On July 19, 1989, United Airlines Flight 232 crashed at Sioux City, Iowa after an uncontained engine failure of the tail engine earlier in the flight disabled all hydraulic systems and rendered most flight controls inoperable. The flight crew, assisted by a deadheading DC-10 flight instructor, performed a partially controlled emergency landing by constantly adjusting the thrust of the remaining two engines; 185 people on board survived, but 111 others died, and the aircraft was destroyed. The DC-10 was designed without backup flight controls because it was considered extremely improbable that all hydraulic systems would fail. However, due to their close proximity under the tail engine, the engine failure ruptured all three, resulting in total loss of control to the elevators, ailerons, spoilers, horizontal stabilizer, rudder, flaps and slats. Following the accident, hydraulic fuses were installed in the #3 hydraulic system below the tail engine on all DC-10 aircraft to ensure that sufficient control remains if all three hydraulic systems are damaged in this area.