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Home > Free Essays & Book Reports > Aviation > Plane Crash

Plane Crash

Instructor: Greg Alston Abstract This paper examines the in-flight separation of the number two pylon and engine from a Boeing 747-121 shortly after takeoff from the Anchorage International Airport on March 31, 1993. The safety issues discussed focus on the inspection of Boeing 747 engine pylons, meteorological hazards to aircraft, the lateral load-carrying capability of engine pylon structures, and aircraft departure routes at Anchorage International Airport during turbulent weather conditions. Shortly after noon on March 31, 1993 the number two engine and pylon separated from Japan Airlines Inc. flight 46E shortly after departure from the Anchorage International Airport. The aircraft, a Boeing 747-121, had been leased from Evergreen International Airlines Inc. The flight was a scheduled cargo flight from Anchorage to Chicago-O'Hare International Airport. On board the airplane was the flight crew and two nonrevenue company employees. The airplane was substantialy damaged during the separation of the engine but no one on board the airplane or on the ground was injured. Flight 46E departed Anchorage about 1224 local time. The flight release and weather package provided to the pilots by Evergreen operations contained a forecast for severe turbulence. As fight 46E taxied onto the runway to await its takeoff clearance, the local controller informed the flight crew that the pilot of another Evergreen aircraft reported severe turbulence at 2,500 feet while climbing out from runway 6R. After takeoff, at an altitude of about 2,000 feet, the airplane experienced an uncommanded left bank of approximately fifty degrees. Although the desired air speed was 183 knots, the air speed fluctuated from a high of 245 knots to a low of 170 knots. Shortly thereafter the flight crew reported the number two throttle slammed to its aft stop, the number two thrust reverse indication showed thrust reverser deployment, and the number two engine electrical bus failed. Several witnesses on the ground reported that the airplane experienced several severe pitch and roll oscillations before the engine separated. Shortly after the engine separated from the airplane, the flight crew declared an emergency, and the captain initiated a large radius turn to the left to return and land on runway 6R. The number one engine was maintained at maximum power. While on the downwind portion of the landing pattern bank angles momentarily exceeded forty degrees alternating with wings level. About twenty minutes after takeoff flight 46E advised the tower they were on the runway. The aircraft was substantially damaged as a result of the separation of the number two engine. Estimated repair costs exceeded twelve million dollars. In addition, several private dwellings, automobiles, and landscaping were damaged by the impact of the number two engine and various parts of the engine pylon and the wing leading edge devices. The National Transportation Safety Board (NTSB) determined the probable cause of this accident was the lateral separation of the number two engine pylon due to an encounter with severe or possibly extreme turbulence. This resulted in dynamic lateral loadings coming from many directions that exceeded the lateral load-carrying capability of the pylon. It was later discovered that the load-carrying capability of the pylon was already reduced by the presence of the fatigue crack near the forward end of the pylon's forward firewall web. As a result of this investigation the NTSB made seven recommendations to the Federal Aviation Administration (FAA), including the inspection of Boeing 747 engine pylons, the potential meteorological hazards to aircraft, an increase in the lateral load capability of engine pylon structures, and the modification of the aircraft departure routes at Anchorage International Airport during periods of moderate or severe turbulence. The NTSB also recommended that the National Weather Service (NWS) use the WSR-88D Doppler weather radar system to document mountain-generated wind fields in the Anchorage area and to develop detailed low altitude turbulence forecasts. In the course of the investigation the NTSB explored virtually every contributing factor contributing to the aircraft accident. These included weather, mechanical failure, design deficiencies, and human factors. The flight crew was properly trained and qualified for this fight. None of the crew members' Federal Aviation Administration (FAA) records contained any history of accidents, incidents, or violations. The flight crew and the mechanics who had worked on the airplane before the flight volunteered to be tested for the presence of alcohol and both lawful and illegal drugs. All of the test results were negative. The investigation revealed that the flight crew was in good health. The airplane, registration N473EV, was a Boeing model 747-121, serial number 19657. The airplane was manufactured in June 1970, and was originally configured to carry passengers. The airplane was acquired by Evergreen International Airlines in December 1988, and was subsequently reconfigured to carry cargo. The airplane had seating for the three flight crew members and two observers or passengers. The airplane was equipped with four Pratt & Whitney JT9D-7 engines and appropriate equipment for Instrument Flight Rules (IFR) operations. At the time of the accident, the airplane had accumulated 83,906 flight hours and 18,387 cycles. The estimated economic design life for the