16:00   NDI, Inspections and Maintenance
Chair: Graham Clark
30 mins
Russell Wanhill, D.J. Platenkamp, T. Hattenberg, A.F. Bosch, P.H. de Haan
Abstract: ABSTRACT The pressure cabin MegaLiner Barrel (MLB) fatigue test was part of the Airbus 380 development programme. The NLR has done teardowns of GLARE (GLAss REinforced aluminium laminate) structural features from several key locations of the MLB: a window area, a beam above the passenger door, and stringer couplings. The teardowns began with Non-Destructive Inspection (NDI), and were followed by fractographic investigation of NDI-indicated cracks in the window and door beam locations. The main objectives were: • verification of NDI techniques and capabilities • determination of fatigue initiation and crack growth behaviour • provision of data to verify crack growth models for GLARE. The overall results demonstrated very good NDI capabilities and high fatigue damage tolerance by the GLARE structural features. The longest aluminium layer crack in the door beam had an almost constant growth rate, which agrees with model predictions.
30 mins
Jon Mowery, Robert Eastin
Abstract: Damage tolerance requirements were first introduced into 14 CFR 25.571 just over 30 years ago with Amendment 45 in 1978. The intent was to increase the level of safety relative to the threats of fatigue, corrosion and accidental damage. The requirements of § 25.571 remained virtually unchanged for 20 years. The current requirements were put into place in 1998 with Amendment 96. Again the intent was to increase the level of safety. The changes made with Amendment 96 were very significant from both a philosophical and practical perspective. This paper focuses on the fatigue threat only and considers three key elements of the requirements relative to where we started in 1978 and where we are today. The first element considered is the role of inspection in assuring continued airworthiness. Safety-by-inspection was the preferred fatigue management strategy of Amendment 45 although there were provisions made for “other procedures” to be used. It was implied, if not explicit, that almost without exception inspections could and should be used by themselves to manage fatigue in primary airframe structure. Consequently, resulting fatigue management programs approved by the FAA for new type designs relied almost exclusively on inspections to insure safety. Few if any replacements or modifications were included in the Supplemental Structural Inspection Documents or Airworthiness Limitation Section of the Instructions for Continued Airworthiness. The heavy reliance on inspection to maintain adequate safety changed dramatically with Amendment 96 in 1998. Amendment 96 along with supporting guidance material effectively states that it is expected that there will be a significant number of areas where inspections cannot be relied on to detect cracking after it has initiated and therefore it is necessary to design those areas so that fatigue cracking is precluded for some period of time. The second element considered is the operational life during which inspections or other procedures are meant to preclude catastrophic failures due to fatigue. Although Amendment 45 specifically states that safety must be insured throughout the operational life of the airplane there was no explicit requirement to set a bound on life. It has been standard practice to leave the life unbounded for structures certified to the damage tolerance requirements of § 25.571(b). This practice implied that it was possible to establish all the inspections and other procedures that would ever be needed to operate an airplane indefinitely. In reality this is not possible nor is it necessary. Amendment 96 did, as previously discussed, recognize that there would be areas of the structure where fatigue cracking could not be managed adequately with inspection alone. For these areas a demonstration, based on full scale fatigue test evidence, that dangerous cracking will not will not occur prior to the design service goal of the airplane is required. However, there is no requirement to set a corresponding bound on the operational life. The unbounded operational life conundrum remains. The last element considered is the concept of allowing a period of time to elapse before starting to inspect (i.e. an inspection threshold). Although it was an industry standard and FAA accepted practice to not require inspections to start until many years after the airplane went into service Amendment 45 was silent on this issue. At Amendment 96 however, prescriptive and problematic requirements for setting thresholds were introduced. The implicit assumptions and logic of these requirements are examined. In closing it is concluded that we haven’t quite got it right. Supporting rationale is provided and recommendations are made for changes needed. It is noted that most of the changes were previously recommended by the FAA chartered Aviation Rulemaking Advisory Committee in 2003 but never acted on by the authorities.
30 mins
Gregory Shoales, Scott A Fawaz, Molly Walters
Abstract: Fleet managers depend upon various tools to assure the safe operation of their aircraft. Aircraft which are managed by damage tolerance depend upon some sort of model to track the progression of damage and to schedule inspections. Various nondestructive inspection (NDI) techniques are employed a specific intervals to help verify the damage growth model predictions. NDI indications may give input to the next inspection interval, dictate a repair action or in more drastic cases support retirement of a particular aircraft. It is therefore vital that the NDI indication is fully understood as it relates to an actual damage state. The fleet manager similarly relies upon the accuracy of continuing damage model predictions. Any uncertainty in the inspection data and/or the model predictions leads to corresponding uncertainty in the actual condition of a given aircraft. Fleet managers can reduce risk to safety by choosing the conservative side of uncertainty. However, such conservatism can increase the risk to aircraft availability. Performing a structural teardown analysis program of one or more aircraft with known service history provides precise damage data resulting from a given usage. These data can then be used to validate NDI methods, update damage models and reduce uncertainty in the damage condition assessment of the remaining fleet. In the past six years the USAF Academy Center for Aircraft Structural Life Extension (CAStLE) has been involved, at various levels, in multiple teardown analysis programs. CAStLE’s level of involvement has ranged from consultant through failure analysis support to planning and executing all elements of an entire program. In each program large regions of selected aircraft were disassembled by precision means, cleaned of all coatings and inspected by a variety of nondestructive inspection (NDI) techniques. NDI indications were then evaluated by failure analysis methods to determine the root cause of failure—the type of damage which resulted in the NDI indication. It is useful to the aircraft manager to distinguish between damage findings as to whether they were a result of aircraft usage or not. Damage resulting from aircraft usage, ether flight hours or the passage of time, can be further grouped into corrosion mechanisms, crack growth mechanisms and combinations of the two. Knowing the precise damage mechanism is crucial to the fleet manager as it goes to the type of continuing damage model used to evaluate that damage. Often times a large number of damage findings show the NDI indication to be the result of artifacts which are not related to aircraft operations but rather aircraft manufacture or maintenance. Such artifacts include poor fastener hole quality and a variety of metallurgical anomalies. Understanding the existence of such findings is also important to the fleet manager. This is because they are called upon to make decisions based on NDI indications without always having the luxury of knowing whether it was a result of operations or not. Damage findings such as these, while they are not managed by any continuing damage model, are also of interest because they can nucleate crack and/or corrosion damage mechanisms. This work presents a summary of findings from teardown programs conducted on more than eight aircraft ranging from small military trainers to large transports. Comparisons are presented for damage types and categories of aircraft. Furthermore, the damage scale is categorized in relationship to the scale of the damaged component. Lastly, damage type and scale is presented categorized by the type of damaged component (skin, stiffener, bulkhead, etc.).