United Airlines Flight 232 was a regularly scheduled United Airlines flight from Denver to Chicago, continuing to Philadelphia. On July 19, 1989, the DC-10 (registered as N1819U) serving the flight crash-landed at Sioux City, Iowa, after suffering a catastrophic failure of its tail-mounted engine, which led to the loss of many flight controls. At the time, the aircraft was en route from Stapleton International Airport to O'Hare International Airport. Of the 296 passengers and crew on board, 111 died in the accident and 185 survived, making the crash the fifth-deadliest involving the DC-10, behind Turkish Airlines Flight 981, American Airlines Flight 191, Air New Zealand Flight 901, and UTA Flight 772. Despite the deaths, the accident is considered a prime example of successful crew resource management because of the large number of survivors and the manner in which the flight crew handled the emergency and landed the airplane without conventional control.
The airplane, a McDonnell Douglas DC-10-10 (registration N1819U), was delivered in 1973 and had been owned by United Airlines since then. Before departure on the flight from Denver on July 19, 1989, the airplane had been operated for a total of 43,401 hours and 16,997 cycles (a takeoff and subsequent landing is considered an aircraft cycle). The airplane was powered by CF6-6D high-bypass-ratio turbofan engines produced by General Electric Aircraft Engines (GEAE).
Captain Alfred Clair Haynes, 57, was hired by United Airlines in 1956. He had 29,967 hours of total flight time with United Airlines, of which 7,190 were in the DC-10.
First Officer William Roy Records, 48, was hired by National Airlines in 1969. He subsequently worked for Pan American World Airways. He estimated that he had approximately 20,000 hours of total flight time. He had 665 hours as a DC-10 first officer.
Second Officer Dudley Joseph Dvorak, 51, was hired by United Airlines in 1986. He estimated that he had approximately 15,000 hours of total flying time. He had 1,900 hours as a second officer in the Boeing 727 and 33 hours as a second officer in the DC-10.
Training Check Airman Captain Dennis Edward Fitch, 46, was hired by United Airlines in 1968. He estimated that, prior to working for United, he had accrued at least 1,400 hours of flight time with the Air National Guard, with a total flight time of approximately 23,000 hours. His total DC-10 time with United was 3,079 hours, of which 2,000 hours were accrued as a second officer, 1,000 hours as a first officer, and 79 hours as a captain. He had learned of the crash of Japan Airlines Flight 123, caused by a catastrophic loss of hydraulic control, and had wondered if it was possible to control an aircraft using throttles only. He had practiced under similar conditions on a simulator.
Flight 232 took off at 14:09 CDT from Stapleton International Airport, Denver, Colorado, bound for O'Hare International Airport in Chicago with continuing service to Philadelphia International Airport.
At 15:16, while the plane was in a shallow right turn at 37,000 feet, the fan disk of its tail-mounted General Electric CF6-6 engine explosively disintegrated. Debris penetrated the tail in numerous places, including the horizontal stabilizer, puncturing the lines of all three hydraulic systems.
The pilots felt a jolt, and the autopilot disengaged. As Records took hold of his control column, Haynes focused on the tail engine, whose instruments indicated it was malfunctioning; he found its throttle and fuel supply controls jammed. At Dvorak's suggestion, a valve cutting fuel to the tail engine was shut off. This part of the emergency took 14 seconds.
Meanwhile, Records found that the plane did not respond to his control column. Even with the control column turned all the way to the left, commanding maximum left aileron, and pulled all the way back, commanding maximum up elevator – inputs that would never be used together in normal flight – the aircraft was banking to the right with the nose dropping. Haynes attempted to level the aircraft with his own control column, then both Haynes and Records tried using their control columns together, but the aircraft still did not respond. Afraid the aircraft would roll into a completely inverted position (an unrecoverable situation), the crew reduced the left wing-mounted engine to idle and applied maximum power to the right engine. This caused the airplane to slowly level out.
The various gauges for all three hydraulic systems were registering zero. The three hydraulic systems were separate, so that failure of any one of them would leave the crew with full control, but lines for all three systems shared the same narrow passage through the tail where the engine debris had penetrated, and thus control surfaces were inoperative. The crew contacted United maintenance personnel via radio, but were told that, as a total loss of hydraulics on the DC-10 was considered "virtually impossible", there were no established procedures for such an event.
