by an

disruption analyze step. The process is not only used

for solving the disruption by a system but also to get

an overview how to process and manage disruptions.

2 RESCHEDULING

FRAMEWORK

(Herrmann, 2006) defines rescheduling as “...the

process of updating an existing production schedule

in response to disruption or other changes” (p.137).

The reactive process will perform, if the schedule is

already invalidated by a disruption. Three goals have

to be solved by the solution: 1) fast reaction against

the disruption, 2) the quality of the schedule shall

not worsen and 3) minimal changes to the original

schedule (Sauer, 2004). A solution of the process is

mainly domain dependent. Several researchers

investigate different domains. (Pinedo, 2005) defines

two domain areas. One area includes the

manufacturing trade, the other one represents the

supply of services. Of course domain-independent

solutions are very welcome but the solutions rely on

abstract and more general models (Ghallab et al.,

2004). A framework for the rescheduling process for

a manufacturing system is introduced by (Vieira et

al., 2003) and (Herrmann, 2006). The intension

behind the framework was to understand the

rescheduling research and the definition of some

terms but it was also used as a control strategy. The

framework includes the following areas:

rescheduling environment, methods and strategies.

The environment describes the type of the

schedule whether it is static or if it is a dynamic

environment. The authors specify three reschedule

methods. The right shift rescheduling delays each

remaining job on the involved machine with the

305

Gebhardt B. and Sauer J..

A Rescheduling Framework for Airline Scheduling.

DOI: 10.5220/0004011803050308

In Proceedings of the 9th International Conference on Informatics in Control, Automation and Robotics (ICINCO-2012), pages 305-308

ISBN: 978-989-8565-21-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

amount of time in order to get a valid schedule. The

partial rescheduling reuse parts of the schedule and

only alter the jobs that were affected directly and

indirectly. The last method which is mentioned in

the framework refers to the complete regeneration.

Every job which is not processed before the

rescheduling point is included in the rescheduling

process. A further repair method which is not

mentioned, changes the original schedule by adding

or removing activities. This will not only influence

the jobs or rather activities, but also the constraints

(van der Krogt and de Weerdt, 2005). The third part

of the framework is the rescheduling strategy. The

authors distinguish between dynamic and predictive-

reactive scheduling. Dynamic scheduling is not

using a production schedule. The frequency of

starting the rescheduling method is defined in three

categories: an event-driven, a periodic procedure and

a hybrid one. The presented framework is based on

(Jones et al., 1998) and (Herrmann et al., 2003). The

process is divided into two phases. The precondition

defines the tasks which shall be processed only once.

The second phase is defined as the rescheduling

process, including tasks which are used during each

trigger. The precondition describes the schedule

based upon the scheme published in (Jones et al.,

1998):

Precondition:

 Requirement generation is the classification

of the schedule

 Processing complexity refers to the

processing steps

 Scheduling criteria matches the performance

criteria

 Parameter variability defines the degree of

uncertainty of the schedule parameter

 Schedule environment defines if it is a static

or dynamic schedule

 Rescheduling strategy defines the policy of

the trigger

The rescheduling process is capsuled from the

precondition while the precondition is more like

describing the domain. This is the reason why the

rescheduling strategy is moved from the

rescheduling process into the precondition phase.

The rescheduling process is an periodic process

used for every disruption which has an influence to

the schedule.

Rescheduling Process:

 Analyze the disruption and the schedule for

possible solution

 Select a repair method and algorithm

 Review the solution

3 PRECONDITION

The precondition describes the domain and the

behavior. The characteristics will help to find a

feasible reactive scheduling process. The first three

conditions: requirement generation, processing

complexity and scheduling criteria are described in

(Graves, 1981). The requirement generation differs

between open shop and closed shop.

The processing complexity displays the property

of the facility. These criteria differ between stages

and available facilities:

 One stage – One facility

 One stage – Multi facilities

 Multi stage – Flow shop

 Multi stage – Job shop

The scheduling criteria are grouped into

schedule cost and performance. These shall also be

described as one of the goals of the rescheduling

process in order to verify the solution.

The parameter variability defines the uncertainty

of the parameters of the schedule problem. The

authors divide the schedule problem into two

groups: deterministic and stochastic. The scheduling

environment is integrated into the presented

frameworks and defines the problem as static or

dynamic. The last precondition entry defines one

part of the rescheduling solution. Most of the

schedules need a reschedule process while the

schedule is invalid due to a disruption. Therefore the

process is defined previously. The strategies divide

into a dynamic and a predictive-reactive one. A

periodic, event-driven and hybrid policy is available.

A periodic policy checks the schedule after a

configured time interval. For real time reactions an

event-driven policy is implemented. A hybrid policy

starts the rescheduling process periodically and only

if a special event takes place an event-driven policy

is chosen.

