AbstractToday no Life Cycle Cost model attempts to quantify the rate of airframe maintenance cost increases due to the effects of aging. It is usually characterized as the point at which both the cost and operational burden associated with repairing fatigue, corrosion, and stress corrosion cracking either by themselves or in combination, result in a significant departure from a newly manufactured aircraft.
Today no industry recognized cost escalation factors exist which represent airframe maintenance cost increases due to aging. The cost to maintain and operate aging aircraft is being reviewed to identify factors, which must be included in the methodology to accurately assess the economic service life of aging aircraft. This study will analyze a large body of literature to confirm maintenance growth as a function of age. Validating these perceived maintenance increases and quantifying the significance of this trend is necessary to accurately forecast future cost of ownership.Introduction and BackgroundPredicting the point at which an aircraft will become uneconomical to support requires a thorough understanding of aging aircraft maintenance trends and forecast modification requirements. Aging aircraft maintenance requirements may combine with FAA mandated avionics or noise compliance requirements (Stage HI or IV) to mark the end of an aircraft’s Economic Service Life (ESL).
(Pyles, 2003) Individual aircraft must be evaluated regarding factors such as historical cyclic utilization, environmental basing history and previous maintenance history to provide an accurate snapshot of today’s economic assessment (Rice, 1998). Today’s economic value must then be projected forward considering future annual fleet cyclic utilization and forecast corrosion growth rates for each specific environmental area of operation (Cooke, 1998). High acquisition costs of replacement equipment are forcing both the military and commercial sectors to focus on Total Ownership Costs (TOC).
Cumulative ownership costs over the 20, 30 and even 40 year life span of an aircraft can totally eclipse the initial acquisition costs and are driving a renewed interest in Life Cycle Cost modeling (Dhillon, 1989).There are many economic factors that contribute to determining when an aircraft should be replaced. Typically the primary cost driver is increasing maintenance requirements due to the age of the aircraft. Increasing maintenance requirements drive up maintenance costs and drive down aircraft availability (NRC, 1997). Since 1988, when an Aloha airlines jet experienced a catastrophic structural failure, airline and industry working groups have developed aging aircraft programs to inspect and modify various portions of an airframe. The costs to comply with “aging program” Air worthiness Directive (AD) requirements may be less than $500,000 for a DC-10, but more than $5 million for a 747-200 (Sweers, 1997). To definitively model cost growth, an understanding of each airframe design’s inherent resistance to fatigue and corrosion is necessary.
A detailed LCC analysis must not only include the initial acquisition costs, but must also include both the implicit and explicit, operational and disposal costs (Blanchard, 1991).Figure 1. Life Cycle Cost Forecasts Should Include Both Implicit and Explicit Costs. (Blanchard, 1991). Competing alternatives should recognize both the explicit monetary benefits such as: reduced fuel consumption and lower maintenance costs, plus attempt to quantify the implicit and rather subjective non-monetary benefits such as customer preference for a newer aircraft. (Pyles, 2003).
The FAA has recognized this trend to keep aircraft in service longer. In 1988 the U.S. large transport fleet totaled nearly 3,700 aircraft and had an average fleet age of 12.7 years. In just ten years, the heavy commercial transport fleet has grown to nearly 5,400 aircraft and the average age is now 15.8 years and now totaled about 6000 aircrafts (U.
S. Navy, 2006). It is estimated that by year 2010 the average U.S. fleet age will be between 18 and 20 years old (Garvey, 1999).
This trend to keep aircraft longer must be reflected in the LCC forecasting methodology. Many of the heavy commercial transport aircraft were designed for between 20,000 and 60,00 flight cycles, and between 30,000 and 60,000 flight hours. The majority of the aircraft were also designed to meet a 20-year life expectancy. (Pyles, 2003)Figure 3.
Failure Rate over the Life Expectancy of a System May Be Categorized into Three Distinct Failure Periods. Operation and Support Costs Closely Follow the System Reliability. (Blanchard, 1991) The “bathtub” hazard rate (or failure rate) curve has been used to represent the failure behavior rate of many items (Figure 3). This curve is divided into major three parts: burn-in period (decreasing hazard rate region), useful life period (constant hazard rate region) and wear out period (increasing hazard rate region).