Boeing 747 is 20,000 flights, 60,000 hours, and 20 years. The number two engine, serial number 662812, had accumulated a total of 56,709.8 hours and 10,923 cycles since new and had accumulated 5,752.5 hours and 1,200 cycles since overhaul two years prior. The maintenance logs had no reports of severe e! ngine vibration on the number two engine. The maintenance records contained no deferred repair items regarding the number two engine pylon structure. The airplane was equipped with a Sundstrand Data Control Mark VI-J4 ground proximity warning system (GPWS). In addition to providing GPWS alerts, this system provides windshear caution, windshear warning, and bank angle warning. The system provides windshear warning and cautions between five feet and 1,500 feet during the initial takeoff and between 1,500 feet and thirty feet during the final approach phases of flight. The bank angle advisory indicates a roll attitude that is excessive for the flight condition. Generally, above 1,500 feet, the callout occurs at forty degrees of bank. The callout occurs again if roll attitude increases by twenty percent. When roll attitude increases to forty percent above the initial callout angle, the callout repeats continuously. Below 1,500 feet, the callout angle is reduced progressively. The windshear caution or windshear warning did not activate because the turbulence was encountered above 1,500 feet, well outside the warning envelope of the system. However, the system did provide bank angle warnings during the turbulence encounter. A significant meteorological advisory (SIGMET) was issued at 1145 and was valid until 1545. This SIGMET advised that moderate and frequent severe turbulence could be encountered from the surface to 12,000 feet. In addition, moderate and frequent severe mountain wave turbulence could be encountered from 12,000 feet to 39,000 feet within an area bounded by Bethel, Johnstone Point, Sitkinak Island, and Dillingham. The northern extent of the SIGMET area was about thirty-six nautical miles south of Anchorage. A correction to the SIGMET was made at 1342 adding the Anchorage area to the list of locations within the advisory area. According to an individual of the NWS forecast office at Anchorage, the delay in issuing the correction (about 2 hours) was due to the workload. The delay caused the omission of Anchorage from the SIGMET location points to go unnoticed. The aviation weather forecaster also stated that turbulence east of the airport was not an infrequent event in the presence of a strong easterly flow near mountain top level. He believed that in addition to the strong easterly flow the turbulence was increased by an upper level trough moving through the area, which, coupled with heating, made the atmosphere unstable. He also stated that in the eighteen years as a forecaster at Anchorage he did not remember previously seeing as many severe turbulence pilot reports as he saw that afternoon. Several other pilots reported severe turbulence encounters about the time of the accident. At 1210, a pilot of another Boeing 747 reported severe turbulence at 2,500 feet and moderate turbulence between 3,000 feet and 10,000 feet during the climbout to the north. The pilot of a U.S. Marshall Service Cessna 310 reported that he took off from runway 15 at Merrill Field to perform a maintenance fight about noon. At 300 feet above the ground, the airplane encountered a downdraft and the airplane's air speed went from 120 knots to 90 knots and lost about 200 feet of altitude. After he emerged from the downdraft, the pilot turned the airplane to a heading of 120 and climbed to 900 feet. Shortly thereafter, the airplane encountered an updraft. The vertical velocity indicator pegged the needle at 4,000 feet per minute upward and that despite reducing the throttles to idle the airspeed would not fall below 160 knots. The pilot stated that as he maneuvered the airplane back to the airport for landing, the airplane encountered severe turbulence with fifty-knot variations in air speed. The pilot concluded in his written report that, in twenty years of flying, this was the worst turbulence he has encountered. The NTSB also inspected the navigation aids and communications within in Anchorage area. No difficulties or problems were found. Damage to the airplane consisted of the loss of the number two engine and its pylon and the loss of most of the left wing leading edge devices between engines number one and two. During the investigation, the fuse pins holding the engine pylons to the wings were removed from the airplane. The two midspar fuse pins for the number two engine were found to be deformed. The aft diagonal brace fuse pin was fractured. The inboard midspar fuse pin for the number one engine was found to be substantially deformed. None of the other fuse pins on the airplane had any indications of damage or deformation. Relatively small areas of impact damage were also noted on the wings and trailing edge flaps. The number two engine, all portions of the number two engine pylon, and most of the leading edge structure between the number one and number two engines were recovered. There was no evidence of an in-flight fire prior to the separation of the number two engine. Several witnesses on the ground reported seeing a flash or ball of fire as the engine separated from the airplane. There were no reported fires on the ground as a result of falling debris. Persons who first saw the engine after it struck the ground reported steam rising from the engine. Firefighters from the Anchorage Fire Department sprayed water on the engine