United Airlines Flight 232 was a scheduled flight operated by United Airlines between Denver and Philadelphia via Chicago. On 19 July 1989, the Douglas DC-10 (Registration ) suffered an uncontained failure of its number 2 engine which destroyed all three of the aircraft's hydraulic systems. With no controls working except the power levers for the two remaining engines, it broke up during an emergency landing on the runway at Sioux City, Iowa, killing 110 of its 285 passengers and one of the 11 crew members.
Owing to the skill of the crew and a DC-10 instructor pilot, 175 passengers and 10 crew members survived the crash. The disaster is famous within the aviation community as a textbook example of successful Crew Resource Management, due to the effective use of all the resources available aboard the plane for help during the emergency.
The flight took off at 14:09 (CDT) from Stapleton International Airport, Denver, Colorado, and was due to fly to Philadelphia International Airport in Philadelphia, Pennsylvania, via O'Hare International Airport in Chicago, Illinois. 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 broke in two. The fan disk and its blades were not contained by the ductwork, but instead punched through the ductwork and fan cowling. Pieces of the engine penetrated the aircraft tail section in numerous places, including both horizontal stabilizers. Pieces of shrapnel which went through the right horizontal stabilizer severed the lines of all three hydraulic systems, allowing the fluid to drain away.
Captain Alfred C. Haynes and his flight crew (First Officer William Records, who was flying, and Second Officer Dudley Dvorak, flight engineer) felt a jolt going through the aircraft, and warning lights showed that the autopilot had disengaged, and the tail-mounted number two engine was malfunctioning. The co-pilot noticed that the airliner was off course, and moved his control column to correct this, but the plane did not respond. The flight crew discovered that the pressure gauges for each of the three hydraulic systems were registering zero, and they realized that the initial failure had left all control surfaces immovable. The three hydraulic systems were separated such that a single event in one system would not disable the other systems, but lines for all three systems shared the same ten-inch wide route through the tail where the engine debris penetrated, and beyond that there was no backup system, a fact which the NTSB later recommended be remedied.
The plane had a continual tendency to turn right, and was difficult to maintain on a stable course. It began to slowly oscillate vertically in a phugoid cycle, which is characteristic of planes in which control surfaces command is lost. With each iteration of the cycle the aircraft lost approximately 1500 feet of altitude. Dennis E. Fitch, a DC-10 flight instructor, was deadheading as a passenger on the plane and offered his assistance. After entering the cockpit, Fitch discovered that the only available method of controlling the aircraft was through adjusting the throttles of the remaining two engines; running one engine faster than the other to turn the plane (differential thrust), and accelerating or decelerating in order to gain or lose altitude. Using this method, it was possible to mitigate the phugoid cycle and make rough steering adjustments. At one point Fitch manually lowered the landing gear in flight, hoping that this would force trapped hydraulic fluid back into the lines allowing some movement of control surfaces. Although the gear lowered successfully, there was no improvement in control response as all the fluid had been lost through the punctured lines.
Haynes kept his sense of humor during the emergency, as recorded on the plane's CVR:
A more serious remark often quoted from Haynes was made when ATC asked the crew to make a left turn to keep them clear of the city:
Landing was originally planned on the 9,000 foot (2743 m) Runway 31. The difficulties in controlling the aircraft made lining up almost impossible. While dumping 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 coming out at the end lined up with runway 31. When they came out they were instead left with an approach on the shorter Runway 22 of 6,600 feet (2012 m), with little capacity to maneuver.
Fire trucks had been placed on runway 22, anticipating a landing on runway 31, and there was a scramble as the trucks rushed out of the way. All the vehicles parked there got out of the way before the plane touched down.
Fitch continued to control the aircraft's descent by adjusting engine thrust. With the loss of all hydraulics, the crew were unable to control airspeed independent from sink rate. On final descent, the aircraft was going 240 knots and sinking at 1850 feet per minute, while a safe landing would require 140 knots and 300 feet per minute. The aircraft began to sink faster while on final approach and veer to the right. 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 sheared off and the main part of the aircraft skidded sideways, rolled over on to its back, and slid to a stop upside down on the right side of runway 22. Witnesses reported that the aircraft cartwheeled but the investigation did not confirm this. News reports that the aircraft cartwheeled were due to misinterpretation of the video of the crash that showed the flaming right wing tumbling end-over-end.
