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科技譯文
題 目 降低商用飛機的直接維護(hù)費用的方法
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降低商用飛機的直接維護(hù)費用的方法
商用飛機的維護(hù)活動是飛機耐飛性能的一個必要組成部分。飛機維護(hù)是令飛機回復(fù)到可使用狀態(tài)下的一個上木。它包括維護(hù)、修理、徹底檢查、檢驗和狀態(tài)測定。它可以分為兩種類型。
修正的維護(hù)。這些活動，即由提供對于某一已知的或疑似的故障及（或）缺陷的方案，來是失敗的結(jié)果回復(fù)到一種令人滿意的情況。修正的維護(hù)大體上可分為過失確認(rèn)、過失隔離、拆卸、替換、重新裝配、對準(zhǔn)或者調(diào)整，以及測試。這一種維護(hù)的類型即是不預(yù)定的維護(hù)，而且受益于診斷的使用以減輕在維護(hù)資源方面的負(fù)擔(dān)。
預(yù)防的維護(hù)。這些活動，即由系統(tǒng)檢驗、探測、疲勞項目的替換、調(diào)整、口徑測定，以及清潔等，來使之保持在可使用狀態(tài)。在飛機和儀器的整個壽命中，它以一種規(guī)定的形式實行。因此，它也被成做預(yù)定的維護(hù)。
維護(hù)通常的目標(biāo)是，在一家航空公司需要維修飛機時，能夠以最低的費用提供一套完整的維護(hù)服務(wù)?，F(xiàn)在商用飛機的維護(hù)費用對飛機費用的所有權(quán)起一個重要作用。維護(hù)費用一般占與飛機操作相關(guān)費用的10%-20%（Maple,2001）。
直接的維護(hù)費用（DMC）被定義為，用于維護(hù)一個飛機或相關(guān)儀器所需的勞動力費用和材料費用（ATA，國際航空運輸協(xié)會和ICCAIA，1992）。DMC不包括勞動和物質(zhì)的開支，如行政、監(jiān)督、使用工具工作、測試儀器、設(shè)備、記錄及保存等活動的費用（Knotts,1999）。航空公司通常會尋求維護(hù)費用的保證，如果DMC超過約定的指定水平，飛機制造者將招致財政上的處罰。
我們的研究目標(biāo)是找出一些為商用飛機減少DMC的方法。本論文首先分析了影響DMC的主要因素，然后討論了可以減少DMC的一些方法。
[DMC的主要影響因素]
依照定義，DMC的公式是：
DMC=（+）LR+MC,
其中，是指飛機維護(hù)人員在飛機上的工作時間；是指飛機維護(hù)人員不在飛機上的凌夷部分工作時間；LR是指勞動費用；MC是指材料的費用。影響DMC的因素可以依下列各項分類。
[設(shè)計因素]
可靠性和可維護(hù)性（R&M）是飛機的固有價值。它只能由設(shè)計決定。雖然像經(jīng)過高度訓(xùn)練的人和一個應(yīng)答的補給系統(tǒng)這樣的其他因素，也能使時間限定在一個絕對的最小量中，但是只有國有的R&M才能決定這一最小量。即使改良訓(xùn)練或技術(shù)支持也不能夠有效的彌補因一架拙劣設(shè)計（根據(jù)R&M）的商用飛機在可用性方面所造成的損失。將支持飛機飛行的費用減少到最小，最大限度的提高籍由最好設(shè)計所生產(chǎn)出的產(chǎn)品的可用性，使之可靠并且可維護(hù)。對于商用飛機整個壽命期所花費用來說，大概有70%-80%的費用是由設(shè)計階段來決定的。
[過失診斷效率]
系統(tǒng)和技術(shù)的復(fù)雜性逐漸增加加大了即使、有效的過失診斷的困難。由此成為系統(tǒng)可維護(hù)性的問題因素。而且，從減少時間周期和費用方面來看，無效的過失診斷可能會很貴。因為“沒有發(fā)現(xiàn)錯誤（NFF）”的情形會對維護(hù)費用產(chǎn)生很大的影響?，F(xiàn)代系統(tǒng)的設(shè)計經(jīng)歷了40%或者更高的儀器錯誤消除率。這些錯誤是有歧義的、勞動密集型的測試程序所造成的。航空電子學(xué)和電氣科學(xué)方面的不可預(yù)定維護(hù)費用占民用飛機DMC的18%，40%與儀器錯誤消除相關(guān)的被歸類為NFF。在1992年，一項對部件轉(zhuǎn)移的審計突出了英國空中航線的機群每平均有8000項被轉(zhuǎn)移走?？v觀所有的工作室，其所有部件中的14%，被發(fā)現(xiàn)有NFF。一臺航空電子學(xué)儀器平均會產(chǎn)生出30%的NFF。在財政上來看，若是考慮到直接和間接費用，那么 這就等于是每年在NFF上的開支總共就需要兩千萬英鎊（Knotts,1999）。
[與組織相關(guān)的可變因素]
這些可變因素跟一家特定的航空公司有關(guān)。他們包括飛機機群的規(guī)模和共通性，飛機的齡和使用率，維修標(biāo)準(zhǔn)和計劃，檢查間隔的頻率，承做轉(zhuǎn)包工作的水平，會計方法，通貨波動，地方勞動力費用，消耗品的可循環(huán)率，以及材料價格（Maple,2001）。
[環(huán)境因素]
這些因素依賴于操作員的位置。舉例來說，它是沙漠的環(huán)境或者海洋性氣候。再舉例來說，由于沙和鹽的腐蝕，將會對引擎的維護(hù)儀器產(chǎn)生重要影響。
