Flight 232 of United Airlines was the scheduled flight between Denver and Philadelphia. On July 19 in 1989 Douglas DC-10 suffered a failure of its second engine (in the tail) which further destroyed all three aircraft’s systems of hydraulics. Without any controls operating except power levers of two remaining engines, the aircraft broke up in the course of emergency landing at Sioux city, Iowa which killed 110 of 285 passengers and 1 of 11 crew staff. To analyze the causes and consequences of the crash creep analysis is the most relevant methodology. It will be made through discussion of technical and structural failures that led to disaster.
Creep model for analysis of aircraft crash
The first problems appeared when the aircraft took shallow right turning at 37. 000 feet, N-1 fan disk of the tail-mounted CF6-6 engine produced by General Electric broke into two parts. The fan’s cowling was destroyed and the pieces of engine appeared in aircraft’s tail in various places. Many pieces of the shrapnel severed the structure of all hydraulic systems which allowed fluid to pour out (Kilroy 2007).
Containment ring
Containment ring is 86 inches in diameter and its axial length is 16 inches. It surrounds the stages of first fan disc blades. It is designed for absorption of energy on that order that is associated with the release of a fan blade and possible damage. The containment ring of the second engine was divided at 1:45 and 7:30 positions. The ring’s upper-left part separated from the plane during the flight. And its other parts were found at the crash place and were then analyzed by specialists (Kilroy,
1989). Examination of 7:30 separation sector of the containment ring revealed great deformation and it was found out that the first fan disc burst into the ring in this sector. The analysis of the containment ring break at 1:45 position found out features peculiar to tensile overstress separations.
Restraint system
The crew found out that pressure gauges for each of hydraulic system registered zero and understood that the failure made control surfaces and restraint system immovable. Hydraulic systems were divided in a way that prevented any single event in certain system disabling other systems but no backup system was in place.
Environment
It was difficult to maintain the plane on the stable course after engines broke and it started to oscillate vertically in the phugoid cycle which is characteristic of environment transformations when hydraulic powers are lost. With each repeat of this cycle the plane lost 1500 feet in altitude. The crew found out the only way to control aircraft was by adjusting its throttles of two remaining engines; and running one engine faster than other to turn the aircraft (differential thrust); and decelerating or accelerating to make the plane turn.
By means of these techniques it was possible to deter a phugoid cycle and make some adjustments. The pilot tried to manually lower the landing gear with the hope that it would enforce hydraulic fluid back to allow restoring the movement of the control surfaces. But it hadn’t caused any improvement since all the fluid was lost (National Transportation Safety Board, 1989).
The observation of weather conditions made at Sioux Airport at 1559 evaluated the ceiling of 4000 feet with many broken clouds and the visibility of 15 miles. The temperature was about 80 F and winds approximately 360 and 14 knots. Also there were cumulus clouds in all the quadrants. The last wind which was reported to the crew at 1558 was from OIO’ (11 knots).
Energy Absorption
The cause of fracture of the fan disc according to creep investigation can be ascribed to so-called fatigue crack. The analysis showed the penetration of fluorescent dye which was used to find out cracks during maintenance which indicated that the crack was already present before the flight and should have been detected and investigated before it in order to prevent the disaster. Of course, this is partly due to the human factor which is always in place during the process of aircraft’s investigation.
It was found that the problems with elimination of gas anomalies occurred during the process of titanium disc ingot purifying. The nitrogen amount exceeded the norm which caused ‘hard alpha inclusion’ which resulted in crack in the course of forging and fell out after final machining that formed the cavity with microscopic breaks (Captain Al Haynes, 1989). During engines’ running phases these cracks grew bigger until they became so large to result in structural failure of the disc. In newer technologies much higher melting temperatures were used and the process of ‘triple vacuum’ which were aimed to eliminate these problems. This investigation caused decommissioning of already operating fan disc in order to prevent other catastrophes.
Post-crash factors
After the pilots were given the line to land they continued controlling system’s environment by making engine thrust. The crew was unable to make control of airspeed independent of sink rate. On the final stage of descent the speed of aircraft
was higher than needed for safe landing (240 knots and the sinking process of 1850 per minute instead of 140 knots and 300 feet. The right wing hit the runway and immediately ignited. The tail sector broke from the impact and the fuselage broke into several pieces.
On the final stage the right wing completely broke off and the main part of the plane sheared off and stopped on the right side of the runway.
Many factors following landing influenced catastrophic consequences – people’s death. Most people were killed due to the injuries inflicted by various physical impacts during landing, but many of the passengers occupying middle fuselage section which was situated directly above the fuel containers died from smoke poisoning in the post-crash fire conditions, which burned for much time during certain delays in firefighting efforts. 185 people from the second-class seating survived in this accident. Some people could get out of the debris but many had to wait until rescue teem released them.
Several post-crash factors influenced the rate of death:
- Actions of crew which were assessed by NTSB positively: “under the circumstances the UAL flight crew performance was highly commendable and greatly exceeded reasonable expectations’.
- The angle of descent was rather shallow which increased the number of survival (if the plane would have got into the spin the number of mortality would be considerably higher).
- The accident occurred during day hours.
- The accident occurred when the medical shift in trauma and combustion center were in place and thus more medical personnel were available to treat injured.
- The specialists from Air National Guard which were on duty at Sioux Airport assisted in triage and evacuation of injured.
Thus, the analysis shows that the crash of the aircraft was difficult to prevent since it was caused by technical problems which were present before the flight.
References
Captain Al Haynes (1989). Eyewitness Report: United Flight 232. 2007. Web.
Charles, M. T., Settle A. K. (1991) United Flight 232: Sioux City’s response to an air disaster. Organization & Environment, Vol. 5, No. 1, 77-90.
Kilroy, C. (1989). Special Report: United Airlines Flight 232. 2007. Web.
National Transportation Safety Board (1989). Aircraft Accident Report PBSO-910406 NTSB/AAR-SO/06. 2007. Web.