Related Accidents / Incidents

The UAL Flight 232 Sioux City accident has links to at least three other airplane accidents/incidents, all of which occurred prior to the Sioux City accident. The first was the National Airlines DC-10 accident that occurred on November 3, 1973 near Albuquerque, New Mexico. During cruise, the fan assembly of the No. 3 engine disintegrated, resulting in loss of the inlet, fan case and many of the fan blades. Engine debris struck the fuselage, right wing root area, and engines No. 1 and No. 2. Engine debris that impacted the airplane caused damage to two of the aircraft's three hydraulic systems and failure of a passenger window. Debris from the failed engine punctured the No. 1 engine oil tank which was close to being empty by the time the airplane had completed its emergency landing.

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The second link is to Eastern Airlines Lockheed L-1011 that occurred on September 22, 1981. During climb out from Newark, New Jersey, the crew noticed an engine vibration indication for the number two (center) engine. The fan bearing on the center engine subsequently failed, liberating the fan assembly inside the tail compartment of the airplane. Three of the four hydraulic systems were lost due to massive system damage caused by the fan liberation. An uneventful diversion to New York's Kennedy Airport was accomplished.

The third link is to the Japan Airlines B-747 accident that occurred on August 12, 1985. The airplane experienced a sudden decompression at 24,000 feet over Japan when the aft pressure bulkhead failed. The accident investigation identified failure of the aft pressure bulkhead due to a repair that had been previously made. The bulkhead failure ruptured part of the fuselage vertical fin and caused the loss of all four aircraft hydraulic lines that resulted in loss of all aircraft primary control functions. The flight crew maintained minimal control of the airplane by manipulating thrust from the engines, but the airplane subsequently crashed into a mountain.

The Japan Airlines accident is linked to the Sioux City accident because the pressure bulkhead failure caused loss of all four aircraft hydraulic systems. The Japan Airlines and United Airlines Sioux City accidents showed that the lack of independence of the hydraulic systems from a single failure event such as a zonal hazard can result in an unsafe condition. System isolation can be comprised by lack of physical separation even though functional isolation exists.

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