Aging of U.S. Air Force Aircraft Final Report (1997) / Chapter Skim
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Part I Problem Definition and Status of the Aging Force
Part I Problem Definition and Status of the Aging Force
Pages 9-34
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From page 9... ...
Chapter 1 describes the background and objectives for the study. Chapter 2 is a summary of force management processes, future force projections, and an assessment of force management, including the Aircraft Structural Integrity Program.
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From page 11... ...
and durability and damage tolerance assessments of their older aircraft, already identified many potential problems, developed individual aircraft tracking programs, developed force structural maintenance plans, and taken many maintenance actions to protect safety and extend aircraft life. The Air Force has also initiated an aging aircraft research and development (R&D)
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From page 12... ...
The important issues related to the aging of aircraft engine structures were not included in the committee's task and would be best addressed separately in the future. This report summarizes the committee's assessment of the adequacy of the Air Force R&D program, assesses the force management process and its needs, identifies key technology issues and R&D needs, identifies and prioritizes R&D opportunities, and identifies and develops an overall strategy that addresses the Air Force's aging aircraft needs.
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From page 13... ...
The Air Force aging aircraft problem can be best understood by examining the existing force management process, future force projections, and the current structural condition of the many types of aging aircraft in the Air Force inventory. In this chapter, force management processes and future force projections are summarized, followed by the committee's assessment of the process, including ASIP.
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From page 14... ...
Data from the full-scale durability tests also assist in the development of the structural maintenance program required throughout the aircraft life cycle. The initial estimated weapon system phase-out point is also established based on estimates of safety limits and economic life considerations discussed in Chapter 4.
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From page 15... ...
This program is based in part on data packages generated by ASIP. The maintenance and sustainment program includes such elements as field maintenance programs depot maintenance programs modification and repair programs Sonic Flight vibration Flutter Interpretation and evaluation of test results spectra survey Structural maintenance Individual aircraft tracking records program · technology transition and insertion · weapon system assessments · ASIP updating It is the responsibility of the system program director to ensure that ASIP is continued on the weapon system throughout its operational life.
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From page 16... ...
The sand charts for each of the major operating commands, along with a brief description of each, are shown in the following sections. Air Combat Command Figures 2-1, 2-2, and 2-3 show the ACC force structure for the next 25 years, broken down into fighter, bomber, and attack; airlift and rescue systems; and other specialized systems.
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From page 17... ...
800 700 600 ~ 500 oh 400 a) O 300 LL 200 100 o | HH-60G Blackhawk| HH-X =7{ 1997 2002 2007 2012 2017 Fiscal year FIGURE 2-2 Force structure projection for the ACC airlift and rescue aircraft.
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1997 2002 2007 2012 2017 Fiscal year FIGURE 2-3 Force structure projection for other ACC aircraft. Source: JACG (1996)
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Continued efforts to employ and refine the AFMPP should ensure that structural economic life considerations are incorporated into the force structure planning. Service Life Projection Currently, there is no clear definition of all the elements that constitute the determination of structural economic life for aircraft systems, or a standard economic model to assist in determining when the costs of operating and maintaining 19 the system reach a level that clearly warrant replacement.
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From page 20... ...
It has been the implementation of the structural inspections and modifications that have been derived from the damage tolerance approach and applied to the aircraft weapon systems by the Air Force's logistics and operational support organizations that has so successfully protected the structural safety of Air Force aircraft for more than two decades. However, the committee is concerned that the extended use of old aircraft coupled with the current trends of reducing military budgets and manpower, increased reliance on contracted maintenance, use of commercial design practices rather than military specifications, and possible complacency of Air Force management (because of the greatly reduced number of aircraft lost due to structural failures in recent years)
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From page 21... ...
Also, sufficient R&D resources should be maintained to support and improve aging aircraft engineering, inspection, and maintenance and repair. Recommended R&D and engineering and management tasks are expanded on in Part II.
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From page 22... ...
The Air Combat Command's (ACC) fighter, bomber, and attack aircraft and the Air Education and Training Com mand's T-37 and T-38 trainer aircraft were either designed to be damage tolerant using the safe crack growth concept or were later analyzed on the basis of crack growth to establish safety limits and inspection requirements.
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From page 23... ...
