: Aircraft engineering ( . .)

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: 1511


Method for the prediction of maintainability measures.

 

Jezdimir Knezevic

 

MIRCE Akademy, Woodbury Park, Exeter, EX5 1JJ, United Kingdom E- Mail: guest(a),mirceakademv. Com accepted July 17, 2007

Summary

 

The biggest challenge for the maintainability engineers is to accurately predict the maintainability measures of the future maintenance task at the early stage of system design, when changes and modifications are possible at almost no extra cost. This paper responds to this challenge by proposing a new metho- dology for the fast and accurate prediction of maintainability measures and the identification of resources needed for the successful completion of maintenance tasks considered. The proposed method is based on the maintainability meas- ures related to the comprising maintenance activities, and the maintenance ac- tivities block diagram which is applicable to maintenance task whose consisting activities are performed: simultaneously, sequentially, and combined. The me- thod presented could be successfully used at the very early stage of design when most of the information available is based on the previous experience, as well as, at the stage when design is completed and tests are performed in order to generate a maintainability data for the adopted configuration of the system.

Key words: Maintainability measures, maintainability prediction, simul- taneous sequential and combined maintenance task, maintainability block dia- gram, maintainability function, maintainability-based design.

 

1. Introduction

 

Experience tells us that the biggest opportunity to make an impact a main- tainability characteristic of any system is at the design stage. Consequently, the biggest challenge for the maintainability engineers is to quickly and accurately predict the maintainability measures of the future maintenance task at the early stage of design, when changes and modifications are possible at almost no extra

cost.

 

This is a very difficult prediction task due to group interaction between the sequences of activities within each task and the arrangements for the sharing of maintenance resources. Thus, the main objective of this paper is to present a methodology for the fast and accurate prediction of maintainability measures, at design stage, for the maintenance tasks of the future systems based on the cor- responding measures related to comprising maintenance activities.

The biggest challenge facing maintainability engineers is to predict main- tainability measures related to maintenance tasks of:

The future products at the early stage of design;

 

The benefit of modifications on existing item/system.

 

This paper responds to this challenge by proposing a new methodology for the fast and accurate prediction of maintainability measures and the identifi- cation of resources needed for the successful completion of maintenance tasks considered [jk]. The proposed method is based on the maintainability measures related to the comprising maintenance activities, and the maintenance activities block diagram which is applicable to maintenance task whose consisting activi- ties are performed: simultaneously, sequentially, and combined. The method presented could be successfully used at the very early stage of design when most of the information available is based on the previous experience, as well as, at the stage when design is completed and tests are performed in order to generate a maintainability data for the adopted configuration of the system.

Concept of maintenance task

 

According to Knezevic: "Maintenance task is a set of activities that need to be performed, in specified manner, by the user in order to maintain the func- tionability of the item/system. In accordance to the Maintenance Program De- velopment Document MSG-3, revision 2, published in 1993, maintenance tasks could be categorized in the following categories:

Servicing: replenishment of consumable fluids, cleaning, washing, painting, etc.;

Lubrication: installing or replenishing lubricant;

 

Inspection: Examination of an item against a defined physical stan-

 

dard;

 

 

General Visual Inspection performs to detect obvious unsatisfactory

 

conditions. It may require the removal of panels and access doors, work stands, ladders, and may be required to gain access;

Detailed Visual Inspection consists of intensive visual search for evi- dence of any irregularity. Inspection aids, like mirrors, special lighting, hand lens, horoscopes, etc. are usually required. Surface cleaning may be required, as well as elaborate access procedure;

Special Visual Inspection: an intensive examination of specific area us- ing special inspection equipment such as radiographic techniques, dye pene- trent, eddies current, high power magnification or other NDT. Elaborate access and detailed disassembly may be required;

Check: a qualitative or quantitative assessment of function;

 

Examination: a quantitative assessment of one/more functions on an item to determine if it performs within acceptable limits;

Operational: a qualitative assessment to determine if an item is fulfil- ling its' intended function. It does not require quantitative tolerances;

Restoration: perform to return an item to a specific standard. This may involve cleaning, repair, replacement or overhaul;

Discard: removal of an item from service.