在本論文中，我們忽略了某一特定的航空公司這一因素，再討論了設(shè)計和過失診斷的影響。
[一項關(guān)于R&M在自由操作期間的維護(hù)的新觀念]
在傳統(tǒng)方式下，R&M設(shè)計的探討是建立在失敗基礎(chǔ)上的。這種探討認(rèn)為，在儀器設(shè)備的整個壽命期間，偶然的失敗是不可避免的，并且這種失敗將導(dǎo)致許多不可預(yù)定的維護(hù)工作在日常工作中產(chǎn)生。由于不可預(yù)定的維護(hù)是不能被計劃出來的，所以，從維護(hù)費用方面來說，不可預(yù)定的維護(hù)可能是做昂貴的。最近的研究表明，給大型的商用噴氣式飛機每一年每一架飛機的不可預(yù)定維護(hù)費用在一百萬英鎊左右（Kumar et al.,1999a）。為了減少費用，一個以維護(hù)的自由操作時期（MFOP）為基礎(chǔ)的新方法已經(jīng)發(fā)展起來了。MFOP被定義為，儀器設(shè)備在沒有任何維護(hù)措施，也沒有因系統(tǒng)錯誤或限制導(dǎo)致的操作員的約束行為就能夠執(zhí)行現(xiàn)已指定的任務(wù)。這個操作的時間段就是MFOP。（Hockley,1998）。
在MFOP的時期，籍由設(shè)計，任何的維護(hù)的必要性應(yīng)該是保持在一個最小量。并且，儀器設(shè)備僅僅允許執(zhí)行如飛行服務(wù)這樣的在計劃內(nèi)的最低限度維護(hù)。一個MFOP之后，緊接著就是一個維護(hù)恢復(fù)時期（MRP）。
MRP被定義為是一段被限定了的時間。在此期間，需要完成適當(dāng)?shù)陌才藕驼_的維護(hù)，使系統(tǒng)回復(fù)到滿負(fù)荷狀態(tài)，這樣才能夠完成接下來的MFOP。由于它們必須包括不同的維護(hù)活動，所以不是所有的MRP將會是相同的期間，因為可替換的單位（LRU）個體排成一行，比如那些使用壽命期滿的，需要做一些徹底檢查，而且不用維護(hù)或正確的檢驗。另外，我們會做出正確的維護(hù)來是那些錯誤的系統(tǒng)回復(fù)到滿負(fù)荷狀態(tài)（Hockley,1998）。
MFOP是一個擔(dān)保時期的延長。操作員正在考慮在系統(tǒng)的整個使用壽命期間擴充這一概念。產(chǎn)品供應(yīng)商或者制造商需要作出如下保證，即在指定的一段操作時間內(nèi)，因為有預(yù)先社頂?shù)馁|(zhì)量保證登記，不可預(yù)定的維護(hù)工作是不需要的。這個質(zhì)量由自由操作時期的存留能力時間（MFOPS）按比例來決定（Kumar et al.,1999b）。
MFOPS被定義為某一項目在MFOP的期間存留下來的可能性。
對于商用飛機的MFOP，當(dāng)考慮到R&M的設(shè)計時，會有兩種減少DMC的方法。
飛機的固有可靠性可以得到句到的改變。更高的可靠性可以減低錯誤的次數(shù)，因此，飛機可靠性是有工時和必需的物質(zhì)決定的，DMC就會相應(yīng)減低。
完成飛機的MFOPS，這就意味著任意的失敗應(yīng)該在MFOP期間被根除。傳統(tǒng)的文化認(rèn)為，錯誤不僅是不可避免的，而且，從某種角度來看，它是可以接受的，這應(yīng)該放棄。一種細(xì)節(jié)詳細(xì)的知識環(huán)境和使用經(jīng)驗，以及對于為什么失敗的非常機制下的理解，會被灌輸給發(fā)展程序員。許多技術(shù)或者解決辦法都以一條更積極的方式設(shè)計其可靠性，不給失敗予任何機會。這些技術(shù)可以有一種緩慢的設(shè)計變化過程，即選擇不同的成分，生成一個改進(jìn)的程序，或者，可以有一種更快速的設(shè)計變化過程。
完成最適宜的維護(hù)計劃。很明顯，飛機所有的系統(tǒng)在某一時間都需要做一些維護(hù)工作，而且，這些工作是在MRP計劃中的。MFOP延期事實上是對MRP所有正確的 維護(hù)，因此，不可預(yù)定的維護(hù)部分其實被轉(zhuǎn)化成了更多的計劃中的維護(hù)，它是建立在有更高可靠性的儀器上的，這才能產(chǎn)生更高的可靠性能。MRP的價值與效率的關(guān)系及其平衡的設(shè)置建立并支持最好的整個MFOP系統(tǒng)。其實際價值可以在設(shè)計期間的系統(tǒng)工程學(xué)中的經(jīng)貿(mào)學(xué)和方法學(xué)來體現(xiàn)。這些能減少維護(hù)計劃中一定量的百分比。而后勤支援可能被集中到一個特定的飛機操作地點。緊急事故處理資源可能會重新分配到幾頂?shù)墓ぷ髦小＿@樣，MFOP就能為操作者帶來靈活性。此時，操作者可以在一定范圍內(nèi)執(zhí)行組織和正確的維護(hù)工作。然后，DMC就會下降，這是因為用于報廢飛機的勞動力和材料減少了。舉例來說，如今一架飛機在整天壽命期中的直線型維護(hù)占所有維護(hù)勞動力的50%（Maple,2001），由MFOP設(shè)計出的飛機常規(guī)工作將會減少到最小量。
[過失診斷]
過失診斷的進(jìn)程一般可氛圍感應(yīng)信號、提取特征以及連續(xù)的診斷論證。當(dāng)對現(xiàn)代的商用飛機診斷失敗時，大部分感應(yīng)信號和提取的特征程序由于感應(yīng)器、動力學(xué)實驗和信號檢測這些技術(shù)的發(fā)展，可以是自動完成的。這樣，診斷論證（即，怎樣找出錯誤的根源）就成為了決定過失診斷的效率的主要因素。