Failures, which occur during the burn-in period, are normally due to design or manufacturing defects. The useful life period is characterized by unpredictable or random failures. This useful life portion remains constant until the onset of the wear-out or “Aging” period indicated by an increasing failure rate. (Blanchard, 1991)Part failure rates are directly proportional to operational support costs.
Many failures during the Burn-in or newness period however are covered by the manufacturers warranty and not the responsibility of the owner. Some of the reasons for this increasing failure rate during the aging phase may be due to inadequate maintenance, aging, corrosion, fatigue, incorrect overhaul practices and most likely a combination of all the above.This study will analyze a large body of literature to confirm maintenance growth as a function of age. Validating these perceived maintenance increases and quantifying the significance of this trend is necessary to accurately forecast future cost of ownership.Statement of the ProblemToday no Life Cycle Cost model attempts to quantify the rate of airframe maintenance cost increases due to the effects of aging.
MIL-HDBK-1530 defines aging aircraft as the point in time in which the aircraft: 1) has over flown it’s design service goal, 2) is corroded, or 3) has reached the time of onset of widespread fatigue damage (WFD). It is further characterized as the point at which both the cost and operational burden associated with repairing fatigue, corrosion, and stress corrosion cracking (SCC) either by themselves or in combination, result in a significant departure from a newly manufactured aircraft. Today no industry recognized cost escalation factors exist which represent airframe maintenance cost increases due to aging.Figure 2. How Aircraft Aging May Relate to Flight Safety, Aircraft Availability, and Support Costs (Kim et al., 2005)The cost to maintain and operate aging aircraft is being reviewed to identify factors, which must be included in the methodology to accurately assess the economic service life of aging aircraft.
Recent GAO reports have been critical of increases in both depot maintenance man-hours (cost) and duration (downtime) of military aircraft. These GAO reports have associated these increases with the irreversible effects of aging. (General Accounting Office, 1996.)To accurately predict the Economic Service Life of an aircraft one must model total aircraft ownership costs of the subject aircraft (including aging considerations), and compare those costs to the costs of acquiring and operating an alternative aircraft.
Aging fleet maintenance requirements must be also combined with estimates to upgrade outdated avionics. These plans may include navigational upgrades required to comply with Future Air Navigation requirements, or Global Air Traffic Management requirements as well as safety modifications required by the Federal Aviation Administration. (Pyles, 2003)Future mandatory navigation upgrades may include a Global Positioning System coupled to Flight Management System and Reduced Vertical Separation Minimum equipment. Safety upgrades may require the addition of an Enhanced Ground Proximity Warning System coupled to a second generation Traffic Alert and Collision Avoidance System.
Also on the drawing board are new 300 channel Digital Flight Data Recorders, Cockpit Voice Recorders, and a more powerful Emergency Locator Transmitters. (Hebert, 2003) The need to comply with future FAA and ICAO mandated Stage HI and future Stage IV noise requirements is also a political issue. Various civil airports (especially in Europe) are assessing the legality of excluding aircraft equipped with American made Hush Kits. (Kim et al., 2005) Flight restrictions may limit the operating hours of hush-kitted aircraft or discouraging their operation through the use of higher landing fees.
Each of these modifications, each with an individual subjective probability, needs to be quantified for possible inclusion in a total Cost of Ownership forecast. (Commission on Engineering and Technical Systems, 2001)Literature ReviewThis review of literature is organized by relative applicability to Life Cycle Cost modeling of aging aircraft. It would begin at the apex representing recent studies, which attempt to quantify the aging effects on military and commercial aircraft, broadening quickly to a base of literature, which guides the novice cost modeler.Pyles (2003) of the Rand Corporation termed his upper and lower bounds the best case and worst case in which the maintenance of aging aircraft in the military could expect to operate.
