United Airlines is disputing U.S. safety investigators' finding that it should have detected the tiny flaw in a critical engine part that disintegrated, causing the 1989 Iowa crash that killed 112 people.

The National Transportation Safety Board said Thursday the probable cause of the crash of United Flight 232 at Sioux City, Iowa, was "human factors limitations" in United's inspection and quality control procedures. The board issued the finding without dissent.Board members said that because none of hundreds of similar tests conducted over many years on similar parts had ever found a defect, laxity on the part of inspectors was to be expected.

Reporting on a yearlong investigation, the safety board said United's inspectors missed a fatigue crack in a critical area of a titanium fan disk in the tail engine of the DC-10 aircraft.

The plane, en route from Denver to Chicago, crashed 40 minutes after metal from its exploding tail engine shredded the hydraulic system used to control flight operations.

But United officials who attended the hearing said blame should have been placed on General Electric Aircraft Engines, which manufactured the part.