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R E D U C E D

Maintenance Costs

FOR THE737-600/-700/-800/-900 FAMILY OF AIRPLANES

STEVEN HAYES

REGIONAL DIRECTOR

MARKETING – AIRPLANE ECONOMICS

BOEING COMMERCIAL AIRPLANES

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Reducing fleet maintenance costs is a key issue affecting any airline’s bottom line.

During the development of the 737-600/-700/-800/-900 family of

airplanes, Boeing initiated a program tolower the airplanes’ overall airframe

maintenance costs by as much as 15 percent,relative to the earlier 737-300/-400/-500family. The program, called Maintenance

Cost Reduction by Design, included participation by 21 airlines from around

the world and provided extensive in-service experience upon which to draw.

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MAINTENANCE COST SAVINGSACHIEVED THROUGH AIRPLANEDESIGN

To reduce maintenance costs by 15 percent, the 737-600/-700/-800/-900airplane program leveraged potentialsavings from three areas (fig. 2):

■ Airplane design improvements.

■ Scheduled maintenance programimprovements.

■ Improved maintenance documentsand training.

Airplane design improvements.During the planning stages of the 737-600/-700/-800/-900 program, abasic design philosophy was followedknown as value-added technology.Essentially, the design team made discretionary design changes to the air-plane only if they offered a better valueto the airline customer in terms ofsuperior economical performance, whichincluded lower maintenance costs.

The design team’s approach focusedon a number of key goals: using digi-tal instead of analog systems, loweringsystem part counts, having more com-mon parts among models, simplifyingsystem designs, improving corrosionprotection, enhancing fault identifica-tion, and providing better access toparts for maintenance.

Many of the specific design changeswere directly attributable to airline customer inputs (fig. 3). Examples ofdesign changes that enhanced maintain-ability include the following:

■ A completely new wing with double-slotted continuous span flaps with 30 percent fewer parts for improvedmaintainability.

■ Wing leading-edge panels designedfor easier access by maintenancecrews.

■ A simplified main landing gearassembly for enhanced maintain-ability and a 30 percent reduction in brake change time.

■ Improved access to line replaceableunits (LRU) and components, maxi-mum use of quick-disconnect line

1A major concern of airlines in today’s com-petitive business environment is lowering their airplane-related operating costs (AROC). Thesecosts directly affect an airline’s cash flow andultimately its financial health. An airline’s totalAROC falls into six categories: flight and cabincrews, fuel, maintenance, navigation and land-ing fees, ownership and spares, and deprecia-tion (fig. 1).

A sizable part of AROC is related to airplanemaintenance. Although maintenance costs, as a percentage of AROC, will vary — dependingon such factors as airplane type, average flightsegment length, and airplane age — typicalmaintenance costs range from approximately 10 to 20 percent of AROC. These percentagesmay seem somewhat small at first glance, but they represent significant sums of money.Large carriers, for example, have maintenancebudgets in excess of $1 billion.

From the early stages of airplane develop-ment, the 737-600/-700/-800/-900 team strove

to lower maintenance costs and give the airlinesthe airplane with the most competitive operatingcost possible. The target for overall reduced air-frame maintenance costs was 15 percent, relativeto the earlier 737-300/-400/-500 family, whichhad set the standard for lowest operating costs in its class. The 15 percent reduction was consid-ered an attainable, though challenging, goal thatwould significantly increase the airplanes’ value to airline customers.

Although airplane type is a major determinantof an airline’s maintenance cost, it is not the only one. Ultimately, the realized improvement inmaintenance costs for a particular airline also will be a function of other factors including thatairline’s maintenance policies and procedures, operating parameters, and labor market.

This article discusses

1. Maintenance cost savings achieved through airplane design.

2. Actual in-service savings realized by airlines.

AIRPLANE-RELATED OPERATING COSTS

FIGURE

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fittings, and improved ground-support equipment, which reducedengine removal and installation timeby more than 50 percent.

■ A redesigned auxiliary power unit(APU) for improved access andmaintainability.

■ Single-point-of-service vacuum lavatories that significantly lowerservicing time at the gate.

■ Redesigned electronics and equip-ment bays for improved access to remove and repair components.

■ New CFM56-7 engines for 15 percentlower maintenance costs when com-pared with the equivalent maximumthrust rating of the CFM56-3 engine.

■ Improved consistency and usability of built-in test equipment (BITE) user interfaces to reduce time anderrors during airplane system troubleshooting.

Another vital aspect of the designimprovements involved corrosion pre-vention. This included increased sealingbetween critical parts, additional pro-tective finishes such as primer appliedto detailed parts, and improved drainagethat included a door threshold gutter redesign. These changes were made notonly to reduce maintenance costs during

AREAS OF POTENTIAL MAINTENANCE COST SAVINGS

FIGURE

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an operator’s heavy maintenance checksand structural inspections but also toextend the service life of the airplane.