He generalized how the future costs of fleet modernization projects, aging material failure mechanisms and part obsolescence may combine in unexpected proportions on aircraft which are being operated 20, 30 or possibly even 40 years beyond their Design Service Objective (DSO). In the worst case scenario, a fleet could experience an “unexpected phenomena which could suddenly jeopardize an entire fleet’s flight safety, mission readiness, or support cost, and that an extended time period may be required to design, test and field a replacement aircraft.” (p.33) In addition, this study measures how the USAF aircraft fleets’ ages relate to modification and maintenance workloads and material consumption. It provides the basis for future estimations of the effects of those activities on aircraft, availability maintenance-resource requirements, and annual operating costs.Sweers and Didonato (1997), of Boeing Commercial Aircraft Group (BCAG), presented a technical paper titled The Economic Considerations of Operating Post Production Aircraft Beyond Their Design Service Objective. Sweers posed a very useful analogy for attempting to model the costs of aging aircraft.
He compared the health care cost of aging humans, to that of aircraft. Typically, how someone took care of themselves during their first 60 years must be known, i.e., to what extent did they stick to a healthy diet, exercise or avoid stress? Also, did they develop any life shortening health habits such as smoking, drinking, or drugs? Were they exposed to toxins or radiation? Did they experience high periods of stress in their lives over a long period? Likewise, one must consider the unique history of each aircraft to develop a meaningful forecast.
Sweers (1997) also explained how the Corrosion Prevention and Control Programs (CPCP) compounded the vicious cycle of corrosion and fatigue damage. Aging program directives required the inspection and removal of all corrosion. During the removal process, metal is removed which makes a structure less tolerant of cyclic fatigue. Sweers (1997) then developed a forecast heavy maintenance growth curve for the Boeing 727, 737 and 747 as well as the Douglas DC-9 and DC-10.Khemaies (1999), from International Air Transport Association (IATA), recognized the value of collecting accurate aircraft maintenance costs. He described several pit falls cost analysts studying historical data may encounter. Khemaies cautioned that the mandatory nature of the DOT Form 41 reporting system insures a very large sample size. It does not, however, insure reporting consistency between airlines.
One airline may calculate the direct maintenance burden (overhead) for a specific operation, while another airline may assign a direct cost, of 80 percent to account for overhead. Additional reporting discrepancies between airlines may also be due to the apportionment of material handling, support equipment and initial spares provisioning. One airline may account for it as being maintenance related another might account for it as part of the initial aircraft capital costs.These inconsistencies between airlines could cause significant error for one attempting to compare airline to airline costs. This study however is comparing like aircraft maintenance costs from several airlines over a period of 25 years.
Khemaies is working to standardize the DOT data collection system and his international system referred to as the Product Performance Measurement (PPM) are comparable tools useful to evaluate aircraft on a global scale.Lincoln and Melliere (1998) published a study titled Economic Life Determination for Military Aircraft. This study provided a methodology for using a Weibull distribution of an equivalent initial flaw size (EIFS) in a structural member to predict crack growth rates due to fatigue. Their study also recognized the combining and even compounding effects of Stress Corrosion Cracking (SCC) and Corrosion. Lincoln also emphasized the concept that determining the Economic Service Life of an aircraft is beyond the realm of the structural engineer. The engineer may determine the economics of a single repair but is not positioned to determine when the cumulative economic burden is unacceptable. The concept of total aircraft economics envelops significantly more economic factors than structural repair. It is a secondary purpose of this study to combine the key cost elements, which must be considered in an Economic Service Life Study.
Life Cycle Costing by Dhillon (1989) and Life-Cycle-Cost and Economic Analysis by Blanchard (1991) both offer the well known “bathtub curve” used to represent failure behavior of many mechanical and electrical items. This curve represents increasing reliability (decreasing failures) during a component’s early life stages, a mid-life period in which the failures are purely random or stable and a third phase, which is marked by an upward trend in failures as a function of wearout. Dhillon (1989) discusses several common aircraft industry LCC models; PRICE (Program Review of Information for Cost and Evaluation), LCOM (Logistics Composite Model), CACE (Cost Analysis Cost Estimating) and BACE (Budgeting Annual Cost) model however, none of these models attempts to quantify the third (or aging) phase of an aircraft life.
