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 Federal investigators have identified the problem in last July's crash of a United Airlines flight in Iowa:
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 a structural flaw 
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that developed during the making of a titanium engine disk.
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 For several months, officials at the Federal Aviation Administration and the National Transportation Safety Board have suspected
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 that a metallurgical flaw in the disk led to a crack
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 that ultimately caused the tail engine to break apart in flight.
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 The explosion sent shards of metal flying,
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 severing the DC-10's hydraulic or control systems,
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 and led to the crash
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 that killed 112 people.
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 But investigators could confirm their theory only after the recent retrieval of a big chunk of Flight 232's tail engine from a cornfield near the Sioux City Airport in Iowa.
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 The safety board will begin four days of hearings on the accident tomorrow in Sioux City.
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 Among the issues
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 the board will examine 
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is whether United Airlines, a unit of UAL Corp., should have been able to detect the cracks through maintenance checks.
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 The engine involved was a CF6-6
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 made by General Electric Co.
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 Anthony Broderick, the FAA's acting executive director for regulatory standards and compliance, said
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 that recent tests of the failed engine disk indicate 
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that a flaw 
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-- known as hard alpha --
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 occurred in the titanium during its production almost 20 years ago.
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 He said
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 there wasn't any way
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 to detect the flaw at that time,
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 and that the process has since been changed 
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to decrease the chance
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 that such flaws would occur.
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 The FAA already has ordered 
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that all 232 disks
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 made by the old process
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 be removed from the planes
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 and subjected to an ultrasonic test in a water-submersion chamber.
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 Such tests make the FAA confident that a Sioux City-type accident won't happen again,
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 said Mr. Broderick.
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 A spokesman for GE said 
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that the company has been working with the FAA all along on this issue
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 and will comply fully with the required inspections.
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 But he also pointed out
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 that the recalls will have no impact on GE's engine production.
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 The CF6-6 series engines aren't being manufactured any more;
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 they are only being used in the DC-10 Series 10 planes currently in service,
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 he said.
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