Other specific design improvementsincluded incorporation of a digital cabinpressure control system (CPCS) in lieu of an analog system, which reduced thenumber of mechanical parts. The CPCSredesign also added the capability forBITE to more quickly identify problemareas, such as individual LRUs orwiring anomalies. This reduced me-chanics’ fault-isolation troubleshootingtime. Also, the BITE capability in-creased the systems’ mean time betweenunscheduled removals (MTBUR) byapproximately 75 percent and reducedthe initial provisioning costs for spareparts by as much as 45 percent. Byincreasing the MTBUR, componentsremain on the airplane for a longertime, thereby reducing the inventoryrequirement for additional spare parts.

Another design improvementinvolved integrating the stall manage-ment and yaw damper computers into asingle unit. Eliminating the yaw dampercomputer reduced spares provisioningand maintenance costs and improvedsystem reliability.

In the flight deck systems, reliability,redundancy, and BITE were enhanced to make the systems less susceptible to errors (i.e., more fault tolerant) and

4.5% AROC

5.0% AROC

7.2% AROC

Maintenance is commonly defined as direct airframe, direct engine, and maintenance overhead. It includes labor and material but not spares or ground-support equipment.

Ownership and spares

Flight/cabincrew

Fuel

Depreciation

Direct airframe

Direct engine

Direct overhead

Maintenance17%

Navigation and landing fees

improve their mean time between fail-ures (MTBF) by 62 percent. Systemoptimization, in part through improvedfault tolerance capability and MTBF,allowed the design team to reduce thetotal number of parts in the flight decksystems by 23 and provide the airlineswith increased dispatch reliability.

For these and many other designimprovements, a detailed analysis wasdone at the system level to ensure thatthe comprehensive cost reductionsreached the 15 percent reduction target.Engineers used three-dimensional com-puter (digital) modeling and employedhuman factors principles for all designwork. The use of digital modeling provided the means to perform detailedmaintainability analysis. The approachemploys maintenance access solids(MAS), which are solid design elementsused to reserve space for accessing,removing, and installing parts. The digital computing data sets verified that airplane components could be readilyaccessed and removed during the designphase and in-service maintenance.

The human interface also was digitally

defined as part of the MAS designapproach. The MAS design was used toexamine the areas of space, or envelopes,needed to conduct various types ofinspections: borescope envelopes, accessenvelopes for tool sweeps and the removaland installation of components, andvisual envelopes for line-of-sight accessto inspect components. Each envelopewas verified using detailed three-dimensional digital reviews to ensure ahigh level of airplane maintainability.

Scheduled maintenance programimprovements.The reduced scheduled maintenanceactivities in the 737-600/-700/-800/-900airplanes are the result of designchanges at the airplane system level and an integration of industrywidemaintenance practices developed by theMaintenance Steering Group (MSG), acommittee comprising airframe manu-facturers, airlines, and U.S. FederalAviation Administration representatives.The latest iteration of these maintenancepractices is MSG Level 3, Revision 2(MSG-3 Rev. 2).

Design changes. Design improvementsat the airplane system level reducedscheduled maintenance activities byextending maintenance intervals oreliminating tasks altogether. This provides the airlines with maximumflexibility and use of both the airplaneand airline maintenance resources. The result is lower costs to airlines forscheduled airframe maintenance checks,which account for 18 to 20 percent ofoverall maintenance costs.

One example of reduced scheduledmaintenance by system redesign is the trailing-edge flap-drive system. The system was redesigned to last the life of the airplane, with no anticipated overhauls, by switching from an oil-filledreservoir to grease lubrication and using100 percent corrosion-resistant steelmaterial in lieu of the original chromeplating to eliminate corrosion wear. As aresult, the scheduled maintenance inter-vals were extended from 20,000 flightcycles to 75,000 flight cycles.

The air-conditioning packs also were redesigned, based on in-serviceexperience and customer inputs, which

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EXAMPLES OF DESIGN CHANGES TO THE 737-600/-700/-800/-900

FIGURE

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accumulated operator experience, andreliability data on similar airplane systems and parts.

In addition, the maintenance tasks for the 737-600/-700/-800/-900 were not prepackaged into more extensivemaintenance inspections performed on a periodic basis (i.e., letter checks such as A-checks and C-checks). Not tyingtasks to a predefined major maintenancecheck allows airlines to more efficientlyimplement tasks into their maintenanceprograms based on their specific air-plane use and operation. The specificmaintenance intervals for individualtasks provided the airlines with greaterflexibility in the establishment of theirmaintenance programs.

Improved maintenance documents and training.Improved manuals and training alsowere integral to the overall plan toreduce maintenance costs by 15 percent.These improvements involved faultreporting and isolation manuals, thestructural repair manual, digital format-ting, and training.