Another common LCC model used by the USAF is CORE model (Cost Oriented Resource Estimating) model. CORE has gained popularity in recent years due to the AF’s interest in lowering costs. The AF has created a Reduction of Total Ownership Cost (R-TOC) program office, which has developed several in-depth cost visibility reports, which allow users to trace detailed costs.The R-TOC program is described very well in a report by Booth (2000).
He describes the R-TOC goal as insuring costs are not merely pushed from one cost account to another, but indeed tracked, to insure the total cost to the AF is reduced. The majority of RTOC cost reports are furnished in CAIG (Cost Analysis Improvement Group) format. This format breaks down aircraft Operation and Support costs (O;S) into seven major cost elements. These seven costs elements correspond directly with the CORE model’s output. This has been the primary cause for the increase in CORE popularity.
In 1997 Lincoln, along with a long and distinguished list of scientists and engineers working for the National Research Council, published a report for the USAF titled Aging of U.S. Air Force Aircraft. In this report, one recurring theme is apparent, the need for an overall economic service life estimation model that integrates the estimates of structural deterioration caused by fatigue, corrosion, and stress corrosion cracking (SCC) with all other operating cost elements. The current lack of such a tool inhibits Air Force planners from establishing a realistic timetable to phase out a current system and begin planning for replacement aircraft. These three basic components of airframe structural deterioration (fatigue, SCC and corrosion) are the same components, which contribute to commercial airframe maintenance growth.
Economics and ReadinessThe economic burden associated with the inspection and repair of fatigue cracks can be expected to increase with age until the task of maintaining aircraft safety could become so overwhelming and the aircraft availability so poor that the continued operation of the aircraft is no longer viable. In addition, corrosion detection, repair, and component replacement can add significantly to or, in some cases, dominate the total structural maintenance burden. The committee concludes that the major emphasis of the Air Force’s technical and force management with regard to corrosion and stress corrosion cracking (SCC) should be focused on the early detection of corrosion and the implementation of effective corrosion control and mitigation practices so as to drastically reduce unscheduled repairs and replacement costs and aircraft downtime.
Key technical issues and operational needs include (Hebert, 2003):• the development of improved NDE techniques for the detection and rough quantification of hidden corrosion;• the classification of corrosion severity to provide guidance for maintenance;• the generalized application of corrosion-preventive compounds and the development of corrosion-preventive compounds that can be applied on external surfaces to protect unsealed joints and fasteners;• the development of a material and process substitution handbook and engineering guidelines for the replacement of components exhibiting corrosion and SCC with more-resistant materials and processes;• the development and application of materials and processes to inhibit SCC;• the development of technologies for the removal, surface preparation, and reapplication of surface finishes with improved corrosion-resistant finishes on existing aircraft; and,• the assessment of the potential use of the dehumidified storage of aircraft, where practical.The committee believes that fatigue cracking will occur eventually on all aging aircraft as flight hours increase. From an economic standpoint, the major impact for a fail-safe-designed structure occurs with the onset of WFD. For safe-crack-growth-designed structures, the major impact occurs when the structure exhibits a rapid increase in the number of fracture-critical areas. In both cases, a choice must be made to undertake major modifications, structural replacement, or retirement. Although it may not be possible to avoid reaching this point for any given aircraft, operational changes such as fuel management, gust avoidance, active or passive load alleviation systems, reduced pressurization, and flight restrictions to minimize flight in severe mission segments can reduce the rate of fatigue damage and delay expensive repair-replace-retire decisions.
For aircraft that are approaching their economic service limit, these options should be considered to allow time for modification or replacement acquisition programs.Force Management and Predicted Economic Service LifeThe Air Force modernization planning process includes the essential elements for force structure planning and management, but, to be completely effective, it should significantly improve estimates of the probable economic service life of aging aircraft systems. There is no clear definition of all of the cost elements that contribute to the economic service life of an aircraft, nor is there a precise methodology for estimating when the costs of operating and maintaining a system will be high enough to warrant replacement. The committee believes that the development of an estimate of economic service life with metrics that integrate the effects of structural deterioration (i.e.