4 RESCHEDULING PROCESS

The iterative rescheduling process is divided into

three parts:

 The analyzing of the invalid schedule

 The selection of the repair method

 The review of the solution

This process is triggered by the rescheduling

strategies and will run through by each disruption.

The following chapters describe the tree parts in

detail.

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4.1 Analysis

The analysis is a new part in the rescheduling

framework. The goal of this step is to analyze the

invalid schedule and provide information for the

repair method. This approach implies several

questions:

 What kind of disruption takes place?

 Which part of the schedule is involved?

 How much time is available for solving the

problem?

 What kind of influence has the disruption to

the schedule?

 What kind of precondition items and

constraints are affected?

These questions can be clustered into disruption,

timing and precondition scopes. (de Snoo, 2011)

arranges the questions into two part after a

disruption has occurred: 1) the time area includes the

time which is necessary to solve the problem and the

time which is available and 2) the consequence

triggers by the disruption and the solution. The cause

of the disruption is divided into extern and intern.

The reason behind the split is the different handling,

communication and cost aspects. The scope is the

main item that influences the rescheduling strategy

and the repair method. The available time defines

the strategy which includes also the knowledge of

the interruption from a running repair method

process. The consequence depends on the

information process and the actions of the

rescheduling process. Each disruption can have

different consequences. The information of the

analysis will store into a journal. The journal

includes also the resources and jobs which are

involved in the rescheduling method. This detailed

information is needed for the interruption of the

rescheduling process if a new disruption takes place.

The journal can answer the question whether the

rescheduling process is interruptible. If the analysis

has done the tasks and logged the information into

the journal the repair of the schedule starts.

4.2 Repair Methods

The repair methods are the background for the

implemented algorithms in an IT system. Five

possibilities exist to solve the problem of an invalid

schedule:

 Right-Shift method

 Repair method

 Plan Reuse

 Reschedule from scratch

 Hybrid

The advantage of the right-shift method is, the

schedule gets a solution very fast. The disadvantage

depends on the cost which is caused by the delays.

Some restrictions have to be set using this method:

 How much delay is acceptable for each

flight?

 How many flights can be affected till the

right-shift shall stop?

 How much cost of this solution is allowed?

The second method works with additions and

cancellations. The cancellation does not only refer to

jobs, but also to the schedule constraints. The idea

behind this repair method is the use of two phases.

The first phase removes the constraints. The second

phase is the planning phase which includes the

extension to satisfy the goal. The advantage is the

use of an independent planer because of the split into

two phases where the first phase is more domain

depended than the second phase. The problem is to

find the best possible additions and cancellations of

jobs and constraints. Planning from the first

principle is called rescheduling from scratch. The

second principle describes the plan reuse (Koehler,

1996). The plan reuse method reuses valid parts of

the initial schedule. The goal is to leave as many

jobs as possible unchanged in order to reduce the

modification into the new valid plan. The plan

stability is the advantage of this method because

minimal changes will be conducted. Generate a plan

from scratch is also called replanning. The planner

constructs a new plan including every job which is

not processed yet. The plan stability is not

considered in this case. (Fox et al., 2006) and

(Koehler, 1996) declare that the speed of plan

production, plan quality, plan stability and the cost is

much better when using the plan reuse method

instead of planning from scratch. The hybrid

combines two or more presented methods in one

repair method (Lim et al., 2005). A time duration

shifting and the insertion and exchange is developed

in the neighborhood search procedure. A hybrid is

useful to get the advantages of each method in order

to solve the flexibility in an invalid schedule.

The result of the execution of the selected repair

method will verify in the last step of the

rescheduling process.

4.3 Review

The review serves the auditing of results. The audit

includes (Sauer, 2004) (Fox et al., 2006):

 Quality

A Rescheduling Framework for Airline Scheduling

307

 Stability

 Robustness

The quality of a schedule depends on the

purpose by the different groups. (Kempf et al., 2000)

discuss the measurement of quality. The authors

differ between:

 individual schedules vs. group of schedules

 absolute measurement vs. relative

comparison

 tradeoffs between multiple metrics

 static vs. dynamic measurement

Plan stability defines the deviation between the

original schedule and the new schedule. More stable

schedules create less stress on execution components

and avoids nervousness. The influence of further

disruptions is reduced if the repair method generates

a robust schedule. That implies less rescheduling in

the future. The review step finishes the introduced

rescheduling framework.

5 CONCLUSIONS

This paper describes the rescheduling framework

phases and steps. The intension of this kind of

research is the understanding of rescheduling and

setup definition and control strategies .The

introduced framework is divided into two phases.

The preconditions define the domain and give

further information to the second phase. The

rescheduling process is an iterative process

including the new step of analyzing the disruption,

the selection of the repair method and the review of

the new schedule.

The next step of the research will be an

evaluation of the rescheduling process using a case

study from an airline company.

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