Optimization in Airline Scheduling : Successes, Challenges, and New Directions Ellis L. Johnson IMA Workshop on Travel and Transportation Nov 14, 2002

Optimization in Airline Optimization in Airline SchedulingScheduling::

Successes, Challenges, Successes, Challenges, and New Directions and New Directions

Ellis L. Johnson

IMA Workshop on Travel IMA Workshop on Travel and Transportationand Transportation

Nov 14, 2002

2 IMA Travel and Transportation, Nov 14 2002IMA Travel and Transportation, Nov 14 2002

Airline Operations ResearchAirline Operations Research

Airlines have been a hotbed of ORAirlines have been a hotbed of OR Planning process over time Planning process over time

• Schedule and complex developmentSchedule and complex development• Fleet assignment and routingFleet assignment and routing• Yield management and passenger flowYield management and passenger flow• Crew pairing and bid lines Crew pairing and bid lines • Operations and recoveryOperations and recovery

Computational issuesComputational issues• All require solving large-scale All require solving large-scale

optimization problemsoptimization problems


Airline OR SuccessesAirline OR Successes

Excess crew costs in the planning Excess crew costs in the planning process has been driven to 0-3%process has been driven to 0-3%• AA was 8-10% 15 yrs ago: now 0-2% AA was 8-10% 15 yrs ago: now 0-2% • Each 1% is worth about $10 million/yrEach 1% is worth about $10 million/yr

• 1997 had 9,000 pilots costing $1.2 billion1997 had 9,000 pilots costing $1.2 billion

• Larger schedules and complex rulesLarger schedules and complex rules

Fleet assignment has increased Fleet assignment has increased profit by $tens of millions/yrprofit by $tens of millions/yr

Yield management maintains Yield management maintains revenue despite low faresrevenue despite low fares


Implementation of Airline ORImplementation of Airline OR

Cost reductions were initially Cost reductions were initially embraced, e.g. crew schedulingembraced, e.g. crew scheduling

All airlines will say that they are All airlines will say that they are unique, have a different culture, etc.unique, have a different culture, etc.

US pilots have strange pay structureUS pilots have strange pay structure• Asian and European pilots are simply Asian and European pilots are simply

(high) salaried employees (high) salaried employees • US pilots get paid for what they fly + US pilots get paid for what they fly +

minimum guarantees + conflictsminimum guarantees + conflicts


Compartmentalization into SilosCompartmentalization into Silos

Making sequential plans is commonMaking sequential plans is common• Schedule -> FAM & Routing -> CrewSchedule -> FAM & Routing -> Crew• Pricing -> YM -> promotions & salesPricing -> YM -> promotions & sales

Has led to optimization within siloHas led to optimization within silo Too little feedback Too little feedback What is needed is integrated What is needed is integrated

planning over function and over timeplanning over function and over time• Operations experience does not feed Operations experience does not feed

back into the planning process back into the planning process


Compartmentalization into SilosCompartmentalization into Silos

Each group thinks they are the most Each group thinks they are the most important planning groupimportant planning group• Pricing and marketing define the airlinePricing and marketing define the airline• YM is needed to keep the pricing guys YM is needed to keep the pricing guys

from giving away the airlinefrom giving away the airline• Schedules and complexes are productSchedules and complexes are product• Crews think the airline ends in cockpitCrews think the airline ends in cockpit

Strategic planning and global Strategic planning and global optimization are impossible in this optimization are impossible in this environment environment


How Has Airline OR SucceededHow Has Airline OR Succeeded

Very optimized in certain silosVery optimized in certain silos• Crew SchedulingCrew Scheduling• Fleet assignmentFleet assignment• Yield managementYield management

Savings and increased revenue has Savings and increased revenue has been very realbeen very real

Fleet assignment has as objective Fleet assignment has as objective revenue - cost of operating airplanesrevenue - cost of operating airplanes

YM is much needed and big successYM is much needed and big success


Where to Find the Next Success?Where to Find the Next Success?

Decision tools in operations and Decision tools in operations and recovery from disruptionsrecovery from disruptions• Begins to integrate over time and can Begins to integrate over time and can

feed back into robust planningfeed back into robust planning

Schedule developmentSchedule development• Is driven by markets and strategyIs driven by markets and strategy

Integrate YM and FAM over timeIntegrate YM and FAM over time• Initiated at Sabre to better model Initiated at Sabre to better model

revenue but is not being done over timerevenue but is not being done over time

Look at entire crew processLook at entire crew process


Operations and RecoveryOperations and Recovery

Integrated recovery must consider Integrated recovery must consider aircraft, airports, crews, passengersaircraft, airports, crews, passengers

Environment is dynamic and Environment is dynamic and uncertainties abounduncertainties abound