根據(jù)過失診斷的觀念，飛機是一個復(fù)雜的系統(tǒng)。它的結(jié)構(gòu)是多樣的階層建筑結(jié)構(gòu)，它包含有許多次級系統(tǒng)，比如飛機主結(jié)構(gòu)、引擎、自動飛行系統(tǒng)、起落架、聯(lián)絡(luò)系統(tǒng)、液壓和飛行系統(tǒng)。每個次級系統(tǒng)是由更低級別的次級系統(tǒng)或者次級單元構(gòu)成的。并且，這些次級系統(tǒng)或次級單元之間通常是有聯(lián)系的。由于飛機結(jié)構(gòu)和功能的復(fù)雜性和多相性，飛機結(jié)構(gòu)水平之間的聯(lián)系是難以定義的。次級系統(tǒng)或次級單元的輸入和輸出之間的數(shù)量關(guān)系往往是無法測知或不正確的。
很多技術(shù)領(lǐng)域中，先進(jìn)的技術(shù)如機械化、電氣化、計算機和自動機械控制，以及電子學(xué)都適用于現(xiàn)代的飛機。越來越多的電機械儀器已經(jīng)用于飛機。這些儀器的機械和電子部分已經(jīng)不僅整合了飛機的控制，還整合了飛機的功能和結(jié)構(gòu)。飛機的過失診斷囊括了各種學(xué)科的知識。
我們從以上的議題中總結(jié)出了商用飛機的診斷論證的困難性，而且很多時候，這需要有專家的參與。然而，我們需要的專家因為調(diào)換、疾病，以及雇傭關(guān)系的改變等原因，經(jīng)常是不到位的。除此之外，很多技術(shù)領(lǐng)域已經(jīng)應(yīng)用于大型商用飛機，而且一個專家不再可能蚩尤所有現(xiàn)有的系統(tǒng)知識。發(fā)展一個包涵系統(tǒng)知識、專長和經(jīng)驗的過失診斷專家系統(tǒng)被視為一個定位困難的方法。這樣不僅可以帶來比人工更正確，更一致的結(jié)果，而且在某種程度上，它可以代替一個專家，使很多使用者可以輕易獲得寶貴的專長，尤其適用于相對不熟練的職工和新來者。
大部分的NFF將會被專家系統(tǒng)避免，如此一種有成本效益和及時的過失診斷將會幫助減少DMC。
[結(jié)論]
MFOP的觀念已經(jīng)作為面向未來所作出的一個大步驟被航空宇航工業(yè)認(rèn)同。一些在較早時間所提出的關(guān)頂已應(yīng)用于A340-600（Cini和Griffith,1999）。過失診斷專家系統(tǒng)已經(jīng)應(yīng)用于波音777的中央計算機維護(hù)系統(tǒng)。毫無疑問，它們能極大地減少DMC。
Methods to reduce direct maintenance costs for commercial aircraft
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Introduction
Commercial aircraft maintenance activities form an essential part of airworthiness. Aircraft maintenance is actions that can restore an item to a serviceable condition, and consist of servicing, repair, modification, overhaul, inspection and determination of condition. It can be classified into two types.
Corrective maintenance. All actions performed as a result of failure to restore an item to a satisfactory condition by providing correction of a known or suspect malfunction and/or defect. Corrective maintenance in general consists of fault verification, fault isolation, disassembly, replacement, reassembly, alignment/adjustment, and test. This type of maintenance is known as unscheduled maintenance, and benefit from the use of diagnostics to ease the burden on the maintenance resource.
Preventive maintenance. All actions performed at defined intervals to retain an item in a serviceable condition by systematic inspection, detection, replacement of wear out item, adjustment, calibration, cleaning etc. It is carried out at prescribed points in an aircraft and equipment’s life. It is also termed as scheduled maintenance.
