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30 Years Later: Remembering United Flight 232
On July 19, 1989, the crew of United Airlines flight 232 from Denver to Chicago was faced with the impossible task of flying a McDonnell Douglas DC-10 with 296 people on board after losing all three hydraulic systems. With the event occurring 30 years ago on Friday, it remains as one of the greatest aviation miracles in history.
United 232 was scheduled to depart the now-closed Stapleton Airport in Denver for Chicago’s O’Hare International Airport, with continuing service to Philadelphia International Airport. The aircraft operating as United 232 that day, a McDonnell Douglas DC-10-10 registered N1819U, was delivered to United in 1973, 16 years prior. The doomed tri-jet had 43,401 total airframe hours and 16,997 cycles powered by three General Electric CF6-6D engines.
The McDonnell Douglas DC-10 entered service in 1970 with a rocky start. Its early days were plagued with bad press and a poor safety record. Several incidents also occurred involving the DC-10’s outward-opening cargo door and in 1974, the Federal Aviation Administration (FAA) issued an airworthiness directive and all DC-10’s underwent mandatory cargo door modifications. In 1979, American Airlines flight 191, a DC-10-10, crashed shortly after takeoff from O’Hare Airport when the number one engine and pylon assembly separated from the wing, killing all on board.
Flight 232 was under the command of Captain Al Haynes, an experienced pilot with 29, 967 hours of flight time, 7,190 of which were in the DC-10. In the right seat was First Officer William Records with approximately 20,000 hours of flight time, 665 of which were as a DC-10 First Officer. Captain Haynes and First Officer Records were joined by Second Officer Dudley Dvorak, who had approximately 15,000 hours of flight time with 33 hours as a Second Officer in the DC-10. Although the crew did not yet know it, behind them in the cabin sat Training Check Airman Captain Dennis Fitch with 3,079 hours on the aircraft.
En route to the Windy City, the flight departed Denver normally at 2:09 p.m. and climbed to its cruising altitude of 37,000 feet. One hour and seven minutes later at 3:16 p.m., the aircraft entered a shallow right turn and suddenly, a loud bang was heard. The DC-10’s airframe shuddered and vibrated as the crew began to investigate the cause. Checking their engine instruments, the flight crew quickly realized the number two tail-mounted aft engine had failed.
Captain Haynes called for the engine shutdown checklist. While performing the checklist, Second Officer Dvorak noticed that the aircraft’s hydraulic pressure and quantity gauges read zero and First Officer Records quickly realized he could not control the damaged DC-10 as it entered a right descending turn.
The aircraft did not respond to Captain Haynes’ control inputs either. With no flight controls to bring the DC-10 out of the increasing bank, the crew had to find a way to stop the bank or the DC-10 would roll over and become unrecoverable. Needing a unique solution, the crew closed the throttle on the number one engine and firewalled the number three engine. The differential thrust caused the DC-10 to yaw left and the wings slowly leveled.
Shortly after the number two engine failed, one of the flight attendants advised the crew that a United Air Lines DC-10 training check airman was on board and volunteered his assistance. Captain Haynes seized the opportunity for more help on the flight deck and Captain Dennis Fitch arrived upfront at approximately 3:29 p.m. When Captain Fitch arrived on the flight deck, Captain Haynes requested that Captain Fitch perform a visual inspection of the DC-10’s wings. Captain Fitch performed the visual inspection and discovered the inboard ailerons on both wings were slightly up and the spoilers were locked down.
The crew contacted the Minneapolis Air Route Traffic Control Center who had been providing air traffic control (ATC) services through the region to request emergency assistance and vectors to the nearest airport. ATC initially suggested Des Moines, Iowa; however, once the controller realized flight 232 was headed toward Sioux City, Iowa, the controller asked the flight crew if they would prefer to go there. The crew agreed.
Down an engine with no way to control the wounded DC-10 other than asymmetric thrust, the crew had no choice but to attempt to use the throttles to steer the aircraft toward Sioux City. Making things worse, the DC-10 had entered what is known as a phugoid cycle – the aircraft would descend and rapidly gain speed. This increased speed also increased lift, causing the aircraft to climb. As it climbed, the DC-10 lost speed, lift decreased, and the cycle would repeat itself. After each cycle, the crew found themselves at a lower altitude.
Using asymmetric thrust, the crew was able to put themselves on a heading to line up with Sioux City’s Runway 22. The original plan was to have Flight 232 land on Sioux City’s Runway 31, approximately 9000 feet long. With no flight controls, however, it was a miracle flight 232 was aligned with any runway. Despite Runway 22 having been closed about a year earlier, it was decided to continue to try for a landing on that runway.
The landing would undoubtedly be the most difficult any of the crew had ever faced. With all three hydraulic systems gone, they would have no flaps for slow flight and no brakes to slow down once on the ground. The landing gear would have to be manually lowered using a handle in the cockpit floor. Captain Haynes announced to the passengers that the crew would attempt to land at Sioux City and that he would give the “brace” signal before landing. He warned them this would be the roughest landing they had ever experienced.
When the crew finally had Sioux City’s Runway 22 in sight, Captain Haynes reported it to the Sioux City Approach controller. Just two minutes out from landing, Captain Haynes still managed to keep his sense of humor. When the controller cleared flight 232 to land on any runway, Captain Haynes replied with a laugh and said: “You want to be particular and make it a runway, huh?”
At 4:00 p.m., the crippled DC-10, traveling at approximately 215 knots, approached Sioux City’s runway 22. The DC-10’s normal landing speed is approximately 140 knots. As the DC-10 crossed the threshold, the right wing suddenly dipped and struck the runway surface, causing it to catch fire and cartwheel down the runway. In the end, 111 of the 296 people on board the aircraft lost their lives.
The accident investigation revealed that a fatigue crack had formed in the number two engine’s stage 1 fan disk, which ultimately caused the fan disk to suffer a catastrophic disintegration during the flight. The debris from the fan disk was hurled from the engine and caused extensive damage to the rear of the airplane, damaging all three hydraulic systems.
The fatigue crack originated from a metallurgical defect that formed during manufacturing of the fan disk. The National Transportation Safety Board (NTSB) determined that the probable cause of the accident was “the inadequate consideration of the human factors limitations in the inspection and quality control procedures used by the [United] engine overhaul facility. These limitations 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 of the engine.”
The accident had a lasting impact on aviation, showing the high value of crew resource management and exemplifying the teamwork of flight 232’s crew as a model for the industry. An airworthiness directive was issued to perform ultrasonic testing on the stage 1 fan disk of CF6-6 engines. In the fall of 1989, the FAA formed the Titanium Rotating Components Review Team, which was tasked with reviewing the design, manufacturing, inspection and life management procedures of titanium rotating components to make recommendations for improvement of their structural integrity.
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