 

It is necessary to stress that some resources are needed to facilitate the successful completion of the maintenance task. As the main task of these re-

sources is to facilitate the maintenance process they will be called maintenance resources, MR. The resources needed for the successful completion of every maintenance task, could be grouped into following categories:

■ Maintenance Supply Support, MSS: is generic name which includes all spares, repair items, consumables, special supplies, and related inventories needed to support maintenance process;

■ Maintenance Test and Support Equipment, MTE: includes all tools, special condition monitoring equipment, diagnostic and check-out equipment, metrology and calibration equipment, maintenance stands and servicing and handling equipment required to support maintenance tasks associated with item/system;

■ Maintenance Personnel, MP: required for the installation, check-out, handling, and sustaining maintenance of the item/system and its associated test and support equipment are included in this category. Formal training for main- tenance personnel required for each maintenance task should be considered;

■ Maintenance Facilities, MFC: refers to all special facilities needed for completion of maintenance tasks. Physical plant, real estate, portable buildings, inspection pits, dry dock, housing, maintenance shops, calibration laboratories, and special repair and overhaul facilities must be considered related to each maintenance task;

■ Maintenance Technical Data, MTD: necessary for check-out proce- dures, maintenance instructions, inspection and calibration procedures, overhaul procedures, modification instructions, facilities information, drawings and spe- cifications that are necessary in the performance of system maintenance func- tions. Such data not only cover the system but test and support equipment, transportation and handling equipment, training equipment and facilities;

■ Maintenance Computer Resources, MCR: refers to all computer equip- ment and accessories, software, program tapes/disks, data bases and so on, ne- cessary in the performance of maintenance functions. This includes both condi- tion monitoring and diagnostics.

■ On the other hand, it is important to remember that each task is per- formed in a specific work environment that could make significant impact on

the safety, accuracy and ease of task completion. The main environmental fac- tors could be grouped as follows:

■ Space impediment (which reflects the obstructions imposed on main- tenance personnel during the task execution which requires them to operate in awkward positions);

■ Climatic conditions (rain/snow, solar radiation, humidity, temperature, and similar situations, which could make significant impact on the safety, accu- racy and ease of task completion;

■ Platform on which maintenance task is performed (board of the ship/submarine, space vehicle and similar).

In order to illustrate the above concept, a very simple maintenance task will be used. It is related to changing a wheel on a small passenger car [4]. Thus, the objective of this task is to restore functionability of a faulty tyre by replacing wheel and tyre assembly with functionable one. Maintenance tasks, like this one, are specified in the user manual that is delivered to the user to- gether with the car, at the beginning of the operation of the system. Also, all maintenance resources needed for the successful completion of the tasks consi- dered to be performed by the user have been provided by the manufacturer of the car to the user as a part of overall package.

 

Boeing's approach to maintainability: chief mechanic

 

Jezdimir Knezevic

 

MIRCE Akademy, Woodbury Park, Exeter, EX5 1JJ, United Kingdom E- Mail: guest(a),miTceakademy.com

accepted August 23, 2007

 

Summary

 

Since, 1916 there has always been a Chief Pilot on every Boeing model. However, the 777, in recognition of the importance of the maintenance process to successful airline operation, is the first Boeing model to have a chief me- chanic. Thus, 1990 the history in aircraft maintenance was made when Jack Hessburg was named as the chief mechanic new airplanes, Boeing Commercial

Airplane Group, and became the first person in commercial aviation history to hold this position.

The major efforts of the chief mechanic were concentrated on making the airplane as mechanic friendly as possible. Consequently, he was proud to say that: 777 was built first for the line mechanic because he's the guy who signs the logbook and has to work in this tremendously time-driven environment". The paper describes that impact on the Chief Mechanic on the design of the aircraft from the maintenance process point of view, which has provided an ex- tremely high level of the inherent availability.

Key words: Chief mechanics, design for maintenance, maintainability impact on availability, built-in-test, accessibility, working together.

Maintenance managers want a clean gate-their report card in line main- tenance is based on having a clean gate and not having pigeons roosting at the airplane 'sfin.

J. Hessburg

 

Introduction

 

The latest commercial aircraft produced by Boeing Company, Boeing

 

777, has been designed for a useful life of 20 years. Boeing recommended and authorities of the FAA and JAA decided what maintenance is required to keep the airplane airworthy while in service. This involved defining what minimum scheduled and unscheduled maintenance must be performed in order to contin- ue flying. Scheduled maintenance is performed at certain intervals that are tied to number of flight hours, number of cycles (such as turn-on/off, take-offs and landings), etc. It consists primarily of inspections followed by maintenance, corrosion prevention, etc. Unscheduled maintenance is performed after a failure occurs. Depending upon the criticality of the failure, maintenance is accom- plished either before the airplane is returned to revenue service or within a spe- cified interval.

When total cost is considered over the life cycle, it is evident that the op- erating and support costs of the airplane will eventually exceed the initial acqui- sition cost. In order for Boeing to make the airplane attractive to the airlines, the engineers must include maintenance cost savings in the design. This was done

by increasing the reliability and maintainability. Increased reliability means fewer failures to fix. Increased maintainability means shorter maintenance times.

