Methods to reduce direct

maintenance costs for

commercial aircraft

Haiqiao Wu

Yi Liu

Yunliang Ding and

Jia Liu

The authors

Haiqiao Wu, Yi Liu, Yunliang Ding and Jia Liu are all in the College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, People’s Republic of China

Keywords

Direct costs, Commercial aircraft, Maintenance costs, Experts, Diagnostic testing

Abstract

Direct maintenance costs (DMC) of commercial aircraft make a significant contribution to an aircraft’s cost of ownership. The aim of our research is to find out some methods to reduce DMC. The paper first points out that design and fault   diagnosis are the key factors to influence DMC, disregarding factors unique to a particular airline. A new concept of R&M design-maintenance free operating period and fault diagnosis expert system are discussed in this paper, in order to reduce DMC.

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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