On July 19, 1989, a United Airlines DC-10 aircraft, traveling from Denver to Philadelphia with a stop in Chicago, encountered a catastrophic failure of its rear engine. The malfunction was caused by an undetected manufacturing defect in the fan disk of the engine — a large, spinning component that compresses air before it enters the combustion chamber. At an altitude of more than 11,000 meters, the fan disk disintegrated, sending debris through the rear of the plane and severing all three of the aircraft’s independent hydraulic systems. These systems are designed to provide redundancy, meaning if one fails, the others should still function. However, the simultaneous failure of all three was considered so unlikely that it had not been addressed in the plane’s design or safety assessments. The incident occurred at 15:16 when the second engine’s fan disk shattered, sending metal fragments through the plane’s rear section. The pilots were suddenly left without any way to control the aircraft’s flight surfaces. The co-pilot found his controls unresponsive, and the captain, Al Haynes, took over. He reduced the power of the first engine, allowing the plane to stabilize into a level flight. However, the aircraft then began to oscillate violently up and down, a phenomenon known as the phugoid cycle, which makes it extremely difficult to control without hydraulic assistance. With no hydraulics, the crew had to rely on adjusting the thrust of the two remaining engines to maneuver the plane. Unexpectedly, a passenger named Denny Fitch, who was a DC-10 instructor on leave, entered the cockpit and offered his expertise. He helped the crew adjust the engine power to turn, climb, and descend — a technique that had not been covered in any training manual or simulator prior to the incident. Air traffic control in Minneapolis became aware of the situation and suggested an emergency landing. After several violent oscillations, the crew decided to divert to Sioux Gateway Airport, which allowed the plane to land on any of its runways. To minimize the risk to the city, the pilots kept the plane away from populated areas and targeted runway 31. They performed a series of right turns to burn off excess fuel before the final approach. Deploying the landing gear was a challenge, as the loss of hydraulics made the normal mechanism inoperative. However, the DC-10’s design allows the landing gear to descend slightly under its own weight if hydraulic pressure is lost. The crew decided to deploy the gear, hoping it would cushion the impact. However, the final approach did not go as planned. The plane descended too quickly and tilted upon landing, causing the right wing to break off and the fuselage to slide into a cornfield. Of the 285 passengers and 11 crew members on board, 111 people lost their lives, including one flight attendant. More than 180 survivors escaped from the flaming wreckage — a survival rate investigators called nearly miraculous given the severity of the failure. The Sioux City accident led to significant changes in aviation safety. New regulations were introduced for turbine manufacturing and aircraft design to prevent similar failures of control systems. Safety valves were added to the remaining DC-10 fleet, and hydraulic piping was reconfigured to ensure that a single breach would not disable an entire system. These changes helped improve the safety of future flights.