What about robust planning?What about robust planning?• Integration over timeIntegration over time• In reality is a huge stochastic programIn reality is a huge stochastic program

• Requires model for integrated recovery Requires model for integrated recovery

SimAir is a tool to study operations, SimAir is a tool to study operations, recovery, and robust planningrecovery, and robust planning


Schedule DevelopmentSchedule Development

Is driven by markets and strategyIs driven by markets and strategy Was considered too important to Was considered too important to

turn over to a mere modelturn over to a mere model Tremendously important and Tremendously important and

complicated problem involving:complicated problem involving:• Market presenceMarket presence• Structure of complexesStructure of complexes• Fleets and crewsFleets and crews


Integrate YM and FAMIntegrate YM and FAM

Has been initiated at Sabre to better Has been initiated at Sabre to better model revenue model revenue

One way of doing it over time is DOne way of doing it over time is D33

• It means demand driven dispatchIt means demand driven dispatch• Involves reassigning crew compatible Involves reassigning crew compatible

sub-fleets so as to better match sub-fleets so as to better match capacity to high revenue demandcapacity to high revenue demand

• Is a close to departure FAM Is a close to departure FAM

Ira Gershkof conceived JITIra Gershkof conceived JIT• Industry was not ready for itIndustry was not ready for it


Crew: The Rest of The Story Crew: The Rest of The Story

Manpower planning, conflicts, Manpower planning, conflicts, overtime flying, and reservesovertime flying, and reserves

In US airlines as high as 30% of the In US airlines as high as 30% of the pilots may be on reserve bid linespilots may be on reserve bid lines• Actual flying is about 50% of usualActual flying is about 50% of usual• Of that flying, more than half is to cover Of that flying, more than half is to cover

conflicts and as little as 1/3 to flying to conflicts and as little as 1/3 to flying to cover disruptionscover disruptions

The “sleeper” issue: fatigue and The “sleeper” issue: fatigue and circadian rhythmcircadian rhythm


Crew: The Rest of The Story Crew: The Rest of The Story

Milind G. SohoniMilind G. Sohoni T. Glenn BaileyT. Glenn Bailey Operations Research, Delta Technology Inc.Operations Research, Delta Technology Inc. Atlanta, GA 30354-1801Atlanta, GA 30354-1801

Helen CarterHelen Carter Crew Resource Analysis, Delta Air Lines Inc.Crew Resource Analysis, Delta Air Lines Inc. Atlanta, GA 30320Atlanta, GA 30320

Ellis L. JohnsonEllis L. Johnson Georgia Institute of Technology ISyEGeorgia Institute of Technology ISyE Atlanta, GA 30332-0205Atlanta, GA 30332-0205


1 - 5 years1 - 5 years 3 - 9 mos.3 - 9 mos. 1 - 1.5 mos.1 - 1.5 mos. DailyDaily

Delta Technologya wholly owned subsidiary

of

Ÿ B lock T im e F orecas tingŸ S chedu le G enera tionŸ F lee t A ss ignm en t

Ÿ N etw ork D eve lopm en tŸ F ligh t P ro fitab ilityŸ M arke t A na lys is

Long Range CrewManpow er Planning

Long Range Planning

Ÿ A irc ra ft R o ta tionŸ S lo t M anagem en t

Ÿ Crew PairingOptim ization

Ÿ Bidline GenerationŸ Bidding and Line

Aw ards

Ÿ V aca tion A ss ignm en tŸ V acancy B idd ing and

A w ards

Daily Operations

Ÿ Involuntary andVoluntary Flying

Ÿ Crew Recoveryduring IROPs

Ÿ A irc ra ft R ecove ryŸ P assenge r R ecovery

Pre-month Planning

Ÿ Initial TrainingScheduling

Ÿ Vacation Scheduling

Ÿ Recurrent TrainingScheduling

Ÿ Flight Instructor Scheduling

Airline Business Process -Time Line

Crew Planning Overview Crew Planning Overview


Integrated Crew Planning

Crew Manpower Planning and Crew StaffingCrew Manpower Planning and Crew Staffing

Crew Pairing Bidline

Generation Bidding

TrainingScheduling

Sim Utilization New Hires

Vacation Scheduling

Daily Operational UtilizationDaily Operational Utilization

Expected UtilizationExpected Utilization

Feed B

ack

to L

ong R

ang

e P

lannin

gFe

ed B

ack

to L

ong R

ang

e P

lannin

g

Operational PlansOperational Plans

CONFLICTSCONFLICTS


Crew Pre-Month Planning

Crew Pairing Optimization

Regular/Reserve Bidline Generation

Bidding and Conflict Resolution

Vacation Scheduling

Initial Training Scheduling

Tim

eTim

e

Recurrent Training Scheduling Flight Instructor Scheduling

Month of OperationMonth of Operation

Supplemental Regular Lines


Reserve Demand

Net Reserve Net Reserve DemandDemand

(1) Vacation Conflict(1) Vacation Conflict (2) Initial Training Conflict(2) Initial Training Conflict (3) Transition Conflict(3) Transition Conflict (4) Recurrent Training (4) Recurrent Training ConflictConflict

