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, 112 died during the accident, while 184 people survived.[a] The crash was the fifth-deadliest one 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 how the flight crew handled the emergency and landed the aeroplane without conventional control.
AIRCRAFT
The aeroplane, 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 aeroplane 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 aeroplane was powered by CF6-6D high bypass-ratio 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 before 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, an air-driven generator could be deployed that would provide 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 aeroplane. 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 era, 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 aeroplane...
CREW
Flight 232's captain, Alfred C. (Al) 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.
Haynes' co-pilot was first officer William R. (Bill) Records, 48, first 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 was hired by United Airlines in 1985 and had accrued 665 hours as a DC-10 first officer while at United.
Flight Engineer Dudley J. 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.
Dennis Edward Fitch, 46, a training-check airman aboard Flight 232 as a passenger, was hired by United Airlines in 1968. He estimated that, before working for United, he had accrued at least 1,400 hours of flight time with the Air National Guard, with a total flight time around 23,000 hours. His total DC-10 time with United was 2,987 hours, including 1,943 hours accrued as a flight engineer, 965 hours as a first officer, and 79 hours as a captain. Fitch had learned of the 1985 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 practised under similar conditions on a simulator.
Eight flight attendants were also aboard the flight.
EVENTS
TAKEOFF AND ENGINE FAILURE
Flight 232 took off at 14:09 Central Daylight Time 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 its cruising altitude of 37,000 feet (11,000 m), the fan disk of its tail-mounted General Electric CF6-6 engine explosively disintegrated. 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 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.
ATTEMPTS TO CONTROL PANNELS
Photo of United Airlines Flight 232 from the NTSB report, with the damage done by the second engine, highlighted 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 the elevator – inputs that would never be used together in normal flight – the aircraft was banking to the right with the nose drops. 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 aeroplane to slowly level out.
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 maintenance personnel via radio but were told that as a total loss of hydraulics on the DC-10 was considered "virtually impossible", no procedures were established 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 about 1,500 feet (460 m) of altitude. Fitch, an experienced United Airlines captain and DC-10 flight instructor, was among the passengers and volunteered to assist. The message was relayed by a flight attendant to the flight crew, who invited Fitch up to the cockpit; he arrived and 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 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.
CRASH LANDING
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, no change in the controllability of the aircraft resulted.
The 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 (turning the plane in this direction was easier) intending to line up with Runway 31. When they came out they were instead lined up with the closed 6,888-foot (2,099 m) Runway 22 and had little capacity to manoeuvre. 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 a four-lane Interstate 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.
The plane landed askew, causing the explosion and fire seen in this still from a video taken by local news station KTIV.
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 both airspeed and sink rate. On final approach, the aircraft's forward speed was 220 knots (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 (260 km/h) and 300 feet per minute (1.5 m/s). 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. Moments before landing, Fitch noticed the high sink rate and that the plane started to yaw and roll right again, and pushed the throttles to full power in an attempt to mitigate the high sink rate and level the plane. It was now 16:00. The CVR recorded these final moments.
Not enough time remained for the engines to respond to Fitch's inputs, nor was there 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 cornfield 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.
INJURIES AND DEATHS
Of the 296 people on board, 112 died. Most were killed by injuries sustained in the multiple impacts, but 35 people in the middle fuselage section directly above the fuel tanks died from smoke inhalation in the post-crash fire. Of those, 24 had no traumatic blunt-force injuries. The majority of the 184 survivors were seated behind first class and ahead of the wings. Many passengers were able to walk out through the ruptures to the structure.
- 35 died because of smoke inhalation (none were in first-class).
- 76 died for reasons other than smoke inhalation (17 in first class).
- One died 31 days after the crash.
- 47 were seriously injured (eight in first class).
- 125 had minor injuries (one in first class).
- 13 had no injuries (none in first class).
The passengers who died for reasons other than smoke inhalation were seated in rows 1–4, 24–25, and 28–38. Passengers who died because of smoke inhalation were seated in rows 14, 16, and 22–30. The person assigned to 20H moved to an unknown seat and died of smoke inhalation.
One crash survivor died one month after the accident; he was classified according to NTSB regulations as a survivor with serious injuries.
Fifty-two children, including four "lap children" without their own seats, were on board the flight because of the United Airlines "Children's Day" promotion. Eleven children, including one lap child, died. Many of the children were travelling alone.
Rescuers did not identify the debris that was the remains of the cockpit, with the four pilots alive inside, until 35 minutes after the crash. All four recovered from their injuries and eventually returned to flight duty.
INVESTIGATION (FAILED COMPONENTS)
The National Transportation Safety Board determines that the probable cause of this accident was the inadequate consideration given to human factors limitations in the inspection and quality control procedures used by United Airlines' engine overhaul facility which resulted in the failure to detect a fatigue crack originating from a previously undetected metallurgical defect located in a critical area of the stage 1 fan disk that was manufactured by General Electric Aircraft Engines. The subsequent catastrophic disintegration of the disk resulted in the liberation of debris in a pattern of distribution and with energy levels that exceeded the level of protection provided by design features of the hydraulic systems that operate the DC-10's flight controls.