The common goal of maintenance is to provide a fully serviceable aircraft when it is required by an airline at minimum cost. For the present, maintenance costs of commercial aircraft make a significant contribution to an aircraft’s cost of ownership. Maintenance costs typically account for 10-20 per cent of aircraft-related operating costs (Maple, 2001).
Direct maintenance costs (DMC) is defined as the labor and material costs directly expended in performing maintenance of an aircraft or related equipment (ATA, IATA and ICCAIA, 1992). DMC do not include the labor and material expenditures, which contribute to activities such as administration, supervision, tooling , test equipment, facilities, record keeping etc. (Knotts, 1999). Airlines usually seek maintenance cost guarantees, where the aircraft manufacturer incurs financial penalties if DMC exceed agreed specified levels.
The aim of our research is to find our some methods to reduce DMC for commercial aircraft. In the continuation, the paper first analyzes the key factors that influence DMC, then discusses some methods that could reduce DMC, and finally draws a conclusion.
Key influence factors of DMCs
According to the definition, the formula for DMC is
DMC = ( +) LR + MC
Where is maintenance man hours off aircraft, LR is labor rate, and MC is material costs.
The factors, which effect on DMC, can be categorized as follows.
Design factor
Reliability and maintainability (R&M) is an inherent property of aircraft. It can be achieved only by design. Although other factors, such as highly trained people and a responsive supply system, can help keep down time to an absolute minimum, it is the inherent R&M that determines this minimum. Improving training or support cannot effectively compensate for the effect on availability of a poorly designed (in terms of R&M) commercial aircraft. Minimizing the cost to support an aircraft and maximizing the availability of that aircraft are best done by designing the product to be reliable and maintainable. R&M design has become an essential art of the development process of modern commercial aircraft life costs are determined during the design stage.