to prevent a possible fire. The pylon is designed to carry the thrust and torque loads of the engine as well as lateral, longitudinal, and vertical loads from maneuvers and gusts. Lateral loads are ultimately absorbed by the midspar fuse pins and side brace. According to Boeing, the fuse pins can withstand an ultimate lateral load of more than 2.8 G on the engine. Additionally, Boeing reported that the portion of the structure of the pylon that is critical under lateral loads is the firewall just aft of the forward engine mount. The Boeing calculations indicated that this firewall will fracture at a lateral load of between 2.35 G and 2.88 G when it contains a fatigue crack of the size found in this structure. The Boeing 747 airplane and its pylon structure were designed in the mid-1960's using the computer capabilities and analytical skills of the time. Boeing's current computer modeling of the pylon structure and the loads applied to it are considerably more complicated and provide greater resolution of the data than would have been possible with the techniques employed when the airplane was designed. The use of modern computer structural design programs allowed considerable modeling of the pylon's response to various load inputs with various structural failures. The number two engine pylon was separated into four pieces as a result of three principal fracture areas. These fractures were located just aft of the forward engine mount bulkhead, among a jagged vertical plane aft of the rear engine mount bulkhead, and around the inboard midspar fuse pin fitting. The two forward pieces of the pylon remained attached to the engine through the forward and rear engine mounts. Examination of the fractures around the perimeter of the break aft of the forward engine mount bulkhead revealed features typical of overstress separations, except for a small flat fracture region in the firewall web. The flat fracture area was approximately in the middle of the web on the outboard side of the web centerline. The fracture was a lateral fracture about two inches long through the thickness of the web and was aft of the third transverse stiffener behind the forward engine mount bulkhead. Investigators cut the flat fracture area from the remainder o! f the firewall and examined it in detail with a bench binocular microscope and a scanning electron microscope. The mating fracture faces had been heavily rubbed. Despite the rubbing, isolated areas of contrasting color, indicative of through-the-thickness propagation, was noted. Compression buckling of the firewall web extended from the fatigue crack area forward to the outboard side of the pylon at the second transverse stiffener. Inspection of the other three pylons on the airplane found no similar cracks. The fuse pin from the underwing fitting for the diagonal brace was the only one that was found broken. The outboard portion of the pin was cocked within the underwing fitting. The inboard piece of this fuse pin was recovered on the ground near the aft portion of the pylon. The fractures on the fuse pin and retainer bolt appeared typical of overstress separations. The investigation found that all of the remaining fractures and buckling of the structure were consistent with deformation of the pylon structure in an outboard and upward direction. Examination of the other fracture surfaces disclosed no evidence of pre-accident damage or cracking. All separations appeared typical of overstress separations. Selected sections from the primary structures of the pylon were returned to the safety board's materials laboratory for examination. The material from the sections was found to be within applicable manufacturer's specification requirements for composition, conductivity, and hardiness. The two fatigue cracks that were found in the number two engine pylon structure were subjected to metallurgical examinations. One of the fatigue cracks was a lateral fracture about two inches long and was in the web of the pylon forward firewall, just aft of the third transverse stiffener behind the forward engine mount bulkhead. This fatigue crack was lateral to the web. Although most of the features of this crack had been obliterated by rubbing, a few isolated areas of fatigue striations were found. The orientation of the grain indicated that the cracking propagated through the thickness of the web. The web material, a nickel alloy, appeared to comply with specification requirements. There was no evidence of damage or defects that may have contributed to initiation of the fatigue cracking. The pieces of the midspar web near the aft end of the web had been deformed into a wave shape, consistent with compression buckling. A fatigue crack was found in this portion of the web, on the only piece of the pylon structure that remained attached to the wing. Almost the entire length of this crack was sandwiched between portions of the inboard midspar fitting and other pieces of structure at the aft end of the midspar. The plane of cracking was oriented forty-five degrees to the fore-and-aft direction. This is consistent with propagation under tensile stresses from shear loading of the web. The cracking initiated from both sides of a fastener hole. Additional disassembly of the inboard midspar fitting and complete removal of the web piece showed extensions of the fatigue cracking. The overall length of the fatigue cracking area including the extensions, was about three inches. There was no evidence of any damage or defects that may have contributed to initiation of! the fatigue cracking. Metallurgical examination of