, from fatigue and corrosion) with economic considerations is essential to force management.Nelson (1997) of the RAND Corporation produced an interesting study titled Life-Cycle Analysis if Aircraft Turbine Engines. This study, of course, focused on jet engines rather than airframe costs, but he developed a cost forecasting methodology based on engines time-of-arrival (TOA).
TOA was a useful predictor because it considered the state of the technology at the time of design and manufacture. TOA was one of many predictive factors, others included thrust to weight ratio and developmental costs. The technology, which constitutes the majority of today’s aging aircraft, is of a homogenous pool. It is suspected that airframe maintenance will also be influenced by the airframe’s TOA.Predicting aircraft damage due to corrosion and corrosion fatigue crack growth has been attempted by Harlow et al. (1998). He has developed a mechanistically based probability model. His report Probability Modeling and Analysis of J-STARS Tear-Down Data from Two B707 Aircraft describes his probabilistic approach.
Their prediction model is based on damage values, statistically estimated from experimental data for the localized corrosion and fatigue crack growth rates adjusted for primary cyclic loading (flight cycles). The predicted probability of occurrence (PoO) was compared against the multiple hole-wall crack data collected from the inspection of numerous fastener holes on the lowers wing panels of two aging Boeing 707 aircraft. Harlow’s proposed mechanistically based probability model for corrosion and corrosion fatigue cracking appear to have great promise as an effective tool in airworthiness assessment and fleet management.
These methods once perfected could be coupled with cost modules to predict and price the cost of maintaining an aging fleet.Duuette, (1997) unveiled the Federal Aviation Administration’s response to the White House’s Commission on Aviation Safety. (Pyles, 2003) The FAA recommends expanding the joint FAA / industry Aging Aircraft programs, which have previously focused on structures, to now cover wiring, hydraulic lines, control cables and pneumatic devices. The Aging Aircraft program was established after the Aloha Airlines accident in 1988.
The challenge is to develop maintenance and inspection practices for aircraft systems that adequately address aging aircraft components. As airplanes age, the requirements for inspections, repairs and parts replacement change, and many times increase. This is compounded by the fact the each transport model has a system design requiring maintenance and inspections unique to that aircraft. Much of the new information on the state of aging systems has come to light over the last several years. Information from accident and incident databases needs to be analyzed to identify trends in aging systems. The FAA thus far has concluded that wiring is extremely difficult to inspect and there is in many instances insufficient inspection criteria for corrosion on flight control and hydraulic components.
The FAA has released a seven step plan enhance the safety of non-structural aircraft components. (Pyles, 2003)Spare parts for aging aircraft are also faced with escalating costs. Moog and Hayes (1999) of the Royal Australian Air Force (RAAF) were faced with a unique aging aircraft problem when the USAF decided to retire the F-111 fighter aircraft. In their study 1998 study Aged Aircraft Life of Time Spares Purchase Risk Management they describe how the RAAF has operated the F-111 for 25 years in partnership with the USAF. The RAAF had planned to continue to operate this aircraft for another 20 years until the USAF’s decision significantly complicated the logistics support environment of the aircraft. Many parts did become available at significantly reduced costs as the USAF reduced inventory, however due to fiscal budget and warehouse space constraints all of those opportunities could not be realized.Additionally, the RAAF spares management was faced with the paradigm shift of repairable parts becoming throwaway – due to both reduced spares cost and the lack of a repair source, and once throwaway parts now becoming repairable due to the lack of a manufacturing facility.
The RAAF now had to define future escalation costs considering the probability of: part availability and production lead-time. While USAF’s decision to retire their F-111 has provided the opportunity to purchase large numbers of excess spares at favorable rates, the reduction in maintenance support and the increased storage requirements, may cause a large rise in overall support costs.Das (1999), of the Boeing Commercial Airplane Group, has studied the economics of consolidating aging aircraft maintenance requirements into an airlines routine maintenance program. The primary interest is to consolidate existing routine maintenance programs with the FAA mandated Corrosion Prevention and Control Program (CPCP) and the fatigue related inspections like the Supplemental Structural Inspection Program (SSIP) under the umbrella of ATA owned MSG-3 rev.2 analysis. In the current competitive environment airlines must integrate their maintenance to optimize their maintenance resources. Actual time for inspection can often be less then 20 percent of the total cost.