Fault diagnosis efficiency
The increasing complexity of systems and technology adds to the difficulty of effective and timely fault diagnosis, thus contributing to the problems of system maintainability. Moreover, ineffective fault diagnosis can be expensive in terms of down time and cost, with “no fault found (NFF)” situations contributing significantly to maintenance costs. Current system designs experience a 40 per cent, or higher, equipment false removal rate as a result of ambiguous and labor intensive test procedures. Avionics and electrical unscheduled maintenance accounts for 18 per cent of a civil aircraft’s DMC, 40 per cent of related equipment removals are classified as NFF. In 1992, an audit of component removals highlighted an average of 8,000 items removed from British Airways’ fleet per month. A total of 14 per cent of components, across all workshops, were found to have NFF. Certain avionics equipment experienced 30 per cent NFF. Financially, considering direct and indirect costs, this equated to an annual NFF expenditure totaling $20 million (Knotts, 1999).
Organization-related variables
These variables are relative to a specific airline. They include fleet size and commonality, aircraft age and utilization, maintain standard and plan, frequency of check intervals level of subcontracting, accounting method, currency fluctuations over time, local labor rates, and material prices (Maple, 2001).
Environmental factors
These factors depend on the location of the operator. For example, it is a desert environment or a maritime climate. For example, corrosion due to sand salt will have a significant influence to engine maintenance equipment.
Disregarding factors unique to a particular airline, impacts of design and fault diagnosis are discussed in this paper.
A new concept of R&M design- maintenance free operating period
The traditional approach pf R&M design, which is based on the meantime between failures (MTBF), acknowledge that random failures are inevitable throughout the equipment’ life, and leads to much unscheduled maintenance to be performed in routine of airline. The unscheduled maintenance tends to be most expensive in terms of maintenance costs because it is unplanned. Recent studies show that the cost of unscheduled maintenance for large commercial jet aircraft is in the range of 1 million pounds per aircraft per year (Kumar et al.,1999a). in order to reduce the costs, a new method based on maintenance free operating period (MEOP) has been developed.
MFOP is defined as a period of operation during which the equipment must be able to carry out all its assigned missions without any maintenance action and without the operator being restricted in any way duo to system faults or limitations (Hockley, 1998).
During MFOP, the necessity for any maintenance should be, by design, kept to a minimum. And the equipment is allowed to carry out only some planned minimal maintenance, such an flight servicing. A maintenance recovery period (MRP) follows immediately after a MFOP.
MRP is defined as the down time during which appropriate scheduled or corrective maintenance is done to recover the system to its fully serviceable state so that it is capable of achieving the next MEOP. Not all MRPs will be of the same duration because they need to encompass different maintenance activities for individual line replaceable unit (LRU), such as those that are life-expired, those that require some overhaul and prevent maintenance or just inspection to be done to restore the full capability of those faulty systems (Hockley, 1998).
MEOP is an extension of warranty period. The operators are considering extending this concept throughout the life of the system. The contractor/manufacture will be expected to guarantee that no unscheduled maintenance activities will be required during each defined period operation with the predefined level of confidence. The confidence is scaled by maintenance free operating period survivability (MFOPS) (Kumar et al., 1999b).
MFOPS is defined as the probability that the item will survive for the duration of the MEOP.
There are two ways to reduce DMC when conducting R&M design with MEOP for commercial aircraft.
Inherent reliability of aircraft can be improved greatly. Higher reliability and therefore, the man-hours and material necessary to fix them, so DMC will be brought down.
To achieve MFOPs of aircraft, it means that random failure should be eradicated during MEOP. The traditional culture, which believes that not only failures are unavoidable but also that are acceptable in a way, should be discarded. A detailed knowledge of the environment and usage to be experienced, together with a more thorough understanding of the very mechanisms of why things fail, will be fed into development programmers. Many technique or solutions will be applied to design for reliability in a more proactive way, so that failure mechanism is not given the opportunity to occur. The techniques could range from a change in physical design, selecting a different component, an improved build process, or a more radical design change.