the fracture in the fuse pin from the aft end of the diagonal brace revealed evidence of a direct shear overstress separation. The retention bolt for this pin was fractured as a result of excessive bending and shear loads. The maintenance records were examined at Evergreen's corporate headquarters in McMinnville, Oregon. This examination included a review of flight log entries, nonroutine work order cards, work order cards generated by all levels of routine checks and inspections, engineering orders, engineering changes and repair authorizations, mechanical reliability report files, airworthiness directive (AD) tracking sheets, major alteration record lists, engine logs, engine status reports, and engine trend monitoring sheets. The records did not reveal any previous encounters with severe turbulence. The three major alterations and repairs involving the wing were either far outboard of the number one pylon or were performed on the right wing. Two overweight landings had been recorded since the aircraft was put into service with Evergreen. In both cases, an inspection of the airplane was accomplished in accordance with the Boeing Maintenance Manual. A D maintenance check was started in April 1992 and completed in September 1992. During the check, a structural inspection was performed on the number two engine pylon. The inspection procedures called for the notation of any structural irregularities, corrosion, loose or missing fasteners, cracks, bulges, deformities, and delaminations. This check specifically called for inspection of the torque bulkhead, particularly in the area of the midspar fittings and diagonal brace fittings. During the D maintenance check, two cracks were found in the skin on the bottom of number two pylon, just aft of the aft engine mount thrust link. The cracks were stop drilled, and two doublers were fabricated and installed. A third crack was found on the diagonal brace upper end outboard clevis lug bushing. The diagonal brace and lug were subsequently replaced. A fourth crack was found six inches from the aft end of the outboard bottom edge of the number two pylon internal lower angle. A new internal lower angle was fabricated and installed. During a B maintenance check performed in November 1990, the entire number two engine pylon was removed from the wing. During the time in which the pylon was removed, extensive inspection and repair work was accomplished on the pylon and its fittings. These maintenance actions included the inspection and rework of the upper link forward lug, the diagonal brace lug, and the midspar attach fitting horizontal clevis, replacement of the upper link fuse pins, inspection of the forward engine mount bulkhead structure, replacement of tile forward support fitting bolts, rework of the rear engine mount bulkhead fitting, and rework of the midspar outboard attach fitting and the inboard pylon attach fitting. The forward engine mount bulkhead had been modified in order to prevent cracking in the firewall web near the bulkhead. At the time of the accident, the Boeing 747 Maintenance Manual did not address inspection of the pylon forward firewall web where the fatigue crack was found on the accident airplane. Boeing had previously issued a service bulletin on February 14, 1986, for operators to inspect for fatigue cracking of an adjacent lower spar web. The service bulletin reported an operator experiencing two cracks approximately six inches long in the aft lower spar web of the number one pylon after 8,500 flight-hours. Following the accident Boeing issued a service bulletin that called for a detailed visual inspection of the horizontal firewall of the inboard engine pylons on Boeing 747 airplanes powered by Pratt and Whitney JT9D-3A or -7 series engines. The service bulletin states that airplanes with over 15,001 flight cycles should be inspected within six months of the release of the service bulletin. Airplanes with between 6,001 and 15,000 flight cycles should be inspected within twelve months, and airplanes with less than 6,000 flight cycles should be inspected at 6,000 flight cycles or within twelve months, whichever is later. There have been no operator reports of finding cracks in the forward web as a result of the inspections from this service bulletin. Additionally, following the accident Boeing requested selected operators of high time Boeing 747s to inspect their airplanes for cracks in the forward web. Boeing reports that the operators found no evidencence of cracking. The investigation found that there were multiple separations in the number two engine pylon that allowed the engine to separate from the wing. There was evidence that the direction of separation was outboard and up. This evidence included the lack of damage on the inboard side of the pylon, the fractures and deformation in the major structural members of the pylon, and a piece of the wing leading edge structure that was embedded in the rear of the engine. The examination of the pylon structure also yielded sufficient clues to determine the sequence of pylon fractures that resulted in the loss of the engine. The rear engine mount fitting in the pylon was intact and, when recovered, a major piece of the pylon was still attached to the engine. However, the fitting was cracked and heavily distorted in relation to the pylon structure around it. This cracking and distortion indicated motion of the forward end of the engine in the outboard and up directions. This damage indicates that the pylon srtucture was intact when the damage occurred. If the pylon had been separated at any location aft of