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INVESTIGATIVE REPORT ON UNITED AIRLINES FLIGHT 232 CRASH, SIOUX CITY; IOWA; USA JULY 19TH 1989; NTSB # AAR – 90-06

April 20, 2012

SUMMARY:

On July 19, 1989, at 1516, a DC-10-10, reg. # N1819U, operated by United Airlines as flight 232 from Stapleton International Airport, in Denver, Colorado, to O’Hare International Airport in Chicago, Illinois., experienced a catastrophic failure of the No. 2 tail-mounted engine during cruise flight. The separation, fragmentation and forceful discharge of stage 1 fan rotor assembly parts from the No. 2 engine led to the loss of the three hydraulic systems that powered the airplane’s flight controls. The flight crew experienced severe difficulties controlling the airplane and with no controls working except the thrust levers for the two remaining engines, the aircraft broke up during an attempted emergency landing at Sioux Gateway Airport, Iowa. There were 285 passengers and 11 crewmembers onboard. One flight attendant and 110 passengers were fatally injured, but 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.

INDEX:

1. Chronology of the flight
2. Causes of the crash
3. Conclusions and Recommendations
4. Illustrations

SOURCES:

NTSB.gov
airsafe.com
airdisaster.com
wikipedia.com

1. Chronology of the flight:
The flight took off at 14:09 (CDT) from Stapleton International Airport, Denver, Colorado, bound for O’Hare International Airport in Chicago, Illinois with ongoing service to Philadelphia International Airport in Philadelphia, Pennsylvania. 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 failed and disintegrated the debris from which was not contained by the engine’s nacelle. Pieces of the structure 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.
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.
It turned out that one of the passengers on board flight 232 was Dennis Fitch, a United trainee and check pilot with over 3,000 hours on the DC-10. Haynes asked Fitch to go back and look out the windows to check for any structural damage. When Fitch returned to the flight deck, he informed Haynes that the both of the inboard ailerons were sticking up, but none of the controls appeared to be damaged or moving.
Haynes asked Fitch to take control of the throttle levers to allow the crew time to sort out the other decisions they were facing. Fitch knelt down in front of the controls and began to work with the throttles to maintain control of the aircraft. During this time, the aircraft had completed two slow right turns while descending.
The crew gave a call to Minneapolis Air Traffic Control Center relaying their problem. The controller initially gave 232 a vector towards Des Moines International, but seeing that the aircraft had continued its turn back towards the west before straightening out, he then gave it a vector towards Sioux City Gateway Airport. Calling Sioux City approach, Haynes requested the ILS (Instrument Landing System) frequency for runway 31 (9,000ft).
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
Fitch continued to control the aircraft’s descent by adjusting engine thrust. With the loss of all hydraulics, the crew was 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.

2. Causes of the crash:
The National Transportation Safety Board investigation reported that  the metallurgical examination showed that the primary fracture had resulted from a fatigued section on the inside diameter of the disk. Further examination showed that the fatiguing had resulted in a small cavity on the surface of the disk. The 17 year old disk had undergone routine maintenance and six times had been subjected to fluorescent penetration inspections. Investigators of the NTSB also stated 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 this fatigue crack originating from a previously undetected metallurgical manufacturing 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. This failure led to a loss of all three hydraulic systems. The flight control systems were powered by the hydraulic system, and as a result were inoperative for the rest of the flight.
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.

3. Conclusions and Recommendations
Subsequent simulator tests showed that other DC-10 crews were unable to repeat the effort of the crew of 232. Investigators concluded that, in its damaged condition, it was not possible to land the aircraft on a runway. While some level of control was possible, no precision could be achieved and a landing under these conditions was stated to be “a highly random event”. As a result, the crew was given much praise for managing to put the aircraft down just off the runway centerline and saving as many lives as they did.
As of the human error in inspecting the crack: the NTSB report states, that analytical procedures performed on the fracture face of the segment of the rotor disk and water washings from this surface showed the presence of di and triphenyl phosphates, compounds present in FPI fluid similar to that used to inspect the disk prior to the failure. This unique combination of 87 chemicals shows that the crack existed at the time of this inspection and that the crack was sufficiently open so that the FPI fluid entered the crack.

Based on this finding and the conclusion from metallurgical analysis that the crack was approximately 0.5 inch long on the surface of the bore of the rotor disk at the time of last inspection, the Safety Board concludes that the crack was detectable at the time of last inspection with FPI fluid.

However, the crack was not detected and consequently the rotor disk was considered to be free of flaws and was accepted as a serviceable part. A review of the inspection process suggests several explanations for the inspector’s failure to detect the crack. It is possible that the inspector did not adequately prepare the part for inspection or that he did not rotate the disk, as it was suspended by a cable, to enable both proper preparation and subsequent viewing of all portions of the disk bore, particularly the area hidden by the suspension cable/hose. It is also possible that loose developer powder, which could have dropped from the suspension cable, obscured the crack sufficiently to prevent its recognition as a flaw. Finally, inspection experience indicates that certain areas of CF-6 disks, because of their geometry, frequently show large FPI indications and that other areas rarely do so.

The NTSB raised these safety issues in this report # AAR-90-06:
1. General Electric Aircraft Engines’ (GEAE) CF6-6 fan rotor assembly design, certification,      manufacturing, and inspection.
2. United Airlines’ maintenance and inspection of CF6-6 engine fan rotor assemblies.
3. DC-10 hydraulic flight control system design, certification and protection from uncontained engine debris.
4. Cabin safety, including infant restraint systems, and airport rescue and firefighting facilities.
(Of the four children deemed too young to have seats of their own (‘lap children’), one died from
smoke inhalations.)

CF6-6 engine failed and disintegrated the debris from which was not contained by the engine’s nacelle. Pieces of the structure 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.
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.
It turned out that one of the passengers on board flight 232 was Dennis Fitch, a United trainee and check pilot with over 3,000 hours on the DC-10. Haynes asked Fitch to go back and look out the windows to check for any structural damage. When Fitch returned to the flight deck, he informed Haynes that the both of the inboard ailerons were sticking up, but none of the controls appeared to be damaged or moving.
Haynes asked Fitch to take control of the throttle levers to allow the crew time to sort out the other decisions they were facing. Fitch knelt down in front of the controls and began to work with the throttles to maintain control of the aircraft. During this time, the aircraft had completed two slow right turns while descending.
The crew gave a call to Minneapolis Air Traffic Control Center relaying their problem. The controller initially gave 232 a vector towards Des Moines International, but seeing that the aircraft had continued its turn back towards the west before straightening out, he then gave it a vector towards Sioux City Gateway Airport. Calling Sioux City approach, Haynes requested the ILS (Instrument Landing System) frequency for runway 31 (9,000ft).
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
Fitch continued to control the aircraft’s descent by adjusting engine thrust. With the loss of all hydraulics, the crew was 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.


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