To achieve an optimum maintenance plan. Obviously, the overall system of aircraft will need some maintenance actions at some point, but there will be performed during the planned MRPs. The MEOP defers virtually all corrective maintenance to MRP, so the “unscheduled” element of maintenance is exchanged for more scheduled maintenance, based on the general improvement of reliability associated with more inherently reliable equipment. A more practical, cost-effective and balanced set of MRPs that build-up and support the best overall system MFOP, can be achieved by means of trade-off and methodology for system engineering during design. This reduce some of the uncertainty present in maintenance planning. Contingency resources could be re-allocated to scheduled work and logistic support could be concentrated in one particular location of aircraft operations. In this way, the MFOP provides the operator with flexibility in where and when it carries out its preventive and corrective maintenance to an extent. Then DMC will be reduced, because of decrease of labor and materials to cope with unserviceable aircraft. For example, line maintenance accounts for 50 per cent of all maintenance labor over the course of an aircraft’s lift cycle today (Maple, 2001), the routine work of an aircraft designed by MFOP will be decreased to minimum.
Fault diagnosis
The process of fault diagnosis can be generally divided into sense signal, feature extraction and diagnostic reasoning in sequence. When diagnosing failures of modern commercial aircraft, most of the procedure of sense signal and feature extraction can be accomplished automatically, due to the technology development of sensor, dynamic test and signal analysis. Then diagnostic reasoning (how to find out the source of failure) is a key factor to contribute to the efficiency of fault diagnosis.
In terms of the concept of fault diagnosis, aircraft is a complicated system. Its structure is a multiple hierarchical architecture, which is comprised of many subsystems, for example, aircraft structure, engine, auto flight system, landing gear, communications system, hydraulic power and navigation system. Each subsystem is formed by subsystem or subunits are lower level. And the subsystems of subunits are usually interactive with each other. Connections between the levels of aircraft structure are usually difficult to define duo to the multiplicity and heterogeneity the structures and functions of aircraft. The quantitative relationships between the input and output of subsystem or unit usually are unavailable or inexact.
Advanced technology of much technosphere has been applied to modern aircraft synthetically, such as machinery, electrics, computer, automatic control and electronics. More and more electromechanical equipments have been used in aircraft. The mechanical and electric components of these equipments have been integrated in the manner not only of control, but also of function and structure. Multidisciplinary knowledge is required to diagnose the fault of aircraft.
Above issues result in the difficulties of diagnostic reasoning for commercial aircraft, and it always needs the expert’s participation. However, the required expert is not often available due to shift, sickness, change of employment and so on. In addition, much technosphere has been utilized in large commercial aircraft, and an expert is unlike to possess all the exiting system knowledge. To develop a fault diagnosis expert system, which could capture system knowledge, expertise and experience, is seen as a way to address the difficulty. It would not only produce more accurate and consistent results than its human counterpart, but also take the place of an expert in a manner and make precious expertise available to many users, in particular to less skilled staff and newcomer.
Most of NFF will be avoided by expert system, thus a cost-effective and timely fault diagnosis will help to reduce DMC.
Conculsion
The concept of MFOP has been acknowledged by aerospace industry as large step for future reliability specifications. Some of the ideas described in the earlier sections are being developed for A340-600 (Cini and Griffith, 1999). Fault diagnosis expert system has been encompassed in central maintenance computer system of Boeing 777. there is no doubt that they can reduce DMC greatly.
References
Cini, P.F. and Griffith, P. (1999), “Designing for MFOP: towards the autonomous aircraft”, journal of Quality in Maintenance Engineering, Vol. 5 No. 4, pp. 296-308.
Hockley, C.J. (1988), “Design for success”, Proc. Instn. Engrs., Part G Vol. 212, pp.371-8.
Knots, R.M.H. (1999), “Civil aircraft maintenance and support fault diagnosis from a business perspective”, Journal of Quality in Maintenance Engineering, Vol. 5 No.4, pp. 335-47.
Kumar, U.D., Crocker, J. and Knezevic, J. (1999a), “Evolutionary maintenance for aircraft engines”, Proceedings Annual Reliability and Maintainability Symposium, pp. 62-8.
Kumar, U.D., Knezevic, J. and Crocker, J. (1999b), “Maintenance free operating period - an alternative measure to MTBF and failure rate for specifying reliability”, Reliability Engineering and System Safety, Vol. 64, pp. 127-31.
Maple, M. (2001), “Understanding maintenance costs for new and existing aircraft”, Airline Fleet and Asset Management, No. 5, pp. 56-62