The figure of merit chosen to measure reduction of the follow-on costs was schedule reliability. In other words, how often will the airplane, or fleet of airplanes, meet the scheduled take-off time? The target for initial delivery is

97.8 \% with improvement to 98.8 \% after fleet maturity. In order for the air- plane to meet such a high number it must be inherently reliable. Double and triple redundancy is used in critical areas, allowing deferral of maintenance to an overnight time while the back-up system or systems keep the plane flying until that time.

Maintenance must be able to be completed during the scheduled down- times, whether it is during a 45-minute turnaround between flights or during an overnight. This implies having good means of identification and isolation of failures, as well as good access to the equipment. Innovative computer aided human models were used to prove good maintenance access without the use of expensive mock-ups. Fault identification and isolation is enhanced with the use of extensive built in testing with fault messages displayed on the computer screens available to the mechanics. Great care was used to ensure that mainten- ance messages are prioritized, understandable, do not give extraneous informa- tion, and are accurate. Accompanying fault isolation and maintenance manuals complement this information.

Reliability requirements were passed along to equipment manufacturers by specifying mean time between failures (MTBF) and target mean time be- tween unscheduled removals (MTBUR). The latter was estimated to be between

0.8 and 0.9 of MTBF, but could be verified only by service experience. It was recognized that unscheduled removals also counted the times that equipment is wrongfully removed because of the haste that a gate mechanic expends in trying to clear a fault during a 45-minute turnaround. The tendency is to replace the first suspected unit or groups of units in order to eliminate the obvious faults from the process. Thus the maintenance messages must give the right informa-

tion that avoids removing good items. Specifying both MTBF and MTBUR

 

mean both inherent reliability and field reliability could be controlled.

 

For fault tolerant systems or items, the reliability index was mean time between maintenance alerts (MTBMA). Maintenance alerts are the maintenance messages that are documented on equipment internal failure that did not imme- diately affect function.

Boeing also documented "lessons learned" data to record service history and feedback from other airplanes in order to avoid the same mistakes in the design of the new airplane. The airline representatives stayed in touch by at- tending design reviews and other meetings of concurrent engineering teams. From time to time their field mechanics visited Boeing to provide their inputs. The result was a working together relationship that benefited both sides and will result in increased reliability and maintainability.

 

1. The chief mechanic

 

The one page document signed by United Airline's Executive Vice Presi- dent Operations, and Boeing represented by Vice President of the new Airplane division and Executive Vice President of Sales stated that in order to launch on- time a truly great airplane we have a responsibility to work together to design, produce, and introduce an airplane that exceeds the expectations of flight crews, and maintenance and support teams and ultimately our passengers and shippers. From day one:

Best dispatch reliability in the industry

 

Greatest customers appeal in the industry

 

User friendly and everything works.

 

To accomplish their vision the general manager of New Airplane Products set out in some very bold directions that would shake up Boeing both from an organizational and technical standpoint. Some of the objectives are listed be-

low:

 

Customers-the airlines-would play an unprecedented role in shaping the plane's design.

Suppliers too, would work more closely than ever with Boeing and be challenged to achieve technical break-thoughts in every aspect, from avionics to controls to propulsion systems.

Boeing's internal organization would be transformed dramatically; the

 

777 would be the company's first jetliner ever to be designed and modeled al- most entirely on computers.

Since, 1916 there has always been a Chief Pilot on every Boeing model. However, the 777, in recognition of the importance of the maintenance process to successful airline operation, is the first Boeing model to have a chief me- chanic. Thus, 1990 the history in aircraft maintenance was made when Jack Hessburg was named as the chief mechanic new airplanes, Boeing Commercial Airplane Group, and became the first person in commercial aviation history to hold this position.

The major efforts of the chief mechanic were concentrated on making the airplane as mechanic friendly as possible. Consequently, he was proud to say that: 777 was built first for the line mechanic because he's the guy who signs the logbook and has to work in this tremendously time-driven environment Airliner (1995). The reason for this is very simple: Maintenance Managers want a clean gate-their report card in line maintenance is based on having a clean gate and not having pigeons roosting at the airplane's fin Beraer (1994).

Chief mechanic was continuously stressing that airlines are not in the air- plane business-they're in the transportation business. So airlines must decide how important to them is to go flying. According to Hessburg (1995) there are several ways that manufacturer can control that. Thus:

1. to design things that are extremely reliable,

 

2. to put in redundancy or fault tolerance,

 

3. to provide a system that, when breaks, is easy to fix.

 

However, in any case there is a need for trade-off because if everything is made easy to fix and everything highly reliable, and everything extremely re- dundant and fault tolerant, the very efficient airplane is created but no one can afford to buy it.