the rear mount fitting, the fitting would not have been distorted as it was because the pylon structure would have moved with the fitting as engine motion attempted to generate the cracking and distortion. The condition of the rear engine mount suggests that the forward end of the engine separated from the main portion of the pylon and moved in the outboard direction while the remainder of the pylon was intact and attached to the wing. The examination of the front of the pylon revealed that the pylon structure was fractured just aft of the forward engine mount bulkhead, and that a small piece of the forward part of the pylon was attached to the engine at the forward engine mount. The fracture on this part of the pylon contained indications of overstress separations except for the two inch fatigue crack in the forward firewall. The firewall contained compression buckling that extended to the area of the fracture. Overstress separations from shear loading were found on both sides of the fatigue area. These overstress separation areas probably occurred immediately after the compression buckling and was the start of the complete fracture of the pylon aft of the forward engine m! ount bulkhead. The front end of the engine was now free to swing to the left under the same lateral loads that produced the initial separation of the pylon. The movement of the front of the engine to the left created the heavy distortion and cracking in the rear mount fitting. As the front end of the engine swung to the left, the pylon structure would have bent in the outboard direction. At the same time, the engine would have been producing thrust at an unusual angle The combination of the bending of the pylon and the unusual thrust angle would account for the damage found on the midspar fuse pins, the large vertical fracture in the middle of the pylon, the shear buckling of the mid spar web, and the direction of fracture of the major structural members of the pylon. Boeing performed a finite element analysis of the forward portion of the pylon structure. This analysis showed that the fatigue crack in the firewall would reduce the stress capacity of the pylon by about ten percent. The computer generated model predicted that in the presence of the cracked web, the number two engine pylon would fail with a lateral load of between 2.35 G and 2.88 G. The separation of the number two engine pylon was due to an encounter with severe or possibly extreme turbulence that resulted in dynamic multi-axis lateral loadings that exceeded the ultimate lateral-load carrying capability of the pylon. The load carrying ability of the pylon was already reduced by the presence of the fatigue crack near the forward end of the pylon. The computer analysis found that encounters with severe turbulence can produce enough lateral loads to separate the pylon from the wing even without the presence of any cracks in the pylon web. Encounters with moderate and seve! re turbulence are considered relatively normal events by pilots and controllers, and operations are not curtailed by the forecast or pilot reports of severe turbulence. Therefore, there is a safety-of-flight concern regarding the lateral design loads for engine pylons during severe turbulent conditions. However, diminishing this concern is the fact that Boeing 747 airplanes, as well as many other makes and models of airplanes, have been operating successfully for many years without engines or pylons separating from the wings solely because of turbulence. ln general, it would appear that airline operating procedures and pilots' actions have been effective in avoiding operations into extreme or very severe turbulence that could damage their airplanes. This is why no structural modifications were required as a result of this accident. However, the NTSB recommended that the FAA should modify the design load requirements of 14 CFR Part 25 to consider multiple axis loading and ! to consider the magnitude of the loads that can be experienced in turbulence conditions. The fatigue cracking found on the midspar web probably resulted from sheet bending due to flexing or vibration of the web material. The crack probably would have been detected if there had been a requirement to inspect this area. Therefore the FAA should require all operators to inspect the entire pylon forward firewall web at specific flight hour intervals. It is not reasonable to suspend operations during turbulence because aircraft have been able to operate safely during such conditions. The most intense turbulence occurs near the mountains at low altitude. Therefore, by staying away from the mountains on departure, aircraft may lessen the chance of encountering severe turbulence. The FAA should consider modifying the departure routes of aircraft at Anchorage during periods of moderate or severe turbulence in order to minimize an aircraft's encounter with mountain-induced low level turbulence. The NTSB conducted a very through investigation of this accident They included areas that an average person with any knowledge of aviation would never have thought. Their final recommendations seem to be logical and have merit. Common sense prevailed and led to sound recommendations. Referrences Vogt, C. W., Coughlin, S., Lauber, J. K., Hart, C. A., & Hammerschmidt, J. (1993). Aircraft accident report, in-flight engine separation, Japan Airlines Inc., flight 46E (National Transportation Safety Board Rep. No. AAR-93/06). Oster, C. V. (1992). Aviation safety in a changing world New York: Oxford University Press.

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