Fault reporting manuals (FRM) and fault isolation manuals (FIM).In-service experience on earlier airplanemodels had shown no standardized,consistent method of isolating andreporting faults. The troubleshootinginformation for mechanics was placedthroughout the maintenance manual.These factors resulted in higher numbers of components being removed

indicated that the 737-300/-400/-500two-wheel (turbine and compressor)ball-bearing air-cycle machine and theram-air-system turbofan required oilservicing. The air-conditioning packswere redesigned to a three-wheel (turbine, compressor, and fan) air-bearing cycle machine, which requiredno scheduled maintenance service.

Design improvements in the airplane’selectrical power system also reducedscheduled maintenance activities. Basedon in-service information, designersswitched from a 50- to a 90-kVA electrical power system and selected an integrated drive generator in placeof the constant speed drive to improveoverall system reliability and extendthe intervals between system oil-levelchecks and filter changes.

Maintenance planning document. The737-600/-700/-800/-900 team analyzedscheduled maintenance activities usingthe MSG-3 Rev. 2 process along withsupporting in-service 737-300/-400/-500scheduled maintenance data. MSG-3Rev. 2 is the same process used on the777. The process uses airplane systemanalysis to identify system redundan-cies, system reliability, past system safety records, and the system safetyfactor. This analysis leads to a more efficient maintenance program.

The 737-600/-700/-800/-900 main-tenance program had a reduction inscheduled maintenance activities com-pared with a 737-300/-400/-500 mainte-nance program in 1998. Historical datafrom the 737-300/-400/-500 mainte-nance program served as a baseline andwere used to improve the 737-600/-700/-800/-900 maintenance program.

The MSG-3 Rev. 2 analysis focusedon the following areas: applicable andeffective tasks, clear distinction betweensafety and economic tasks, system-levelanalysis of reliability and redundancycapabilities, integration of corrosion prevention and control and structuralinspections, and preclusion of systemstasks by zonal inspection where applic-able. The analysis optimized maintenanceintervals for each task based on designand manufacturing improvements,

by maintenance crews that later werefound not to have any faults (i.e.,no-fault-found component removals).

To remedy the situation, new FRMsand FIMs were written to the latest Air Transport Association (ATA) specification, which is based on theimproved understanding of the main-tenance crews’ informational needs. These manuals made for more preciseidentification of faults between flightand maintenance crews, reduced troubleshooting lookup time, and standardized reporting of faults.These manuals, combined with flight crews’ advance notification to groundcrews of incoming faults, reduced no-fault-found component removals.

Structural repair manual (SRM).In conjunction with improved FRMsand FIMs, the SRM was enhanced.Developed in accordance with the newATA specification, the SRM reflects the updated ATA chapter designationsfor each airplane system and subsystem.The SRM includes more detailedstructural identification, resulting in lessresearch time for mechanics. It also con-tains more comprehensive, user-friendlyreports and sets larger allowable damagelimits on systems and parts. In addition,the SRM reduces or eliminates areas of the airplane once considered critical to airplane safety and performance (i.e.,critical zones). The SRM also documentsmore temporary repairs and extends temporary repair time intervals.

New wing with simplified leading- and trailing-edge slats

Simplified landing gear design features; brake changed in 30% less time

New corrosion-resistant materials and treatments (throughout airplane)

New design-build process and tools (throughout airplane)

New interior

New engines, improved access, remove and replace in half the time

Simplified APU installation

Improved rudder and elevator

Vacuum lavatories

New AC electrical system

Improved avionics, fewer LRUs

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Digital format. Data from publishedstudies and airline surveys showed thatmechanics on average spent 25 percentof their time researching troubleshootingand repairs in the FIMs and aircraftmaintenance manuals (AMM). Thisactivity was hampered because docu-mentation only was available on paperor microfilm, where copy quality attimes was poor. The manual process ofturning pages or advancing microfilmcartridges to locate the desired informa-tion was time consuming. Also, therewas no online access to engineeringdrawings. To address this issue, Boeingdocuments were made available in adigital format retrievable from a CD-ROM.Software products such as the PortableMaintenance Aid (PMA) allow quickaccess to data through hyperlinks whilefeaturing enhanced search and naviga-tion capabilities. Online access to main-tenance documentation and engineeringdrawings and documents was madeavailable through the MyBoeingFleet.comweb site to reduce the time mechanicsspent researching, viewing, and printingdocuments.

Training. The 737-600/-700/-800/-900team also made improvements in main-tenance training, in part to take fulladvantage of the enhanced service man-uals. Mechanics are trained more effi-ciently using new maintenance-orientedcourses and line-oriented simulator sessions. The training uses the samesystems description section contained in the AMM. Classes are conductedthrough computer-based training,which is student paced, more frequentlyrevised, and more interactive. Coursematerials were developed jointly withmajor suppliers and can be used forrecurring training or as a refresher.