Of the 296 people on board, 111 died in the crash. Most were killed by injuries sustained in the multiple impacts, but many in the middle fuselage section directly above the fuel tanks died from smoke inhalation in the post-crash fire, which burned for longer than it might have due to delays in the firefighting efforts. The majority of the 185 survivors were seated behind first class and ahead of the wings (one of the crash survivors died a month later of his injuries). Many passengers were able to walk out through the ruptures to the structure, and in many cases got lost in the high field of corn adjacent to the runway until rescue workers arrived on the scene and escorted them to safety.
Of all of the passengers:
The passengers who died for reasons other than smoke inhalation were seated in rows 1-4, 24-25, and 28-38. Passengers who died due to smoke inhalation were seated in rows 14, 16, and 22-30. A person assigned to 20H moved to an unknown seat and died due to smoke inhalation. One person died 31 days after the incident; the NTSB classified his injuries as "serious."
Fifty-two children, including four lap children, were on board the flight due to the United Airlines "Children's Day" promotion. Eleven children, including one lap child, died. Many of the children had traveled alone.
Investigation attributed the cause of the fracture of the fan disk to a failure of United Airlines maintenance processes to detect an existing fatigue crack. Post-crash analysis of the crack surfaces showed the presence of the penetrating fluorescent dye used to detect cracks during maintenance, indicating that the crack was present and should have been detected at a prior inspection. The detection failure arose from poor attention to human factors in United Airlines' specification of maintenance processes.
The crack in the fan disk was traced back to the Alcoa foundry from which the engine part was sourced. It turned out that there was a defect in elimination of gaseous anomalies during the purifying of the titanium disk ingot. An excess amount of nitrogen was in the material, causing a 'hard alpha inclusion' which cracked during forging and then fell out during final machining, forming a cavity with microscopic cracks at the edges. During the engine's normal running cycle, one of these cracks grew slowly each time the engine was powered up and brought to operating temperature, until it grew large enough for the disk to fail structurally. Newer batches used much higher melting temperatures and a 'triple vacuum' process to eliminate these impurities. The subsequent investigation and Airworthiness Directive also revealed several other fan disks already in service from the same batch of ingots which had started to exhibit the initial cracking symptoms of part failure.
Some portions of the aircraft that had broken away when the fan disk failed were later found in farm fields along the flight path.
Because this type of aircraft control is difficult for humans to achieve, some researchers have attempted to integrate this control ability into the computers of fly-by-wire aircraft. Early attempts to add the ability to real airplanes were not very successful, the software having been based on experiments conducted in flight simulators where jet engines are usually modeled as "perfect" devices with exactly the same thrust on each engine, a linear relationship between throttle setting and thrust, and instantaneous response to input. Later, computer models were updated to account for these factors, and planes have been successfully flown with this software installed.
The odds against all three hydraulic systems failing simultaneously had previously been calculated as high as a billion to one. A similar failure had occurred four years previously when Japan Airlines flight 123 suffered a structural failure that left it without any hydraulic controls.
Newer aircraft designs such as the MD-11 have incorporated hydraulic fuses to isolate a punctured section and prevent a total loss of hydraulic fluid; this was also partially implemented on DC-10 models after the accident.
Of the four children deemed too young to have seats of their own ('lap children'), one died from smoke inhalation. Despite the survival rate for the lap children being a little higher than that of the passengers (75% versus 62.5%), the NTSB added a safety recommendation to the FAA on its "List of Most Wanted Safety Improvements," the response to which was flagged on NTSB's website as an "acceptable response, progressing slowly." It also sparked a campaign, led by United Flight 232's senior flight attendant, Jan Brown Lohr, for all children to have seats on aircraft.
"Had any of those things not been there," Haynes said later, "I'm sure the fatality rate would have been a lot higher."
As with the Eastern Air Lines Flight 401 crash of a similarly-sized Lockheed L-1011 in 1972, the relatively shallow angle of descent likely played a large part in the relatively high survival rate. The National Transportation Safety Board concluded that under the circumstances, "a safe landing was virtually impossible."
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