United Airlines Flight 232 was a regularly scheduled United Airlines flight from Stapleton International Airport in Denver to O'Hare International Airport in Chicago, continuing to Philadelphia International Airport in Philadelphia, United States. On July 19, 1989, the McDonnell Douglas DC-10 serving the flight crash-landed at Sioux Gateway Airport in Sioux City, Iowa, after suffering a catastrophic failure of its tail-mounted engine due to an unnoticed manufacturing defect in the engine's fan disk, which resulted in the loss of all flight controls. Of the 296 passengers and crew on board, 112 died, while 184 people survived. Thirteen passengers were uninjured.
Despite the fatalities, the accident is considered a good example of successful crew resource management, still a relatively new concept at the time. Contributing to the outcome was the crew's decision to recruit the assistance of a company check pilot, on board as a passenger, to assist controlling the aircraft and troubleshooting of the problem the crew was facing. A majority of those aboard survived; experienced test pilots in simulators were unable to reproduce a survivable landing. It has been termed "The Impossible Landing", as it is considered one of the most impressive landings performed in the history of aviation.
Aircraft
The airplane, a McDonnell Douglas DC-10-10 (registration N1819U), was delivered in 1971 and 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 (takeoff-landing pairs). The aircraft was equipped with three General Electric CF6-6D high-bypass turbofan engines produced by General Electric Aircraft Engines (GEAE). The aircraft's No. 2 (tail-mounted) engine had accumulated 42,436 hours and 16,899 cycles of operating time immediately prior to the accident flight.
The DC-10 used three independent hydraulic systems, each powered by one of the aircraft's three engines, to power movement of the aircraft's flight controls. In the event of loss of engine power or primary pump failure, a ram air turbine could provide emergency electrical power for electrically powered auxiliary pumps. These systems were designed to be redundant, such that if two hydraulic systems were inoperable, the one remaining hydraulic system would still permit the full operation and control of the airplane. However, at least one hydraulic system must have fluid present and the ability to hold fluid pressure to control the aircraft. Like other widebody transport aircraft of the time, the DC-10 was not designed to revert to unassisted manual control in the event of total hydraulic failure. The DC-10's hydraulic system was designed and demonstrated to the Federal Aviation Administration (FAA) as compliant with regulations that "no single [engine] failure or malfunction or probable combination of failures will jeopardize the safe operation of the airplane..."
Crew
Flight 232's captain, Al Haynes, 57, had been hired by United Airlines in 1956. He had 29,967 hours of total flight time with United, of which 7,190 were in the DC-10.
Haynes' co-pilot was Bill Records, 48. He estimated that he had 20,000 hours of total flight time. He was hired first by National Airlines in 1969, and had worked for Pan American World Airways after that. He was hired by United in 1985, and had accrued 665 hours as a DC-10 first officer while at United.
Flight engineer Dudley Dvorak, 51, was hired by United Airlines in 1986. He estimated that he had about 15,000 hours of total flying time. While working for United, he had accumulated 1,903 hours as a flight engineer in the Boeing 727 and 33 hours as a flight engineer in the DC-10.
Denny Fitch, 46, was a training check pilot who was on Flight 232 as a passenger. He had studied the crash of Japan Air Lines Flight 123 which suffered a total hydraulic system failure and loss of flight controls, and he had practiced similar situations in a flight simulator. He was hired by United in 1968 and he estimated that, prior to working for United, he had accrued at least 1,400 hours of flight time with the Air National Guard, making his total flight time around 23,000 hours. His total DC-10 time with United was 2,987 hours, including 1,943 hours as a flight engineer, 965 hours as a first officer, and 79 hours as a captain.
There were eight flight attendants on board.
Accident
Takeoff and engine failure
Flight 232 departed from Stapleton International Airport in Denver at 14:09 Central Daylight Time, en route to O'Hare International Airport in Chicago with continuing service to Philadelphia.
At 15:16, while the airplane was making a slight right turn at its cruising altitude of 37,000 feet (11,000 m), the fan disk of its tail-mounted General Electric CF6 engine disintegrated explosively. The uncontained failure resulted in the engine's fan disk departing the aircraft, tearing out components including parts of the No. 2 hydraulic system and supply hoses in the process; these were later found near Alta, Iowa. Engine debris penetrated the aircraft's tail section in numerous places, including the horizontal stabilizer, severing the No. 1 and No. 3 hydraulic system lines where they passed through the horizontal stabilizer.
