The accidents were difficult to explain at the time because the airframe

had flown fewer hours than any comparable aircraft in service.

had been certified without any structural testing at all.

had shown strength margins several times greater than any load a flight would impose.

was built from a material that had never before been used in aviation.

According to the passage, a large margin of static strength gives no protection against fatigue because

static tests are conducted at temperatures unlike those encountered in flight.

fatigue cracks form only in structures that have already been overloaded once.

pressurization imposes larger loads than any static test can reproduce.

The passage indicates that a fail-safe structure is one designed so that

a crack that begins is arrested by the surrounding structure rather than spreading unchecked.

no crack can begin anywhere in the airframe.

the aircraft can complete a flight after losing pressurization.

inspection is unnecessary between scheduled overhauls.

The author calls one part of the episode "uncomfortable" chiefly because

the fleet was returned to service before the cause had been established.

the water-tank test destroyed an otherwise serviceable aircraft.

the investigation took longer than the accidents warranted.

the engineers followed the accepted standard correctly and the standard was incomplete.

It can be inferred that the change from square-cornered openings to rounded ones was adopted because

rounded openings are cheaper to manufacture and inspect.

square corners weaken the material immediately around them during assembly.

stress concentrates at a sharp corner, which is where a fatigue crack is likely to begin.

rounded openings allow a larger window area for the same structural weight.

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Fatigue and the Lesson of the Comet

PRACTICE

100:00+4 −1MCQ Single Answer

Reading Passage

The de Havilland Comet entered service in 1952 as the world's first jet airliner, and for two years it was the most advanced aircraft flying. In January and April of 1954, two Comets broke apart in the air within months of each other, and the fleet was grounded. What made the accidents so difficult to explain was that the airframe was not, by the standards then applied, a marginal design. Tested against the loads it would carry, the structure had shown margins of strength several times greater than anything a flight would impose on it. The investigation that followed took an unusual form. A complete fuselage was submerged in a water tank and pressurized and depressurized repeatedly, simulating one flight after another. It failed after roughly three thousand cycles, and the fracture began at a rivet hole near the corner of an opening in the skin. The mechanism was fatigue. Stresses far below the level a structure can withstand once will, if applied and released many thousands of times, open cracks too small to see, and each cycle extends them a little further until the remaining material can no longer carry the load. A large margin of static strength offers no protection against this, because resistance to a single load and resistance to repeated loading are different properties of a material and are not predictable from one another. The consequences reshaped the discipline. Design questions shifted from whether a structure would hold to how many cycles it would survive; airframes were built to be fail-safe, so that a crack which did start would be arrested by the structure around it rather than running unchecked; inspection at fixed intervals became mandatory rather than prudent; and square-cornered openings, at whose corners stress concentrates, gave way to rounded ones. The uncomfortable part of the episode is the part usually omitted. The Comet's engineers were not negligent. They applied the accepted standard of their time correctly, and the standard itself was incomplete — which is a harder failure to guard against than carelessness, and a more common one.

The passage is primarily concerned with