Thus, if several inspections are scheduled at the same time when the area is accessed to conduct one type of inspection considerable expenses can be saved. This process typically involves performing some maintenance early to achieve access economy with other mandatory maintenance. Even with the performance of some maintenance early, or out of cycle, the total cost to the operator is often reduced.Johnson (1999), working with the Aircraft Structural Integrity Program (ASIP) office, has developed a software tool called FLEETLIFE. This software tool is designed to automate the maintenance data collection and trend analysis tasks involved with maintaining aging aircraft. As an engineering tool, FLEETLIFE is designed to provide a means to effectively evaluate annual fleet usage data, structural safety and performance, and overall fleet integrity. The goal is to allow timely management and detect potential negative trends, and pro-actively make corrections to insure future structural integrity of the fleet.
Johnson is developing modules that will interface with the Air Force Total Cost of Ownership program office to add the new dimension cost as a fleet performance indicator. A logical next step in the evolution of this tool would be to introduce the capability of cost forecasting based on historical trends.FLEETLEFE was designed to solve several problems typically found with aging aircraft programs. The foremost problem involves collecting and organizing and analyzing tremendous amounts of maintenance, data associated with a fleet of aircraft. These data are used on a continual basis to assess the current structural health of the fleet. The problem with most PC based analysis tools is that the source data necessary to accomplish these tasks are usually stored on several disjointed computer systems. FLEETLIFE’s strength is in its ability to communicate with multiple electronics data sources compiling and integrating maintenance and operational data for an accurate fleet health check.The US AF has recently contracted with The Boeing Company to perform an Economic Service Life Study on the KC-135 aircraft.
This study by Keating and Dixon (2003) focused on the Economic considerations of operating an aircraft for potentially eighty-years. Costs of aircraft non-availability as well as several priced options to improve availability is included in the study. They revealed that operating an accessible aircraft for one more year results in some aircraft availability level at the cost of the requisite labor, maintenance, and fuel. On the contrary, purchasing a new aircraft results in a stream of both costs and aircraft availability. They also find that the Air Force should repair, rather than replace, an aging system if and only if the availability-adjusted marginal cost of the offered aircraft is less than the replacement’s average cost per existing year.
Cost Estimation MethodologiesSeveral cost estimation methodologies methods should be used during the estimation process. No single methodology is necessarily better than the other, in fact, their strengths and weaknesses are often complimentary to each other. Five cost estimating methods discussed in Boehm’s (1997) book Software Engineering Economics are the analogy, bottom-up, top-down, expert judgment, and algorithms (parametrics). These methods are often used in conjunction with each other.Analogy MethodEstimating by analogy means comparing the proposed project to previously completed similar projects where cost information is known.
Actual data from the completed projects are extrapolated to estimate the proposed project. Estimating by analogy can be done either at the system level or the component level. The main strength of this method is that the estimates are based on actual project data and past experience. Differences between completed projects and the proposed project can accounted for manually.Bottom-Up MethodBottom-up estimation involves identifying and estimating each individual component separately, then combining the results to produce an estimate of the entire project. It is often difficult to perform a bottom-up estimate early in the life cycle process because the necessary information may not be available. This method is also commonly referred to as the engineering approach.Top-Down MethodThe top-down method of estimation is based on overall characteristics of the project.
This method is more applicable to cost estimates when only global properties are known. The top-down method is usually faster, easier to implement and requires minimal project detail.Expert Judgment MethodExpert judgment involves consulting with subject matter experts to use their experience and understanding of a proposed project to provide an estimate for the cost of the project. The obvious advantage of this method is the expert can factor in differences between past project experiences and requirements of the proposed project.Parametric or Algorithmic MethodThe algorithmic method involves the use of equations to perform cost estimates. The equations are based on research and historical data and use such cost per flying hour, or annual support costs.