At Boeing, this was addressed through Design Build Teams, DBTs, whose members were from engineering, customer support, tooling, manufactur- ing, airlines, suppliers and similar Sabbagh (1996). This was fundamental de- parture from the traditional way where engineering would design the airplane, then throw it over the fence to manufacturing and say You figure out, how to build it. Manufacturing would throw it over the fence to customer service and say, Go support the customer. Then it's thrown over the fence to the airline-

OK, there's your airplane. This time there's no throwing it over the fence be- cause we're all in the same room together. The DBTs include three, four, five or more people from launch customer airline, and they stayed with us throughout the design process. Chief mechanic was always insisting to work with mechan- ics that perform maintenance tasks themselves. Hessburg reasoning is very sim- ple I don't need the chairman of the airline. I need the gay who works the ter- minal at gate B3 in Chicago. I like to stress that the mechanics were joy to work with Hessburg (1998).

 

2. Maintainability impact on availability

 

The majority of users state that the equipment availability is equally im- portant to them as its safety, because they cannot tolerate having equipment out of operation. There are several ways that designers can control that. One is to build items/systems that are extremely reliable, and consequently, costly. The second is to provide a system that, when it fails, is easy to restore it. Thus, if everything is made highly reliable and everything is easy to repair, the producer has got a very efficient system, which no one can afford to buy. Consequently, the question is how much a utility of the system is needed, and how much is one prepared to pay for it? For example, how important for the aircraft operator is to move its plain, when 300 fair paying passengers expect to leave the gate at 6.25 am? Clearly, the passengers are not interested what the problem is, or is that de- signer's error, manufacturers, maintainers, operators or somebody else's prob- lem. They are only interested in leaving at 6.25 am in order to arrive at chosen destination at 7.30 am. Thus, if any problem develops, it needs to be rectified as soon as possible.

Consequently, maintainability is one of the main factors in achieving a high level of operational availability, which in turn increases users or customers satisfaction.

Another area to be considered under maintainability is trouble-shooting the various modules within the allowed time. For the airlines, this is usually on- ly about one hour at the gate prior to its departure to the next destination. An easily manageable device is needed for the diagnostic of all different modules in order to determine their state and identify the failed one within it. Practice shows that false removals cost about the same as an actual failure when the component under investigation is removed and replaced. Reducing this would be a big cost saver. Device of such capabilities have been developed in aero- space industry, as a result of maintainability studies and research. For example, the design of Boeing 777 includes On-Board Maintenance System with the objective to assist the airlines with a more cost-effective and time-responsive device to avoid expensive gate delays and flight cancellations (Proctor, (1995) Journal Aviation Week & Space Technology).

 

3. Accessibility vs. maintainability

 

Hessburg (1995) stated that one of the common perceptions is that main- tainability is simply the ability to reach a component to change it. However, that is only a small aspect, according to him. Maintainability is actually just one di- mension of system design and a system's maintenance management policy. For example it could be required from the designer that only three screws are ac- ceptable on a certain partition panel in order to get speedy access inside. How- ever, this request has to be placed into larger context and it becomes a trade-off. If the item behind that panel needs to be checked once in every five-six years, it does not make much sense to concentrate much intellectual effort and spend project money on quick access. Thus, a lot of fasteners and connectors could be tolerated and the item may not be quickly accessible, but all of that has to be traded off against the cost and operational effectiveness of the system.

Additionally, decision-makers have to be aware of the environment in which maintainers operate. It is much easier to maintain an item on the bench,

than at the airport gate, war theatre, busy morning traffic, or any other result- oriented and schedule-driven environment. Thus, the trade off process has to take into account the operational environment and the significance of the con- sequences if the task is not completed satisfactorily, when the trade off is made. According to Hessburg, the chief mechanic of new airplanes from Boeing:

Maintenance managers want a clean gate, their report card in line main- tenance based on having a clean gate and not having pigeons roosting on the airplane's fin. So it is necessary to try to influence the design that way, and say,

here's what mechanics have to do at the gate [May 1994, Aviation Equipment

 

Maintenance].

 

4. Education of design engineers

 

According to Hessburg (1995) part of chief mechanic job is education. It is his task to make people aware of the environment in which mechanics oper- ate. It's not that designers are stupid, but they're inexperienced on this side of business. For example, they have to learn that there are different types of main- tenance. Anyone can maintain an airplane component or system on the bench. However, the gate environment is very much result and schedule driven. That's different type of maintenance Knezevic (1998).

Chief mechanic's further challenge is to remind design community that there are more regulations that govern maintenance than govern design. Conse- quently, even the most junior mechanic can keep an airplane in the hangar if something isn't right.