(1) Weather Disruptions(1) Weather Disruptions (2) Aircraft Maintenance(2) Aircraft Maintenance (3) Sick Leave(3) Sick Leave

Open Time TripsOpen Time Trips

Voluntary FlyingVoluntary Flying Premium FlyingPremium Flying ReservesReserves

Pre-Month PlanningPre-Month Planning Irregular OperationsIrregular Operations

Could be up to 25%Could be up to 25%total trips builttotal trips built

Higher ReserveHigher ReserveAvailability DesiredAvailability Desired

Could cover 10% of the open trip flying


Integrated Planning - Function & Time

Integrated Planning - Function & Time

Integrated planning breaks down silos Integrated planning breaks down silos

Integrated planning over time horizon?Integrated planning over time horizon?

• Current crew planning ends with bid linesCurrent crew planning ends with bid lines

• leaves it and goes on to the next monthleaves it and goes on to the next month

Is feedback on operations built in?Is feedback on operations built in?

Isn’t planning a multi-stage problem?Isn’t planning a multi-stage problem?

• Shouldn’t the plan at any stage depend on Shouldn’t the plan at any stage depend on

options and scenarios at the next stage?options and scenarios at the next stage?

• What is value of making decisions later?What is value of making decisions later?


Overview – Crew Schedule ProcessOverview – Crew Schedule Process

Manpower planning 2-3 years out Manpower planning 2-3 years out

Pairings and bidlines for monthly planPairings and bidlines for monthly plan

Swaps, conflicts, and transitionSwaps, conflicts, and transition

• Pairings fall out and must be reassigned Pairings fall out and must be reassigned

Operations involves rescheduling Operations involves rescheduling

disrupted pairingsdisrupted pairings

• Voluntary overtime flying and reservesVoluntary overtime flying and reserves


Manpower PlanningManpower Planning

Manpower planning 2-3 years out Manpower planning 2-3 years out

Pilots and positions by fleet and basePilots and positions by fleet and base

• Involves transitions – who requests moveInvolves transitions – who requests move

• Includes new hiresIncludes new hires

• Sensitive to changes in fleet plan Sensitive to changes in fleet plan

Vacation plan and recurrent training Vacation plan and recurrent training

Reserve staffing level included Reserve staffing level included


Pairings and BidlinesPairings and Bidlines

Pairings and bidlines during month Pairings and bidlines during month

Pairings are in Pairings are in fleetfleet and for and for crew basecrew base

Swaps take place Swaps take place

Vacations, recurrent training, and Vacations, recurrent training, and

transition conflicts cause transition conflicts cause pairings to pairings to

drop outdrop out of the bidline of the bidline

In operations, In operations, pairings may be pairings may be

disrupted or dropped outdisrupted or dropped out


Swaps, Conflicts, and TransitionSwaps, Conflicts, and Transition

Conflicts: vacation, training, transitionConflicts: vacation, training, transition

Uncovered pairingsUncovered pairings go to reserves go to reserves

• Reserve crew bidlinesReserve crew bidlines already assigned already assigned

• Reserves used in this way will be Reserves used in this way will be

unavailable in operationsunavailable in operations

Recurrent training may be assigned by Recurrent training may be assigned by

the airline after bidlines are assignedthe airline after bidlines are assigned

• Pairings that drop out are thus controlled Pairings that drop out are thus controlled


Operations and RecoveryOperations and Recovery

Operations involves rescheduling for disruptions Operations involves rescheduling for disruptions

Reserves cover parts of pairings (usually ends) Reserves cover parts of pairings (usually ends)

and whole pairings typically from unprotected and whole pairings typically from unprotected

bidlines bidlines

However, voluntary overtime takes precedence However, voluntary overtime takes precedence

Premium overtime may be only option if no Premium overtime may be only option if no

reservesreserves

Crew recovery can swap crews (move-ups), cover Crew recovery can swap crews (move-ups), cover

part of a pairing with reserves, or send home part of a pairing with reserves, or send home


