American Eagle Flight 4184 had been holding for 32 minutes in a chilly drizzle last October when air traffic controllers in Chicago cleared the pilots to make a routine descent from 10,000 to 8,000 feet. Although the pilots did not know it, a dangerous ridge of ice had built up on the wings, and in a fraction of a second, to their complete and ultimately final terror, the pilots lost control of the ATR-72 turbo prop.

The plane’s controls moved on their own, tilting the right wing almost perpendicular to the ground. The pilots wrestled with the aircraft, but the wing tilted violently again, the plane flipped on its back and, without hope of recovery, the plane plunged toward earth.

All 68 people aboard were killed when the plane slammed into a soybean field near Roselawn, Ind., a crash so violent that few recognizable pieces of the plane were left intact.

It was a crash that did not have to happen.

A New York Times investigation has found that the Federal Aviation Administration had for years brushed aside repeated warnings from pilots and experts, and from the behavior of the plane itself, that something was awry. The failure to heed those warnings raises troubling questions that go beyond the Roselawn crash, questions about the procedures and safeguards of the agency itself.

The agency does not routinely monitor crashes abroad of foreign-made planes operating in this country — as happened with the ATR. It also relies heavily on manufacturers’ evaluations of their own airplanes — as with the ATR — rather than conducting independent tests. As a result, its own experts increasingly lack the hands-on knowledge to ask the right questions.

The F.A.A.’s bureaucratic culture keeps it from being aggressive, as does its reluctance to impose costly safety modifications on airlines. There have been assertions for years that issues like wind shear — and de-icing — become priorities only after a fatal crash brings them to public attention. The experts call it “tombstone technology,” and the ATR experience provides a compelling example.

The ATR-72 and the smaller ATR-42 were involved in a string of incidents in Europe and the United States, including a 1987 crash in the Alps that killed 37 people. In at least 20 incidents the planes faltered in icy conditions. Yet the F.A.A. failed to conduct a far-reaching, independent review of the plane’s design until after 68 people were killed in the United States in October.

“It’s a story of a series of linked incidents, and at least one accident, that all point basically in the same direction — that there is a problem in certain conditions with the aircraft,” said William D. Waldock, a professor of aeronautical science at Embry-Riddle Aeronautical University in Arizona and a leading safety expert. “But until the blood gets deep enough, there is a tendency to either ignore a problem or live with it.”

For their part, F.A.A. officials vigorously defend the agency’s actions in certifying the plane, and continuing to allow it to fly. The pattern, they insist, became clear only after the Roselawn crash. It is only in hindsight, they say, that the ATR crash seems as if it could have been prevented. ATR officials say much the same. The plane, they say, fully meets the standards set by American rules, and exhibits its flaw only when subjected to much more severe conditions.

When the F.A.A. certified the ATR to operate in this country, it meant that the plane met the agency’s standards for flying in icy conditions. The continuing problems with ice, however, inevitably prompted concerns — not only about the design of the plane but also about the stringency of the standards themselves. And while the agency signed off on design changes in the ATR, it never reconsidered its own ice standards.

But by the late 1980’s, experts inside and outside the F.A.A. were pressing the agency to do just that. Since the turbo props used by the growing commuter airlines flew at lower altitudes where freezing rain and drizzle are more prevalent, the agency needed to adopt new, more demanding standards, they argued. For the F.A.A., however, new standards would require new rules, new tests and new costly demands on the airlines.

Few in the agency’s top ranks favored such an undertaking.

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