 

5. Built -In-Test-Equipment (BITE)

 

Built-In-Test-Equipment, commonly known as BITE, is a common term in the industry, referred to the part of the system that performs the maintenance function. In most digital avionics the equipment part of BITE includes some hardware and much software. For software purposes this is an important dis- tinction. The maintenance function, or BITE, is classed as non-essential for safety; unlike the fault detection function (BIT) which is an integrated part of a

system classed as essential or critical and must be certified to the same standard as that system.

According to Hessburg (Airliner/Jan-Mar 1995), good troubleshooting is nothing more than good deductive reasoning. At the center of that reasoning is a careful collection and evaluation of physical evidence. Unfortunately, many air- craft devices use computer chips to provide a function formerly fulfilled by substantial mechanical parts or subsystems. Consequently, troubleshooting, in the traditional sense of searching for physical evidence of failure, is hindered. You can't troubleshoot a computer chip by looking for physical evidence of failure. A broken chip does not look any different than a healthy one. Although it can be argued that broken chips occasionally make smoke, evidence of mal- function is seldom readily apparent. Broken chips do not leak, vibrate, or make noise. Bad software within them does not leave puddles or stains as evidence of its misbehavior. Ones and zeros falling off the end of a connector pin are diffi- cult to see. The Aircraft Information Management System (ATMS) is the first commercial avionics system based on Integrated Modular Avionics (IMA) technology and provides the Boeing 777 with a quantum leap forward into avionics capabilities. AIMS not only provides an unparalleled level of systems integration and functionability but also offers significant cost of ownership ben- efits to the airlines, particularly in the area of avionics maintenance.

While there are many significant maintenance enhancement features pro- vided by AIMS, the focus of this analysis is on AIMS platform BITE and its af- fect on hardware and software fault insulation. Although there is no question that the transition to digital avionics from analog systems, which began in the late 1970's, has resulted in dramatic improvements in equipment reliability, the ratio of unscheduled removals to confirmed failures (MTBUR/MTBF) has not varied significantly. In complex avionics computers it is not unusual to see MTBUR/MTBF ratios in the range of 0.33 to 0.50. Increased complexity and functional capabilities of avionics systems, combined with the corresponding increase in software contained in these systems, have contributed to keeping this ratio relatively unchanged. These same trends have made it increasingly

difficult to isolate anomalous software events from intermittent hardware faults and correlate to the resulting Flight Deck Effects (FDE).

According to Hessburg (1995) the Aircraft Information Management Sys- tem for the Boeing 777 airplane is developed around the principles of Integrated Modular Avionics (IMA). The concept behind Integrated Modular Avionics is to provide a system that allows the integration of multiple systems functions in- to a set of shared common hardware and software resources (e.g. processor, memory, I/O and operating system).

The AIMS system for the Boeing 777 is a highly integrated avionics ar- chitecture that incorporates the following airplane functions:

Flight Management,

 

Displays,

 

On Board Maintenance,

 

Airplane Condition Monitoring ,

 

Communications Management,

 

Information Management.

 

The development of AIMS focused on two primary objectives:

 

1. Enhance functionality and performance,

 

2. Reduce airline cost of ownership,

 

Enhanced system functionality with a corresponding decrease in cost per function is a result of the transition from a federated architecture to integrated modular avionics architecture. At the center of this IMA architecture is the AIMS cabinet. The AIMS cabinet provides a hardware and software platform that allows multiple avionics functions, such as FMS or Displays, to execute on shared resources. The AIMS cabinet allows functions of different system certi- fication levels (critical, essential and non-essential) to operate using shared pro- cessor, memory, I/O and software resources. Each function is allocated to one or more software partitions for execution in the AIMS cabinet. Undesired inte- raction between partitions is prevented through rigorous implementation of time and space partitioning.

The entire 777 AIMS system was designed from the onset with mainten- ance and airline cost of ownership in mind. Several examples of maintenance features are incorporated into AIMS are:

1) improved dispatch reliability through fault tolerant designs and de- ferred maintenance capabilities,

2) reduced spares cost through common hardware part number compo-

 

nents,

 

 

3) On-Board Maintenance System (OMS) with airplane-wide flight deck

 

effects correlation of BITE detected events,

4) the use of Liquid Crystal Display (LCD) flat panel displays to replace traditional CRTs resulting in reduced weight and power with increased reliabili- ty,

5) improved MTBUR/MTBF ration through isolation of hardware and software faults.

The key objective in the development of AIMS platform BITE was to provide a step increase in the MTBUR/MTBF ratio. The basic AIMS architec- ture and BITE software represent a major evolution in the ability to isolate and contain software anomalies and hardware faults and provides unparalleled transparent recovery from most single event upset events. AIMS lock step processing architecture and high integrity monitoring approach will detect vir- tually any hardware fault condition. While all of these items will contribute sig- nificantly to an improved MTBUR/MTBF ratio, the maximum benefit will not be achieved without proper training and co-ordination with airline maintenance personnel and changes to existing maintenance practices.