Advantages of this method include being able to generate repeatable results, easily modifying input data, easily refining and customizing formulas, and better understanding of the overall estimating methods since the formulas can be analyzed.Rice (1999) presented an approach to Economic Service Life modeling. His primary focus is attempting to model the USAF FCC-135 aircraft. This aircraft is a unique case study in aging aircraft in that the aircraft is over forty years old, yet has accumulated the flying hours equivalent to a five-year-old commercial aircraft. Growing maintenance and repair costs for the FCC-135 fleet are only weakly dependent on the structural aging of these aircraft relative to their design service goals. The growing problem appears to due to “environmental aging”, as compared to “structural aging”. Additional depot maintenance downtime has caused a significant reduction of aircraft available to for operational service.ConclusionsThompson, of the Lexington Institute addressed the Military Procurement Subcommittee in 1999.
His remarks focused on the aging fleet of military aircraft. Thompson began his speech by pointing out the fact many Americans do not realize that only 3% of the federal budget is spent on weapons procurement and modernization and we currently spend about twice that (6%) on gambling. He pointed out in his very effective speech the gamble we are currently taking with our military. At stake here is sending our military personnel to war in obsolete, aging aircraft.
“The Navy needs to find a common airframe for all of the carrier-based support missions and start planning for its acquisition. But the administration’s fiscal year 2000-2005 spending plan contained almost nothing for a so-called “Common Support Aircraft.” So this is an aging aircraft problem unlikely to be resolved anytime soon.”The classic aircraft were constructed from aluminum alloys popular twenty to thirty years ago which have poor corrosion and Stress Corrosion Cracking (SCC) resistance properties. New aircraft are constructed from alloys, whose exfoliation-resistant tempers exhibit both corrosion and stress corrosion cracking resistance (NRC 1998).
Applications of aging airframe growth rates are therefore limited to aircraft constructed with similar materials and construction techniques. This concept of insuring similar period materials therefore excludes broad-brush application to modern aircraft, which tend to use composites extensively.Commercial airlines also typically have an ongoing fleet modernization program, constantly tailoring their fleet mix to present day flight routes and load factors. This selective retention also tends to reduce overall fleet mix costs.
This is a luxury the military is unable to match as they typically must maintain a fleet size specified by a higher command. Commercial airframe maintenance growth trends would tend to be lower than their military counterparts due to both their ongoing fleet modernization and selective retention.Basing locations for commercial aircraft also consider the relative environmental effects on aircraft when deciding where aircraft are to be based. Again, the military cannot be as selective due to military basing requirements. Fleet utilization is also a significant discriminator. Military Tanker / Transport aircraft may average 300 — 800 annual flying hours, whereas their commercial counterpart may average 3,500 flying hours annually.
For aging considerations where corrosion is the predominant consideration utilization is not a concern. However, on aircraft with high cyclic fatigue, adjustments would be necessary to insure comparability.Many of the aging aircraft represented in this study were from an era, which did not initially apply corrosion preventative compounds (CPC) during the manufacturing process. Aircraft, which remain in commercial service, however, must comply with the intent of Airworthiness Directive (AD91-07-19), which requires the inspection, and continued application of CPC compounds during scheduled heavy maintenance checks. Assessing the effectiveness of CPCs applied after the manufacturing process is difficult.
Effectiveness is very dependent on compound penetration into tight crevices such as lap joints. CPC effectiveness is also dependent on how well any existing corrosion was removed prior to CPC application. Additionally, due to the newness of the Airworthiness Directive (AD) driven CPC program, many aircraft are only now returning for their second heavy maintenance check since initial application of CPC. Measuring the effectiveness of the CPC program will take several repeat heavy maintenance visits.The problem this study focused on was trying to improve aging aircraft cost forecasting methodology. Currently no recognized cost models account for cost growth as a function of aging. No industry recognized factors have been developed which accurately predict airframe maintenance cost growth as a function of time.
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