ACTUAL IN-SERVICE SAVINGSREALIZED BY AIRLINES

Early indications are that the 737-600/-700/-800/-900 family of airplanes is meeting the projected goal of a 15 percent reduction in airframe main-tenance costs. With airlines moving into their second and third years of 737-600/-700/-800/-900 operation,

actual cost data are becoming available.(First-year operators are excluded frommaintenance cost analysis because an airplane’s initial introductory period can skew results.)

Boeing performed its analysis usingactual airline maintenance costs. Wherepossible, airlines were selected that operated both the earlier 737 models and737-600/-700/-800/-900s to normalizelabor rates, line accounting practices, andoperating efficiencies, and to give a truecomparative picture of costs.

Looking at cost data reported to the U.S. Department of Transportation,737-800 total airframe maintenancecosts were nearly 10 percent lower thanthose of the comparable 737-400 model(fig. 4a). On a per-seat basis, the 737-800maintenance costs were 18 percentlower than those of the 737-400 (fig. 4b).Assuming an annual airplane utilizationof approximately 3,000 flight-hours,the cost saving equates to approximatelyUS$50,000 per airplane per year.

Reviewing similar data reported tothe International Air TransportationAssociation’s Production Performance

Measurement group (IATA/PPM),the maintenance costs for the 737-700were nearly 14 percent lower than that of the comparable 737-300 (fig. 5).Because the 737-600/-700/-800/-900airplanes were designed as a family,they share the same design principles and virtually identical systems. Assuch, this comparison analysis can be applied to all new 737 models.

A recent Maintenance CostProtection Program (MCPP) reviewwith a European operator supports theprojected reduced costs. MCPP is aBoeing program offered to airlines tomitigate maintenance cost risk. The program provides for corrective actionby Boeing if airframe maintenancecosts exceed target levels. The speci-fied target level was approximately 15 percent lower than established costsfor the earlier 737 models operated by the airline. A detailed maintenancecost review by both Boeing and airline personnel was performed, whichincluded the material consumptionlist, rotable and repairable parts, andwarranty claims. The analysis revealed

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–10%

170

165

160

155

150

145

140

Comparative total adjusted airframe maintenance costs for U.S. airlines operating 737-400s and 737-800s based on U.S. Department of Transportation (Form 41) data.

737-400 (147 seats) 737-800 (162 seats)

Note: Values shown are for labor and material, are based on a weighted fleet average,and are adjusted for a common flight time, airplane aging, and contract costs.

U.S.

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per

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2000

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$167.60

$151.12

AIRFRAME MAINTENANCE COSTS — U.S. DOT

FIGURE

4a

–14%

145

140

135

130

125

120

115

110

Comparative total adjusted airframe maintenance costs for an airline operating both 737-300s and 737-700s based on International Air Transportation Association Production Performance Measurement data.

737-300 (126 seats) 737-700 (126 seats)

Note: Values shown are for labor and material and are adjusted for a common flight time, airplane aging, and contract costs.

U.S.

dol

lars

per

flig

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1999

dat

a)

$140.34

$121.30

AIRFRAME MAINTENANCE COSTS — IATA/PPM

FIGURE

5

that actual airframe maintenance costs were 18 percent lower than the established MCPP target for the one-yearperiod July 1998 to June 1999.

As airlines gain more experience withthe new 737 models, reported costs areexpected to be even lower than the pre-dicted cost reductions. It is important tonote that an airline’s actual maintenance costs will vary depending on its specific scheduled maintenance plan, contractmaintenance agreements, local laborrates, cost accounting practices, operatingenvironment, and airplane utilization.

–18%

Base

20

10

0

-10

-20

Comparative total adjusted airframe maintenance costs per seat for U.S. airlines operating 737-400s and 737-800s, based on U.S. Department of Transportation (Form 41) data.

737-400 (147 seats) 737-800 (162 seats)

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AIRFRAME MAINTENANCE COSTS , PER-SEAT BASIS — U.S. DOTFIGURE

4b

SUMMARY Maintenance activities forthe 737-600/-700/-800/-900family of airplanes were reduced through improved airplane design, reducedscheduled maintenance,enhanced training, improvedtechnical manuals, andimproved access to technicaldata. Reduced maintenanceactivities translate directly into lower operating costs forthe airlines.

The goal of the new air-plane program was to reduceairframe maintenance costsby 15 percent, relative to the737-300/-400/-500, which hadset the standard for lowestoperating costs in its class.

Although realized improve-ments in maintenance costs are a function of individual airlines’ maintenance policies,operating parameters, and labor market, early indicationsare that the 737-600/-700/-800/-900 overall is well alongtoward meeting the projectedcost-reduction goal.