When Airplanes Feel Fatigued

When it comes to things that fly, metal fatigue is a bad thing. How safe are our airplanes, and what does the future hold?



One of the time-honored conventions of aviation journalism is that every article about metal fatigue has to start with mention of the "ill-fated Comet."

That requirement has now been met.

Metal fatigue did not begin with the Comet. By the middle of the 19th century ferrous metals-irons and steels-were in common use in industrial machinery, and it had become apparent that components that were amply strong for the work they were required to do were nevertheless liable to break after long use. In 1849 researchers conducted the first systematic investigations of what would come to be termed metal fatigue, and confirmed what countless shop workers already knew: that metal parts that undergo many repeated strains eventually fail at a loading far below their original ultimate strength. By the 1870s, systematic tests of various alloys had been conducted under repeated loads of different types, and the behavior of metals in fatigue-producing conditions had been well documented. What was happening was apparent; why and how it was happening was not yet understood.

The basic facts were simple. There is, to begin with, a level of loading below which no appreciable fatigue takes place in ferrous metals. Called the fatigue limit, it is around a third to a half of the tensile yield strength, depending on the material. Repeated applications of higher stresses lead to eventual failure, with the intensity of the stress, number of cycles, order of high- and low-stress events, alloy types, and many other factors, including even surface finish, corrosion and minor manufacturing variations, determining the "fatigue life" of a component. Unlike iron and steel, aluminum does not have a clear fatigue limit; it is assumed to accumulate fatigue at any level of stress.

Paradoxically, fatigue does not cause a progressive loss of strength; a partly fatigued part remains just as strong as a new one until, at some point, cracks develop. Furthermore, until cracking begins there are no detectable signs of fatigue, which, according to the current hypothesis, build up at the invisible atomic or molecular level, not, as was originally guessed, at the microscopic (but visible) level of the metal's crystal structure. Fatigue tests typically involve inflicting millions, or hundreds of millions, of load reversals upon a sample part of standard shape and surface finish, and generating a curve of stress levels against cycles-to-failure. Data collected from generic test items are applied to all sorts of parts, using computational "finite-element" stress analysis to predict the stress at any location. Stress concentrations in complex parts can be predicted with reasonable accuracy, but since uncontrollable variables like bolt fits and the tightness of nuts and rivets can significantly affect stresses, the judgment and experience of the structural engineer remain an essential part of design.

Fatigue could be minimized simply by making load-bearing parts large enough. But flight vehicle structures need to be light. Instead, assumptions about service life and the frequency of various kinds of loadings are used to decide what fraction of its maximum strength a given part can be allowed to use, and how often.

One type of fatigue that is so routine that we don't even think of it as fatigue is cracking in parts like engine baffles or mounts. Because of the high frequency of their vibrations, engines produce fatigue much faster than turbulence or maneuvering do. Other highly stressed parts, like wing or tail spars or pressurized structures, accumulate cycles more slowly; but they may be subject to much higher stresses and so exhaust their fatigue lives at a higher rate.

When we talk about the aging of the civil aviation fleet, we are talking about two slow destroyers of structures: corrosion and fatigue. Although there are types of corrosion that remain hidden, most can be found by careful inspection. Fatigue, on the other hand, commonly goes undetected until it manifests itself by some catastrophic event like the loss of a wing or the bursting of a pressurized fuselage. Events of this kind are rare-fewer than three percent of aircraft accidents involve structural failures, and only a fraction of those involve fatigue -but we have seen a number of them, including several bursting Boeings, and their psychological impact is tremendous. Nevertheless, most pilots are under the impression that airplanes that have not had identified fatigue problems are "fatigue free" and as good as new, regardless of their age. This is an illusion. Until cracking begins, it is for all practical purposes impossible to tell whether it will begin in 20 years, or tomorrow.

The entire fleet of Beech T-34s-tandem two-seat trainers based on the Bonanza airframe, most of which are now in civilian hands-has languished for months under an FAA grounding order after three airplanes lost wings while maneuvering. All three were being used by commercial operators for simulated air combat. The fleetwide grounding has aroused the ire of T-34 owners who argue that daily dogfights obviously subject airplanes to far higher loads than average users do. The FAA, on the other hand, has taken the position that failures in airplanes subject to heavy use may presage similar fatigue problems in the entire fleet.

All metal airframes are subject to metal fatigue, because it is impossible to operate in the Earth's atmosphere without incurring structural stresses from turbulence. Turbulence-related stresses seldom exceed a couple of Gs, however. Ordinary wear and tear takes most airplanes out of the fleet before fatigue becomes a safety issue.

Still, there is reason to expect that fatigue may affect airplanes increasingly in the future. For one thing, there is little difference in performance and utility between airplanes built just after World War II and those built 10 years ago, and so used airplanes, even quite old ones, retain their value well. The high and rising price of new aircraft makes old workhorses, like the Cessna 402, attractive to small operators. The average age of the entire civil fleet, excluding airliners, therefore tends to increase. In the quest for productivity, utilization rates rise. At the same time, ex-military aircraft, such as C-130s, find their way into various civil applications, like firefighting, where they are subject to high stress levels but may no longer receive the systematic maintenance that they got during their military careers. Finally, more large aircraft are being designed in accordance with "damage tolerance principles" under which some fatigue cracking is expected, but is supposed to be detected and corrected under a program of regular inspections.

At present, most owners of personal and business aircraft can look upon the T-34 and 400-series Cessna situations with interested indifference, since it is not yet their ox that is gored. In time, however, other types will approach the end of their fatigue lives, and the perplexing administrative problem of how to determine a fair and reasonable balance between risk of failure and cost of remediation will grow. The problem of metal fatigue will certainly become more common in the future, not less.