The recognition that software anomalies exist in complex avionics sys- tems, combined with an unprecedented ability to isolate software and hardware faults, requires a new though process on how to deal with these conditions in airline operations. New procedures are required to handle the case where the OMS correlates a flight deck effect to a software fault event. Under these condi- tions, removing a CPM will result in a no fault found condition since an actual hardware fault does not exist. Airlines and certification authorities will need to agree on practices that allow the maintenance personnel to download the BITE

memory for analysis and then run the return to service a test via the MAT. If the test passes, the LRM can be returned to service safely without the need to re- move and/or replace the LRM. Not only will this allow the airlines to keep the hardware in service but it will also provide very valuable data in isolating the root cause of any software anomaly that does occur. The only time an LRM should be replaced is when an actual hardware fault is logged in the BITE histo- ry and the maintenance procedure requires the LRM to be removed for that fault condition.

Hopkins (1995) stated that introduction into service of the Boeing 777 and introduction of AIMS represents a significant area of new technology intro- duction to airline maintenance operations. A certain level of familiarity and confidence needs to be established with maintenance and flight personnel be- fore significant gains are observed in the MTBUR/MTBF ratio. However, the AIMS platform BITE architecture and its extensive capabilities will lead to an extremely rapid product maturity cycle. The BITE architecture has already demonstrated tremendous benefits in the development cycle over previous BITE architectures and manufacturers believe that these benefits will very quickly carry over into airline operations.

 

6. Working together example

 

According to Johnson (1993) airlines have in the past introduced new air- craft to the fleet with small groups of aircraft engineers. These engineers were based at aircraft manufactures main assembly line to inspect the aircraft during final assembly. These inspectors would arrive just before final assembly began and would begin to inspect selected areas of the aircraft as they made their way along the assembly line. This late arrival to a new aircraft programmer meant that all the design work had been completed and the design frozen. In essence, the airline took delivery of an aircraft that the manufacturer believed to be what the customer required. This was not always the case and a request for a design change i.e. MC (Master Change) at this relatively late stage in productions could mean lengthy debate around:

1. The time the re-design would take; were there sufficient design staff available?

2. Could the re-design be incorporated into the production line, without disruption to the aircraft build process?

3. Could the airline's aircraft be modified before delivery?

 

4. Would a retro fit programmer have to be managed for those aircraft that could not be retro fitted before delivery?

These MCs (Master Changes) could be expensive, and the costs incurred would be passed on to the airline requesting the change.

There is also a certain amount of conflict between the aircraft designer and manufacturing processes and the aircraft maintenance engineer. For ease of assembly the manufacturer will install components at convenient times during assembly. This may be convenient to the manufacturer but to the airline engi- neer this can result in a relatively simple task taking several hours to complete. Hydraulic, pneumatic and electrical systems may need to be broken down for access. Other LRUs (Line Replaceable Units) may also need to be removed to gain the vital access to the LRU that needs to be replaced. All this time and ef- fort wasted, for once the units have been replaced and the system reconnected there will be hours and hours of function, leak checks and engines running to check the system that has been disturbed.

Hence, on previous aircraft introductions the line engineer has had little or no involvement with the design of the aircraft. Normally seeing the aircraft for the first time when it arrives at the airline's engineering base, the airline engi- neer finds out how maintainable this new aircraft type will be. However, the in- troduction of the chief mechanic, the Boeing has placed a great emphasis on maintainability, which came from the concept of Working Together and

Service Readiness Programmes. As a consequence, the aircraft line engineers became involved from the early days of the design process. With the design now closed, Working Together continues with the airline's involvement in the 777 Cycle Validation Programme.

For example, in 1992 British Airways based a team consisting of one se- nior manager, two principal airframe engineers, one principal propulsion engi-

neer and one maintainability engineer in Seattle, supported by a London based project team. This team could draw on representatives from the airline's line engineers to support 777 maintainability reviews. These reviews took place at the prime sub-contractors' sites. Some of the real life experiences regarding fol- lowing companies are cited below:

*Garrett: Auxiliary Power Unit 331-500: during the APU maintainabil- ity reviews, together with British Airways were representatives from: ANA (Ja- pan) and United Airlines from North America along with Boeing personnel at- tended these reviews.

The following is an extract form the August 1992 review.

 

Design concern regarding the Air Turbine Starter Motor and Air Turbine

 

Starter Control Valve:

 

a) When refitting the APU starter motor, it is difficult to align the aligning pin on the gearbox with the hole on the motor flange. An index mark on the flanges (gearbox and motor) would assist alignment.

b) When spare motors are supplied, a label should be fitted/tied to the shaft directing the maintenance engineer to service the motor with oil before fit-

ting.

