The U.S. Air Force has many old aircraft that form the backbone of the total operational force structure. The oldest are the more than 500 jet tanker aircraft, the KC-135, that were first introduced into service more than 40 years ago. The B-52H bomber, the C-130 airlifter, the T-38 trainer, and the T-37A primary trainer all became operational 35 to 40 years ago; the C-141 and C-5A airlifters, 25 to 35 years ago; the F-15 air superiority fighter, the A-10 close air support aircraft, and the E-3 (AWACS), 20 to 25 years ago; and the F-16 multirole fighter and the KC-10 jet tanker, 15 to 20 years ago. Of these, only the C-141 is currently being replaced (by the C-17). Other replacements are in various stages of development for the T-37A (by the JPATS), the F-15 (by the F-22), and the F-16 (by the Joint Strike Fighter). For the most part, these replacements are a number of years away, and the program schedules continue to be constrained by and subject to the vagaries of annual funding cycles. For example, at best, it will be at least another 15 to 20 years before there will be a significant number of replacements for the F-16 combat fighter force. The remainder of the aircraft mentioned above have no planned replacements and are expected to remain in service an additional 25 years or more.
To varying degrees, all of these older aircraft either have encountered, or can be expected to encounter, aging problems such as fatigue cracking, stress corrosion cracking, corrosion, and wear. The challenge to the Air Force management and technical community is to meet the following objectives:
Objective A. Identify and correct problems that could threaten aircraft safety.
Objective B. Prevent or minimize problems that could become an excessive economic burden or adversely affect force readiness.
Objective C. For the purpose of future force planning, have the methodology to predict when the maintenance burden will become so high, or the aircraft availability so poor, that it will no longer be viable to retain the aircraft in the inventory.
The Air Force has been aware of these objectives for a number of years and has, through their Aircraft Structural Integrity Program (ASIP) 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) program that is intended to support ASIP and address identified needs in the areas of widespread fatigue damage, corrosion-fatigue interactions, structural repairs, dynamics, health monitoring, non-destructive evaluation and inspection (NDE/I), and various aircraft subsystems (Rudd, 1996).
The Air Force requested that the National Research Council, through the National Materials Advisory Board, conduct this study with the following specific objectives:
identify an overall strategy that addresses the Air Force aging aircraft needs
recommend and prioritize specific technology opportunities in (1) fatigue, corrosion-fatigue interactions, and stress corrosion cracking; (2) corrosion prevention and mitigation; (3) nondestructive inspection; (4) maintenance and repair; and (5) failure analysis and life prediction technologies
complement, rather than duplicate, the efforts of industry, the Federal Aviation Administration, the National Aeronautics and Space Administration, other military services, and international organizations
To address the overall objectives, the committee performed the following tasks:
reviewed and analyzed the structural histories, problems, and force management procedures employed on the older Air Force aircraft to assist in identifying research needs and in developing a recommended overall aging aircraft strategy
reviewed and analyzed critical degradation and failure mechanisms associated with aging aircraft that have been experienced to date or can be expected to be experienced in the future