Since the 1970s it appears that the Air Force's San Antonio Air Logistic Center and their prime contractor, Northrop-Grumman, have done a good job of maintaining structural safety and preventing wing failures through the use of safety inspections,4 structural modifications, design changes, and lower wing surface replacements. Additional full-scale wing fatigue testing has also been performed to identify critical areas in the new and modified structure.
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From page 24... ...
E, and H models. As pointed out in Appendix A, the major uncertainty about the C-130E/H airframe is the probable service life of the fuselage and the associated future structural maintenance needs.
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, the use in terms of both total flight hours and number of flights is low compared with their commercial counterparts and thus would not be expected to experience the onset of widespread fatigue cracking in many more years of operations. The high-time C-9s on the other hand have exceeded the contractor's original design life goals in terms of numbers of flights and total flight hours, but is still considerably less than the 102,000 flights and 78,000 hours that the aircraft manufacturer has now verified by retest and tear-down inspection of a high-time commercial aircraft.
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From page 26... ...
It is the team' s intent to identify aircraft with the highest potential for structural failure, define further damage tolerance analysis requirements, and consider other actions that may be necessary to AGING OF U.S. AIR FORCE AIRCRAFT minimize the potential for failure.
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From page 27... ...
The issues and needs identified in this chapter are the basis for the recommended engineering and management actions and near-term and longterm research and development presented in Part II of this report. The three primary mechanisms that can affect the structural health and longevity of the Air Force' s metallic aircraft structures are · corrosion · stress corrosion cracking · fatigue cracking (including low-cycle and high-cycle fatigue)
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From page 28... ...
CPCs are a critical part of maintenance programs to prevent and AGING OF U.S. AIR FORCE AIRCRAFT · generalized use of CPCs and development of CPCs that can be applied on external surfaces and that will penetrate and protect unsealed joints and around fastener heads on older aircraft structures2 · guidance for the application of advances in alloys and processes offering improved corrosion protection improved techniques to discover and roughly quantify hidden corrosion without requiring disassembly of the aircraft classification of corrosion severity, similar to current commercial aircraft practice, to provide guidance to maintenance actions improved understanding of the probable rates of corrosion and corrosion trends for specific operational aircraft for use in planning maintenance actions dehumidified storage of aircraft or dehumidification of susceptible areas of particular aircraft With improved detection methods and the implementation of improved corrosion prevention and control actions, the committee does not believe that physical corrosion damage per se will limit the structural life of Air Force aircraft.
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From page 29... ...
durability and damage tolerance assessment of the aircraft With continued vigilance, improvement in prevention and control procedures, and replacement of susceptible components with corrosion-resisting alloys with minimized residual 29 stresses, the committee believes that SCC problems in older Air Force aircraft can be managed and that SCC need not be a life-limiting damage mechanism. FATIGUE CRACKING Unlike corrosion and SCC, which the committee believes can be controlled and thus would not, in themselves, physi cally limit structural life, fatigue cracking is a direct result of aircraft use (i.e., load or stress cycles)
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From page 30... ...
in this environment to avoid superposition of fatigue and stress corrosion crack growth rates.3 Although the damage tolerance requirements and the ap proach for accounting for potential environmental effects have served the Air Force well over the past two decades, the committee continues to be concerned that, as structures age, as corrosion protection systems continue to deteriorate, and as materials corrode, there may be effects that have not been adequately considered. Specific corrosion concerns or issues that could affect safety limits and repeat inspection intervals include .
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From page 31... ...
Such a capability requires the development of an EIF database, correlated with full-scale structural test articles, for cracks that initiate because of fretting, very small defects, scratches, dings, and corrosion damage. Among the greatest NDI challenges is to develop methods that can reliably, rapidly, and cost effectively determine, without fastener removal or disassembly, if an aircraft has widespread fatigue cracking.
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From page 32... ...
In addition, NDE methods protect structural safety by detecting, providing quantifiable characterization, and screening fatigue cracking, stress corrosion cracking, and corrosion conditions that are, or could become, a flight-safety concern. However, field practices and implementation of NDE methodology to meet many aging aircraft problems are inadequate and often inconsistent with current technical capabilities (e.g., field systems often do not take advantage of technology used during production of the structural components)
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From page 33... ...
33 STRUCTURAL MAINTENANCE AND REPAIRS An effective airframe structural maintenance program evaluates (1) sources (root causes)
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From page 34... ...
. Commercial airframe manufacturers have been active in updating obsolete materials and process specifications in their core design and manufacturing practices and in their maintenance programs (Goranson, 1997~.
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