 

c) On the starter motor and valve, different size clamps are used. All four V band clamps should be a common type and removable with the same size socket.

The following action were taken:

 

a) Index marks on the motor and adapter flanges are being incorporated to improve alignment.

b) Vendor has been directed to include a label indicating the motor re- quires oil servicing before installation.

c) On production parts, V-band clamps will be removable with same size socket. The clamps used during demonstration were not production hardware.

Also the changing of an aluminum alloy horoscope access plug to stain- less steel to prevent the plug threads being stripped when being removed, prior to horoscope inspection being carried out.

*General Electric: GE90: six British Airways line engineers were sent to Cincinnati on three occasions for maintainability reviews. Their task was the removal and replacement of all the power plant LRUs, using type designed tool- ing, to ensure maintainability. Also demonstrated was the fan case removal pro- cedure using specially designed GSE (Ground Support Equipment). Design changes have been incorporated in the engine stand, fan case removal dolly as recommended by the line engineers for ease of operation in the field. Several engine changes have been demonstrated using the GE90 mock-up engine, with the Boeing engine change bootstrap equipment and all the engine change hard- ware developed for the task.

*Boeing Company: the British Airways maintainability engineer, who is permanently based in Seattle, has had a continuing dialogue with the Boeing design team and the 777 Chief Mechanic Jack Hessburg. The issues cover High Intensity Radiated Field to the more mundane removal of the 13-kg Electronic Control Boxes in the overhead roof space.

*GE90 Engine 3000 cycle validation programme: carried out at the GE engine test facility at Peebles, Ohio. During this test, at planned engine shut downs all the GE90 LRUs and Boeing Engine Build Unit, EBU, line replacable units will be removed and replaced in accordance with maintenance manual procedures and IPC Illustrated Parts Catalogue, IPC. For defect rectification the trouble shooting charts and Fault Isolation Manual, FIM, will be used, along with the systems lock-out procedures in the Mandatory Minimum Equipment List, MMEL. The purpose of using all the manuals and tooling is to verify the correct maintenance practices and procedures.

In comparison to previous aircraft introduced, when the 777 was intro- duced in British Airways on its first revenue service the line engineer would have been involved in maintainability reviews, MMEL steering meetings, and reviewed a large proportion of the maintenance manuals. With the cycle valida- tion programme mechanics have gained invaluable hands-on experience and, with the constant Working Together focus on 777 Service Readiness, Brit- ish Airways was prepared to introduce the new generation 777 into worldwide airline operation.

7. Conclusion

 

In the conclusion, it is worth pointing out that there has always been a Chief Pilot on every Boeing model, but the 777 is the first Boeing model with a Chief Mechanic. This certainly illustrates the recognition of the importance of maintenance to successful air carrier operation. The history was made in 1990, when Jack Hessburg, took this job. His major efforts were concentrated on making the airplane as technician friendly as possible. Consequently, he proudly stresses that 777 was built first for the line mechanic because he's the guy who signs the logbook and has to work in this tremendously time-driven environment.

Both Communities, design and operation, considered the introduction of the chief mechanic job on the 777, so brilliant that the future new airplanes, build by Boeing, will benefit from this, now permanent position.

According to reports, the 777 were proved to be remarkably user-friendly, and just about everything does work. This is attributed largely to the fact that the chief mechanic was the integral part of the design and development process. Consequently, the final words in this paper should be left to the person who made it possible, namely Jack Hessburg, the chief mechanic:

Maintenance Managers want a clean gate-their report card in line main- tenance is based on having a clean gate and not having pigeons roosting at the airplane 'sfin.

*General Electric: GE90: six British Airways line engineers were sent to Cincinnati on three occasions for maintainability reviews. Their task was the removal and replacement of all the power plant LRUs, using type designed tool- ing, to ensure maintainability. Also demonstrated was the fan case removal pro- cedure using specially designed GSE (Ground Support Equipment). Design changes have been incorporated in the engine stand, fan case removal dolly as recommended by the line engineers for ease of operation in the field. Several engine changes have been demonstrated using the GE90 mock-up engine, with the Boeing engine change bootstrap equipment and all the engine change hard- ware developed for the task.

*Boeing Company: the British Airways maintainability engineer, who is permanently based in Seattle, has had a continuing dialogue with the Boeing design team and the 777 Chief Mechanic Jack Hessburg. The issues cover High Intensity Radiated Field to the more mundane removal of the 13-kg Electronic Control Boxes in the overhead roof space.

