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 …