The plane was tending to pull right, and slowly oscillated vertically in a phugoid cycle – characteristic of planes in which control surface command is lost. With each iteration of the cycle, the aircraft lost approximately 1,500 feet (460 m) of altitude. On learning that Fitch, an experienced United Airlines captain and DC-10 flight instructor, was among the passengers, the crew called him into the cockpit for assistance.
Haynes asked Fitch to observe the ailerons through the passenger cabin windows to see if control inputs were having any effect. Fitch reported back that the ailerons were not moving at all. Nonetheless, the crew continued to manipulate their control columns for the remainder of the flight, hoping for at least some effect. Haynes then asked Fitch to take over control of the throttles so that Haynes could concentrate on his control column. With one throttle in each hand, Fitch was able to mitigate the phugoid cycle and make rough steering adjustments.
As the crew began to prepare for arrival at Sioux City, they questioned whether they should deploy the landing gear or belly-land the aircraft with the gear retracted. They decided that having the landing gear down would provide some shock absorption on impact.The complete hydraulic failure left the landing gear lowering mechanism inoperative. Two options were available to the flight crew. The DC-10 is designed so that if hydraulic pressure to the landing gear is lost, the gear will fall down slightly and rest on the landing gear doors. Placing the regular landing gear handle in the down position will unlock the doors mechanically, and the doors and landing gear will then fall down into place and lock due to gravity. An alternative system is also available using a lever in the cockpit floor to cause the landing gear to fall into position. This lever has the added benefit of unlocking the outboard ailerons, which are not used in high-speed flight and are locked in a neutral position. The crew hoped that there might be some trapped hydraulic fluid in the outboard ailerons and that they might regain some use of flight controls by unlocking them. They elected to extend the gear with the alternative system. Although the gear deployed successfully, there was no change in the controllability of the aircraft.
Landing was originally planned on the 9,000-foot (2,700 m) Runway 31. Difficulties in controlling the aircraft made lining up almost impossible. While dumping some of the excess fuel, the plane executed a series of mostly right-hand turns (it was easier to turn the plane in this direction) with the intention of lining up with Runway 31. When they came out they were instead lined up with the shorter (6,888 ft) and closed Runway 22, and had little capacity to maneuver. Fire trucks had been placed on Runway 22, anticipating a landing on nearby Runway 31, so all the vehicles were quickly moved out of the way before the plane touched down. Runway 22 had been permanently closed a year earlier.
ATC also advised that I-29 ran North and South just East of the airport which they could land on if they did not think they could make the runway. The pilot opted to try for the runway instead.
The plane landed askew, causing the explosion and fire seen in this still from local news station video.
Fitch continued to control the aircraft's descent by adjusting engine thrust. With the loss of all hydraulics, the flaps could not be extended and since flaps control both the minimum required forward speed and sink rate, the crew were unable to control both airspeed and sink rate. On final descent, the aircraft was going 220 knots and sinking at 1,850 feet per minute (approximately 407 km/h forward and 34 km/h downward speed), while a safe landing would require 140 knots and 300 feet per minute (approximately 260 km/h and 5 km/h respectively). Fitch needed a seat for landing; Dvorak offered up his own, as it could be moved to a position behind the throttles. Dvorak sat in the cockpit's jump seat for landing. Fitch noticed the high sink rate and that the plane started to yaw right again, and pushed the throttles to full power in an attempt to mitigate the high sink rate and level the plane.
There was not enough time for the flight crew to react. The tip of the right wing hit the runway first, spilling fuel, which ignited immediately. The tail section broke off from the force of the impact, and the rest of the aircraft bounced several times, shedding the landing gear and engine nacelles and breaking the fuselage into several main pieces. On the final impact, the right wing was shorn off and the main part of the aircraft skidded sideways, rolled over onto its back, and slid to a stop upside-down in a corn field to the right of Runway 22. Witnesses reported that the aircraft "cartwheeled" end-over-end, but the investigation did not confirm this. The reports were due to misinterpretation of the video of the crash that showed the flaming right wing tumbling end-over-end and the intact left wing, still attached to the fuselage, rolling up and over as the fuselage flipped over.