*GE90 Engine 3000 cycle validation programme: carried out at the GE engine test facility at Peebles, Ohio. During this test, at planned engine shut downs all the GE90 LRUs and Boeing Engine Build Unit, EBU, line replacable units will be removed and replaced in accordance with maintenance manual procedures and IPC Illustrated Parts Catalogue, IPC. For defect rectification the trouble shooting charts and Fault Isolation Manual, FIM, will be used, along with the systems lock-out procedures in the Mandatory Minimum Equipment List, MMEL. The purpose of using all the manuals and tooling is to verify the correct maintenance practices and procedures.

In comparison to previous aircraft introduced, when the 777 was intro- duced in British Airways on its first revenue service the line engineer would have been involved in maintainability reviews, MMEL steering meetings, and reviewed a large proportion of the maintenance manuals. With the cycle valida- tion programmer mechanics have gained invaluable hands-on experience and, with the constant Working Together focus on 777 Service Readiness, Brit- ish Airways was prepared to introduce the new generation 777 into worldwide airline operation.

 

AIRCRAFT MAINTAINABILITY

 

In order to understand the approach to maintainability assessment it is important to consider what maintainability assessment seeks to address. Optim- al maintainability is where maintenance can be conducted with simplicity and speed, ensuring the safety of all those who come into contact with the product and where it can be conducted within the capabilities and limitations of main- tenance personnel.

Striving for optimal maintainability within the design process ensures that the maintenance task is as simple as possible, considering design issues such as

accessibility, interchangeability of components and the use of standard tools, materials, ground support equipment and maintenance techniques. Components should be accessible for the full range of tasks that need to be performed to keep the main system operational by the specified range of personnel using appropri- ate tools. Maintenance tasks including examination, replenishment, test, diag- nosis, removal and replacement should be well designed and maintenance engi- neers should also have access to well written, unambiguous maintenance docu- mentation.

There are three aspects of safety that need to be considered during the de- sign of an aircraft - the safety of the aircraft passengers and crew, the safety of third parties and the safety of the maintenance engineer. The maintenance engi- neer should be protected from system defects such as sharp edges, insufficient foot or handholds and so on during the maintenance task. The consequence of maintenance error on the maintenance engineer should also be bonsidered. When making an error however, the consequence can also affect third parties such as other maintenance personnel working on the aircraft and can, in the most serious cases, lead to a loss of the aircraft. These too must be considered.

The maintenance environment has a huge bearing on the ease of mainten- ance. Maintenance tasks should be designed in such a way that they can be achieved in any conditions under which they could be performed. For example, when maintenance engineers are working under time pressures, in the dark, or in difficult weather conditions such as heat, cold, damp or high wind.

Achieving optimal maintainability also ensures that the capabilities and limitations of those conducting maintenance have been considered, as this dic- tates the ability of the item to be retained in, or restored to operational functio- nality. These Human Factors (HF) are inherently linked with maintainability. Tasks should be designed with an understanding of the capabilities and limita- tions of humans to optimise performance and avoid human error. Human error is a natural condition of being human, and it is crucial mechanism to help hu- mans learn, but the maintenance task should be designed with this realisation in mind to ensure that error does not lead to safety. The consideration of maintai- nability during the aircraft design process consists of a number of stages. Once

the maintainability targets have been set and requirements specified, the aims of maintainability should be integrated into the design. However, the design must be assessed to ensure that such requirements are met and targets achieved. The characteristics that are investigated during maintainability assessment are de- pendent on many factors. These include the focus of the assessment (i.e. a zone of the aircraft or a specific component), the type of component (i.e. structural components have to be dealt with differently to systems components), and the stage of design (i.e. in early designmany detailed aspects of maintainability cannot be assessed). Within military aircraft design, Defence Standard 05-123 (UK Ministry of Defence, 1983) states that any prototype used for maintainabil- ity assessment will allow investigation of accessibility (for servicing, testing, removal and replacement) and clearances for loading and off-loading of wea- pons and armament. These are quite limited, but they imply the requirement for other functions. For instance, accessibility assumes some assessment of anthro- pometric characteristics, range of user movement (and the limitations imposed by standard and protective clothing), and visual and physical access.

Maintainability of a product is demonstrated to the customer before ac- ceptance. This concentrates on the corrective maintenance of a sample of units often determined randomly, or at the request of the customer. These timings can then be compared to the criteria outlined in the specification. However, as a general rule, Def Stan 00-41 (UK Ministry of Defence, 1993) states that the fol- lowing should be considered in military maintainability assessment; fault detec- tion capability, fault isolation capability, times to repair-by-replacement, inter- changeability of Line Replaceable Units (LRU) and accuracy of maintenance manuals.

Once the aircraft is delivered to the customer and is in operation, it will undergo preventative and corrective maintenance in order to keep it in a servi- ceable state. Information on the problems encountered in service should be fed back to the manufacturers to make improvements on maintainability for that and future aircraft.

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