Damage tolerance, or the design philosophy that allows for crack growth up to certain limits, depends ultimately on feedback from the fleet to assure that actual cracks on planes in service match predictions. In this respect, service difficulty reports (SDRs) are fundamental to damage tolerance, because they provide regulators with examples of cracking, the problems encountered, the fixes, and the ages of the aircraft.
While SDRs are key to managing fatigue, the Federal Aviation Administration (FAA) does not demand comprehensive or uniform reporting by the airlines via the SDR system (see ASW, Sept. 26). Yet, as co-authors Robert Eastin of the FAA and Steve Swift of Australia’s Civil Aviation Safety Authority (CASA) point out in a recent paper, every report must be investigated and analyzed to have confidence in the fatigue management system, or to correct it as required. Extracts of the paper are presented below with permission. It is the finest discourse on damage tolerance we have seen in recent years. The paper represents the personal views of the authors, not those of their respective regulatory agencies.
The authors liken fatigue management to facets of a diamond, in which the various facets represent aspects of the problem. The three Ds they talk about – Detectable, Duration, Dangerous – are vital. All three must be known to assure safety. They point out that advisory circular (AC) 91-56A, “Continuing Structural Integrity Program for Large Transport Category Airplanes,” does in fact mention the three Ds. However, AC 91-60, “The Continued Airworthiness of Older Airplanes,” does not mention them. Yet the interdependence of the three D’s, and the other facets of the diamond, are vital for assured safety.
The following excerpts are from “Rough Diamond: Two Regulators Review Damage Tolerance,” by Bob Eastin, FAA, and Steve Swift, CASA, Australia:
“In 1978, the FAA adopted ‘damage tolerance’ as the preferred choice for managing fatigue in airliners. …
“However, as we do our best to regulate ‘damage tolerance,’ we see problems serious enough to convince ourselves that we must do better. …
“Passengers buy tickets on aircraft 20 years (and 20,000 hours) old, their safety resting on inspection programs based on guesswork. …
“We propose five ‘facets’ [which they call the ‘diamond’ of damage tolerance]:
“Site. Where could cracks start? We often get this wrong. …
“Scenario. How will the cracks grow? For example, will there be one or many? Will they interact? Will cracks in one part start cracks in another? Wrong answers give a very rough diamond.
“Detectable. As a crack grows, this is the smallest size detectable, consistently, considering the inspection method and other factors. So, conservatively, it is the largest crack any single inspection could miss. Beware of optimism. For every lucky find of a small crack, there are many more misses of large ones. ‘Probability of detection’ curves from field trials can help.
“Dangerous. As the crack continues to grow, this is the size that is starting to be ‘dangerous,’ because the structure is about to lose the strength we want to assure.
“Duration. This is the time it will take a crack to grow from ‘detectable’ to ‘dangerous.’ It is our ‘safety window.’ …
“Against the trend, we think retirement needs reinstating as a viable (and sometimes necessary) alternative to inspection. It is more forgiving if we get a crack site wrong, as we often do. …
“The rules generally require ‘ultimate’ strength. The damage tolerance rules only require ‘limit’ strength, two-thirds of that. Is there a conflict? If cracking is predicted, but not yet reported in service, we hope there are a few, if any, weak aircraft. So, we accept the possibility that the strength of a few aircraft may fall to limit. Once cracking is reported, we try to act to assure fleet wide ultimate strength. … What do we do? We redesign either the inspections or the structure to assure ultimate strength. Experience indicates that it is usually more prudent to do the latter … Whichever, we want quick reporting to responsive designers. We want that for modification and repairs, not just basic aircraft. Unfortunately, in the United States and Australia our rules are weak …
“We are not getting the most from our expensive reporting systems and databases. We must investigate every report, using the diamond. Sometimes we only get one warning. Sometimes the next report comes from the crash site. …
“The common definition of an ‘aging aircraft’ as one 15 or more years old causes two problems. The first is complacency. Safety problems can start earlier, as airworthiness directives show. For example, the wings of two types of agricultural aircraft experienced dangerous fatigue cracking after only one year of flying. The second problem is paralysis. Some think managing fatigue in ‘aging aircraft’ is so mysterious, they don’t know where to start. So they don’t. More need to know the diamond [approach] is simple and versatile enough for all the ‘aging’ process: from when the aircraft leaves the factory, until it finally leaves the register.
“Finally, we still seem a long way from analyzing fatigue and corrosion together …
“Damage tolerance is not yet delivering to its full safety potential. It is not used as well as it should. It is not used as often as it could. … We hope the diamond stimulates debate on outstanding issues, because there is still much to do.
“[This] review will have been worthwhile if, one day, in a travel agency, we overhear:
“Client: ‘I want a hotel with 4 stars, and an airplane with diamond-standard maintenance.’
“Agent: ‘I know just the hotel. And, don’t worry, all airplanes have that safety feature now.’ “
Underpinning the “diamond standard” assurance of damage tolerance is rigorous SDR reporting, by which the location of crack propagation and crack growth can be tracked. As the co-authors of this paper indicate, the next report could come from a crash site.
The authors argue that to follow good practice, like the diamond, the rules “need rewriting not just revising.” Their paper includes three pages of proposed rules that are written in plain English and are a model of clarity. (For the full paper, see http://www.casa.gov.au/airworth/papers/roughdiamond.pdf; Eastin, e-mail [email protected])