The pilots felt a jolt, and the autopilot disengaged. As First Officer Records took hold of his control column, Captain Haynes concentrated on the tail engine, the instruments for which indicated it was malfunctioning; he found its throttle and fuel supply controls jammed. At Dvorak's suggestion, a valve for fuel to the tail engine was shut off. This part of the emergency took 14 seconds.
Attempts to control the plane
Meanwhile, Records found that the airplane 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 (a nearly 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 level slowly.
While Haynes and Records performed the engine shutdown checklist for the failed engine, Dvorak observed that the gauges for fluid pressure and quantity in all three hydraulic systems were indicating zero. The loss of all hydraulic fluid meant that control surfaces were inoperative. The flight crew deployed the DC-10's air-driven generator in an attempt to restore hydraulic power by powering the auxiliary hydraulic pumps, but this was unsuccessful. The crew contacted United Airlines maintenance personnel via radio, but were told that the possibility of a total loss of hydraulics in a DC-10 was considered so remote that no procedure had been established for such an event.
The airplane was tending to pull right and oscillated slowly vertically in a phugoid cycle – characteristic of planes in which control surface command is lost. With each iteration of the cycle, the aircraft lost about 1,500 feet (460 m) of altitude. Dennis Fitch, an experienced United Airlines captain and DC-10 flight instructor, was among the passengers and volunteered to assist. The message was relayed by senior flight attendant Jan Brown Lohr to the flight crew, who invited Fitch into the cockpit; he began assisting at about 15:29.
Haynes asked Fitch to observe the ailerons through the passenger cabin windows to see if control inputs were having any effect. Fitch reported 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 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.
Air traffic control (ATC) was contacted and an emergency landing at nearby Sioux Gateway Airport was organized. The conversation was recorded by the airplane's cockpit voice recorder (CVR), including:
Sioux City Approach: "United Two Thirty-Two Heavy, the wind's currently three six zero at one one; three sixty at eleven. You're cleared to land on any runway."
Haynes: "[laughter] Roger. [laughter] You want to be particular and make it a runway, huh?"
ATC asked the crew to make a left turn to keep them clear of the city:
Haynes: "Whatever you do, keep us away from the city."
Haynes later noted, "We were too busy [to be scared]. You must maintain your composure in the airplane, or you will die. You learn that from your first day flying."
Crash landing
As the crew began to prepare for arrival at Sioux Gateway Airport, 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, no change of the controllability of the aircraft resulted.
Landing was originally planned for 9,000-foot (2,700 m) Runway 31. Difficulties in controlling the aircraft made alignment with the runway almost impossible. While dumping some of the excess fuel, the airplane executed a series of mostly right-hand turns (turning the airplane in this direction was easier) with the intention of aligning with Runway 31. When they finished they were instead aligned with the closed 6,888-foot (2,099 m) Runway 22, and had little ability to maneuver. Runway 22 had been closed permanently a year earlier. 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 airplane touched down.
ATC also advised that a four-lane Interstate highway ran north and south just east of the airport, which they could land on if they did not think they could make the runway. Captain Haynes replied that they were passing over the interstate at that time and they would try for the runway instead.
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 was unable to control either the airspeed or the sink rate. On final approach, the aircraft's forward speed was 220 knots (250 mph; 410 km/h) and it had a sink rate of 1,850 feet per minute (9.4 m/s), while a safe landing would require 140 knots (160 mph; 260 km/h) forward speed and 300 feet per minute (1.5 m/s) sink rate. Moments before landing, the roll to the right suddenly worsened significantly and the aircraft began to pitch forward into a dive; Fitch realized this and pushed both throttles to full power in a desperate, last-ditch attempt to level the plane. It was now 16:00. The CVR recorded these final moments:
First Officer Records: "Close 'em off."
Captain Haynes: "Left turn, close 'em off."
First Officer Records: "Pull 'em all off."
Captain Fitch: "Nah, I can't pull 'em off or we'll lose it, that's what's turning ya."
First Officer Records: "Okay."
Captain Fitch: "Back, Al!"
Captain Haynes: "Left, left throttle, left, left, left, left, left, left, left, left, left, left, left!"
Ground Proximity Warning System: "Whoop whoop pull up. Whoop whoop pull up. Whoop whoop pull up."
Captain Haynes: "Everybody stay in brace!"
GPWS: "Whoop whoop pull up."
Captain Haynes: "God!"
[Sound of impact]
End of recording.