Reserve Crew PlanningReserve Crew Planning

Spans the planning timelineSpans the planning timeline

Manpower planning allows for Manpower planning allows for

reserves ~25% reserves ~25%

Reserve bidlinesReserve bidlines assigned before assigned before

conflicts and need to cover conflicts and need to cover conflictsconflicts

and and disrupted pairingsdisrupted pairings

• Reserves needed less if Reserves needed less if recoveryrecovery protects protects

future trips future trips


Scheduling Recurrent TrainingScheduling Recurrent Training

Between planning and operationsBetween planning and operations

Determines conflicts Determines conflicts

• Schedule reserves to cover pairings Schedule reserves to cover pairings

• Schedule deadheads, classes, and Schedule deadheads, classes, and

simulators for continuing qualification simulators for continuing qualification

Mainly feasibility constraints but costsMainly feasibility constraints but costs

• Trips that fall outTrips that fall out

• Instructors to fill in on simulators Instructors to fill in on simulators


Crew Scheduling IssuesCrew Scheduling Issues

Modeling and optimization have been Modeling and optimization have been

successfully applied to:successfully applied to:

• Pairing optimization Pairing optimization

• Bidlines and rosteringBidlines and rostering

The rest of the storyThe rest of the story

• Manpower planning and trainingManpower planning and training

• Reserve crew bidlines and utilizationReserve crew bidlines and utilization

• Operations and recoveryOperations and recovery


More of the Rest of the StoryMore of the Rest of the Story

Numbers of pilots for future fleets and Numbers of pilots for future fleets and

schedules schedules

Quality of life issues Quality of life issues

Fatigue: circadian rhythm in planning Fatigue: circadian rhythm in planning

and recoveryand recovery

• Weekly patterns of days offWeekly patterns of days off

““Overtime” flying and reserve crewsOvertime” flying and reserve crews

Crew compatibility, cabin crew, weeklyCrew compatibility, cabin crew, weekly


FatigueFatigue

Research shows shortening the 24-Research shows shortening the 24-hour clock causes fatigue, slower hour clock causes fatigue, slower reactions, and insomnia reactions, and insomnia • Even for domestic fleets due to red-eyesEven for domestic fleets due to red-eyes

• Also tendency for short duty + short restAlso tendency for short duty + short rest

• Jet lag is a familiar but serious problem Jet lag is a familiar but serious problem • Even international business travelers suffer Even international business travelers suffer

insomniainsomnia

Maintaining sleep window in planningMaintaining sleep window in planning• E.g. 9pm - 4 am in pairing and bid linesE.g. 9pm - 4 am in pairing and bid lines

Operations can lead to longer dutiesOperations can lead to longer duties


Robust Scheduling Robust Scheduling

Crew-compatible fleet assignment Crew-compatible fleet assignment • Pure city at small spokesPure city at small spokes

• Helps with crew, maintenance, plane swapsHelps with crew, maintenance, plane swaps

• Critical presence and mid-day breakoutsCritical presence and mid-day breakouts

Crew basesCrew bases• Pure city gives more swapsPure city gives more swaps

• Mainly for overnighting crewsMainly for overnighting crews

Critical presence of crew-compatible Critical presence of crew-compatible fleets at complexesfleets at complexes

• Plane swaps for D3 and crew swaps Plane swaps for D3 and crew swaps


Solving These ProblemsSolving These Problems

Many, many column LPs and IPsMany, many column LPs and IPs Station decomposition for FAM Station decomposition for FAM Non-negative least squares methods Non-negative least squares methods

• Primal-dual subproblem methodsPrimal-dual subproblem methods• Dual-primal and d-p subproblem Dual-primal and d-p subproblem • Combined objective function methodsCombined objective function methods• Network flow specialized methodNetwork flow specialized method

Use of parallel clusterUse of parallel cluster• MARS code for stochastic problems MARS code for stochastic problems • Parallel SimAir (so far just a dream)Parallel SimAir (so far just a dream)


SimAir PurposeSimAir Purpose

Allows running different schedules Allows running different schedules

and different recovery algorithms and and different recovery algorithms and

checking performancechecking performance

Allows testing different planning Allows testing different planning

models with different recoverymodels with different recovery

Improvement from better recoveryImprovement from better recovery

• Initial recovery was pushbackInitial recovery was pushback

• Cancel short cycles, reroute planesCancel short cycles, reroute planes


National University of SingaporeNational University of Singapore

Temasek Project in Air Logistics and Temasek Project in Air Logistics and TransportTransport• Air logistics business process and ITAir logistics business process and IT• Air cargo terminals - efficient operation Air cargo terminals - efficient operation • Multi-commodity flow and cargo flowMulti-commodity flow and cargo flow• Crew scheduling in A-P long-haul Crew scheduling in A-P long-haul

environment environment • Air cargo YM, booking, and trackingAir cargo YM, booking, and tracking• SimAir including passenger and cargo SimAir including passenger and cargo

reroutingrerouting

Documents

Optimization in Airline Scheduling : Successes, Challenges, and New Directions Ellis L. Johnson IMA Workshop on Travel and Transportation Nov 14, 2002